Types and characteristics of data transmission media in information networks. Concept of data transmission medium Two main types of data transmission medium
Selection and justification of the data transmission medium
1. General characteristics of the data transmission medium
Data transmission media are divided into two categories. Cable transmission medium (media) - with a central conductor enclosed in a plastic sheath.
Cables are widely used in small local area networks. The cable typically transmits signals at the lower end of the electromagnetic spectrum, which is normal electrical current and sometimes radio waves.
Wireless data transmission media involves the use of higher frequencies of the electromagnetic spectrum.
These are radio waves, microwaves and infrared rays. Such an environment is necessary for mobile computers or networks that transmit data over long distances. It is usually used in enterprise networks and wide area networks (a cell phone uses a microwave signal to transmit the signal).
Networks spanning multiple geographic locations often use a combination of cable and wireless transmission media.
When choosing the optimal media type, you should know the following characteristics of the data transmission medium:
Price;
Difficulty of installation;
Bandwidth;
Signal attenuation;
Susceptibility to electromagnetic interference (EMI, Electro-Magnetic Interference);
Possibility of unauthorized eavesdropping.
Price. The cost of each data transmission medium should be compared with its performance and available resources.
Difficult to install. The complexity of the installation depends on the specific situation, but some general comparison of transmission media can be done. Some types of media install with simple tools and don't require much training, while others require extensive training and are best left to professionals to install.
Bandwidth. The capabilities of a data transmission medium are usually measured in terms of bandwidth. In communications, the concept of "bandwidth" refers to the range of frequencies transmitted by a data transmission medium. In networks, it is estimated by the number of bits that can be transmitted through a given medium per second. Cable bandwidth is also affected by signal transmission methods.
Number of nodes. An important characteristic of a network is the number of computers that can be easily connected to network cables. Each network cable system has a natural number of nodes, exceeding which requires the use of special devices: bridges, routers, repeaters and hubs to expand the network.
Signal attenuation. Electromagnetic signals weaken during transmission. This phenomenon is called attenuation.
Electromagnetic interference. Electromagnetic interference (EMI) affects the transmitted signal. They are caused by external electromagnetic waves that distort the desired signal, making it difficult for the receiving computer to decode. Some communication media are more susceptible to electromagnetic interference than others. Interference is also called noise.
The following can be used as a data transmission medium in electronic communications:
· coaxial cable;
· twisted pair of wires (twistedpair);
· fiber optic cable;
· infrared radiation;
· microwave radio range;
· radio range.
Today, the vast majority of computer networks in most cases use wires or cables for connections.
Thus, Belden, a leading cable manufacturer, publishes a catalog where it offers more than 2,200 types. Fortunately, most networks use only three main groups of cables:
1. coaxial cable;
2. twisted pair (twisted pair):
Unshielded Twisted Pair (UTP);
Shielded Twisted Pair (STP);
3. fiber optic cable.
2. Twisted pair cables
Twisted pairs of wires are used in the cheapest and today, perhaps, the most popular cables.
A twisted-pair cable consists of several pairs of twisted insulated copper wires in a single dielectric (plastic) sheath. It is quite flexible and easy to lay.
Typically the cable contains two or four twisted pairs. Unshielded twisted pairs are characterized by poor protection from external electromagnetic interference, as well as poor protection from eavesdropping for the purpose of, for example, industrial espionage.
Interception of transmitted information is possible both using the contact method (using two needles stuck into the cable) and using the non-contact method, which boils down to radio interception of electromagnetic fields emitted by the cable. To eliminate these shortcomings, shielding is used.
In the case of shielded twisted pair STP, each of the twisted pairs is placed in a metal braided shield to reduce cable emissions, protect against external electromagnetic interference and reduce the mutual influence of pairs of wires on each other (crosstalk). Naturally, shielded twisted pair is much more expensive than unshielded twisted pair, and when using it it is necessary to use special shielded connectors, so it is much less common than unshielded twisted pair.
The main advantages of unshielded twisted pair cables are the ease of installation of connectors at the ends of the cable, as well as the ease of repairing any damage compared to other types of cable. All other characteristics are worse than other cables.
According to the EIA/TIA568 standard, there are five categories of unshielded twisted pair (UTP) cables.
3. Coaxial cables
A coaxial cable is an electrical cable consisting of a central wire and a metal braid, separated by a layer of dielectric (internal insulation) and placed in a common outer sheath.
Until recently, coaxial cable was the most widely used cable, due to its high noise immunity (due to metal braiding), as well as higher permissible data transfer rates than in the case of twisted pair (up to 500 Mbit/s) and large permissible transmission distances ( up to 1 km and above).
It is more difficult to mechanically connect to it for unauthorized wiretapping of the network, and it also produces noticeably less electromagnetic radiation to the outside.
However, installation and repair of coaxial cable is much more complicated than twisted pair cable, and its cost is higher (it is approximately 1.5-3 times more expensive compared to twisted pair cable). Installing connectors at the ends of the cable is also more difficult. Therefore, it is now used less frequently than twisted pair.
Coaxial cable is mainly used in networks with a bus topology.
If the braid is grounded at two or more points, not only the network equipment, but also the computers connected to the network may fail. Terminators must be matched with the cable, that is, their resistance must be equal to the characteristic impedance of the cable.
For example, if a 50 ohm cable is used, only 50 ohm terminators are suitable for it.
There are two main types of coaxial cable:
1) thin cable, having a diameter of about 0.5 cm, is more flexible;
2) thick (thick) cable, having a diameter of about 1 cm, is much more rigid. It is a classic version of coaxial cable, which has almost completely been replaced by more modern thin cable.
