Basic concepts of network technologies. Network technologies of local networks Modern network technologies

Network technologies of local networks

In local networks, as a rule, a shared data transmission medium (mono-channel) is used and the main role is played by protocols of the physical and data link layers, since these levels best reflect the specifics of local networks.

Network technology is an agreed upon set of standard protocols and software and hardware that implement them, sufficient to build a computer network. Network technologies are called core technologies or network architectures.

Network architecture determines the topology and method of access to the data transmission medium, the cable system or data transmission medium, the format of network frames, the type of signal encoding, and the transmission speed. In modern computer networks, such technologies or network architectures as: Ethernet, Token-Ring, ArcNet, FDDI have become widespread.

Network technologies IEEE802.3/Ethernet

Currently, this architecture is the most popular in the world. Popularity is ensured by simple, reliable and inexpensive technologies. A classic Ethernet network uses two types of standard coaxial cable (thick and thin).

However, the version of Ethernet that uses twisted pairs as a transmission medium has become increasingly widespread, since their installation and maintenance are much simpler. Ethernet networks use bus and passive star topologies, and the access method is CSMA/CD.

The IEEE802.3 standard, depending on the type of data transmission medium, has modifications:

 10BASE5 (thick coaxial cable) - provides a data transfer rate of 10 Mbit/s and a segment length of up to 500 m;

 10BASE2 (thin coaxial cable) - provides a data transfer rate of 10 Mbit/s and a segment length of up to 200 m;;

 10BASE-T (unshielded twisted pair) - allows you to create a network using a star topology. The distance from the hub to the end node is up to 100m. The total number of nodes should not exceed 1024;

 10BASE-F (fiber optic cable) - allows you to create a network using a star topology. The distance from the hub to the end node is up to 2000m.
In development of Ethernet technology, high-speed options have been created: IEEE802.3u/Fast Ethernet and IEEE802.3z/Gigabit Ethernet. The basic topology used in Fast Ethernet and Gigabit Ethernet networks is passive star.

Fast Ethernet network technology provides a transmission speed of 100 Mbit/s and has three modifications:

 100BASE-T4 - uses unshielded twisted pair (quad twisted pair). The distance from the hub to the end node is up to 100m;

 100BASE-TX - uses two twisted pairs (unshielded and shielded). The distance from the hub to the end node is up to 100m;

 100BASE-FX - uses fiber optic cable (two fibers in a cable). Distance from the hub to the end node is up to 2000m; .

Gigabit Ethernet – provides a transfer speed of 1000 Mbit/s. The following modifications of the standard exist:

 1000BASE-SX - uses fiber optic cable with a light signal wavelength of 850 nm.

 1000BASE-LX - uses fiber optic cable with a light signal wavelength of 1300 nm.

 1000BASE-CX – uses shielded twisted pair cable.

 1000BASE-T – uses quad unshielded twisted pair cable.
Fast Ethernet and Gigabit Ethernet networks are compatible with networks based on the Ethernet standard, so it is easy and simple to connect Ethernet, Fast Ethernet and Gigabit Ethernet segments into a single computer network.

The only drawback of this network is the lack of a guarantee of access time to the medium (and mechanisms providing priority service), which makes the network unpromising for solving real-time technological problems. Certain problems are sometimes created by the limitation on the maximum data field, equal to ~1500 bytes.

Different encoding schemes are used for different Ethernet speeds, but the access algorithm and frame format remain unchanged, which guarantees software compatibility.

The Ethernet frame has the format shown in Fig.

Ethernet Frame Format (the numbers at the top of the figure indicate the field size in bytes)

Field preamble contains 7 bytes 0xAA and serves to stabilize and synchronize the environment (alternating signals CD1 and CD0 with the final CD0), followed by the field SFD(start frame delimiter = 0xab), which is intended to detect the start of the frame. Field EFD(end frame delimiter) specifies the end of the frame. Checksum field ( CRC- cyclic redundancy check), as well as the preamble, SFD and EFD, are generated and controlled at the hardware level. Some modifications of the protocol do not use the efd field. The fields available to the user are starting from recipient addresses and ending with the field information, inclusive. After crc there is an interpacket gap (IPG - interpacket gap) of 9.6 μsec or more in length. The maximum frame size is 1518 bytes (preamble, SFD and EFD fields are not included). The interface scans all packets traveling along the cable segment to which it is connected, because it is possible to determine whether the received packet is correct and to whom it is addressed only by receiving it in its entirety. The correctness of the packet according to CRC, length and multiplicity of an integer number of bytes is made after checking the destination address.

When the computer is connected to the network directly using a switch, the restriction on the minimum frame length is theoretically removed. But working with shorter frames in this case will become possible only by replacing the network interface with a non-standard one (both for the sender and the recipient)!

If in the frame field protocol/type If the code is less than 1500, then this field characterizes the frame length. Otherwise, it is the protocol code whose packet is encapsulated in the Ethernet frame.

Access to the Ethernet channel is based on the algorithm CSMA/CD (carrier sense multiple access with collision detection).In Ethernet, any station connected to the network can attempt to start transmitting a packet (frame) if the cable segment to which it is connected is free. The interface determines whether a segment is free by the absence of a “carrier” for 9.6 μsec. Since the first bit of the packet does not reach the rest of the network stations simultaneously, it may happen that two or more stations attempt to transmit, especially since delays in repeaters and cables can reach quite large values. Such matches of attempts are called collisions. A collision is recognized by the presence of a signal in the channel, the level of which corresponds to the operation of two or more transceivers simultaneously. When a collision is detected, the station interrupts transmission. The attempt can be resumed after a delay (a multiple of 51.2 μs, but not exceeding 52 ms), the value of which is a pseudo-random variable and is calculated independently by each station (t= RAND(0.2 min(n,10)), where n - contents of the attempt counter, and the number 10 is backofflimit).

Typically, after a collision, time is divided into a number of discrete domains with a length equal to twice the packet's propagation time in the segment (RTT). For the maximum possible RTT, this time is 512 bit cycles. After the first collision, each station waits for 0 or 2 time domains before trying again. After the second collision, each station can wait 0, 1, 2 or 3 time domains, etc. After the nth collision, the random number lies in the range 0 - (2 n - 1). After 10 collisions, the maximum random shutter speed stops increasing and remains at 1023.

Thus, the longer the cable segment, the longer the average access time.

After waiting, the station increases the attempt counter by one and begins the next transmission. The default retries limit is 16; if the retries are reached, the connection is terminated and a corresponding message is displayed. The transmitted long frame helps to “synchronize” the start of packet transmission by several stations. Indeed, during the transmission time, with a noticeable probability, the need for transmission at two or more stations may arise. The moment they detect packet completion, the IPG timers will be enabled. Fortunately, information about the completion of packet transmission does not reach the stations of the segment at the same time. But the delays this entails also mean that the fact that one of the stations has started transmitting a new packet is not immediately known. If several stations are involved in a collision, they can notify the other stations by sending a jam signal (jam - at least 32 bits). The contents of these 32 bits are not regulated. This arrangement makes a repeat collision less likely. The source of a large number of collisions (in addition to information overload) can be the prohibitive total length of the logical cable segment, too many repeaters, a cable break, the absence of a terminator (50-ohm cable termination) or a malfunction of one of the interfaces. But collisions in themselves are not something negative - they are a mechanism that regulates access to the network environment.

In Ethernet, with synchronization, the following algorithms are possible:

A.

1. If the channel is free, the terminal transmits a packet with probability 1.
2. If the channel is busy, the terminal waits for it to become free and then transmits.

B.

1. If the channel is free, the terminal transmits the packet.
2. If the channel is busy, the terminal determines the time of the next transmission attempt. The time of this delay can be specified by some statistical distribution.

IN.

1. If the channel is free, the terminal transmits the packet with probability p, and with probability 1-p it postpones the transmission for t seconds (for example, to the next time domain).
2. When the attempt is repeated with a free channel, the algorithm does not change.
3. If the channel is busy, the terminal waits until the channel is free, after which it acts again according to the algorithm in point 1.

Algorithm A seems attractive at first glance, but it contains the possibility of collisions with a probability of 100%. Algorithms B and C are more robust against this problem.

The effectiveness of the CSMA algorithm depends on how quickly the transmitting side finds out about the fact of a collision and interrupts the transmission, because continuation is pointless - the data is already damaged. This time depends on the length of the network segment and delays in the segment equipment. Twice the delay value determines the minimum length of a packet transmitted in such a network. If the packet is shorter, it can be transmitted without the sending party knowing it was damaged by the collision. For modern Ethernet local networks, built on switches and full-duplex connections, this problem is irrelevant

To clarify this statement, consider the case when one of the stations (1) transmits a packet to the most remote computer (2) in a given network segment. Let the signal propagation time to this machine be equal to T. Let us also assume that machine (2) tries to start transmitting just at the moment the packet arrives from station (1). In this case, station (1) learns about the collision only 2T after the start of transmission (the signal propagation time from (1) to (2) plus the collision signal propagation time from (2) to (1)). It should be taken into account that collision registration is an analog process and the transmitting station must “listen” to the signal in the cable during the transmission process, comparing the reading result with what it is transmitting. It is important that the signal encoding scheme allows collision detection. For example, the sum of two signals with level 0 will not allow this to be done. You might think that transmitting a short packet with corruption due to a collision is not such a big deal; delivery control and retransmission can solve the problem.

