FDDI technology. Main characteristics. Features of the access method. Fault tolerance of technology. Physical layer of FDDI technology. fddi technology fddi networks are mainly used for

Due to the fact that the FDDI standard is used mainly in the construction of highways, this section will pay some attention to such concepts bridge And router. In addition, to understand the general concept of a LAN, the following is mentioned in more detail and about hub(hub).

Operating principle of the FDDI network

The FDDI network is a fiber optic token ring with a data transfer rate of 100 Mbps.

The FDDI standard was developed by the American National Standards Institute (ANSI) Committee X3T9.5. FDDI networks are supported by all leading network equipment manufacturers. The ANSI X3T9.5 committee has now been renamed X3T12.

Using fiber optics as a distribution medium can significantly expand the cable bandwidth and increase the distance between network devices.

Let's compare the throughput of FDDI and Ethernet networks for multi-user access. The acceptable level of utilization of an Ethernet network lies within 35% (3.5 Mbit/s) of the maximum throughput (10 Mbit/s), otherwise the probability of collisions becomes not too high and the cable throughput will sharply decrease. For FDDI networks, acceptable utilization can reach 90-95% (90-95 Mbit/s). Thus, the throughput of FDDI is approximately 25 times higher.

The deterministic nature of the FDDI protocol (the ability to predict the maximum delay when transmitting a packet over the network and the ability to provide guaranteed bandwidth for each station) makes it ideal for use in real-time network automated process control systems and in time-critical applications (for example, video transmission and audio information).

FDDI inherited many of its key properties from Token Ring networks. First of all, this is a ring topology and a token method of access to the medium.

However, FDDI also has a number of fundamental differences from Token Ring, making it a faster protocol. For example, the data modulation algorithm at the physical level has been changed. Token Ring uses a Manchester encoding scheme, which requires doubling the transmitted signal bandwidth relative to the transmitted data. FDDI implements a “five out of four” coding algorithm – 4B/5B, which ensures the transmission of four information bits with five transmitted bits. When transmitting 100 Mbits of information per second, 125 Mbits/sec is physically transmitted to the network, instead of 200 Mbits/sec, which would be required when using Manchester encoding.

Environment access management has also been optimized. In Token Ring it is based on a bit basis, and in FDDI it is based on parallel processing of a group of four or eight transmitted bits. This reduces the requirements for equipment speed.

Physically, the FDDI ring is formed by a fiber optic cable with two light-conducting fibers. One of them forms the primary ring, is the main one and is used for circulating data tokens. The second fiber forms a secondary ring, is a backup fiber and is not used in normal mode.

Stations connected to the FDDI network are divided into two categories.

1. Class A stations have physical connections to the primary and secondary rings (Dual Attached Station);
2. Stations of class B have a connection only to the primary ring (Single Attached Station - a one-time connected station) and are connected only through special devices called hubs.

The ports of network devices connected to the FDDI network are classified into 4 categories: A ports, B ports, M ports and S ports. Port A is the port that receives data from the primary ring and transmits it to the secondary ring. Port B is the port that receives data from the secondary ring and transmits it to the primary ring. The M (Master) and S (Slave) ports transmit and receive data from the same ring. The M port is used on the hub to connect Single Attached Station via the S port.

The X3T9.5 standard has a number of limitations. The total length of the double fiber-optic ring is up to 100 km. Up to 500 class A stations can be connected to the ring. The distance between nodes when using a multimode fiber-optic cable is up to 2 km, and when using a single-mode cable is determined mainly by the parameters of the fiber and receiving and transmitting equipment (can reach 60 km or more).

FDDI - (Fiber Distributed Data Interface) - a standardized specification for a network architecture for high-speed information transportation over fiber optic lines. Transportation speed is 100 Mbit/s. Logical topology - ring (double), access method - deterministic, with token transport. The access token is transported from station to station along the ring. A station that has a marker has the right to transmit information. The technology allows transportation of asynchronous and synchronous traffic. When transporting synchronous traffic, the ring initialization stage determines the bandwidth that is given to each station for transport. The entire remaining ring bandwidth can be allocated for asynchronous traffic. The real ring throughput can be 95 Mbit/s, but with significant delays in service. When minimizing latency, throughput can drop to 20 Mbps.

The maximum number of stations in the network is 500 with a double ring and 1000 with a single ring. The length between stations is up to 2 km with multimode and up to 45-60 km with single-mode cable, the length of a single ring is 200 km, a double ring is 100 km. FDDI technology can be analyzed as an improvement that manifests itself in increased fault tolerance, performance and an increase in network size relative to the number of nodes and the distance between them. Fault tolerance is increased by the second ring, which closes in the event of a break in the first ring. FDDI technology easily integrates with Token Ring and Ethernet, which makes it widely used in high-speed backbones.

The FDDI standard defines 4 components: SMT, MAC, PHY, PMD (Fig. 1).

SMT (Station Management) - indicates the configuration of rings and stations, algorithms for including a station in the ring and disconnecting it, etc. It generates diagnostic frames, controls access to the network and implements the integrity of the ring, redirects data traffic to a secondary ring in case of problems in the first. You can also use a secondary ring to increase throughput up to 200 Mbps.
MAC (Media Access Control) - specifies frame formats, addressing, CRC calculation algorithm, error handling. Corresponds to MAC - a sublayer of the OSI data link layer. Exchanges information with the higher-level LLC - sublevel.
PHY - (Physical) - indicates encoding and decoding, synchronization, framing of traffic. Refers to the physical layer of the OSI model.
PMD (Physical Medium Dependent) - determines the parameters of optical or electrical elements (cables, transceivers, connectors) characteristics of communication channels. Refers to the physical layer of the OSI model.

Figure - 1

The electrical implementation of the FDDI twisted pair architecture is called CDDI or TPDDI. SDDI defines the implementation of shielded STP Type 1 cable. Compared to the optical version, these technologies are cheaper. but the permitted length of communication channels between nodes is reduced to 100 m. Compared to optical, electrical versions are less standardized and compatibility between equipment from different manufacturers is not guaranteed.

Physical Layer Technologies

FDDI hardware ports have transceivers that implement separate lines for the received (Rx) and transmitted (Tx) signals. logical 4B/5B is used here, where each four bits of the source data is encoded with a 5-bit symbol. An effective transport speed of 100 Mbit/s is realized by a clock frequency of bit intervals of 125 MHz.

The transmission medium is twisted pair or optical fiber:

SMF-PMD is a single-mode fiber with laser sources. The permissible length of the canal is 40 -60 km.
MMF-PMD - implements multimode fiber as a transmission medium, the radiation source is an LED. The permissible channel length is 2 km.
LCF-PMD is a low-cost multimode fiber where the communication channel length is limited to 500 m.
TP PMD - twisted pair STP type 1 or UTP category 5, connectors Rj - 45. Two pairs of wires are implemented, length - 100 m.

For all optical options, the wavelength is 1300 nm, which is why MMF, LCF, SMF ports can be combined if the connection introduces acceptable attenuation. Physical topology FDDI networks - hybrid or ring, partial inclusion of star or tree subnets into the main network through a hub. Figure 2 shows an example in which the following connection types are implemented:

SAS - single connection station (only to the primary ring)
DAS - dual connection station (to both rings)
SAC - single connection concentrator, implements connections of single connection nodes
DAC - dual connection concentrator, implements connection to a double ring with single connection nodes

Figure - 2

Double (DAS) and single (SAS) connection stations have different ways of connecting to the ring (Fig. 3). DAS connection stations (class A) have two transceivers and can be built directly into the core network, into rings. In normal mode, the signal arriving at the Pri_In input is translated to the Pri_Out output, and during transport, the frame transported by the current station is wedged into this chain. The Sec_In - Sec_Out connection is implemented as a backup one. Single connection SAS stations, also known as Class B stations, have one transceiver and are built into the primary ring. There is only one In-Out connection for them. They can connect to the core network through a hub or bypass switch.