A thin cable is used for transmission over shorter distances than a thick one, since the signal attenuates more strongly in it. But a thin cable is much more convenient to work with: it can be quickly routed to each computer, while a thick cable requires rigid fixation on the wall of the room.
Connecting to a thin cable (using BNC bayonet type connectors) is simpler and does not require additional equipment, but to connect to a thick cable you need to use special, rather expensive devices that pierce its shell and establish contact - both with the central core and with the screen.
A thick cable is approximately twice as expensive as a thin one. Therefore, thin cable is used much more often.
Cost per place. Thin coaxial cable has a lower price per workstation, about $25. You can purchase these cables with connectors already attached.
Anyone can lay such cables - they are simply connected in a chain from computer to computer.
Laying thick coaxial cable usually costs about $50 per station. In addition, transceivers will be required for each station (about $100).
Distance restrictions. The total bus length on thin coaxial cable is limited to 185 m. Thick coaxial cable has a total limit of 500 m (in non-repeater structures).
4. Fiber optic cables
Fiber optic (aka fiber optic) cable is a fundamentally different type of cable compared to the two types of electrical or copper cable considered.
Information on it is transmitted not by an electrical signal, but by a light one. Its main element is transparent fiberglass, through which light travels over vast distances (up to tens of kilometers) with insignificant attenuation.
The structure of a fiber optic cable is very simple and similar to the structure of a coaxial electric cable, only instead of a central copper wire, thin glass fiber (with a diameter of about 1-10 microns) is used, and instead of internal insulation, a glass or plastic shell is used, which does not allow light to escape beyond the fiberglass.
Fiber optic cable has exceptional characteristics in terms of noise immunity and secrecy of transmitted information.
In principle, no external electromagnetic interference can distort the light signal, and this signal itself does not in principle generate external electromagnetic radiation.
It is almost impossible to connect to this type of cable for unauthorized network eavesdropping, as this requires breaking the integrity of the cable.
The information transmission medium is those communication lines (or communication channels) through which information is exchanged between computers. The vast majority of computer networks (especially local ones) use wired or cable communication channels, although there are also wireless networks that are now increasingly used, especially in laptop computers.
There are 4 types of data transmission media:
· Twisted pair cables
· Coaxial cables
Fiber optic cables
· Wireless communication channels
Twisted pairs of wires are used in cheap and today, perhaps, the most popular cables. A twisted-pair cable consists of several pairs of insulated copper wires twisted in pairs in a single dielectric (plastic) sheath. It is quite flexible and easy to lay. Twisting the wires allows you to minimize the inductive coupling of cables to each other and reduce the influence of transient processes.
Typically, the cable includes two (Fig. 4.1) or four twisted pairs.
Rice. 4 ,1.
Unshielded twisted pairs are characterized by poor protection from external electromagnetic interference, as well as from eavesdropping, which can be carried out for the purpose of, for example, industrial espionage. Moreover, interception of information transmitted over the network is possible both using the contact method (for example, using two needles stuck into the cable), and using the non-contact method, which boils down to radio interception of electromagnetic fields emitted by the cable. Moreover, the effect of interference and the amount of radiation outside increases with increasing cable length. To eliminate these shortcomings, cable shielding is used.
In the case of shielded twisted pair STP, each of the twisted pairs is placed in a metal braided shield to reduce cable emissions, protect against external electromagnetic interference and reduce the mutual influence of pairs of wires on each other (crosstalk). In order for the screen to protect against interference, it must be grounded. Naturally, shielded twisted pair cable is noticeably more expensive than unshielded cable. Its use requires special shielded connectors. Therefore, it is much less common than unshielded twisted pair.
The main advantages of unshielded twisted pair cables are the ease of installing connectors at the ends of the cable, as well as repairing any damage compared to other types of cable. All other characteristics are worse than other cables. For example, for a given transmission speed, signal attenuation (the decrease in signal level as it travels through the cable) is greater than that of coaxial cables. If we take into account the still low noise immunity, it is clear why communication lines based on twisted pairs are usually quite short (usually within 100 meters). Currently, twisted pair cables are used to transmit information at speeds up to 1000 Mbit/s, although the technical problems encountered at such speeds are extremely complex.
A coaxial cable is an electrical cable consisting of a central copper wire and a metal braid (screen), separated from each other by a layer of dielectric (internal insulation) and placed in a common outer shell (Fig. 4.2).
Figure 4.2
Until recently, coaxial cable was very popular, which is due to its high noise immunity (thanks to metal braiding), wider bandwidths than twisted pair cables (over 1 GHz), and large permissible transmission distances (up to a kilometer). It is more difficult to mechanically connect to it for unauthorized wiretapping of the network; it also produces noticeably less electromagnetic radiation to the outside. However, installation and repair of coaxial cable is much more complicated than twisted pair cable, and its cost is higher (it is approximately 1.5 - 3 times more expensive). Installing connectors at the ends of the cable is also more difficult. Nowadays it is used less frequently than twisted pair. The EIA/TIA-568 standard includes only one type of coaxial cable used in an Ethernet network.
Coaxial cable is mainly used in networks with a bus topology. In this case, terminators must be installed at the ends of the cable to prevent internal signal reflections, and one (and only one!) of the terminators must be grounded. Without grounding, the metal braid does not protect the network from external electromagnetic interference and does not reduce the radiation of information transmitted over the network into the external environment. But when the braid is grounded at two or more points, not only network equipment can fail, but also computers connected to the network. Terminators must be matched with the cable; their resistance must be equal to the characteristic impedance of the cable. For example, if a 50 ohm cable is used, only 50 ohm terminators are suitable for it.