It should only be taken into account that retransmission in the event of a collision registered by the interface is carried out by the interface itself, and retransmission in the case of control of response delivery is performed by the application process, requiring the resources of the workstation's central processor.

Double rotation time and collision detection

Clear recognition of collisions by all network stations is a necessary condition for the correct operation of the Ethernet network. If any transmitting station does not recognize the collision and decides that it transmitted the data frame correctly, then this data frame will be lost. Due to the overlap of signals during a collision, the frame information will be distorted, and it will be rejected by the receiving station (possibly due to a checksum mismatch). Most likely, the corrupted information will be retransmitted by some upper-layer protocol, such as a connection-oriented transport or application protocol. But the retransmission of the message by upper-level protocols will occur after a much longer time interval (sometimes even after several seconds) compared to the microsecond intervals that the Ethernet protocol operates. Therefore, if collisions are not reliably recognized by Ethernet network nodes, this will lead to a noticeable decrease in the useful throughput of this network.

For reliable collision detection, the following relationship must be satisfied:

T min >=PDV,

where T min is the transmission time of a frame of minimum length, and PDV is the time during which the collision signal manages to propagate to the farthest node in the network. Since in the worst case the signal must travel twice between the stations of the network that are most distant from each other (an undistorted signal passes in one direction, and a signal already distorted by a collision propagates on the way back), this time is called double revolution time (Path Delay Value, PDV).

If this condition is met, the transmitting station must have time to detect the collision caused by its transmitted frame even before it finishes transmitting this frame.

Obviously, the fulfillment of this condition depends, on the one hand, on the length of the minimum frame and network capacity, and on the other hand, on the length of the network cable system and the speed of signal propagation in the cable (this speed is slightly different for different types of cable).

All parameters of the Ethernet protocol are selected in such a way that during normal operation of network nodes, collisions are always clearly recognized. When choosing parameters, of course, the above relationship was taken into account, connecting the minimum frame length and the maximum distance between stations in a network segment.

The Ethernet standard assumes that the minimum length of a frame data field is 46 bytes (which, together with service fields, gives a minimum frame length of 64 bytes, and together with the preamble - 72 bytes or 576 bits). From here a limit on the distance between stations can be determined.

So, in 10 Mbit Ethernet, the minimum frame length transmission time is 575 bit intervals, therefore, the double turnaround time should be less than 57.5 μs. The distance that the signal can travel during this time depends on the type of cable and for a thick coaxial cable it is approximately 13,280 m. Considering that during this time the signal must travel along the communication line twice, the distance between two nodes should not be more than 6,635 m In the standard, the value of this distance is chosen significantly less, taking into account other, more stringent restrictions.

One of these restrictions is related to the maximum permissible signal attenuation. To ensure the required signal power when it passes between the most distant stations of a cable segment, the maximum length of a continuous segment of a thick coaxial cable, taking into account the attenuation it introduces, was chosen to be 500 m. Obviously, on a 500 m cable, the conditions for collision recognition will be met with a large margin for frames of any standard length, including 72 bytes (the double turnaround time along a 500 m cable is only 43.3 bit intervals). Therefore, the minimum frame length could be set even shorter. However, technology developers did not reduce the minimum frame length, keeping in mind multi-segment networks that are built from several segments connected by repeaters.

Repeaters increase the power of signals transmitted from segment to segment, as a result, signal attenuation is reduced and a much longer network can be used, consisting of several segments. In coaxial Ethernet implementations, designers have limited the maximum number of segments in the network to five, which in turn limits the total network length to 2500 meters. Even in such a multi-segment network, the collision detection condition is still met with a large margin (let us compare the distance of 2500 m obtained from the permissible attenuation condition with the maximum possible distance of 6635 m in terms of signal propagation time calculated above). However, in reality, the time margin is significantly less, since in multi-segment networks the repeaters themselves introduce an additional delay of several tens of bit intervals into the signal propagation. Naturally, a small margin was also made to compensate for deviations in cable and repeater parameters.

As a result of taking into account all these and some other factors, the ratio between the minimum frame length and the maximum possible distance between network stations was carefully selected, which ensures reliable collision recognition. This distance is also called the maximum network diameter.

As the frame transmission rate increases, which occurs in new standards based on the same CSMA/CD access method, such as Fast Ethernet, the maximum distance between network stations decreases in proportion to the increase in transmission rate. In the Fast Ethernet standard it is about 210 m, and in the Gigabit Ethernet standard it would be limited to 25 meters if the developers of the standard had not taken some measures to increase the minimum packet size.

PDV calculation

To simplify calculations, IEEE reference data is typically used to provide propagation delay values ​​for repeaters, transceivers, and various physical media. In table Table 3.5 provides the data necessary to calculate the PDV value for all physical Ethernet network standards. The bit interval is designated bt.

Table 3.5.Data for calculating PDV value

The 802.3 Committee tried to simplify the calculations as much as possible, so the data presented in the table includes several stages of signal propagation. For example, the delays introduced by a repeater consist of the input transceiver delay, the repeater delay, and the output transceiver delay. However, in the table all these delays are represented by one value called the segment base. To avoid the need to add the delays introduced by the cable twice, the table gives double the delay values ​​for each type of cable.

The table also uses concepts such as left segment, right segment and intermediate segment. Let us explain these terms using the example of the network shown in Fig. 3.13. The left segment is the segment in which the signal path begins from the transmitter output (output T x in Fig. 3.10) of the end node. In the example, this is a segment 1 . The signal then passes through intermediate segments 2-5 and reaches the receiver (input R x in Fig. 3.10) of the most distant node of the most distant segment 6, which is called the right one. It is here that, in the worst case, frames collide and a collision occurs, which is what is implied in the table.

Rice. 3.13.Example of an Ethernet network consisting of segments of different physical standards

Each segment has a constant delay associated with it, called the base, which depends only on the type of segment and on the position of the segment in the signal path (left, intermediate or right). The base of the right segment in which the collision occurs is much larger than the base of the left and intermediate segments.

In addition, each segment is associated with a signal propagation delay along the segment cable, which depends on the segment length and is calculated by multiplying the signal propagation time along one meter of cable (in bit intervals) by the cable length in meters.

The calculation consists of calculating the delays introduced by each cable segment (the signal delay per 1 m of cable shown in the table is multiplied by the length of the segment), and then summing these delays with the bases of the left, intermediate and right segments. The total PDV value should not exceed 575.

Since the left and right segments have different base latency values, in the case of different types of segments at remote edges of the network, it is necessary to perform the calculations twice: once taking a segment of one type as the left segment, and a second time taking a segment of another type. The result can be considered the maximum PDV value. In our example, the extreme network segments belong to the same type - the 10Base-T standard, so double calculation is not required, but if they were segments of different types, then in the first case it would be necessary to take the segment between the station and the hub as the left one 1 , and in the second, consider the segment between the station and the hub to be left 5 .

The network shown in the figure in accordance with the rule of 4 hubs is not correct - in the network between segment nodes 1 and 6 there are 5 hubs, although not all segments are lOBase-FB segments. In addition, the total network length is 2800 m, which violates the 2500 m rule. Let's calculate the PDV value for our example.

Left segment 1 / 15.3 (base) + 100 * 0.113= 26.6.

Intermediate segment 2/ 33,5 + 1000 * 0,1 = 133,5.

Intermediate segment 3/ 24 + 500 * 0,1 = 74,0.

Intermediate segment 4/ 24 + 500 * 0,1 = 74,0.

Intermediate segment 5/ 24 + 600 * 0,1 = 84,0.

Right segment 6 /165 + 100 * 0,113 = 176,3.

The sum of all components gives a PDV value of 568.4.

Since the PDV value is less than the maximum permissible value of 575, this network passes the double signal turnaround time criterion despite the fact that its total length is more than 2500 m and the number of repeaters is more than 4

PW calculation

To recognize the network configuration as correct, it is also necessary to calculate the reduction in the interframe interval by repeaters, that is, the PW value.

To calculate PW, you can also use the values ​​of the maximum values ​​for reducing the interframe interval when passing through repeaters of various physical environments, recommended by IEEE and given in Table. 3.6.

Table 3.6.Reducing the interframe interval by repeaters

In accordance with these data, we will calculate the PVV value for our example.

Left segment 1 10Base-T: 10.5 bt reduction.

Intermediate segment 2 10Base-FL: 8.

Intermediate segment 3 10Base-FB: 2.

Intermediate segment 4 10Base-FB: 2.

Intermediate segment 5 10Base-FB: 2.

The sum of these values ​​gives a PW value of 24.5, which is less than the 49-bit interval limit.

As a result, the network shown in the example complies with Ethernet standards in all parameters related to both segment lengths and the number of repeaters

Maximum Ethernet Performance

The number of Ethernet frames processed per second is often specified by bridge/switch and router manufacturers as the primary performance characteristic of these devices. In turn, it is interesting to know the net maximum throughput of an Ethernet segment in frames per second in an ideal case when there are no collisions in the network and no additional delays introduced by bridges and routers. This indicator helps to assess the performance requirements of communication devices, since each device port cannot receive more frames per unit of time than the corresponding protocol allows.