Figure - 3, a - single connection (SAS), b - double connection (DAS)

Hubs can also be single (SAC) or double (DAC) connections (Fig. 4). Their tasks include implementing the integrity of the logical ring, regardless of the parameters of the line and nodes connected to its ports. DAC implements the inclusion of SAS stations and SAC concentrators in a double logical ring, SAC - includes in a single one. With a 100% tree or star topology, without an explicit ring, the root concentrator implements a null-attachment concentrator.

Figure - 4, a - single connection (SAC), b - double connection (DAC)

Repeater— implements intermediate amplification of the optical signal; in some cases, a transition from single-mode to multimode fiber can be realized. Attenuator— implement a reduction in power at the receiver input to the nominal level.

Bypass switch— double or single, implements node bypass in case of its failure or shutdown. Such a device is placed between the ring and the station and implements one of two options for possible light flow switching schemes (Fig. 5). The switch connects the station to the ring if available allowing ready signal. When implementing bypass switches, you need to consider:

the implementation of such a switch is possible only when connecting stations with neighbors of the same type (only MM or SM) fibers. Otherwise, the connection between single-mode and multimode fiber is inoperable.
The total length of cables coming to the switch from neighboring stations should not exceed the limit for a given type of cables and ports with the attenuation parameter introduced by the switch (~ 2.5 dB).
The number of bypass switches is limited due to attenuation and cable length.

Figure - 5, a - station on, b - off

Splitters— devices that implement combining/branching of optical signals.

FDDI Interfaces and Ports

The standard describes 4 types of ports:

port A - reception from the primary ring, transport to the secondary (for dual connection devices)
port B - reception from the secondary ring, transportation to the primary (-//-)
port M (master) - reception and transmission from one ring. Connects to hubs to connect SAC or SAS.
port S (slave) - reception and transmission from one ring. Connects to hubs and single connection stations.

For a typical ring, there are rules for connecting ports:

port A is connected only from ports B and vice versa
port M is connected only with port S

Table 1 shows the port connection options. V - valid connections are marked, U - unwanted connections that can lead to unexpected topologies. X - absolutely unacceptable. P - connection of ports A and B with ports M, active connection of only port B (while it is alive).

Port 1	Port 2
	A	B	S	M
A	V,U	V	V,U	V,P
B	V	V,U	V,U	V,P
S	V,U	V,U	V,U	V
M	V,P	V,P	V	X

For FDDI technology, special optical duplex connectors have been developed, taking into account the multi-variant connection of transmitters and receivers, FDDI MIC (Media Interface Connector). The plugs on the cables have slots, and the sockets have protrusions; this system eliminates port switching errors (Fig. 6).

Figure - 6, a - for double connection, b - for single connection

Frame formats

Two types of packets can be transmitted in the FDDI ring: a token and each data/command (MAC Data/frame frame) (Fig. 7). Element lengths are in 5-bit characters (due to 4B/5B). The frame length cannot be more than 9000 characters.

Figure - 7

Frames and markers consist of:

Pre - Preamble, a special set of characters with which the station is synchronized and prepared for frame processing
SD - start separator, combination JK
ED - trailing delimiter, one or two T characters
FC - packet control byte.
DA - 2 or 6 byte destination address - unique, multicast or broadcast
SA - frame source address, similar to DA
Info - data field up to 4478 bytes long. Has superior level information (LLC) or control information
FCS - 4-byte CRC code
FS - frame status (12 bits)

Command frames (MAC frames) have the same structure as data frames, but the info field is always zero length. The command code is transmitted in the FC field, and the FS field is implemented to transmit the results.

Based on the contents of the Info field, two types of frames are distinguished - FDDI SNAP, FDDI 802.2. They are similar, with minor exceptions:

FDDI has two frame control bytes carrying its parameters and the frame status field. There are no analogues in Ethernet
Ethernet frames have a length field that is not implemented in FDDI (it is not needed)

Figure 8 shows the frame formats FDDI SNAP, FDDI 802.2.

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Coursework

Name of discipline: scomputers and telecommunications

Topic: HarFDDI technology acting

Introduction
1 FDDI technologies

1.4 Recommendation for using FDDI technology

2 Types of FDDI technology layers
Conclusion
Glossary
List of sources used
List of abbreviations

Applications

Introduction

This course work will discuss issues related to FDDI technology: its main characteristics, features of the access method, fault tolerance and recommendations for its use. Currently, this technology is the safest, but expensive. FDDI technology - fiber optic distributed data interface - is the first local network technology in which the data transmission medium is a fiber optic cable. Work on the creation of technologies and devices for the use of fiber-optic channels in local networks began in the 1980s, shortly after the start of industrial operation of such channels in territorial networks. The HZT9.5 problem group of the ANSI Institute developed in the period from 1986 to 1988. initial versions of the FDDI standard, which provides frame transmission at a speed of 100 Mbit/s over a double fiber-optic ring up to 100 km long. Although FDDI implementations are not as common today as Ethernet or Token Ring, FDDI has gained a significant following, which is increasing as the cost of the FDDI interface decreases. FDDI is often used as a technology backbone and as a means to connect high-speed computers in a local area. The relevance of this topic is that currently high-speed highways (100 Mbit/s) are built only on the basis of FDDI and ATM. All other well-known networks (for example, 100BaseT) operate over distances too short to be used as a corporate backbone. The objectives of this topic are to understand FDDI technology: its main characteristics, features of the access method, fault tolerance and recommendations for its use. The purpose of this work is that FDDI is the first local network technology in which the data transmission medium is a fiber optic cable. Next, the physical layer of FDDI technology will be considered. The physical layer is divided into two sublayers: the environment-independent PHY (Physical) sublayer and the environment-dependent PMD (Physical Media Dependent) sublayer. Next, the MAC layer will be considered. Let's find out what functions this level and operations perform. Using MAC layer operations, stations access the ring and transmit their data frames. In addition to the specifications of the PHY, PMD and MAC levels, the course work will consider the specification of the station management level (SMT), defined by the FDDI standard.

1 FDDI technologies

1.1 Main characteristics of FDDI technology

FDDI (Fiber Distributed Data Interface) technology - a fiber-optic distributed data interface - is the first local network technology in which the data transmission medium is a fiber-optic cable. Work on the creation of technologies and devices for the use of fiber-optic channels in local networks began in the 1980s, shortly after the start of industrial operation of such channels in territorial networks. The HZT9.5 problem group of the ANSI Institute developed in the period from 1986 to 1988. initial versions of the FDDI standard, which provides frame transmission at a speed of 100 Mbit/s over a double fiber optic ring up to 100 km long. FDDI technology is largely based on Token Ring technology, developing and improving its basic ideas. 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 Thru mode - “end-to-end” or “transit”. The secondary ring (Secondary) is not used in this mode. In the event of some type of failure where part of the primary ring cannot transmit data, the primary ring is combined with the secondary ring again to form a single ring. This mode of network operation is called Wrap, that is, “folding” or “folding” of 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. 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. 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 the Token Ring technology with mechanisms for reconfiguring the data transmission path in the network, based on the presence of redundant connections 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 access method changes only affect asynchronous traffic, which is not critical to small delays in frame transmission. For synchronous traffic, the token holding time remains a fixed value. A frame priority mechanism similar to that adopted in Token Ring technology is absent in FDDI technology. FDDI supports real-time network bandwidth allocation, which is ideal for a number of different types of applications. FDDI provides this support by designating two types of traffic: synchronous and asynchronous. Synchronous traffic can consume a portion of the total FDDI network bandwidth equal to 100 Mbps; the rest can be consumed by asynchronous traffic. Synchronous bandwidth is allocated to those stations that require continuous transmission capability. For example, the presence of such a feature helps with the transmission of voice and video information. Other stations use the rest of the bandwidth asynchronously. The SMT specification for the FDDI network defines a distributed bidding scheme for FDDI bandwidth. Asynchronous bandwidth is allocated using an eight-level priority scheme. Each station is assigned a certain level of priority for using asynchronous bandwidth. FDDI also allows long conversations where stations can temporarily use all asynchronous bandwidth. The FDDI priority mechanism can effectively block stations that cannot use synchronous bandwidth and have too low a priority for using asynchronous bandwidth. FDDI stations use an early token release algorithm, like Token Ring networks with a speed of 16 Mbit/s. 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. 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, despite the fact that FDDI technology was developed and standardized by the ANSI Institute, and not by the IEEE committee, it fully fits into the structure of the 802 standards. FDDI is defined by independent technical specifications: 1. Media Access Control (MAC) defines the method of accessing the media, including packet format, token handling, addressing, CRC (Cycle Redundancy Check) algorithm, and error recovery mechanisms. 2.Physical Layer Protocol (PHY) (Physical Layer Protocol) - defines information encoding/decoding procedures, requirements for synchronization, framing and other functions. 3. Station Management (SMT) - defines the configuration of FDDI stations, ring network configuration and ring network management features, including insertion and deletion of stations, initialization, isolation and troubleshooting, scheduling and collection of statistics. 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.