Less commonly, coaxial cables are used in networks with a star topology (for example, a passive star in an Arcnet network). In this case, the matching problem is significantly simplified, since external terminators at the free ends are not required.
There are two main types of coaxial cable:
· thin cable, having a diameter of about 0.5 cm, is more flexible;
· thick cable, about 1 cm in diameter, much more rigid. It is a classic version of coaxial cable, which has almost completely been replaced by modern thin cable.
A thin cable is used for transmission over shorter distances than a thick one, since the signal in it is more attenuated. But a thin cable is much more convenient to work with: it can be quickly routed to each computer, while a thick cable requires rigid fixation on the wall of the room. Connection to a thin cable (using bayonet-type BNC connectors) is simpler and does not require additional equipment. And to connect to a thick cable, you need to use special, rather expensive devices that pierce its shell and establish contact with both the central core and the screen. A thick cable is approximately twice as expensive as a thin cable, so thin cable is used much more often.
As with twisted pair cables, an important parameter of a coaxial cable is the type of outer sheath it has. Similarly, in this case, both non-plenum (PVC) and plenum cables are used. Naturally, Teflon cable is more expensive than PVC cable. Typically, the type of sheath can be distinguished by color (for example, Belden uses yellow for PVC cable, and orange for Teflon cable).
Typical signal propagation delay values in a coaxial cable are about 5 ns/m for a thin cable, and about 4.5 ns/m for a thick cable.
There are options for double-shielded coaxial cable (one shield is located inside the other and is separated from it by an additional layer of insulation). Such cables have better noise immunity and eavesdropping protection, but they are a little more expensive than regular ones.
Currently, coaxial cable is considered obsolete; in most cases, it can be replaced by twisted pair or fiber optic cable. And new standards for cable systems no longer include it in the list of cable types.
Fiber optic (aka fiber optic) cable is a fundamentally different type of cable compared to the two types of electrical or copper cable discussed. Information on it is transmitted not by an electrical signal, but by a light one. Its main element is transparent fiberglass, through which light travels over vast distances (up to tens of kilometers) with insignificant attenuation.
Drawing. 4.3.
The structure of a fiber optic cable is very simple and similar to the structure of a coaxial electrical cable (Figure 4.3). Only instead of a central copper wire, thin fiberglass (about 1 - 10 microns in diameter) is used here, and instead of internal insulation, a glass or plastic shell is used, which does not allow light to escape beyond the fiberglass. In this case, we are talking about the mode of so-called total internal reflection of light from the boundary of two substances with different refractive indices (the glass shell has a much lower refractive index than the central fiber). There is usually no metal braiding on the cable, since shielding from external electromagnetic interference is not required. However, sometimes it is still used for mechanical protection from the environment (such a cable is sometimes called an armored cable; it can combine several fiber optic cables under one sheath).
Fiber optic cable has exceptional characteristics in terms of noise immunity and secrecy of transmitted information. In principle, no external electromagnetic interference can distort the light signal, and the signal itself does not generate external electromagnetic radiation. It is almost impossible to connect to this type of cable for unauthorized network eavesdropping, since this would compromise the integrity of the cable. The theoretically possible bandwidth of such a cable reaches 1012 Hz, that is, 1000 GHz, which is incomparably higher than that of electrical cables. The cost of fiber optic cable is constantly falling and is now approximately the same as the cost of thin coaxial cable.
However, fiber optic cable also has some disadvantages.
The most important of them is the high complexity of installation (micron precision is required when installing connectors; the attenuation in the connector greatly depends on the accuracy of the fiberglass cleaved and the degree of its polishing). To install connectors, welding or gluing is used using a special gel that has the same refractive index of light as fiberglass. In any case, this requires highly qualified personnel and special tools. Therefore, most often, fiber optic cable is sold in the form of pre-cut pieces of different lengths, at both ends of which the required type of connectors are already installed. It should be remembered that poor installation of the connector sharply reduces the permissible cable length, determined by attenuation.
We must also remember that the use of fiber optic cable requires special optical receivers and transmitters that convert light signals into electrical signals and vice versa, which sometimes significantly increases the cost of the network as a whole.
Fiber optic cables allow signal branching (special passive splitters (couplers) for 2-8 channels are produced for this), but, as a rule, they are used to transmit data only in one direction between one transmitter and one receiver. After all, any branching inevitably greatly weakens the light signal, and if there are many branches, then the light may simply not reach the end of the network. In addition, the splitter also has internal losses, so that the total signal power at the output is less than the input power.
Fiber optic cable is less durable and flexible than electrical cable. The typical allowable bend radius is about 10 - 20 cm, with smaller bend radii the central fiber may break. Does not tolerate cables and mechanical tension, as well as crushing influences.
The fiber optic cable is also sensitive to ionizing radiation, which reduces the transparency of the glass fiber, that is, increases the attenuation of the signal. Sudden changes in temperature also have a negative impact on it, and the fiberglass can crack.
Fiber optic cable is used only in networks with a star and ring topology. There are no coordination or grounding problems in this case. The cable provides ideal galvanic isolation of network computers. In the future, this type of cable is likely to replace electrical cables, or at least greatly displace them. Copper reserves on the planet are depleting, but there are more than enough raw materials for glass production.