For communications equipment, the most difficult mode is processing frames of minimal length. This is explained by the fact that a bridge, switch or router spends approximately the same time processing each frame, associated with viewing the packet forwarding table, forming a new frame (for the router), etc. And the number of frames of the minimum length arriving at the device per unit time, naturally more than frames of any other length. Another performance characteristic of communications equipment - bits per second - is used less frequently, since it does not indicate what size frames the device was processing, and it is much easier to achieve high performance, measured in bits per second, with frames of the maximum size.

Using the parameters given in table. 3.1, we calculate the maximum performance of an Ethernet segment in units such as the number of transmitted frames (packets) of minimum length per second.

NOTEWhen referring to network capacity, the terms frame and packet are usually used interchangeably. Accordingly, the units of performance measurement frames-per-second, fps and packets-per-second, pps are similar.

To calculate the maximum number of frames of minimum length passing over an Ethernet segment, note that the size of a frame of minimum length together with the preamble is 72 bytes or 576 bits (Fig. 3.5), so its transmission takes 57.5 μs. By adding the interframe interval of 9.6 μs, we obtain that the period of frames of minimum length is 67.1 μs. Hence, the maximum possible throughput of an Ethernet segment is 14,880 fps.

Rice. 3.5.Towards calculating the throughput of the Ethernet protocol

Naturally, the presence of several nodes in a segment reduces this value due to waiting for access to the medium, as well as due to collisions leading to the need to retransmit frames.

The maximum length frames of Ethernet technology have a field length of 1500 bytes, which together with service information gives 1518 bytes, and with the preamble it amounts to 1526 bytes or 12,208 bits. The maximum possible throughput of an Ethernet segment for maximum length frames is 813 fps. Obviously, when working with large frames, the load on bridges, switches and routers is quite noticeably reduced.

Now let's calculate the maximum useful throughput in bits per second that Ethernet segments have when using frames of different sizes.

Under useful protocol bandwidth refers to the transmission rate of user data carried by the frame data field. This throughput is always less than the nominal bit rate of the Ethernet protocol due to several factors:

· frame service information;

· interframe intervals (IPG);

· waiting for access to the environment.

For frames of minimum length, the useful throughput is:

S P =14880 * 46 *8 = 5.48 Mbit/s.

This is much less than 10 Mbit/s, but it should be noted that frames of the minimum length are used mainly for transmitting receipts, so this speed has nothing to do with the transfer of actual file data.

For frames of maximum length, the usable throughput is:

S P = 813 * 1500 * 8 = 9.76 Mbit/s,

which is very close to the nominal speed of the protocol.

We emphasize once again that such speed can be achieved only in the case when two interacting nodes on an Ethernet network are not interfered with by other nodes, which is extremely rare,

Using medium-sized frames with a data field of 512 bytes, the network throughput will be 9.29 Mbps, which is also quite close to the maximum throughput of 10 Mbps.

ATTENTIONThe ratio of the current network throughput to its maximum throughput is called network utilization factor. In this case, when determining the current throughput, the transmission of any information over the network, both user and service, is taken into account. The coefficient is an important indicator for shared media technologies, since with the random nature of the access method, a high value of the utilization coefficient often indicates low useful network throughput (that is, the rate of transmission of user data) - nodes spend too much time on the procedure for gaining access and retransmitting frames after collisions.

In the absence of collisions and access waits, the network utilization factor depends on the size of the frame data field and has a maximum value of 0.976 when transmitting frames of maximum length. Obviously, in a real Ethernet network, the average network utilization can differ significantly from this value. More complex cases of determining network capacity, taking into account access waiting and handling collisions, will be discussed below.

Ethernet Frame Formats

The Ethernet technology standard, described in IEEE 802.3, describes a single MAC layer frame format. Since the MAC layer frame must contain an LLC layer frame, described in the IEEE 802.2 document, according to IEEE standards, only a single version of the link layer frame can be used in an Ethernet network, the header of which is a combination of the MAC and LLC sublayer headers.

However, in practice, Ethernet networks use frames of 4 different formats (types) at the data link level. This is due to the long history of the development of Ethernet technology, dating back to the period before the adoption of IEEE 802 standards, when the LLC sublayer was not separated from the general protocol and, accordingly, the LLC header was not used.

A consortium of three firms Digital, Intel and Xerox in 1980 submitted to the 802.3 committee their proprietary version of the Ethernet standard (which, of course, described a specific frame format) as a draft international standard, but the 802.3 committee adopted a standard that differed in some details from DIX offers. The differences also concerned the frame format, which gave rise to the existence of two different types of frames in Ethernet networks.

Another frame format emerged as a result of Novell's efforts to speed up its Ethernet protocol stack.

Finally, the fourth frame format was the result of the 802.2 committee's efforts to bring the previous frame formats to some common standard.

Differences in frame formats can lead to incompatibility in the operation of hardware and network software designed to work with only one Ethernet frame standard. However, today almost all network adapters, network adapter drivers, bridges/switches and routers can work with all Ethernet technology frame formats used in practice, and frame type recognition is performed automatically.

Below is a description of all four types of Ethernet frames (here, a frame refers to the entire set of fields that relate to the data link layer, that is, the fields of the MAC and LLC layers). The same frame type can have different names, so below for each frame type are several of the most common names:

· 802.3/LLC frame (802.3/802.2 frame or Novell 802.2 frame);

· Raw 802.3 frame (or Novell 802.3 frame);

· Ethernet DIX frame (or Ethernet II frame);

· Ethernet SNAP frame.

The formats of all these four types of Ethernet frames are shown in Fig. 3.6.

Conclusions

· Ethernet is the most common local network technology today. In a broad sense, Ethernet is an entire family of technologies that includes various proprietary and standard variants, of which the most famous are the proprietary DIX Ethernet variant, 10-Mbit variants of the IEEE 802.3 standard, as well as the new high-speed Fast Ethernet and Gigabit Ethernet technologies. Almost all types of Ethernet technologies use the same method of separating the data transmission medium - the CSMA/CD random access method, which defines the appearance of the technology as a whole.

· In a narrow sense, Ethernet is a 10-megabit technology described in the IEEE 802.3 standard.

· An important phenomenon in Ethernet networks is collision - a situation when two stations simultaneously try to transmit a data frame over a common medium. The presence of collisions is an inherent property of Ethernet networks, resulting from the random access method adopted. The ability to clearly recognize collisions is due to the correct choice of network parameters, in particular, compliance with the ratio between the minimum frame length and the maximum possible network diameter.

· The network performance characteristics are greatly influenced by the network utilization factor, which reflects its congestion. When this coefficient is above 50%, the useful network throughput drops sharply: due to an increase in the intensity of collisions, as well as an increase in the waiting time for access to the medium.

· The maximum possible throughput of an Ethernet segment in frames per second is achieved when transmitting frames of the minimum length and is 14,880 frames/s. At the same time, the useful network throughput is only 5.48 Mbit/s, which is only slightly more than half the nominal throughput - 10 Mbit/s.

· The maximum usable throughput of an Ethernet network is 9.75 Mbps, which corresponds to a maximum frame length of 1518 bytes transmitted over the network at 513 fps.

· In the absence of collisions and access waits utilization rate network depends on the size of the frame data field and has a maximum value of 0.96.

· Ethernet technology supports 4 different frame types that share a common host address format. There are formal characteristics by which network adapters automatically recognize the type of frame.

· Depending on the type of physical medium, the IEEE 802.3 standard defines various specifications: 10Base-5, 10Base-2, 10Base-T, FOIRL, 10Base-FL, 10Base-FB. For each specification, the cable type, the maximum lengths of continuous cable sections are determined, as well as the rules for using repeaters to increase the network diameter: the “5-4-3” rule for coaxial network options, and the “4-hub” rule for twisted pair and fiber optics.

· For a "mixed" network consisting of different types of physical segments, it is useful to calculate the total network length and the allowable number of repeaters. The IEEE 802.3 Committee provides input data for these calculations that indicate the delays introduced by repeaters of various physical media specifications, network adapters, and cable segments.

Network technologies IEEE802.5/Token-Ring

Token Ring networks, like Ethernet networks, are characterized by a shared data transmission medium, which in this case consists of cable segments connecting all network stations into a ring. The ring is considered as a common shared resource, and access to it requires not a random algorithm, as in Ethernet networks, but a deterministic one, based on transferring the right to use the ring to stations in a certain order. This right is conveyed using a special format frame called marker or token.

Token Ring networks operate at two bit rates - 4 and 16 Mbit/s. Mixing stations operating at different speeds in one ring is not allowed. Token Ring networks operating at 16 Mbps have some improvements in the access algorithm compared to the 4 Mbps standard.

Token Ring technology is a more complex technology than Ethernet. It has fault tolerance properties. The Token Ring network defines network operation control procedures that use feedback of a ring-shaped structure - the sent frame always returns to the sending station. In some cases, detected errors in the network operation are eliminated automatically, for example, a lost token can be restored. In other cases, errors are only recorded, and their elimination is carried out manually by maintenance personnel.

To control the network, one of the stations acts as a so-called active monitor. The active monitor is selected during ring initialization as the station with the maximum MAC address value. If the active monitor fails, the ring initialization procedure is repeated and a new active monitor is selected. In order for the network to detect the failure of an active monitor, the latter, in a working state, generates a special frame of its presence every 3 seconds. If this frame does not appear on the network for more than 7 seconds, then the remaining stations on the network begin the procedure for electing a new active monitor.