1.2 Features of the FDDI access method

signal optical fiber coding

To transmit synchronous frames, the station always has the right to capture the token upon arrival. In this case, the marker holding time has a predetermined fixed value. If the FDDI ring station needs to transmit an asynchronous frame, then to determine the possibility of capturing the token the next time it appears, the station must measure the time interval that has passed since the previous arrival of the token. This interval is called the token rotation time (TRT). The TRT interval is compared with another value - the maximum permissible time for the marker to turn around the T_Org ring. If in Token Ring technology the maximum allowable token rotation time is a fixed value (2.6 s based on 260 stations in the ring), then in FDDI technology stations agree on the value of T_Org during ring initialization. Each station can offer its own T_Org value; as a result, the minimum time of the times proposed by the stations is set for the ring. This allows the needs of the applications running on the stations to be taken into account. In general, synchronous applications (real-time applications) need to send data to the network in small chunks more often, while asynchronous applications need to access the network less often, but in larger chunks. Preference is given to stations transmitting synchronous traffic. Thus, the next time a token arrives to transmit an asynchronous frame, the actual token rotation time TRT is compared with the maximum possible T_Org. If the ring is not overloaded, then the token arrives before the T_Org interval expires, that is, TRT< Т_Оpr. В этом случае станции разрешается захватить маркер и передать свой кадр (или кадры) в кольцо. Время удержания маркера ТНТ равно разности Т_Оpr - TRT, и в течение этого времени станция передает в кольцо столько асинхронных кадров, сколько успеет. Если же кольцо перегружено и маркер опоздал, то интервал TRT будет больше Т_Оpr. В этом случае станция не имеет права захватить маркер для асинхронного кадра. Если все станции в сети хотят передавать только асинхронные кадры, а маркер сделал оборот по кольцу слишком медленно, то все станции пропускают маркер в режиме повторения, маркер быстро делает очередной оборот и на следующем цикле работы станции уже имеют право захватить маркер и передать свои кадры. Метод доступа FDDI для асинхронного трафика является адаптивным и хорошо регулирует временные перегрузки сети.

1.3 Resiliency of FDDI technology

FDDI is characterized by a number of fault tolerance features. The main feature of fault tolerance is the presence of a double ring network. If any station connected to the double ring network fails, or loses power, or if the cable is damaged, the double ring network automatically “collapses” (“bends” inward) into a single ring. Simultaneous connection to the primary and secondary rings is called a double connection - Dual Attachment, DA. Connecting only to the primary ring is called a single connection - Single Attachment, SA. As FDDI networks increase in size, the likelihood of more ring network failures increases. If there are two ring failures, the ring will collapse in both cases, effectively segmenting the ring into two separate rings that cannot communicate with each other. Subsequent failures will cause additional segmentation of the ring. Failure-critical devices, such as routers or mainframes, can use another fault tolerance technique called dual homing to provide additional redundancy and improve uptime assurance. In dual wiring, the fault-critical device is connected to two hubs. One pair of hub channels is considered an active channel; the other pair is called the passive channel. The passive link remains in support mode until it is determined that the primary link (or the hub to which it is connected) has failed. If this happens, the passive channel is automatically activated. The FDDI standard provides for the presence of end nodes in the network - stations (Station), as well as concentrators (Concentrator). For stations and hubs, any type of connection to the network is acceptable - both single and double. Accordingly, such devices have the appropriate names: SAS (Single Attachment Station), DAS (Dual Attachment Station), SAC (Single Attachment Concentrator) and DAC (Dual Attachment Concentrator). In the event of a single cable break between dual-connected devices, the FDDI network can continue to operate normally by automatically reconfiguring internal frame paths between hub ports. A cable break twice will result in two isolated FDDI networks. To maintain operation during a power outage in dual-connection stations, that is, DAS stations, the latter must be equipped with optical bypass switches (Optical Bypass Switch), which create a bypass path for the light fluxes when the power they receive from the station disappears. Finally, DAS stations or DAC hubs can be connected to two M ports of one ELE two hubs, creating a tree structure with primary and backup connections. By default, port B supports primary communication and port A supports backup communication. This configuration is called a Dual Homing connection. Fault tolerance is maintained by constantly monitoring the SMT level of hubs and stations for the time intervals of token and frame circulation, as well as the presence of a physical connection between adjacent ports on the network. There is no dedicated active monitor in an FDDI network - all stations and hubs are equal, and when abnormalities are detected, they begin the process of reinitializing the network and then reconfiguring it. Reconfiguration of internal paths in hubs and network adapters is performed by special optical switches that redirect the light beam and have a rather complex design.

A feature of FDDI technology is the combination of several properties that are very important for local networks:

High degree of fault tolerance;

Ability to cover large territories, up to the territories of large cities;

High data exchange speed;

Ability to support synchronous multimedia traffic;

Flexible mechanism for distributing ring capacity between stations;

Ability to operate with a ring load factor close to unity;

The ability to easily translate FDDI traffic into traffic of such popular protocols as Ethernet and Token Ring due to the compatibility of station address formats and the use of a common LLC sublayer. So far, FDDI is the only technology that has managed to combine all of the listed properties. In other technologies these properties are also found, but not in combination. Thus, Fast Ethernet technology also has a data transfer speed of 100 Mb/s, but it does not allow the network to be restored after a single cable break and does not make it possible to work with a large network load factor. One of the most important characteristics of FDDI is that it uses a light guide as the transmission medium. Fiber optics provide a number of advantages over traditional copper wiring, including data security (fiber optics does not emit electrical signals that can be intercepted), reliability (fiber optics is resistant to electrical interference), and speed (fiber optics has much higher potential bandwidth than copper cable). FDDI specifies two types of optical fiber used: single-mode (sometimes called monomode) and multimode. Modes can be thought of as beams of light rays entering an optical fiber at a certain angle. Single-mode fiber allows only one mode of light to propagate through an optical fiber, while multimode fiber allows multiple modes of light to propagate through an optical fiber. Because While the many modes of light propagating along an optical cable may travel different distances (depending on the angle of entry), and therefore reach their destination at different times (a phenomenon called modal dispersion), a single-mode fiber is capable of providing greater bandwidth and cable run lengths longer distances than multimode fibers. Because of these characteristics, single-mode fibers are often used as the backbone of university networks, while multimode fibers are often used to connect workgroups. Multimode fiber uses light-emitting diodes (LEDs) as light generators, while single-mode fiber typically uses lasers. You have to pay for this unique combination of properties - FDDI technology is the most expensive 100 MB technology today. Therefore, its main areas of application are campus and building backbones, as well as connecting corporate servers. In these cases, the costs turn out to be justified - the network backbone must be fault-tolerant and fast, the same applies to a server built on an expensive multiprocessor platform and serving hundreds of users. Many modern corporate networks are built using FDDI technology on the backbone in combination with Ethernet, Fast Ethernet and Token Ring technologies in floor and department networks.