In addition to cable channels, computer networks sometimes also use cable-free channels. Their main advantage is that no wiring is required (no need to make holes in walls, secure cables in pipes and gutters, lay them under raised floors, above suspended ceilings or in ventilation shafts, search for and repair damage). In addition, network computers can be easily moved within a room or building, since they are not tied to anything.
The radio channel uses information transmission over radio waves, so theoretically it can provide communication over many tens, hundreds and even thousands of kilometers. The transmission speed reaches tens of megabits per second (much depends on the selected wavelength and encoding method).
The peculiarity of the radio channel is that the signal is freely emitted into the air, it is not locked into a cable, so compatibility problems arise with other sources of radio waves (radio and television broadcasting stations, radars, amateur radio and professional transmitters, etc.). The radio channel uses transmission in a narrow frequency range and modulation of a carrier frequency signal by an information signal.
The main disadvantage of the radio channel is its poor protection from eavesdropping, since radio waves propagate uncontrollably. Another big drawback of the radio channel is poor noise immunity.
For local wireless networks (WLAN - Wireless LAN), radio connections are currently used over short distances (usually up to 100 meters) and within line of sight. The two most commonly used frequency ranges are 2.4 GHz and 5 GHz. Transfer speed - up to 54 Mbit/s. A common option is 11 Mbit/s.
WLANs allow wireless network connections to be established within a limited area (usually inside an office or university building or in public places such as airports). They can be used in temporary offices or other locations where cabling is not feasible, or as an addition to an existing wired LAN to allow users to work while moving around the building.
The popular Wi-Fi (Wireless Fidelity) technology allows you to organize communication between computers from 2 to 15 using a hub (called an access point, Access Point, AP), or several hubs if there are from 10 to 50 computers. In addition, this technology gives the ability to connect two local networks at a distance of up to 25 kilometers using powerful wireless bridges. For example in Fig. 4.4 shows the connection of computers using one access point. It is important that many mobile computers (laptops) already have a built-in Wi-Fi controller, which greatly simplifies their connection to a wireless network.
Figure 4.4
The radio channel is widely used in global networks for both terrestrial and satellite communications. In this application, the radio channel has no competitors, since radio waves can reach anywhere in the world.
If we talk about possible topologies, then most naturally all wireless communication channels are suitable for a bus-type topology, in which information is transmitted simultaneously to all subscribers. But when using narrowly directed transmission and/or frequency division across channels, it is possible to implement any topology (ring, star, combined topologies) both on the radio channel and on the infrared channel.
A communication line generally consists of a physical medium through which electrical information signals, data transmission equipment and intermediate equipment are transmitted. Synonymous with the term communication line is the term communication channel.
Physicaltransmission medium (medium) can be a cable, that is, a set of wires, insulating and protective sheaths and connecting connectors, as well as the earth's atmosphere or outer space through which electromagnetic waves propagate.
Depending on the data transmission medium, communication lines are divided into the following:
wired (aerial);
cable (copper and fiber optic);
radio channels of terrestrial and satellite communications.
Wired (overhead) communication lines are wires without any insulating or shielding braiding, laid between poles and hanging in the air. Such communication lines traditionally carry telephone or telegraph signals, but in the absence of other options, these lines are also used to transmit computer data. The speed and noise immunity of these lines leave much to be desired. Today, wired communication lines are quickly being replaced by cable lines.
Cable lines are quite complex structures. The cable consists of conductors enclosed in several layers of insulation: electrical, electromagnetic, mechanical, and also, possibly, climatic. In addition, the cable can be equipped with connectors that allow you to quickly connect various equipment to it. There are three main types of cable used in computer networks: twisted pair copper cables, copper coaxial cables, and fiber optic cables.
A twisted pair of wires is called twisted pair. Twisted pair exists in a shielded version (Shielded Twistedpair, STP), when a pair of copper wires is wrapped in an insulating shield, and unshielded (Unshielded Twistedpair, UTP) when the insulating wrap is missing. Twisting the wires reduces the effect of external interference on the useful signals transmitted along the cable. Coaxial cable has an asymmetrical design and consists of an internal copper core and braid, separated from the core by a layer of insulation. There are several types of coaxial cable, differing in characteristics and areas of application - for local networks, for wide area networks, for cable television, etc. Optical fiber cable consists of thin (5-60 microns) fibers through which light signals travel. This is the highest quality type of cable - it provides data transmission at very high speeds (up to 10 Gbit/s and higher) and, moreover, better than other types of transmission media, protects data from external interference.
Terrestrial and satellite radio channels are formed using a transmitter and receiver of radio waves. There are a large number of different types of radio channels, differing both in the frequency range used and in the channel range. The short, medium and long wave bands (KB, MW and LW), also called amplitude modulation (AM) bands based on the type of signal modulation method used in them, provide long-distance communication, but at a low data transfer rate. The fastest channels are those operating in the ultra-short wave (VHF) range, which is characterized by frequency modulation (FM), as well as in the ultra-high frequency range (microwaves). In the microwave range (above 4 GHz), signals are no longer reflected by the Earth’s ionosphere and stable communication requires direct visibility between the transmitter and receiver. Therefore, such frequencies are used either by satellite channels or radio relay channels, where this condition is met.
In computer networks today, almost all described types of physical data transmission media are used, but the most promising are fiber optic ones. Today, both backbones of large territorial networks and high-speed communication lines of local networks are built on them. Twisted pair is also a popular medium, characterized by an excellent quality-to-cost ratio and ease of installation. Using twisted pair cables, end users of networks are usually connected at distances of up to 100 meters from the hub. Satellite channels and radio communications are most often used in cases where cable communications cannot be used - for example, when a channel passes through a sparsely populated area or to communicate with a mobile network user, such as a truck driver, a doctor making a round, etc.