Token Ring Frame Formats

There are three different frame formats in Token Ring:

· marker;

· data frame;

· interrupt sequence

Physical layer of Token Ring technology

The IBM Token Ring standard initially provided for the construction of connections in the network using hubs called MAU (Multistation Access Unit) or MSAU (Multi-Station Access Unit), that is, multiple access devices (Fig. 3.15). The Token Ring network can include up to 260 nodes.

Rice. 3.15.Physical configuration of the Token Ring network

A Token Ring hub can be active or passive. A passive hub simply interconnects ports so that stations connected to those ports form a ring. The passive MSAU does not perform signal amplification or resynchronization. Such a device can be considered a simple crossover unit with one exception - MSAU provides bypass of a port when the computer connected to this port is turned off. This function is necessary to ensure ring connectivity regardless of the state of the connected computers. Typically, port bypass is accomplished using relay circuits that are powered by DC power from the AC adapter, and when the AC adapter is turned off, normally closed relay contacts connect the port's input to its output.

An active hub performs signal regeneration functions and is therefore sometimes called a repeater, as in the Ethernet standard.

The question arises: if the hub is a passive device, then how is high-quality transmission of signals over long distances, which occurs when several hundred computers are connected to a network, ensured? The answer is that in this case each network adapter takes on the role of a signal amplifier, and the role of a resynchronization unit is performed by the network adapter of the active ring monitor. Each Token Ring network adapter has a repeater unit that can regenerate and resynchronize signals, but only the active monitor repeater unit performs the latter function in the ring.

The resynchronization unit consists of a 30-bit buffer that receives Manchester signals with intervals slightly distorted during the round trip. With the maximum number of stations in the ring (260), the variation in the delay of bit circulation around the ring can reach 3-bit intervals. An active monitor “inserts” its buffer into the ring and synchronizes the bit signals, outputting them at the required frequency.

In general, the Token Ring network has a combined star-ring configuration. End nodes are connected to the MSAU in a star topology, and the MSAUs themselves are combined through special Ring In (RI) and Ring Out (RO) ports to form a backbone physical ring.

All stations in the ring must operate at the same speed - either 4 Mbit/s or 16 Mbit/s. The cables connecting the station to the hub are called lobe cables, and the cables connecting the hubs are called trunk cables.

Token Ring technology allows you to use different types of cable to connect end stations and hubs: STP Type I, UTP Type 3, UTP Type 6, as well as fiber optic cable.

When using shielded twisted pair STP Type 1 from the IBM cable system range, up to 260 stations can be combined into a ring with a drop cable length of up to 100 meters, and when using unshielded twisted pair, the maximum number of stations is reduced to 72 with a drop cable length of up to 45 meters.

The distance between passive MSAUs can reach 100 m when using STP Type 1 cable and 45 m when using UTP Type 3 cable. Between active MSAUs, the maximum distance increases respectively to 730 m or 365 m depending on the cable type.

The maximum ring length of a Token Ring is 4000 m. The restrictions on the maximum ring length and the number of stations in a ring in Token Ring technology are not as strict as in Ethernet technology. Here, these restrictions are largely related to the time the marker turns around the ring (but not only - there are other considerations that dictate the choice of restrictions). So, if the ring consists of 260 stations, then with a marker holding time of 10 ms, the marker will return to the active monitor in the worst case after 2.6 s, and this time is exactly the marker rotation control timeout. In principle, all timeout values ​​in the network adapters of the Token Ring network nodes are configurable, so it is possible to build a Token Ring network with more stations and a longer ring length.

Conclusions

· Token Ring technology is developed primarily by IBM and also has IEEE 802.5 status, which reflects the most important improvements being made to IBM technology.

· Token Ring networks use a token access method, which guarantees that each station can access the shared ring within the token rotation time. Because of this property, this method is sometimes called deterministic.

· The access method is based on priorities: 0 (lowest) to 7 (highest). The station itself determines the priority of the current frame and can capture the ring only if there are no higher priority frames in the ring.

· Token Ring networks operate at two speeds: 4 and 16 Mbps and can use shielded twisted pair, unshielded twisted pair, and fiber optic cable as the physical media. The maximum number of stations in the ring is 260, and the maximum length of the ring is 4 km.

· Token Ring technology has elements of fault tolerance. Due to the feedback of the ring, one of the stations - the active monitor - continuously monitors the presence of the marker, as well as the rotation time of the marker and data frames. If the ring does not operate correctly, the procedure for its reinitialization is launched, and if this does not help, then the beaconing procedure is used to localize the faulty section of the cable or the faulty station.

· The maximum data field size of a Token Ring frame depends on the speed of the ring. For a speed of 4 Mbit/s it is about 5000 bytes, and at a speed of 16 Mbit/s it is about 16 KB. The minimum size of the frame data field is not defined, that is, it can be equal to 0.

· In the Token Ring network, stations are connected into a ring using hubs called MSAUs. The MSAU passive hub acts as a crossover panel that connects the output of the previous station in the ring to the input of the next one. The maximum distance from the station to the MSAU is 100 m for STP and 45 m for UTP.

· An active monitor also acts as a repeater in the ring - it resynchronizes signals passing through the ring.

· The ring can be built on the basis of an active MSAU hub, which in this case is called a repeater.

· The Token Ring network can be built on the basis of several rings separated by bridges that route frames based on the “from the source” principle, for which a special field with the route of the rings is added to the Token Ring frame.

Network technologies IEEE802.4/ArcNet

The ArcNet network uses a “bus” and a “passive star” as its topology. Supports shielded and unshielded twisted pair and fiber optic cable. The ArcNet network uses a delegation method to access the communication medium. The ArcNet network is one of the oldest networks and has been very popular. Among the main advantages of the ArcNet network are high reliability, low cost of adapters and flexibility. The main disadvantage of the network is the low speed of information transfer (2.5 Mbit/s). The maximum number of subscribers is 255. The maximum network length is 6000 meters.

Network technology FDDI (Fiber Distributed Data Interface)

FDDI–a standardized specification for a network architecture for high-speed data transmission over fiber optic lines. Transfer speed – 100 Mbit/s. This technology is largely based on the Token-Ring architecture and uses deterministic token access to the data transmission medium. The maximum length of the network ring is 100 km. The maximum number of network subscribers is 500. The FDDI network is a very highly reliable network, which is created on the basis of two fiber optic rings that form the main and backup data transmission paths between nodes.

Main characteristics of the technology

FDDI technology is largely based on Token Ring technology, developing and improving its basic ideas. The developers of FDDI technology set themselves the following goals as their highest priority:

· increase the bit rate of data transfer to 100 Mbit/s;

· increase the fault tolerance of the network through standard procedures for restoring it after various types of failures - cable damage, incorrect operation of a node, hub, high levels of interference on the line, etc.;

· make the most of potential network bandwidth for both asynchronous and synchronous (latency-sensitive) traffic.

The FDDI network is built on the basis of two fiber optic rings, which form the main and backup data transmission paths between network nodes. Having two rings is the primary way to increase fault tolerance in an FDDI network, and nodes that want to take advantage of this increased reliability potential must be connected to both rings.

In normal network operation mode, data passes through all nodes and all cable sections of the Primary ring only; this mode is called the Thru- “end-to-end” or “transit”. The Secondary ring is not used in this mode.

In the event of some type of failure where part of the primary ring cannot transmit data (for example, a broken cable or node failure), the primary ring is combined with the secondary ring (Fig. 3.16), again forming a single ring. This mode of network operation is called Wrap, that is, the "folding" or "folding" of the rings. The collapse operation is performed using FDDI hubs and/or network adapters. To simplify this procedure, data on the primary ring is always transmitted in one direction (in the diagrams this direction is shown counterclockwise), and on the secondary ring in the opposite direction (shown clockwise). Therefore, when a common ring of two rings is formed, the transmitters of the stations still remain connected to the receivers of neighboring stations, which allows information to be correctly transmitted and received by neighboring stations.

Rice. 3.16.Reconfiguration of FDDI rings upon failure

FDDI standards place a lot of emphasis on various procedures that allow you to determine if there is a fault in the network and then make the necessary reconfiguration. The FDDI network can fully restore its functionality in the event of single failures of its elements. When there are multiple failures, the network splits into several unconnected networks. FDDI technology complements the failure detection mechanisms of Token Ring technology with mechanisms for reconfiguring the data transmission path in the network, based on the presence of redundant links provided by the second ring.

Rings in FDDI networks are considered as a common shared data transmission medium, so a special access method is defined for it. This method is very close to the access method of Token Ring networks and is also called the token ring method.

The differences in the access method are that the token retention time in the FDDI network is not a constant value, as in the Token Ring network. This time depends on the load on the ring - with a small load it increases, and with large overloads it can decrease to zero. These changes in the access method only affect asynchronous traffic, which is not critical to small delays in frame transmission. For synchronous traffic, the token hold time is still a fixed value. A frame priority mechanism similar to that adopted in Token Ring technology is absent in FDDI technology. The technology developers decided that dividing traffic into 8 priority levels is redundant and it is sufficient to divide the traffic into two classes - asynchronous and synchronous, the latter of which is always serviced, even when the ring is overloaded.

Otherwise, frame forwarding between ring stations at the MAC level is fully compliant with Token Ring technology. FDDI stations use an early token release algorithm, similar to Token Ring networks with a speed of 16 Mbps.