A group of central servers is also usually connected to the FDDI backbone ring directly, using FDDI network adapters. Due to the emergence of cheaper technologies than FDDI 10 MB, such as Fast Ethernet and iOOVG-AnyLAN, FDDI technology will obviously not find widespread use when connecting workstations and creating small local networks, even with an increase in the speed of these stations and the availability of networks of multimedia information.

2 Types of technology levels FDDI

2.1 Description of the physical layer

FDDI technology uses 4V/5V logical coding combined with NRZI physical coding to transmit light signals over optical fibers. This circuit results in transmission of signals along the link at a clock frequency of 125 MHz. Since out of 32 combinations of 5-bit symbols, only 16 combinations are needed to encode the original 4-bit symbols, several codes were selected from the remaining 16 and are used as service codes. The most important service symbols include the Idle symbol, which is constantly transmitted between ports during pauses between the transmission of data frames. Due to this, stations and hubs of the FDDI network have constant information about the state of the physical connections of their ports. If there is no Idle symbol flow, a failure of the physical link is detected and the internal path of the hub or station is reconfigured, if possible. When two nodes are initially connected by cable, their ports first perform a physical connection establishment procedure. This procedure uses sequences of service symbols of the 4B/5B code, with the help of which a certain physical layer command language is created. These commands allow ports to find out from each other the port type (A, B, M or S) and decide whether the connection is valid. If the connection is correct, then a channel quality test is performed when transmitting 4B/5B code symbols, and then the functionality of the MAC layer of the connected devices is checked by transmitting several MAC frames. If all tests are successful, then the physical connection is considered established. The work of establishing a physical connection is controlled by the SMT station control protocol. The physical layer is divided into two sublayers: the environment-independent PHY (Physical) sublayer and the environment-dependent PMD sublayer. PMD sublevel: The PMD (physical layer medium) level determines the characteristics of the transport medium, including optical channels, power levels, regulates error rates, and specifies requirements for optical components and connectors. FDDI technology currently supports two PMD sublayers: for fiber optic cable and for Category 5 UTP. The latter standard appeared later than the optical one and is called TP-PMD. The optical fiber PMD sublayer provides the necessary means for transmitting data from one station to another over an optical fiber. Its specification defines: -the use of a 62.5/125 micron multimode fiber-optic cable as the main physical medium; requirements for the power of optical signals and maximum attenuation between network nodes. For a standard multimode cable, these requirements lead to a maximum distance between nodes of 2 km, and for a single-mode cable the distance increases to 10-40 km depending on the quality of the cable; - requirements for optical bypass switches and optical transceivers; - parameters of optical connectors MIC (Media Interface Connector), their markings; -- use for transmitting light with a wavelength of 1300 nm; representation of signals in optical fibers in accordance with the NRZI method. The TP-PMD sublayer determines the ability to transmit data between stations over twisted pair cables in accordance with the MLT-3 physical encoding method, which uses two potential levels: +V and -V to represent data in the cable. To obtain a signal spectrum that is uniform in power, the data passes through a scrambler before physical encoding. The maximum distance between nodes according to the TP-PMD standard is 100 m. The maximum total length of the FDDI ring is one hundred kilometers, the maximum number of dual-connected stations in the ring is 500. PHY sublayer: The PHY sublayer defines coding and modulation methods, as well as isolation rules inoperative station, which we will consider further. The FDDI optical path uses a 4B/5B code in which a group of 4 bits is encoded into a group of 5 bits called a symbol. Characters of 5 bits are selected in such a way that they contain no more than two consecutive “O”. FDDI uses 8 of the 16 symbols not used for data encoding as control words. These control words are used as delimiters and signal words.

Groups of 5 bits are transmitted using a potential code without returning to zero with inversion (NRZI - nonreturn to zero with inversion). In this encoding method, bits are represented as a signal that has two meanings. The signal changes its value when i appears in the original binary signal and does not change its value when o appears. Thus, the 4V/5V + NRZI signal changes value at least i times during the transmission of 3 bits. Phase-locked loop uses this signal feature to synchronize the 125 MHz oscillator in the signal receiver timer with a 16-bit preamble. Each node uses a u-bit elastic buffer. Note that the hopping frequency when transmitting a 4V/5V + NRZI signal is 125 MHz, while in the Manchester code the hopping would occur at a frequency of 200 MHz. 2.2 MAC layer In accordance with IEEE 802 standards, the channel layer in local networks consists of two sublevels - LLC and MAC. The FDDI standard does not introduce its own definition of the LLC sublayer, but uses its services described in the IEEE 802.2 LLC document. The MAC sublayer performs the following functions in FDDI technology: Supports services for the LLC sublayer. Forms a frame of a certain format. Manages the token transfer procedure. Controls the station's access to the medium. Addresses stations on the network. Copies frames destined for a given station to a buffer and notifies the LLC sublayer and the SMT station control unit of the frame's arrival. Generates a frame check sequence (CRC) and checks it for all frames circulating around the ring. Removes from the ring all frames generated by this station. Manages timers that control the logical operation of the ring - token retention timer, token turnover timer, etc. Maintains a number of event counters to help detect and isolate faults. Defines the mechanisms used by the ring to respond to error situations - frame corruption, frame loss, token loss, etc. Let's consider the operation of the MAC level using stations with a double connection and one MAC block, that is, a DA/SM station. In each MAC block, two processes operate in parallel: the process of transmitting symbols - MAC Transmit and the process of receiving symbols - MAC Receive. Due to this, the MAC can simultaneously transmit symbols of one frame and receive symbols of another frame. Over the FDDI network, information is transmitted in the form of two data blocks: a frame and a token. Let's consider the purpose of the frame fields: Preamble (PA). Any frame must be preceded by a preamble consisting of at least 16 Idle (I) characters. This sequence is designed to synchronize the RCRCLK generator, which ensures the reception of subsequent frame symbols. Starting Delimiter (SD). Consists of a pair of JK characters that uniquely define the boundaries for the remaining characters in the frame. Control field (Frame Control, FC).

Identifies the type of frame and details of working with it. It has an 8-bit format and is transmitted using two characters. It consists of subfields designated as CLFFZZZZ, which have the following purpose: C - indicates what type of traffic the frame carries - synchronous (value 1) or asynchronous (value o). L - determines the length of the frame address, which can consist of 2 bytes or 6 bytes. FF - frame type, can have a value of 01 to denote an LLC (user data) frame or oo to denote a MAC layer service frame. MAC-level service frames are frames of three types - frames of the Claim Frame ring initialization procedure, frames of the Beacon Frame logical fault signaling procedure, and frames of the SMT Frame ring control procedure. ZZZZ - details the frame type. Destination Address (DA) - identifies the station (unique address) or group of stations (group address) to which the frame is intended. May consist of 2 or 6 bytes. Source address (Source Address, SA) - identifies the station that generated this frame. The field must be the same length as the destination address field. Information (INFO) - contains information related to the operation specified in the control field. The field can be from 0 to 447S bytes in length (from 0 to 8956 characters). The FDDI standard allows the routing information of the Source Routing algorithm defined in the 802.5 standard to be placed in this field.