Classification of networks by type of medium for data transmission
Based on the type of medium for data transmission, networks are divided into wired(copper coaxial cable, twisted pair, optical fiber, etc.) and wireless(radio channels, data transmission in the infrared range, etc.).
Classification of networks by information transmission speed
Based on the information transmission speed, networks can be divided into low-speed (up to 10 Mbit/s), medium-speed (up to 100 Mbit/s) and high-speed (over 100 Mbit/s).
Classification of networks by transmission method
According to the method of data transmission, we can distinguish:
circuit switching networks;
packet switching networks.
In circuit switching networks it is assumed that there is a dedicated route between the source and the destination, a typical example being the telephone network. It is ineffective, since the channel is reserved for the entire duration of the connection; the advantage of this technology is its transparency, since the channel is established for the entire duration of the connection.
In packet switching networks long messages are broken into short packets. Each packet moves from the sender to the recipient through intermediate network nodes. The main advantage is flexibility, sharing of the same communication channels, the ability to change the priority of transmitted information, the disadvantage is the inability to guarantee timely delivery of packets.
Classification of networks according to the role of computers in them
Based on the role of computers in networks, the following types of networks can be distinguished:
peer-to-peer network (p2p) - peer-to-peer network;
client\server network (server-based network) - network with a dedicated server;
mixed networks.
Server- a specially dedicated high-performance computer that controls the operation of the network and/or provides its resources (software, services, files, devices) to other computers on the network and responds to customer requests.
Client computer (client, workstation)- a computer of an ordinary network user that gains access to the resources of the server (servers).
Network Administrator- a person with authority to manage computers, users and resources on a network.
Network administration- network management: setting up network equipment, providing access to data, security, working with users.
Peer-to-peer networks
In a peer-to-peer network, all computers have equal rights. Each of them can act as both a server and a client, each user is the administrator of his own computer, as a result, in such networks, chaos often becomes the norm.
Advantages:
ease of installation and configuration;
independence of individual computers and their resources from each other;
inexpensive to deploy and maintain;
no administrator needed.
Flaws:
users must remember as many passwords as there are network resources;
backup for each computer;
finding information is difficult;
low security.
The number of computers in peer-to-peer networks usually does not exceed 10. Examples include home networks and small office networks.
Dedicated server network
Networks with a dedicated server are usually created in large organizations.
Advantages:
centralized management of user accounts, security and access;
the user only needs one password.
Flaws:
A server malfunction can render the entire network inoperable;
availability of qualified personnel to maintain the network;
high cost.
Physical structure of networks
The physical structure of the network is determined primarily by the medium that will be used for data transmission. It depends on the environment what network equipment will be chosen to create it, and what topology the resulting network will have.
27. Network equipment.
Equipment (terminal equipment)
To create a network environment using cables, special connectors, fixed at their ends. Then the cable is inserted at one end into network adapter(network card) installed in a computer and allowing it to be connected to a network, and others to any communication device(hub, bridge, switch, router, gateway, etc.) If used wireless network adapter, then interaction with the network occurs due to signal transmission between the adapter and access point connected to the local network.
Network adapters (network cards) required to connect to a network environment. Modern computers are usually equipped with Ethernet and Wi-Fi adapters . The network adapter must have the required connector for connecting the connector and a unique physical address (MAC address) used to uniquely identify the computer in a given network segment. To determine the MAC address, you can use, for example, the command:
You can also find information about the “Physical Address” in the properties of the network adapter.
Repeaters and Amplifiers(at the physical level) amplify the transmitted signal.
Hubs organize a working group, represents the active central element of the star. They work on a physical level. Their main task is to receive, amplify and relay the signal received from one computer to all other active ports. No signal processing is performed.
Bridges and switches (bridge and switch) connect two or more network segments, separating the traffic in them, serve to connect networks of the same type (using the same protocols). They help reduce the number of collisions on the network, as they maintain a table of correspondence between their ports and computer MAC addresses. These devices operate not only at the physical, but also at the network layer of the OSI model. The difference between bridges and switches is that a bridge can only transmit one frame at a time, while a switch works with several ports in parallel. Most modern networks are built on switches.
Routers work at the network level. It is used in networks with complex configurations that use different data transmission methods to effectively handle traffic. Their task is to analyze addresses and determine the best route for delivering a data packet. Of course, routers also work at lower levels of the OSI model - they restore the level and shape of the transmitted signal, like bridges and switches - they help avoid collisions. At the same time, routers change transmitted frames, filter network traffic, keep statistics on transmitted data, authorize the user, allow the construction of virtual local networks, etc. Gateways- devices that allow you to combine heterogeneous systems using different network architectures and working with different protocols.
Modems(modulator-demodulator) connect the transmitting device with communication channels; it operates at the data link level, for example, it allows computers to transmit data over telephone wires.
28. Access to the transmission medium.
Closely related to network topology is the concept method of access to the transmission medium, which defines how computers should send and receive data over the network. An example would be:
multiple access with carrier sense and collision detection; If the cable is free, any computer can start transferring data, the rest wait for the transfer to complete. If a collision occurs, transmission is suspended for a random amount of time, after which another attempt is made to transmit data. This method is used in Ethernet networks.
multiple access with carrier sense and collision avoidance; This method differs from the previous one in that before transmitting data, the computer sends a special packet to the network, informing other computers of its intention to start broadcasting. Throughput is reduced. Used in wireless networks.
token passing. A block of data called marker.