MAC level addresses are in a standard format for IEEE 802 technologies. The FDDI frame format is close to the Token Ring frame format; the main differences are the absence of priority fields. Signs of address recognition, frame copying and errors allow you to preserve the procedures for processing frames available in Token Ring networks by the sending station, intermediate stations and the receiving station.

In Fig. Figure 3.17 shows the correspondence of the protocol structure of FDDI technology to the seven-layer OSI model. FDDI defines the physical layer protocol and the media access sublayer (MAC) protocol of the data link layer. Like many other local area network technologies, FDDI technology uses the LLC data link control sublayer protocol defined in the IEEE 802.2 standard. Thus, although FDDI technology was developed and standardized by ANSI and not by IEEE, it fits entirely within the framework of the 802 standards.

Rice. 3.17.Structure of FDDI technology protocols

A distinctive feature of FDDI technology is the station control level - Station Management (SMT). It is the SMT layer that performs all the functions of managing and monitoring all other layers of the FDDI protocol stack. Each node in the FDDI network takes part in managing the ring. Therefore, all nodes exchange special SMT frames to manage the network.

Fault tolerance of FDDI networks is ensured by protocols of other layers: with the help of the physical layer, network failures due to physical reasons, for example, due to a broken cable, are eliminated, and with the help of the MAC layer, logical network failures are eliminated, for example, the loss of the required internal path for transmitting a token and data frames between hub ports .

Conclusions

· FDDI technology was the first to use fiber optic cable in local area networks and operate at 100 Mbps.

· There is significant continuity between Token Ring and FDDI technologies: both are characterized by a ring topology and a token access method.

· FDDI technology is the most fault-tolerant local network technology. In case of single failures of the cable system or station, the network, due to the “folding” of the double ring into a single one, remains fully operational.

· The FDDI token access method operates differently for synchronous and asynchronous frames (the frame type is determined by the station). To transmit a synchronous frame, a station can always capture an incoming token for a fixed time. To transmit an asynchronous frame, a station can capture a token only if the token has completed a rotation around the ring quickly enough, which indicates that there is no ring congestion. This access method, firstly, gives preference to synchronous frames, and secondly, regulates the ring load, slowing down the transmission of non-urgent asynchronous frames.

· FDDI technology uses fiber optic cables and Category 5 UTP as the physical medium (this physical layer option is called TP-PMD).

· The maximum number of dual connection stations in a ring is 500, the maximum diameter of a double ring is 100 km. The maximum distances between adjacent nodes for multimode cable are 2 km, for twisted pair UPT category 5-100 m, and for single-mode optical fiber depend on its quality

Today, networks and network technologies connect people in every corner of the world and provide them with access to the greatest luxury in the world - human communication. People can communicate and play with friends in other parts of the world without interference.

The events taking place become known in all countries of the world in a matter of seconds. Everyone is able to connect to the Internet and post their piece of information.

Network information technologies: the roots of their origin

In the second half of the last century, human civilization formed its two most important scientific and technical branches - computer and About a quarter of a century, both of these branches developed independently, and within their framework computer and telecommunication networks were created, respectively. However, in the last quarter of the twentieth century, as a result of the evolution and interpenetration of these two branches of human knowledge, what we call the term “network technology” arose, which is a subsection of the more general concept of “information technology”.

As a result of their appearance, a new technological revolution occurred in the world. Just as several decades earlier the land surface was covered with a network of expressways, at the end of the last century all countries, cities and villages, enterprises and organizations, as well as individual homes found themselves connected by “information highways.” At the same time, they all became elements of various data transfer networks between computers, in which certain information transfer technologies were implemented.

Network technology: concept and content

Network technology is a sufficient set of rules for the presentation and transmission of information, implemented in the form of so-called “standard protocols”, as well as hardware and software, including network adapters with drivers, cables and fiber-optic lines, and various connectors (connectors).

“Sufficiency” of this set of tools means its minimization while maintaining the possibility of building an efficient network. It should have the potential for improvement, for example, by creating subnets in it that require the use of protocols of various levels, as well as special communicators, usually called “routers.” After improvement, the network becomes more reliable and faster, but at the cost of adding add-ons to the main network technology that forms its basis.

The term “network technology” is most often used in the narrow sense described above, but it is often broadly interpreted as any set of tools and rules for building networks of a certain type, for example, “local computer network technology.”

Prototype of network technology

The first prototype of a computer network, but not yet the network itself, began in the 60-80s. last century multi-terminal systems. Representing a set of monitor and keyboard, located at great distances from large computers and connecting to them via telephone modems or dedicated channels, the terminals left the premises of the computer information center and were dispersed throughout the building.

At the same time, in addition to the operator of the computer itself on the computer information center, all users of the terminals were able to enter their tasks from the keyboard and observe their execution on the monitor, carrying out some task management operations. Such systems, implementing both time-sharing and batch processing algorithms, were called remote job entry systems.

Global networks

Following multi-terminal systems in the late 60s. XX century The first type of networks was created - global computer networks (GCN). They connected supercomputers, which existed in single copies and stored unique data and software, with mainframe computers located at distances of up to many thousands of kilometers, through telephone networks and modems. This network technology has previously been tested in multi-terminal systems.

The first GCS in 1969 was ARPANET, which worked in the US Department of Defense and united different types of computers with different operating systems. They were equipped with additional modules to implement communication systems common to all computers on the network. It was on it that the foundations of network technologies that are still used today were developed.

The first example of the convergence of computer and telecommunications networks

GCS inherited communication lines from older and more global telephone networks, since it was very expensive to lay new long-distance lines. Therefore, for many years they used analog telephone channels to transmit only one conversation at a time. Digital data was transmitted over them at a very low speed (tens of kbit/s), and the capabilities were limited to the transfer of data files and email.

However, having inherited telephone communication lines, GCS did not take their basic technology, based on the principle of circuit switching, when each pair of subscribers was allocated a channel at a constant speed for the entire duration of the communication session. The GCS used new computer network technologies based on the principle of packet switching, in which data in the form of small portions of packets at a constant speed is issued into a non-switched network and received by their recipients on the network using address codes built into the packet headers.

Predecessors of local area networks

Appearance in the late 70s. XX century LSI led to the creation of minicomputers with low cost and rich functionality. They began to really compete with large computers.

Minicomputers of the PDP-11 family have gained wide popularity. They began to be installed in all, even very small production units to manage technical processes and individual technological installations, as well as in enterprise management departments to perform office tasks.

The concept of computer resources distributed throughout the enterprise emerged, although all minicomputers still operated autonomously.

The emergence of LAN networks

By the mid-80s. XX century technologies for combining minicomputers into networks were introduced, based on switching data packets, as in the GCS.

They turned the construction of a single enterprise network, called a local (LAN) network, into an almost trivial task. To create it, you only need to buy network adapters for the selected LAN technology, for example, Ethernet, a standard cable system, install connectors (connectors) on its cables and connect the adapters to the minicomputer and to each other using these cables. Next, one of the operating systems intended for organizing a LAN network was installed on the computer server. After that, it began to work, and the subsequent connection of each new minicomputer did not cause any problems.

The inevitability of the Internet

If the advent of mini-computers made it possible to distribute computer resources evenly across the territories of enterprises, then the appearance in the early 90s. PC led to their gradual appearance, first in every workplace of any mental worker, and then in individual human dwellings.

The relative cheapness and high reliability of PCs first gave a powerful impetus to the development of LAN networks, and then led to the emergence of a global computer network - the Internet, which today covers all countries of the world.

The size of the Internet is growing by 7-10% every month. It represents the core that connects various local and global networks of enterprises and institutions around the world with each other.

If at the first stage data files and email messages were mainly transmitted via the Internet, today it mainly provides remote access to distributed information resources and electronic archives, to commercial and non-commercial information services in many countries. Its freely accessible archives contain information on almost all areas of knowledge and human activity - from new trends in science to weather forecasts.

Basic network technologies of LAN networks

Among them are the basic technologies on which the basis of any specific network can be built. Examples include such well-known LAN technologies as Ethernet (1980), Token Ring (1985) and FDDI (late 80s).

At the end of the 90s. Ethernet technology has become the leader in LAN network technology, combining its classic version with up to 10 Mbit/s, as well as Fast Ethernet (up to 100 Mbit/s) and Gigabit Ethernet (up to 1000 Mbit/s). All Ethernet technologies have similar operating principles that simplify their maintenance and the integration of LAN networks built on their basis.

During the same period, their developers began to build network functions into the kernels of almost all computer operating systems that implement the above-mentioned network information technologies. Even specialized communication operating systems like IOS from Cisco Systems have appeared.

How GCS technologies developed

GKS technologies on analog telephone channels, due to the high level of distortion in them, were distinguished by complex algorithms for monitoring and data recovery. An example of them is the X.25 technology developed in the early 70s. XX century More modern network technologies are frame relay, ISDN, ATM.

ISDN is an acronym that stands for Integrated Services Digital Network and allows remote video conferencing. Remote access is provided by installing ISDN adapters in PCs, which work many times faster than any modems. There is also special software that allows popular operating systems and browsers to work with ISDN. But the high cost of equipment and the need to lay special communication lines hinder the development of this technology.

WAN technologies have progressed along with telephone networks. After the advent of digital telephony, a special technology, Plesiochronous Digital Hierarchy (PDH), was developed, supporting speeds of up to 140 Mbit/s and used by enterprises to create their own networks.