In this case, the combination 102 is placed in the two most significant bits of the source address field SA - group address, a combination that does not make sense for the source address, but indicates the presence of routing information in the data field. Check sequence (Frame Check Sequence, FCS) - contains a 32-bit sequence calculated using the standard CRC-32 method, adopted for other IEEE 802 protocols. The check sequence covers the FC, DA, SA, INFO and FCS fields. Ending Delimiter (ED) - contains a single Terminate (T) character indicating the frame boundary. However, behind it there are also signs of the frame status. Frame status (FS). The first three signs in the status field should be indicators of error (Error, E), address recognition (Address recognized, A) and frame copying (Frame Copied, C). Each of these indicators is encoded by one symbol, with the zero state of the indicator indicated by the symbol Reset (R), and the single state by Set (S). The standard allows equipment manufacturers to add their own indicators after the three required ones. The token consists essentially of one meaningful field - the control field, which in this case contains i in the C field and oooo in the ZZZZ field. Using MAC layer operations, stations access the ring and transmit their data frames. The cycle of frame transmission from one station to another consists of several stages: capture of a token by the station to which it is necessary to transmit the frame, transmission of one or more data frames, release of the token by the transmitting station, retransmission of the frame by intermediate stations, recognition and copying of the frame by the receiving station and removal of the frame from network by the sending station. Let's look at these operations. Token capture. If a station has the right to capture a token, then, after relaying the PA and SD token symbols to the output port, it removes the FC symbol from the ring, by which it recognized the token, as well as the final delimiter ED. It then transmits, following the already transmitted SD symbol, the symbols of its frame, thus forming it from the initial symbols of the token. Frame transmission. After removing the FC and ED fields of the token, the station begins to transmit frame symbols that the LLC layer provided it with for transmission.

The station can transmit frames until the token hold time expires. FDDI networks provide for the transmission of frames of two types of traffic - synchronous and asynchronous. Synchronous traffic is designed for applications that require guaranteed bandwidth for voice, video, process control, and other real-time applications. For such traffic, each station is given a fixed portion of the FDDI ring bandwidth, so a station is entitled to transmit synchronous traffic frames whenever it receives a token from the previous station. Asynchronous traffic is normal local network traffic that does not have high service latency requirements. A station can transmit asynchronous frames only if there is some unused bandwidth left to do so during the last round of the token around the ring. The time interval during which a station can transmit asynchronous frames is called the Token Holding Time (TNT). Each station independently calculates the current value of this parameter using the algorithm discussed below. During the transmission of symbols of its own frame, the station removes from the ring all symbols arriving from the previous station. This process is called MAC replacement (MAC Overwriting). The original source of the frame being removed from the network does not matter - it could be the given MAC node that previously placed this frame in the ring, or another MAC node. The process of deleting frames during transmission never results in the deletion of still unprocessed frames: if the network is operating correctly, then only truncated frames that are generated either when the token is captured or when the source station deletes its frame are deleted.

In any case, a truncated frame is a frame that has a start delimiter, but no end delimiter, and Idle characters are inserted instead of it and perhaps some other fields. If the characters to be deleted belong to a frame previously generated by this MAC node, then simultaneously with the removal of the frame from the ring, signs of the frame status from the FS field are checked - address recognition, copying and errors. If the error flag is set, then the MAC layer does not retransmit the frame, leaving this to the LLC layer or other upper layers of the communication protocol stack. The station stops transmitting frames in two cases: either when the TNT token retention time expires, or when all of its frames are transmitted before this period expires. After transmitting its last frame, the station generates a token and passes it on to the next station. Frame repetition. If the frame is not addressed to a given MAC node, then the latter must simply repeat each character of the frame on the output port. Each MAC node must count the number of complete frames it has received. Each station checks the repeated frame for errors using a check sequence. If an error is detected and the error flag is not set in the FS field, then the MAC node sets this flag in the frame and also increases the counter of erroneous frames recognized by this MAC node. Frame processing by the destination station. The destination station, having recognized its address in the DA field, begins to copy the frame characters into the internal buffer while repeating them on the output port. In this case, the destination station sets the address recognition flag. If the frame is copied to the internal buffer, then the copying flag b is also set. An error sign is also set if it was detected by a check using the control sequence. Removing a frame from the ring. Each MAC node is responsible for removing frames from the ring that it previously placed in it. This process is known as Frame Stripping. If the MAC node, when receiving its frame, is busy transmitting subsequent frames, then it deletes all the characters of the frame returned along the ring. If it has already released the token, then it repeats several fields of this frame at the output before recognizing its address in the SA field. In this case, a truncated frame appears in the ring, in which the SA field is followed by Idle symbols and there is no end delimiter. This truncated frame will be removed from the ring by some station that received it in its own transmission state.

3 Network management using the SMT specification

General characteristics of network management functions according to the SMT specification This specification defines the functions that each node in the FDDI network must perform. SMT monitors and manages all data link and physical layer processes occurring in a single station. In addition, each station's SMT process interacts with other stations' SMT processes to monitor and coordinate all operations in the FDDI ring. In this case, the SMT takes part in distributed peer-to-peer management of the ring. SMT includes three groups of functions

* Connection management - Connection Management (CMT);

* Ring management - Ring Management (RMT);

* Frame-Based Management (FBM). The main functions of SMT connection management are monitoring and managing physical connections organized by the physical layer. RMT ring management functions are to manage local MACs and the rings to which they are attached. The RMT functions are responsible for detecting duplicate addresses, as well as for launching the Claim Token ring initiation procedure and Beacon and Trace emergency procedures. FBM frame-based management functions allow a node to obtain information from other network nodes about their status and statistics about the traffic passing through them. This information is stored in the Management Information Base (MIB). - RMT ring management functions To perform its functions, the RMT node interacts with the local MAC node, the CMT connection management node, as well as other SMT nodes of the station. The RMT node performs the following functions: Notification of the status and presence of the local MAC node. The RMT is responsible for notifying other SMT nodes of: - the availability of a MAC node to transmit and receive frames and tokens; - the beginning or completion of the Beacon process in the local node; - detection of MAC address duplication; - start of the Trace function, which allows the node to exit the state of constant generation of fault signaling frames (Stuck Beacon state); - inoperability of the ring for a long time. Beacon process and exit from it. The Beacon process (alarm process) is used to isolate serious ring faults. The MAC node starts the Beacon process in the following situations: - the Claim Token ring initialization process has not completed within the allotted time; - the SMT node sent the MAC node a command to initiate the Beacon process. If a node enters the Beacon process, then it begins to transmit Beacon frames to the next node in the ring, in which either o or the address of the previous station, received in this case from SMT, is indicated as the destination address. One byte of the reason for starting the Beacon process is sent in the data field.

If a node receives a Beacon frame from another station, then it stops transmitting its Beacon frames and goes into frame repetition mode. Some time after an emergency occurs in the ring, all stations stop generating Beacon frames, except for one, which is located in the ring directly behind the station or cable section that is causing the emergency in the ring. A station that continues to generate Beacon frames enters the Stuck Beacon state. Each station's RMT process, when the station enters the Beacon process, starts a TRM (Ring Management) timer, which measures the period of time during which that station generates Beacon frames. If it exceeds the T_Stuck limit, the RMT process considers that the station has entered a permanent Stuck Beacon signaling state and that the configuration management node was unable to cope with the problem that arose in the ring. In this situation, the RMT node sends a so-called Directed Beacon along the ring to the ring control station. The destination address in the Directed Beacon frame specifies a special group address that the management station must recognize. The information field must contain the address of the previous station - the potential culprit of the problem. After transmitting a few Direct Beacon frames (for reliability), the RMT process initiates the Trace process. The Trace process is used to detect a fault domain - that is, a group of stations that are not operating correctly. The station that initiates the Trace process sends a signal about it to the station immediately preceding it in the ring - that is, to the previous neighbor. The Trace signal is transmitted in the form of a sequence of Halt and Quiet symbols.