The data transfer is carried out by the computer that “captured” the token. There are no collisions.
Typically, the network topology and access to the transmission medium are determined by the network equipment on which the network is built.
29. Topology.
In the context of a computer network, the concept of topology means the way network devices (end systems, stations, hosts) and cable infrastructure are connected to each other. Common network topologies are bus, ring, and star.
Common bus is a network topology in which stations are connected to a common transmission medium, which is a linear cable. The transmitted signal propagates along the entire length of the cable and is received by all stations, but it is processed only by the computer whose hardware MAC address of the network adapter is written in the frame as the recipient address.
This topology is easy to implement and low cost. The disadvantages include:
difficulty scaling, it is difficult to increase the number of computers in a segment of such a network;
at any given time the transmission can be carried out only one of the computers. If two or more computers start transmitting at the same time, problems arise. collisions, leading to the fact that the data has to be transmitted again. The performance of such a network decreases with a large volume of transmitted information and the number of computers;
If a bus is damaged, the entire network stops working.
Currently, this topology is rarely used.
Ring
Ring - a network topology in which stations are connected to repeaters forming a closed loop. Transmitted signals propagate around the ring in one direction and can be received by all stations.
Based on this topology, it is possible to build long-distance networks, since each computer acts as a repeater. Due to the absence of collisions, the network is resistant to overloads. The disadvantages include:
the time of information transmission increases, as it is transmitted along the ring;
adding new computers requires stopping the entire network;
failure of at least one computer or cable segment disrupts the operation of the entire network;
Therefore, two rings are usually laid, which increases the cost of the network.
Star
Star is a local network topology in which all stations are connected to a central switch. In this case, the central node is called a hub, or concentrator.
The hub functions as a repeater, restores incoming signals and forwards them to all other computers and devices connected to it.
This network organization is more reliable. Used quite often. If “smart” network devices (bridge, switch, router) are installed instead of a hub, this allows not only relaying, but also control of transmitted signals.
Mesh
In such networks, there are several routes for delivering information. They have high fault tolerance. Deploying such networks based on cable connections is quite expensive, as it requires an increased amount of cable and more complex setup of network equipment.
More often this topology is implemented in wireless networks.
Mixed (hybrid) networks
Real networks are constantly expanding and modernizing, so usually the network topology is a combination of several basic topologies.
Star-Bus (star on a bus)
Star-Ring (star on a ring)
Hybrid Mesh (hybrid cellular structure)
Tree (tree, star on star)
The choice of topology depends on a number of factors, such as reliability, scalability and performance, cost, and is usually determined by the medium used for data transmission.
30. Wired technologies.
AC Wires
Can be used when transmitting data over short distances.
Telephone wires
Modem, digital/analog communication, baud.
public switched telephone network (PSTN);
digital network of integrated services (ISDN - Integrated Services Digital Network);
digital communication (ADSL - Asymmetric Digital Subscriber Line).
"Twisted pair"
A twisted pair consists of two insulated copper wires twisted together, representing one communication channel; several twisted pairs are combined into a cable wrapped in a dense protective sheath. Twisting reduces crosstalk from adjacent wires in a pair. Used in telephone networks and for networks inside buildings. It is susceptible to interference, so shielding using a metal braid or shell is more often used in networks; unshielded for telephone lines.
Speed up to 100 Gbps, up to 2 km without repeaters.
The most common type of cable for creating computer networks.
Coaxial cable
Similar to twisted pair, it consists of two conductors, but differs in design and can operate over a wider frequency range. Coaxial cable consists of a hollow outer cylindrical conductor with an inner wire inside. The inner conductor is in an insulator, the outer one is covered with a sheath or screen. Diameter from 1 to 2.5 cm. Can be used for transmitting data over long distances, in particular for transmitting television signals, international telephony, and computer networks.
Thin - speed up to 10 Mbit/s over a distance of up to 185 m.
Thick - speed up to 10 Mbps at a distance of up to 500 m
Currently used quite rarely for creating networks.
Fiber optic cable
An optical fiber is a thin medium (from 2 to 125 microns in diameter) capable of transmitting a light beam. Various types of glass and plastic are used to make optical fiber. The lowest losses are achieved in ultrapure fused silica fiber. It consists of three concentric sections, the two inner ones are made of glass with different refractive indices, and there is a light-absorbing shell on top. The fibers are collected into optical cables. It has greater throughput, less attenuation, and electromagnetic isolation.
Speed up to 10 Gbit/sec, segment length up to 40,000 m, operating wavelength in the range from 850 to 1300 nm.
Disadvantages include the high cost of the cable, complex installation, and the need to use additional transceivers that convert light signals into electrical signals and vice versa.
Advantages of cable connection:
high throughput;
noise immunity.
Flaws:
difficulties during installation (access to the sewerage system, installation inside finished buildings, assignment of workplaces);
cable management requires maintenance.
The Ethernet architecture actually combines a set of standards that have both common features and differences. Data transfer speed up to 10 Gbit/s. Ethernet technology uses almost any type of cable and allows for scaling and increasing network capacity. Therefore, today the Ethernet architecture is the most common in local networks.