New Synchronous Digital Hierarchy (SDH) technology in the late 80s. XX century expanded the capacity of digital telephone channels up to 10 Gbit/s, and Dense Wave Division Multiplexing (DWDM) technology - up to hundreds of Gbit/s and even up to several Tbit/s.

Internet technologies

Network ones are based on the use of hypertext language (or HTML language) - a special markup language that is an ordered set of attributes (tags) that are pre-implemented by website developers into each of their pages. Of course, in this case we are not talking about text or graphic documents (photos, pictures), which have already been “downloaded” by the user from the Internet, are in the memory of his PC and are viewed through text or images. We are talking about so-called web pages viewed through programs -browsers.

Developers of Internet sites create them in HTML language (now many tools and technologies have been created for this work, collectively called “website layout”) in the form of a set of web pages, and site owners place them on Internet servers on a rental basis from the owners of their memory servers (the so-called “hosting”). They work on the Internet around the clock, servicing the requests of its users to view the web pages loaded on them.

Browsers on user PCs, having received access through the server of their Internet provider to a specific server, the address of which is contained in the name of the requested Internet site, gain access to this site. Further, by analyzing the HTML tags of each page being viewed, browsers form its image on the monitor screen in the way it was intended by the site developer - with all the headings, font and background colors, various inserts in the form of photos, diagrams, pictures, etc. .

A computer network is an association of several computers to jointly solve information and computing problems.

The key concept of network technologies is a network resource, which can be understood as hardware and software components involved in the sharing process - in the process of network interaction. Access to network resources is provided by network services (network services)

The basic concepts of network technologies include such concepts as server, client, communication channel, protocol and many others. However, the concept of a network resource and a network service (service) are fundamental, since the need to organize work based on the sharing of computer resources, and therefore the creation of network resources and corresponding network services, is the root cause of the creation of computer networks themselves.

Highlight five types of network services: file, print, messages, application databases.

File service — implements centralized storage and sharing of files. This is one of the most important network services; it requires the presence of some network file storage (local network file server, ftp server, etc.), as well as the use of various security mechanisms (access control, file version control, information backup).

Print service — provides opportunities for centralized use of printers and other printing devices. This service accepts print jobs, manages the job queue, and organizes user interaction with network printers. Network printing technology is very convenient in a wide variety of computer networks, as it makes it possible to reduce the number of printers required, which ultimately allows you to reduce costs or use better equipment.

Messaging service — allows you to organize information exchange between users of a computer network. In this case, both text messages (e-mail, network instant messenger messages) and media messages of various voice and video communication systems should be considered as messages.

Database service — is designed to organize centralized storage, search processing and ensure data protection of various information systems. Unlike simply storing and sharing files, a database service also provides management, which includes creating, modifying, deleting data, ensuring its integrity and protecting it.

Application Service — provides a method of operation in which the application is launched on the user's computer not from a local source, but from a computer network. Such applications may use server resources for data storage and computation. The advantage of using network applications is the ability to use them from any connection point to a computer network without the need to install the application on a local computer, the ability for multiple users to collaborate, “transparent” software updates, and the ability to use commercial software on a subscription basis.

Application services are the newest and fastest growing type of network service. A good example here is the office network applications of the online services Google Drive and Microsoft Office 365.

Topic 4 NETWORK TECHNOLOGIES TO SUPPORT SOLUTION OF MANAGEMENT PROBLEMS AT ENTERPRISES

Any enterprise is a collection of interacting elements (divisions), each of which can have its own structure. The elements are interconnected functionally, i.e. they perform certain types of work within the framework of a single business process, as well as information, exchanging documents, fax messages, written and oral orders. In addition, these elements interact with external systems, and their interaction can be both informational and functional. Thus, in the process of functioning of various enterprises, a very complex multi-level system is involved with developed connections not only between the hierarchical levels of the enterprises themselves, but also with the credit system, the state tax service system, clients, partners and other business participants.

The complexity of this system is aggravated by the fact that it is deployed over large territories, covering a large number of participants belonging to various departments, which affects the peculiarities of their information interaction.

In such conditions, the priority tasks are: organizing effective interaction of all business participants through the use of computing and telecommunications tools that form a network technology for information processing in enterprises and organizations.

Network technology- a set of software, hardware and organizational tools that provide communication and distribution of computing resources of PCs connected to the network.

Network technology is an effective business tool, as it provides managers with the necessary service for collective solution of assigned tasks, significantly increases the degree and order of use of resources available on the network, provides remote access to them, and allows organizing a single information space for all participants in business processes.

In terms of creating a unified information space, the organization of network technology is focused on the following areas:

Integration of various hardware and software systems of all business participants. At the initial stage of development of the data transmission system, the problem of information interaction was solved by connecting individual user terminals to information servers with data transmission via dial-up or dedicated channels and telephone lines. Today there is a need to unite remote local computer networks of business participants through high-speed communication channels.

Creation of an electronic document management subsystem, which includes not only the transfer of electronic documents from one user to another, but also the automation of their processing (accounting, storage, technology for collective development of documents, etc.) and the creation of a convenient graphical environment.

Use of high-performance hardware and software, development of applications based on the introduction of modern client-server technology.

Ensuring data security during the processing and transmission of information in the process of implementing business tasks.

Modern network technologies continue those that emerged in the late 1970s. trend towards the development of distributed data processing. The initial stage in the development of such methods of information processing were multi-machine systems, which were a collection of computers of varying performance, integrated into a system using communication channels. The highest stage of distributed data processing technologies has become computer networks of various levels - local and large-scale, which were the basis for organizing network technology to support the solution of management problems in enterprises and organizations.

In general, a computer network is a system of interconnected and distributed PCs focused on the collective use of hardware, software and information network resources.

Network information resources They are databases of general and individual use, focused on problems solved on the network.

Network hardware resources consists of computers of various types, means of territorial communication systems, communication equipment and coordination of the operation of networks of the same level or different levels.

Network software resources are a set of programs for planning, organizing and implementing collective user access to network-wide resources, automating information processing processes, dynamic distribution and redistribution of network-wide resources in order to increase the efficiency and reliability of meeting user requests.

Purpose of computer networks:

Ensure reliable and fast user access to network resources and organize the collective operation of these resources;

Ensure the ability to quickly move information over any distance in order to obtain timely data for making management decisions.

Computer networks allow you to automate the management of individual organizations, enterprises, and regions. The ability to concentrate large amounts of information in computer networks, the general availability of this data, as well as software and hardware processing tools, and high reliability of operation - all this makes it possible to improve information services to users and dramatically increase the efficiency of using computer technology.

The use of computer networks provides the following opportunities:

Organize parallel data processing by several PCs;

Create distributed databases located in the memory of various computers;

Specialize individual computers to effectively solve certain classes of problems;

Automate the exchange of information and programs between individual computers and network users;

Reserve computing power and data transmission facilities in case of failure of individual network resources in order to quickly restore normal operation of the network;

Redistribute computing power between network users depending on changes in their needs and the complexity of the tasks being solved;

Combine work in different modes: interactive, batch “request-response” mode, mode of collecting, transmitting and exchanging information.

Thus, it can be noted that a feature of the use of computer networks is not only the approach of hardware directly to the places where information originates and is used, but also the division of processing and control functions into separate components for the purpose of their effective distribution between several personal computers, as well as ensuring reliable user access to computing and information resources and organization of collective exploitation of these resources. At the same time, certain requirements are imposed on computer networks:

1. Performance computer network is assessed from different positions:

Computer network response time, which refers to the time between the moment the request occurs and the moment the response is received. The response time depends on many factors, such as the services used and the degree of congestion of the network or its individual segments, etc.

Network bandwidth determined by the amount of information transmitted through a network or its segment per unit of time. Network throughput characterizes how quickly a computer network can transfer information.

LAN segment- a) a group of devices (for example, PCs, servers, printers, etc.) that are connected using network equipment; 6) a section of a LAN separated from other sections by a repeater, hub, bridge or router. All stations on a segment support the same media access protocol and share its total throughput.

2. Reliability The operation of a computer network is determined by its following characteristics:

- fault tolerance all its components. To increase the reliability of hardware operation, duplication is usually used, when if one of the elements fails, the others will ensure the functioning of the network;

Ensuring the safety of information and protecting it from distortion;

Data security, which is ensured by the protection of information from unauthorized access, implemented through the use of specialized software and hardware.

3. Controllability- this is the ability to monitor the state of computer network nodes, identify and resolve problems that arise during its operation, analyze and plan the operation of the network.

4. Extensibility characterizes the possibility of adding new connections and nodes to a computer network, the possibility of its physical expansion without a significant decrease in performance.

5. Transparency computer network involves hiding the features of the network from the end user in such a way that a specialist can access network resources as ordinary local resources of the personal computer on which he works.

6. Integrability means the ability to connect different types of equipment and software from different manufacturers to a computer network.

As practice shows, by expanding the capabilities of data processing, better loading of resources and increasing the reliability of IT operations in general, the cost of processing information in computer networks is no less than one and a half times lower compared to processing similar data on autonomous (local) personal computers.

Currently, three main types of computer networks are most widespread - local, corporate, and global.