The station that received the Trace signal and the station that transmitted the Trace signal are disconnected from the ring for a while and perform an internal path test, the so-called Path Test. The details of the Path Test are not defined by the SMT specification. Its general purpose is that a station must autonomously check the transmission of symbols and frames between all its internal nodes to ensure that it is not the cause of ring failure. If the internal path Path Test is successful, the SMT process sends a PC_Start signal to the configuration management units, which causes them to begin restoring the physical connections of the ports.

If Path Test is not performed, then the station remains disconnected from the ring. 3-3 Frame-Based Management Functions This part of the SMT function, called FBM9, is the most high-level because it requires the ring to be operational and able to transmit frames between stations. The FBM specification defines a large number of frame types that exchange stations: Neighborhood Information Frames (NIF) allow a station to find out the addresses of its previous and subsequent neighbors, determine the presence of duplicate addresses, and also check the performance of its MAC node in the absence of other traffic . Neighbor address information can be collected by the control station to construct a logical ring map. Status Information Frames (SIFs) are used by a station to send a request for configuration and operational parameters to another station. Using SIF frames, for example, data on station status, frame counter value, frame priorities, and manufacturer identifier are requested and transmitted.

Status Report Frames (SRF) allow a station to periodically send information about its status around the ring that may be of interest to the ring control station. This could be, for example, information about a change in the state of the station, about unwanted connections, or about an excessively high rate of erroneous frames. Parameter Management Frames (PMFs) are used by the station to read or write parameter values in the SMT MIB management information database. Echo Frames (ECF) allow a station to check communication with any station on the ring. The SMT frame has its own header of a rather complex format, which is embedded in the information field of the MAC frame.

The header is followed by the SMT information field, which contains data about several station parameters. Each parameter is described by three fields - the parameter type field, the parameter length field, and the parameter value field. Using PMF frames, a management station can access the values of parameters stored in the station's Management Information Base, MIB. The SMT specification defines the composition of SMT MIB objects and their structuring. The SMT MIB consists of 6 subtrees. Subtree 5 is reserved for the future. The Internet community has developed a standard MIB for FDDI networks. The RFC 1285 standard defines the objects that are needed to manage FDDI stations using the SNMP protocol. The Internet FDDI MIB is a subtree of the Transmission branch of the MIB-II base. The objects defined in RFC 1285 are identical to the SMT MIB objects. However, the object names and their syntax differ from the SMT MIB specification. These differences must be taken into account by manufacturers of hardware and control software. Typically, compatibility between these two specifications is achieved through FDDI/SNMP mediation agents built into the equipment, as well as through specification translation functions in network management systems. 3.4 Properties of FDDI networks 1) Synchronous and asynchronous transmission Connection to the FDDI network stations can transmit their data to the ring in two modes - synchronous and asynchronous. Synchronous mode works as follows. During the network initialization process, the expected time for the token to traverse the ring is determined - TTRT (Target Token Rotation Time).

Each station that has captured the token is given a guaranteed time to transmit its data to the ring. After this time, the station must finish transmitting and send the token into the ring. Each station, at the moment of sending a new token, turns on a timer that measures the time interval until the token returns to it - TRT (Token Rotation Timer). If the token returns to the station before the expected TTRT bypass time, then the station can extend the time it transmits its data to the ring after the end of the synchronous transmission. This is what asynchronous transmission is based on. The additional time interval for transmission by the station will be equal to the difference between the expected and actual time for the marker to traverse the ring. From the algorithm described above, it can be seen that if one or more stations do not have enough data to fully use the time slot for synchronous transmission, then the unused bandwidth immediately becomes available for asynchronous transmission by other stations. Asynchronous bandwidth is allocated using an eight-level priority scheme. Each station is assigned a certain level of priority for using asynchronous bandwidth. FDDI also allows long conversations where stations can temporarily use all asynchronous bandwidth. The FDDI priority mechanism can effectively block stations that cannot use synchronous bandwidth and have too low a priority for using asynchronous bandwidth. 2) Cable system The FDDI PMD (Physical medium-dependent layer) substandard defines a multimode fiber-optic cable with a light guide diameter of 62.5/125 microns as the basic cable system. It is possible to use cables with other fiber diameters, for example: 50/125 microns. Wavelength -1300 nm. The average power of the optical signal at the station input must be at least -31 dBm. With such an input power, the probability of a bit error when relaying data by a station should not exceed 2.5*10-10. With an increase in the input signal power by 2 dBm, this probability should decrease to 10-12.

The standard defines the maximum permissible level of signal loss in a cable as equal to dBm. The FDDI substandard SMF-PMD (Single-mode fiber Physical medium-dependent layer) defines the requirements for the physical layer when using single-mode fiber optic cable. In this case, a laser LED is usually used as a transmitting element, and the distance between stations can reach 6o and even 100 km. FDDI modules for single-mode cable are produced, for example, by Cisco Systems for its Cisco 7000 and AGS+ routers. Singlemode and multimode cable segments in an FDDI ring can be interleaved. For these Cisco routers, it is possible to select modules with all four port combinations: multimode-multimode, multimode-singlemode, singlemode-multimode, singlemode-singlemode. Cabletron Systems Inc. produces Dual Attached repeaters - FDR-4000, which allow you to connect a single-mode cable to a class A station with ports designed to operate on a multimode cable. These repeaters make it possible to increase the distance between FDDI ring nodes to 40 km. The physical layer substandard CDDI (Copper Distributed Data Interface - distributed data interface over copper cables) defines the requirements for the physical layer when using shielded (IBM Type 1) and unshielded (Category 5) twisted pairs. This greatly simplifies the installation process of the cable system and reduces the cost of it, network adapters and hub equipment. Distances between stations when using twisted pairs should not exceed 100 km. Lannet Data Communications Inc. produces FDDI modules for its hubs, which allow operation either in standard mode, when the secondary ring is used only for fault tolerance in the event of a cable break, or in extended mode, when the secondary ring is also used for data transmission. In the second case, the bandwidth of the cable system is expanded to 200 Mbit/s. 3) Character encoding. FDDI encodes information using symbols. Symbol - 5 bit sequence. Two characters make up one byte. This 5-bit encoding provides 16 data symbols (o-F), 8 control symbols (Q, H, I, J, K, T, R, S) and 8 violation symbols (V).

Conclusion

In this course work, the following issues were considered: the main characteristics of FDDI technology, its functions, recommendations for using FDDI technology; FDDI physical layer, its PMD and PHY sublayers; MAC level, its functions, operations. FDDI technology was the first to use fiber optic cable in local networks, as well as operate at speeds of 10 Mbps. It should be noted that there is a connection between the Token Ring and FDDI technologies: both are characterized by a ring topology and a token access method. Today, FDDI technology is the most fault-tolerant local network technology. Fiber Distributed Data Interface technology is the first local network technology that used fiber optic cable as a data transmission medium.

Today, most network technologies support fiber optic cables as a physical layer option, but FDDI remains the most mature high-speed technology, with standards that have stood the test of time and established standards so that equipment from different manufacturers shows a good degree of compatibility. FDDI is one of the most common backbone technologies and has been used as such for quite some time.

The effectiveness of FDDI highways is due to the impartial distribution of access to the medium based on the transfer of tokens and high resistance to failures and damage. FDDI uses variable length packets, unlike ATM. Because ATM technology provides higher levels of scalability and guaranteed quality of service, its use is rapidly expanding. This is especially clear in networks with high load and different types of traffic (voice, data, video). Therefore, its main areas of application are campus and building backbones, as well as connecting corporate servers. In these cases, the costs turn out to be justified - the network backbone must be fault-tolerant and fast, the same applies to a server built on an expensive multiprocessor platform and serving hundreds of users. Many modern corporate networks are built using FDDI technology on the backbone in combination with Ethernet, Fast Ethernet and Token Ring technologies in floor and department networks. Due to the emergence of cheaper technologies than FDDI 10 MB, such as Fast Ethernet and iOOVG-AnyLAN, FDDI technology will obviously not find widespread use when connecting workstations and creating small local networks, even with an increase in the speed of these stations and the availability of networks of multimedia information.