31. Wireless technologies.
For telecommunications, electromagnetic waves can be used that propagate through the atmosphere or in a vacuum, namely (in order of increasing throughput and increasing frequency of wave oscillation):
radio communications (cellular, satellite) (from 30 MHz to 1 GHz). Provides a high range of information transmission;
communications in the microwave range (from 2 to 40 GHz) (Bluetooth, WLAN);
infrared communication (from 3 1011 to 2 1014 Hz). Used to transmit data over short distances, for example, to interact with portable (mobile) devices. The source and receiver must be in direct line of sight;
light radiation in the visible range. Rarely used.
Typically, low frequency signals propagate from the antenna in all directions; higher frequency signals can be focused into a directional beam.
If a directional antenna is not used and there are no obstacles in the path, radio waves propagate evenly in all directions and the signal strength drops in proportion to the square of the distance between the transmitter and the receiver. They are used where cable channels do not exist or their creation for some reason is impossible or too expensive for transmitting television, radio and other analog signals.
Advantages
the ability to create in hard-to-reach places;
do not require support or maintenance.
Flaws:
are not noise-resistant;
less protected from eavesdropping than wired networks (WEP and WPA security level).
Wi-Fi (Wireless Fidelity, wireless accuracy)- technology that provides connection of mobile users to the Internet. Combines several standards based on the IEEE 802.11 (a, b, g) specification. Low data transmission range.
WiMAX (Worldwide Interoperability for Microwave Access) is the commercial name for the 802.16 wireless communications standard, adopted in January 2003 and supported by an industry group. Unlike the already quite popular Wi-Fi wireless access, WiMAX is less tied to specific bands - its variants are designed for frequencies from 2 to 11 GHz and from 10 to 66 GHz. The channel width occupied on the air by two devices can be selected within wider limits than Wi-Fi - from 1.5 to 28 MHz. “Sophisticated” modulation allows the radio spectrum to be used with an efficiency of 5 bits per hertz (Wi-Fi has 2.7 bits per hertz), so speeds reach 134 Mbps (in a 28 MHz channel). But the main advantage of WiMAX is its range: the maximum distance between devices can reach 50 km. In addition, there may be no direct line of sight between the source and the receiver. Signal strength and greater resistance to reflections allow WiMAX to work even where Wi-Fi is powerless.
Bluetooth technology(IEEE 802.15.1) uses a 2.4 GHz radio signal. It has low power consumption, allows devices to establish interaction with minimal user participation, low range and throughput.
32. Protocols.
A protocol is the rules (agreements, standards) for transmitting information on a network. The protocol defines the format and order of messages exchanged between two or more devices, as well as the actions performed when sending and/or receiving messages or when other events occur.
Since different systems enter into the interaction process, it makes no sense to implement a network connection in the form of a single, monolithic block; the concept of protocol architecture is introduced, when instead of one module serving the interaction of computers, there is a structured set of modules that implement communication functions.
The following analogy can be drawn: when the director of one enterprise writes a letter to the director of another enterprise, then, having written the letter and indicating the person to whom it is addressed, he gives it to the secretary. The secretary finds the recipient's address, puts the letter in the envelope, makes a note about the outgoing in his documents, and takes the letter to the post office. The post office ensures the delivery of the letter, which the secretary receives, makes a note in the inbox, that is, you can always check if the letter is missing, prints it out and places it on the director’s desk. Each level of interaction does not care about what happens below it, is confident that it will work correctly, but can also check that it is working correctly. At each level, additional identifying information specific to that level is added to the letter.
Thus, we can consider a simplified architecture of network communication protocols. The process of network interaction involves: applications, computers and networks, taking this into account, it is natural to solve the problem of interaction at three independent levels:
network access level;
transport layer;
application level.
The network access layer ensures the exchange of data between the computer and the network; the computer transmitting the data tells the network the address of the computer to which this data is intended, and the type of network can be very different.
All tasks related to transmission reliability are performed by the transport layer, checking that all data reaches the recipient and is received by him in the right order.
At the application level, applications perform the actions they need, interact with the user, if necessary, request the network environment from the transport layer, for example, to transfer files.
At each level, service information necessary for data transmission (headers) is added; each level can have its own division into exchange units (packets).
At each level, information is required to identify the recipient, so at the application level this will be the access point to the service (port), the transport level the logical name of the computer, and at the network level the network interface name (MAC address).
Different manufacturers use different data formats and different data exchange protocols so that they can communicate with each other, and common standards are being developed. There are several common protocol architectures:
TCP/IP protocol stack;
OSI reference model;
IBM network architecture tied to this company's equipment.
33. TCP/IP protocol stack.
Although there is no official model for this model, it is by far the most common, and it can be divided into five layers of protocols that form a protocol stack:
application layer;
transport layer;
network layer;
link layer;
physical level.
The physical layer is responsible for the physical interface between the device and the data transmission medium; it works with the characteristics of the transmission medium, the nature of the signals, the data transfer rate, etc. Supports basic local network technologies - Ethernet, Wi-Fi, Token Ring, Bluetooth, etc.
The data link layer organizes data transmission in the existing physical environment.
The network layer is responsible for routing messages as they pass through the Internet Protocol (IP).
The transport layer is responsible for the reliability of data transmission. Supports two protocols:
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Question Evolution of computing systems
1) Batch processing systems:
1950s – the appearance of the first computers.
Batch processing systems were built on the basis of the mainframe - a powerful and reliable universal-purpose computer. Users had punched cards containing data and program commands, operators entered these cards into the computer, and printed results were received the next day.
Maximizing the efficiency of computing power
Neglect of user interests
2)Multi-terminal system
Distributed data input/output.