Network computer technologies are rapidly developing. If previously the main concern of a network administrator was the local computer network of an enterprise or organization, now this network is increasingly becoming geographically distributed. Users must be able to access enterprise network resources from virtually anywhere. At the same time, they want not only to view and send e-mail, but also to be able to access files, databases and other resources on the enterprise network. Within an organization, remotely located branches are often created with their own local networks, which must be connected to the network of the main division using reliable, secure and transparent communications for users. Such networks are called corporate. Taking into account today's realities, users of an enterprise's corporate network also need to be given the opportunity to access the resources of the global Internet, while protecting the internal network from unauthorized access from the outside.

Thus, a corporate network is a hardware and software system that ensures reliable transfer of information between various applications used in an organization. Often corporate network nodes are located in different cities. The principles by which such a network is built are quite different from those used when creating a local network, even covering several buildings. The main difference is that geographically distributed networks use fairly slow (today it is often tens and hundreds of kilobits per second, sometimes 2 Mbit/s and higher) leased communication lines. If when creating a local network the main costs are for the purchase of equipment and laying cables, then in geographically distributed networks the most significant element of the cost is the rental fee for the use of channels, which grows rapidly with the increase in the quality and speed of data transmission. Otherwise, the corporate network should not impose restrictions on which applications and how they process the information transferred over it. The main problem that has to be solved when creating a corporate network is the organization of communication channels. If within one city you can count on renting dedicated lines, including high-speed ones, then when moving to geographically distant nodes, the cost of renting channels becomes very high, and their quality and reliability often turn out to be very low. A natural solution to this problem is to use already existing wide area networks. In this case, it is enough to provide channels from offices to the nearest network nodes. The global network will take on the task of delivering information between nodes.

The ideal option for a corporate network would be to create communication channels only in those areas where it is necessary, and transfer over them any network protocols that are required by running applications. At first glance, this is a return to leased communication lines. However, there are technologies for constructing data transmission networks that make it possible to organize channels within them that appear only at the right time and in the right place. Such channels are called virtual. A system that connects remote resources using virtual channels can naturally be called a virtual network. Today, there are two main virtual network technologies - circuit-switched networks and packet-switched networks. The first includes the regular telephone network, ISDN and a number of other more exotic technologies. Packet switching networks are represented by X.25, Frame Relay and, more recently, ATM technologies. Other types of virtual (in various combinations) networks are widely used in the construction of corporate information systems. Circuit-switched networks provide the subscriber with multiple communication channels with a fixed bandwidth per connection. A regular telephone network provides one communication channel between subscribers. If it is necessary to increase the number of simultaneously available resources, additional telephone numbers must be installed. Even if we forget about the low quality of communication, it is clear that the limited number of channels and long connection establishment times do not allow using telephone communications as the basis of a corporate network. For connecting individual remote users, this is quite convenient and often the only available method.

An alternative to circuit-switched networks is packet-switched networks. When using packet switching, one communication channel is used in a time-sharing mode by many users - much the same as on the Internet. However, unlike networks like the Internet, where each packet is routed separately, packet switching networks require a connection to be established between end resources before information can be transmitted. After establishing a connection, the network “remembers” the route (virtual channel) along which information should be transmitted between subscribers, and remembers it until it receives a signal to break the connection. For applications running on a packet switching network, virtual circuits look like regular communication lines - the only difference is that their throughput and introduced delays vary depending on the network load. Let's consider the main technologies that are used to build corporate networks.

ISDN

A widely used example of a circuit-switched virtual network is ISDN(digital network with integration of services). ISDN provides digital channels (64 Kbps) that can carry both voice and data. A basic ISDN (Basic Rate Interface) connection includes two such channels and an additional control channel with a speed of 16 Kbps (this combination is designated as 2B+D). It is possible to use a larger number of channels - up to thirty (Primary Rate Interface, 30B+D). This significantly increases the bandwidth, but leads to a corresponding increase in the cost of equipment and communication channels. In addition, the costs of renting and using the network increase proportionally. In general, the limitations on the number of simultaneously available resources imposed by ISDN lead to the fact that this type of communication is convenient to use mainly as an alternative to telephone networks. In systems with a small number of nodes, ISDN can also be used as the main network protocol. You should just keep in mind that access to ISDN in our country is still the exception rather than the rule.

X.25

The classic packet switching technology is the X.25. Today there are virtually no X.25 networks operating at speeds higher than 128 Kbps, which is quite slow. But the X.25 protocol includes powerful error correction facilities, ensuring reliable delivery of information even on poor lines and is widely used where there are no high-quality communication channels. (In our country they are not available almost everywhere.) Naturally, you have to pay for reliability - in this case, the speed of network equipment and relatively large, but predictable delays in the distribution of information. At the same time, X.25 is a universal protocol that allows you to transfer almost any type of data. “Natural” for X.25 networks is the operation of applications using the protocol stack OSI. These include systems that use standards X.400(email) and FTAM(file sharing), as well as some others. Tools are available that allow you to implement the interaction of Unix systems based on OSI protocols. Another standard feature of X.25 networks is communication through regular asynchronous COM ports. Figuratively speaking, the X.25 network “extends” the cable connected to the serial port, bringing its connector to remote resources. Thus, almost any application that can be accessed through a COM port can be easily integrated into an X.25 network. Examples of such applications include not only terminal access to remote host computers, such as Unix machines, but also the interaction of Unix computers with each other (cu, uucp), Lotus Notes-based systems, cc:Mail and MS e-mail Mail, etc. To combine LANs in nodes connected to an X.25 network, there are methods of encapsulating packets of information from the local network into X.25 packets. Some of the service information is not transmitted, since it can be unambiguously restored on the recipient's side. The standard encapsulation mechanism is considered to be that described in RFC 1356. It allows various local network protocols (IP, IPX, etc.) to be transmitted simultaneously through one virtual connection. This mechanism (or the older IP-only RFC 877 implementation) is implemented in almost all modern routers. There are also transmission methods over X.25 and other communication protocols, in particular SNA, used in IBM mainframe networks, as well as a number of proprietary protocols from various manufacturers. Thus, X.25 networks offer a universal transport mechanism for transferring information between virtually any application. In this case, different types of traffic are transmitted over one communication channel, without “knowing” anything about each other. When connecting local networks via X.25, you can isolate separate fragments of the corporate network from each other, even if they use the same communication lines.

Today there are dozens of public global X.25 networks in the world; their nodes are located in almost all major business, industrial and administrative centers. In Russia, X.25 services are offered by Sprint Network, Infotel, Rospak, Rosnet, Sovam Teleport and a number of other providers. In addition to connecting remote nodes, X.25 networks always provide access facilities for end users. In order to connect to any X.25 network resource, the user only needs to have a computer with an asynchronous serial port and a modem. At the same time, there are no problems with access authorization in geographically remote nodes; If your resource is connected to an X.25 network, you can access it both from your provider's nodes and through nodes on other networks - that is, from almost anywhere in the world. The disadvantage of X.25 technology is the presence of a number of fundamental speed restrictions. The first of them is connected precisely with the developed capabilities of correction and restoration. These tools cause delays in the transmission of information and require a lot of computing power and performance from X.25 equipment, as a result of which it simply “cannot keep up” with fast communication lines. Although there is equipment that has high-speed ports, the actual speed they provide does not exceed 250-300 Kbps per port. At the same time, for modern high-speed communication lines, X.25 correction tools turn out to be redundant and when they are used, equipment power often runs idle. The second feature that makes X.25 networks considered slow is the peculiarities of encapsulation of local network protocols (primarily IP and IPX). All other things being equal, the connection of local networks via X.25 is, depending on the network parameters, 15-40% slower than when using HDLC over a leased line.

Still, on low-quality communication lines, X.25 networks are quite effective and provide a significant advantage in price and capabilities compared to leased lines.

Frame Relay

Frame Relay technology emerged as a means to realize the benefits of packet switching on high-speed communication lines. The main difference between Frame Relay networks and X.25 is that they eliminate error correction between network nodes. The tasks of restoring the flow of information are assigned to the terminal equipment and software of users. Naturally, this requires the use of sufficiently high-quality communication channels. It is believed that to successfully work with Frame Relay, the probability of an error in the channel should be no higher than 10-6-10-7. The quality provided by conventional analog lines is usually one to three orders of magnitude lower. The second difference between Frame Relay networks is that currently almost all of them implement only the mechanism of permanent virtual connections ( PVC ). This means that when you connect to a Frame Relay port, you must determine in advance which remote resources you will have access to. The principle of packet switching - many independent virtual connections in one communication channel - remains here, but you cannot select the address of any network subscriber. All resources available to you are determined when you configure the port. Thus, on the basis of Frame Relay technology, it is convenient to build closed virtual networks used to transmit other protocols through which routing is carried out. A virtual network's "closedness" means that it is completely inaccessible to other users on the same Frame Relay network. For example, in the USA, Frame Relay networks are widely used as backbones for the Internet. However, your private network can use Frame Relay virtual circuits on the same lines as Internet traffic - and be completely isolated from it. Like X.25 networks, Frame Relay provides a universal transmission medium for virtually any application. The main application of Frame Relay today is the interconnection of remote LANs. In this case, error correction and information recovery are carried out at the level of LAN transport protocols - TCP, SPX, etc. Losses for encapsulating LAN traffic in Frame Relay do not exceed two to three percent. The absence of error correction and complex packet switching mechanisms characteristic of X.25 allows information to be transmitted over Frame Relay with minimal delays. Additionally, it is possible to include a prioritization mechanism that allows the user to have a guaranteed minimum information transfer rate for the virtual channel. This capability allows Frame Relay to be used to transmit latency-critical information such as voice and video in real time. This relatively new capability is becoming increasingly popular and is often the main reason for choosing Frame Relay as the backbone of an enterprise network. It should be remembered that today Frame Relay network services are available in our country in no more than one and a half dozen cities, while X.25 is available in approximately two hundred. There is every reason to believe that as communication channels develop, Frame Relay technology will become more common - primarily where X.25 networks currently exist. Unfortunately, there is no single standard that describes the interaction of different Frame Relay networks, so users are locked into one service provider. If it is necessary to expand the geography, it is possible to connect at one point to the networks of different suppliers - with a corresponding increase in costs. There are also private Frame Relay networks operating within the same city or using long-distance (usually satellite) dedicated channels. Building private networks based on Frame Relay allows you to reduce the number of leased lines and integrate voice and data transmission.