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FDDI network

The FDDI (Fiber Distributed Data Interface) standard was proposed by the American National Standards Institute ANSI (ANSI specification X3T9.5). The ISO 9314 standard was then adopted, conforming to ANSI specifications.

The FDDI standard was initially focused on high transmission speeds (100 Mbit/s) and the use of the most promising fiber optic cable. The choice of optical fiber as a transmission medium determined such advantages of the new network as high noise immunity, maximum secrecy of information transmission and excellent galvanic isolation of subscribers. High transmission speeds make it possible to solve many tasks that are inaccessible to slower networks, for example, transmitting images in real time. In addition, fiber optic cable easily solves the problem of transmitting data over a distance of several kilometers without relaying, which makes it possible to build large networks that even cover entire cities and have all the advantages of local networks (in particular, a low error rate). All this determined the popularity of the FDDI network, although it is not yet as widespread as Ethernet and Token-Ring.

The FDDI standard has significant advantages over all previously discussed networks. For example, a Fast Ethernet network with the same 100 Mbps bandwidth cannot match FDDI in terms of network size allowance. In addition, the FDDI token access method, unlike CSMA/CD, provides guaranteed access time and the absence of conflicts at any load level.

Main technical characteristics of the FDDI network.

The maximum number of network subscribers is 1000.

The maximum length of the network ring is 20 (100) kilometers.

The maximum distance between network subscribers is 2 kilometers.

Transmission medium - multimode fiber optic cable (possibly using electrical twisted pair).

The access method is token.

Information transfer speed – 100 Mbit/s (200 Mbit/s for duplex transmission mode).

It is also possible to use single-mode cable, in which case the distance between subscribers can reach 45 kilometers, and the total ring length can be 200 kilometers.

Frame formats

Rice. Frame format and Token format

Purpose of fields:

Preamble is used for synchronization. Initially it contains 64 bits, but the subscribers through which the packet passes can change its size.

The start delimiter (SD-Start Delimiter) serves as a sign of the beginning of the frame.

The control byte (FC - Frame Control) contains information about the packet (address field size, synchronous/asynchronous transmission, packet type - service or information, command code).

The receiver and source addresses (SA – Source Address and DA – Destination Address) can be 6-byte (similar to Ethernet and Token-Ring) or 2-byte.

The Data field has a variable length (from 0 to 4478 bytes). In service (command) packets, the data field has a zero length.

The Frame Check Sequence (FCS) field contains the packet's 32-bit cyclic checksum (CRC).

The end delimiter (ED – End Delimiter) defines the end of the frame.

The Frame Status byte (FS) includes an error detection bit, an address recognition bit, and a copy bit (similar to Token-Ring).

Format of the FDDI network control byte (Fig. 3):

The packet class bit determines whether the packet is synchronous or asynchronous.

The address length bit sets which address (6-byte or 2-byte) is used in this packet.

The packet type field (two bits) determines whether it is a control or information packet.

The command code field (four bits) indicates which command the receiver should execute (if it is a control packet).

Rice. 3. Control byte format

Network building

The FDDI standard was based on the token access method provided for by the international standard IEEE 802.5 (Token-Ring). The FDDI network topology is a double ring, where the network uses two multi-directional fiber optic cables. Using two rings is the primary way to improve fault tolerance in an FDDI network, and nodes that want to use it must be connected to both rings. In normal network operation mode, data passes through all nodes and all cable sections of the Primary ring, which is why this mode is called Thru mode - “end-to-end” or “transit”. The Secondary ring is not used in this mode. These rings provide redundancy for each other's transmission, that is, if some problems arise on one ring, the other will be included in the transmission. FDDI itself will recognize and eliminate problems that arise. This mode of network operation is called "folding" or "folding" the rings. The collapse operation is performed by FDDI hubs and/or network adapters. To simplify this procedure, data is always transmitted counterclockwise on the primary ring, and clockwise on the secondary ring. 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.

This solution also allows the use of full-duplex information transmission (simultaneously in two directions) with double the effective speed of 200 Mbit/s (with each of the two channels operating at a speed of 100 Mbit/s). A star-ring topology with hubs included in the ring (as in Token-Ring) is also used.

To achieve high network flexibility, the FDDI standard provides for the inclusion of two types of subscribers in the ring:

Class A subscribers (stations) (dual connection subscribers, DAS) are connected to both (internal and external) network rings. At the same time, the possibility of exchange at speeds of up to 200 Mbit/s or network cable redundancy is realized (if the main cable is damaged, a backup one is used). Equipment of this class is used in the most critical parts of the network in terms of performance.

Class B subscribers (stations) (single connection subscribers, SAS –) are connected to only one (external) network ring. They are simpler and cheaper than Class A adapters, but do not have their capabilities. They can only be connected to the network through a hub or bypass switch, which turns them off in the event of an emergency.

In addition to the subscribers themselves (computers, terminals, etc.), the network uses communication hubs, the inclusion of which allows you to collect all connection points in one place in order to monitor the operation of the network, diagnose faults and simplify reconfiguration. When using different types of cables (for example, fiber optic cable and twisted pair), the hub also performs the function of converting electrical signals into optical signals and vice versa. Hubs also come in dual connection (DAC) and single connection (SAC).

An example of an FDDI network configuration is shown in Fig. 4

Rice. 4. Example of FDDI network configuration

Principle of information transfer

FDDI uses what is called multiple token passing.

A station can only start transmitting its own data frames if it has received a token (access token) from the previous station. It can then transmit its frames, if it has any, for a time called token hold time (THT). After the THT time has expired, the station must complete the transmission of its next frame and transfer the access token to the next station. If, at the moment of accepting the token, the station does not have frames to transmit over the network, then it immediately broadcasts the token to the next station.

A subscriber wishing to transmit waits for a token that follows each packet.

When the token arrives, the subscriber removes it from the network and transmits its packet.

Immediately after transmitting his packet, the subscriber sends a new token.

If the address of the frame matches the address of the station, then it copies the frame to its internal buffer, checks its correctness (mainly by checksum), and then transmits the original frame over the network to the subsequent station. In the frame transmitted to the network, the destination station notes three signs: recognition of the address, copying of the frame, and the absence or presence of errors in it.

Having received his packet back through the ring, the sending subscriber destroys it. In the packet status field it has information about whether there were errors and whether the receiver received the packet.

In conclusion, it should be noted that despite the obvious advantages of FDDI, this network has not become widespread, which is mainly due to the high cost of its equipment. The main area of application of FDDI now is basic, core (Backbone) networks that combine several networks. FDDI is also used to connect powerful workstations or servers that require high-speed communication.

FDDI network. Speeds of 10 Mbps are insufficient for many modern networking applications. Therefore, technologies and specific implementations of high-speed LANs are being developed.

FDDI (Fiber Distributed Data Interface) is a LAN of a ring structure, using fiber optic lines and a specific version of the token access method.

The main version of the network uses a double ring on a fiber-optic line. An information speed of 100 Mbit/s is provided. The distance between the extreme nodes is up to 200 km, between neighboring stations - no more than 2 km. The maximum number of nodes is 500. FOCL uses waves with a length of 1300 nm.

Two FOCL rings are used simultaneously. Stations can be connected to one of the rings or to both at once. The use of both rings by a particular node allows this node to have a total throughput of 200 Mbit/s. Another possible use of the second ring is to bypass the damaged area with it (Fig. 4.5).

Rice. 4.5. FOCL rings in the FDDI network

FDDI uses the original code and access method. A code of the NRZ (non-return to zero) type is used, in which a change in polarity in the next time step is perceived as 1, the absence of a change in polarity as 0. In order for the code to be self-synchronizing, after every four bits the transmitter generates a synchronizing edge.