Centralized processing.
1960s: emergence of multi-terminal time-sharing systems.
LAN prototype.
The computer was put at the disposal of several users at once, each with a terminal, the reaction time of the aircraft was quite short.
Computer networks
A computer is a collection of computers connected by communication lines (cables, network adapters, telecommunications equipment).
Classification of networks by territorial basis
LAN - MAN - WAN
Global networks - Wide Area Networks (WAN).
Data transmission over hundreds and thousands of kilometers
Chronologically appeared first (50s-60s)
Evolved from telephone networks
Initially were slow and unreliable
Today WAN:
Represent rings or backbone
Basic speed 2.5 Gbit/s
10-Gbit/s, 40-Gbit/s solutions are common
Complex data monitoring and recovery procedures are applied
Local networks - Local Area Networks (LAN).
Concentrated on an area of 1-2 km.
Speed up to 10 Gbps
Wide range of services
The most important stage of development is the formation of standard LAN technologies: Ethernet, Token Ring, FDDI.
Metropolitan Area Networks (MAN)
Distances of several tens of kilometers
Cheaper compared to WAN
Connection speeds 1-40 Gbit/s
Used to combine existing LANs and connect to the WAN
Current trends
Global networks are close in quality to local ones
2) LANs began to use switches, routers, gateways => the ability to build complex networks
Question. Seven-layer OSI model.
Physical layer
The physical layer defines the electrical, mechanical, procedural and
functional characteristics of activation, maintenance and deactivation of a physical channel between end systems. Physical layer specifications define characteristics such as voltage levels, voltage timing, physical information transfer rates, maximum information transfer distances, physical connectors, and other similar characteristics. Unit of data: Bit
Data Link Layer
The data link layer ensures reliable transit of data across a physical channel. In performing this task, the data link layer deals with issues of physical addressing, network topology, line discipline (how the end system should use the network link), fault notification, orderly delivery of data blocks, and information flow control. Unit of data: Frame
Network layer
The network layer is a complex layer that provides connectivity and route selection between two end systems connected to different "subnets" that may be located in different geographic locations.
In this case, a "subnet" is essentially an independent network cable (sometimes called a segment).
Because two end systems wishing to communicate may be separated by significant geographic distance and multiple subnets; the network layer is the routing domain. Routing protocols select optimal routes through a sequence of interconnected subnets. Traditional network layer protocols transmit information along these
Routes. Unit of data: Packet
Transport layer
The transport layer is concerned with issues such as performing reliable transport of data across the internetwork. By providing reliable services, the transport layer provides mechanisms for establishing, maintaining, and orderly termination of virtual circuits, transport fault detection and recovery systems, and information flow control (to prevent a system from becoming flooded with data from another system). Unit of data: Datagram/Block of data (datagramm)
Session layer
As its name indicates, the session layer establishes, manages, and terminates sessions between application tasks. Sessions consist of a conversation between two or more view objects. The session layer synchronizes the dialogue between objects of the representative layer and manages the exchange of information between them. The session layer provides the means to send information, class of service, and exception notification about problems at the session, presentation, and application layers. Unit of data: Message
Representative level
The representative layer is responsible for ensuring that information sent from the application layer of one system is readable by the application layer of another system. If necessary, the representative layer translates between multiple information representation formats by using a common information representation format.
Unit of data: Message
Application layer
The application layer is the OSI layer closest to the user. It differs from the other layers in that it does not provide services to any of the other OSI layers; however, it provides them for application processes that lie outside the scope of the OSI model. Examples of such application processes include spreadsheet programs, word processing programs, bank terminal programs, etc.
Unit of data: Message
As a data packet moves through the levels from top to bottom, each new level adds its own service information to the packet in the form of a header and, possibly, a trailer (information placed at the end of the message). This operation is called encapsulation top-level data in a lower-level packet
question. Classification of data transmission media.
Under data transmission medium understand the physical substance through which electrical signals are transmitted, used to transfer certain information presented in digital form.
The natural environment is the environment existing in nature - Unnatural. – specially created (cables, etc.)
Natural environments
- Atmosphere Electromagnetic waves are the most widely used data carriers in the atmosphere.
- Radio waves - electromagnetic waves with a frequency less than 6000 GHz (with a wavelength greater than 100 microns).
- Infrared radiation and visible light (laser)
Built Environments The main types of cables are: fiber optic, coaxial and twisted pair. In this case, both coax and twisted pair use a metal conductor to transmit signals, and a fiber-optic cable uses a light guide made of glass or plastic.
Coaxial cable
An important advantage is its ability to transmit many signals at the same moment. Each such signal is called a channel. All channels are organized at different frequencies, so they do not interfere with each other. It has a wide bandwidth; this means that it can transmit traffic at high speeds. It is also resistant to electromagnetic interference and is capable of transmitting signals over long distances.
twisted pair
A cable in which an insulated pair of conductors is twisted with a small number of turns per unit length. Twisting is carried out to reduce external interference.
Advantages: thinner, more flexible, easier to install, inexpensive.
Disadvantages: strong influence of external electromagnetic interference, possibility of information leakage,
strong signal attenuation.
Unshielded Twisted Pair (UTP)
CAT5 (frequency band 100 MHz) - 4 pairs, up to 100 Mbit/s when using 2 pairs and up to 1000 Mbit/s when using 4 pairs, is the most common network media still used in computer networks.
Shielded Twisted Pair (STP)
Foil twisted pair (FTP)
Foil Shielded Twisted Pair (SFTP)
Related information.