Ethernet/Fast Ethernet

Ethernet is the most popular local network topology. It is based on the IEEE 802.3 standard. Ethernet has evolved significantly over the years to support new media and features that were not included in the original standard. Available bandwidth can either be shared among multiple users using hubs, or provided entirely to individual PCs using switches. Not long ago, a clear trend has emerged towards providing users of desktop stations with full-duplex communication channels of 10 Mbit/s. This trend was able to take root thanks to the advent of low-cost Ethernet switches, which made it possible to create high-performance, multifunctional networks without high costs.

Fast Ethernet technology was developed to provide more bandwidth to the devices that needed it, primarily servers and desktop switches. Fast Ethernet is based on the Ethernet standard; This means that implementing this high-speed technology does not require restructuring the existing infrastructure, replacing the management system, or retraining the IT department staff. It is now one of the most popular high-speed technologies - it is inexpensive, stable and fully compatible with existing Ethernet networks. Fast Ethernet networks can use fiber optic (100Base-FX) or copper (100Base-TX) cables. Full duplex communication is supported.

All information system administrators are faced with the challenge of providing Fast Ethernet channels to connect the most powerful desktop stations and servers without disrupting the work of those users who have enough Ethernet 10Base-T. This is precisely why technology for automatically recognizing the speed of an Ethernet/Fast Ethernet network is needed. With this technology, the same device supports both 10Base-T and 100Base-TX. The same switch will provide support for Ethernet and Fast Ethernet, providing desktop stations with more bandwidth, combining 10 and 100 Mbps hubs, and without introducing any changes to the experience of those users who are completely satisfied with 10 Mbps links. In addition, when working with a switch that automatically detects the data transfer rate, there is no need to configure each of the ports separately. This is one of the most effective ways to selectively increase bandwidth in areas where congestion occurs, while still fully maintaining the potential for further bandwidth expansion in the future.

Gigabit Ethernet

Gigabit Ethernet technology fully retains the traditional simplicity and manageability of Ethernet and Fast Ethernet, making it easy to integrate into existing local area networks. The use of this technology makes it possible to increase the bandwidth of the backbone network by an order of magnitude compared to Fast Ethernet. The additional bandwidth allows you to cope with the challenges associated with unplanned changes to the network structure and the addition of new devices to the network, and eliminates the need for constant adjustments to the network. Gigabit Ethernet is ideal for network backbones and server links because it provides high bandwidth at low cost, does not require a change from the traditional Ethernet frame format, and is supported by existing network management systems.

The emergence of the 802.3ab standard, which allows the use of copper cable as a Gigabit Ethernet medium (though at distances of no more than 100 meters), is another important argument in favor of this technology. It should also be noted that IEEE is working on a new 10 Gbit/s standard.

ATM

ATM is a popular technology for local area network backbones. Its use promises significant benefits for large organizations, since it provides close integration between local and geographically distributed networks and is characterized by a high level of fault tolerance and redundancy. To transmit data over the network, communication channels OC-3 (155 Mbit/s) and OC-12 (622 Mbit/s) are used. Just to compare the numbers, these numbers are less than Gigabit Ethernet, but ATM uses alternative methods for allocating bandwidth; By setting one or another level of Quality of Service (QoS), you can guarantee the provision of the bandwidth necessary for the operation of the application. The traffic control capabilities provided by ATM technology enable complete application certainty and service delivery across complex networks. ATM technology has important advantages over existing methods of data transmission in local and global networks, which should lead to its widespread use throughout the world. One of the most important advantages of ATM is providing high speed information transfer (wide bandwidth). ATM eliminates the differences between local and wide area networks, turning them into a single, integrated network. Combining the scalability and efficiency of hardware information transmission inherent in telephone networks, the ATM method provides a cheaper expansion of network capacity. This is a technology solution that can meet future needs, so many users often choose ATM more for its future than today's relevance. ATM standards unify procedures for accessing, switching and transmitting information of various types (data, speech, video, etc.) in one communication network with the ability to operate in real time. Unlike earlier LAN and WAN technologies, ATM cells can be transmitted over a wide range of media - from copper wire and fiber optic cable to satellite links, at any transmission speeds reaching today's limit of 622 Mbit/s. ATM technology provides the ability to simultaneously serve consumers with different requirements for the throughput of a telecommunication system. ATM technology has been gradually making its way into corporate infrastructures for several years now. Users build an ATM network in stages, operating it in parallel with their existing systems. Of course, first of all, ATM technology will have an impact on global networks, and to a lesser extent on trunk communication lines connecting several local area networks. A recent Sege Research survey of 175 users asked which technologies they intended to use on their networks in 1999. ATM has overtaken Ethernet in popularity. More than 40% of users would like to install Ethernet at 100 Mbit/s, and about 45% plan to use ATM at 155 Mbit/s. Quite unexpectedly, it turned out that 28% of respondents intend to use ATM at 622 Mbit/s. A few words about the relationship between ATM and Gigabit Ethernet. Each of these technologies has its own, fairly clearly defined niche. For ATM, these are the backbone networks of a group of buildings integrated into a corporate network, and the backbones of global networks. For Gigabit Ethernet, these are local network backbones and communication lines with high-performance servers. The problems of traffic exchange between Gigabit Ethernet and ATM and the problems of transparent routing are successfully solved. Cisco Systems recently developed a special ATM module for the Catalyst 8500 routing switch. This module allows routing between ATM and Ethernet ports.

Building a corporate network

When building a geographically distributed corporate network, all the technologies described above can be used. At the local network level, there is no alternative to Ethernet technologies, including Fast Ethernet and Gigabit Ethernet; Category 5 twisted pair cable is preferable as a physical transmission medium. To connect remote users, the simplest and most affordable option is to use telephone communication. Where possible, ISDN networks may be used. To connect network nodes in most cases, global data networks are used. Even where it is possible to lay dedicated lines, the use of packet switching technologies makes it possible to reduce the number of necessary communication channels and, importantly, ensure compatibility of the system with existing global network equipment. Connecting your corporate network to the Internet makes sense if you need access to relevant services. Using the Internet as a data transmission medium makes sense only when other methods are not available and financial considerations outweigh the requirements for reliability and security. If you will use the Internet only as a source of information, it is better to use the “connection on demand” technology, that is, a connection method where a connection to an Internet node is established only on your initiative and for the right time. This dramatically reduces the risk of unauthorized entry into your network from the outside. The simplest way to provide such a connection is to dial into the Internet via a telephone line or, if possible, via ISDN. Another more reliable way to provide on-demand connectivity is to use a leased line and Frame Relay protocol. In this case, the router on your end should be configured to break the virtual connection when there is no data for a certain time and re-establish it when access to data is required. Widespread connection methods using PPP or HDLC do not provide this opportunity. If you want to provide your information on the Internet (for example, set up a WWW or FTP server), the on-demand connection is not applicable. In this case, you should not only use access restriction using a Firewall, but also isolate the Internet server from other resources as much as possible. A good solution is to use a single point of connection to the Internet for the entire geographically distributed network, the nodes of which are connected to each other using virtual X channels. 25 or Frame Relay. In this case, access from the Internet is possible to a single node, while users in other nodes can access the Internet using an on-demand connection. To transfer data within a corporate network, it is also worth using virtual channels of packet switching networks. The main advantages of this approach are versatility, flexibility, and safety. When building a corporate information system, both X.25 and Frame Relay or ATM can be used as a virtual network. The choice between them is determined by the quality of communication channels, the availability of services at connection points and, last but not least, financial considerations. Today, the costs of using Frame Relay for long-distance communications are several times higher than for X.25 networks. At the same time, higher information transfer speeds and the ability to simultaneously transmit data and voice may be decisive arguments in favor of Frame Relay. In those areas of the corporate network where leased lines are available, Frame Relay technology is more preferable. In addition, telephone communication between nodes is possible via the same network. For Frame Relay, it is better to use digital communication channels, but even on physical lines or voice-frequency channels you can create a quite effective network by installing the appropriate channel equipment. Where it is necessary to organize broadband communications, for example when transmitting video information, it is advisable to use ATM. To connect remote users to the corporate network, access nodes of X.25 networks, as well as their own communication nodes, can be used. In the latter case, the required number of telephone numbers (or ISDN channels) must be allocated, which can be prohibitively expensive.

In preparing this article, materials from the sites www.3com.ru and www.race.ru were used

ComputerPress 10"1999