This special Manchester encoding is called 4b/5b. Recording 4b/5b means a code in which 5 bits are used to self-clock when transmitting 4 bits of binary code so that there cannot be more than two zeros in a row, or another mandatory edge is added after 4 bits, which is what is used in FDDI.

With this code, the encoding and decoding blocks become somewhat more complicated, but the transmission speed over the communication line increases, since the maximum switching frequency is almost halved compared to the Manchester code.

In accordance with the FDDI method, a packet consisting of a token and information frames circulates around the ring. Any station ready to transmit, having recognized the packet passing through it, inserts its frame at the end of the packet. She also eliminates it after the frame returns to her after turning around the ring and provided that it was perceived by the recipient. If the exchange occurs without failures, then the frame returning to the sending station is already the first in the packet, since all previous frames must be eliminated earlier.

An FDDI network is typically used to combine many separate LAN subnets into a single network. For example, when organizing an information system for a large enterprise, it is advisable to have an Ethernet or Token Ring LAN in the premises of individual project departments, and communicate between departments via the FDDI network.

Fiber Distribution Data Interface or FDDI was created in the mid-80s specifically to connect the most critical sections of the network. Although the data transfer speed of 10 Mbit/s was excellent for a workstation, it was clearly insufficient for inter-server communications. Based on these needs, FDDI was designed for communication between servers and other critical parts of the network and provided the ability to control the transmission process and ensure high reliability. This is the main reason why it still occupies such a prominent place in the market.

Unlike Ethernet, FDDI uses a ring structure, where devices are combined into a large ring and transmit data sequentially to each other. A packet may travel through more than 100 nodes before reaching its destination. But don't confuse FDDI with Token Ring! Token Ring uses only one token, which is passed from one machine to another. FDDI uses a different idea - the so-called time marker. Each machine sends data to the next for a certain period of time, which they agree on in advance when they connect to the ring. Stations can send packets simultaneously if time permits.

Because other machines do not have to wait for the transmission medium to become available, packet sizes can be as large as 20,000 bytes, although most use 4,500-byte packets, only three times the size of an Ethernet packet. However, if the packet is destined for a workstation connected to the ring using Ethernet, then its size will not exceed 1516 bytes.

One of the biggest advantages of FDDI is its high reliability. It usually consists of two or more rings. Each machine can receive and send messages to its two neighbors. This scheme allows the network to function even if the cable is broken. When a cable is broken, the devices on both ends of the break begin to act as a plug and the system continues to function as a single ring that passes through each device twice. Since each specific path is unidirectional and devices transmit data at a specified time, this scheme completely eliminates collisions. This allows FDDI to achieve almost full theoretical throughput, which is actually 99% of the theoretically possible data rate. The high reliability of the double ring, subject to all of the above, forces consumers to continue to buy FDDI equipment.

Operating principle of the FDDI network The FDDI network is a fiber-optic marker ring with a data transfer rate of 100 Mbit/s. The FDDI standard was developed by the American National Standards Institute (ANSI) Committee X3T9.5. FDDI networks are supported by all leading network equipment manufacturers. The ANSI X3T9.5 committee has now been renamed X3T12. Using fiber optics as a distribution medium can significantly expand the cable bandwidth and increase the distance between network devices. Let's compare the throughput of FDDI and Ethernet networks for multi-user access. The acceptable level of utilization of an Ethernet network lies within 35% (3.5 Mbit/s) of the maximum throughput (10 Mbit/s), otherwise the probability of collisions becomes not too high and the cable throughput will sharply decrease. For FDDI networks, acceptable utilization can reach 90-95% (90-95 Mbit/s). Thus, the throughput of FDDI is approximately 25 times higher. The deterministic nature of the FDDI protocol (the ability to predict the maximum delay when transmitting a packet over the network and the ability to provide guaranteed bandwidth for each of the stations) makes it ideal for use in real-time network automated control systems and in time-critical applications (for example, video and audio information). FDDI inherited many of its key properties from Token Ring networks (IEEE 802.5 standard). First of all, this is a ring topology and a token method of access to the medium. A marker is a special signal rotating around a ring. The station that receives the token can transmit its data. However, FDDI also has a number of fundamental differences from Token Ring, making it a faster protocol. For example, the data modulation algorithm at the physical level has been changed. Token Ring uses a Manchester encoding scheme, which requires doubling the transmitted signal bandwidth relative to the transmitted data. FDDI implements a “five out of four” coding algorithm - 4B/5B, which ensures the transmission of four information bits with five transmitted bits. When transmitting 100 Mbits of information per second, 125 Mbits/sec is physically transmitted to the network, instead of 200 Mbits/sec, which would be required when using Manchester encoding. Medium Access Control (VAC) has also been optimized. In Token Ring it is based on a bit basis, and in FDDI it is based on parallel processing of a group of four or eight transmitted bits. This reduces the requirements for equipment speed. Physically, the FDDI ring is formed by a fiber optic cable with two light-conducting fibers. One of them forms the primary ring, is the main one and is used for circulating data tokens. The second fiber forms a secondary ring, is a backup fiber and is not used in normal mode. Stations connected to the FDDI network are divided into two categories. Class A stations have physical connections to the primary and secondary rings (Dual Attached Station); 2. Class B stations are connected only to the primary ring (Single Attached Station - a one-time connected station) and are connected only through special devices called hubs. The ports of network devices connected to the FDDI network are classified into 4 categories: A ports, B ports, M ports and S ports. Port A is the port that receives data from the primary ring and transmits it to the secondary ring. Port B is the port that receives data from the secondary ring and transmits it to the primary ring. The M (Master) and S (Slave) ports transmit and receive data from the same ring. The M port is used on the hub to connect Single Attached Station via the S port. The X3T9.5 standard has a number of limitations. The total length of the double fiber-optic ring is up to 100 km. Up to 500 class A stations can be connected to the ring. The distance between nodes when using a multimode fiber-optic cable is up to 2 km, and when using a single-mode cable is determined mainly by the parameters of the fiber and receiving and transmitting equipment (can reach 60 km or more). Topology. The flow control mechanisms used to build a LAN are topologically dependent, which makes it impossible to simultaneously use Ethernet IEEE 802.x, FDDI ANSI, Token Ring IEEE 802.6 and others within a single distribution environment. Despite the fact that Fiber Channel to some extent may resemble the LANs so familiar to us, its flow control mechanism is in no way related to the topology of the distribution medium and is based on completely different principles. Each N_port, when connected to the Fiber Channel mesh, goes through a registration procedure (log-in) and receives information about the address space and capabilities of all other nodes, based on which it becomes clear which of them it can work with and under what conditions. And since the flow control mechanism in Fiber Channel is the prerogative of the lattice itself, it is completely unimportant for the node what topology underlies it. Point-to-point The simplest scheme, based on a serial full-duplex connection of two N_ports with mutually acceptable physical connection parameters and the same classes of service. One of the nodes receives address 0, and the other - 1. In essence, such a scheme can be considered a special case of a ring topology, where there is no need for access control through arbitration. As a typical example of such a connection, we can cite the most common connection between a server and an external RAID array. Loop with arbitrated access The classic connection scheme for up to 126 ports, with which it all began, judging by the abbreviation FC-AL. Any two ports on a ring can communicate over a full-duplex connection just like a point-to-point connection. At the same time, all the others act as passive repeaters of FC-1 level signals with minimal delays, which, perhaps, is one of the main advantages of FC-AL technology over SSA. The fact is that addressing in SSA is based on knowing the number of intermediate ports between the sender and the recipient, therefore the address header of the SSA frame contains a hop count. Each port encountered along the frame path reduces the contents of this counter by one and then regenerates the CRC, thereby significantly increasing the transmission delay between ports. To avoid this undesirable effect, the FC-AL developers preferred to use absolute addressing, which ultimately made it possible to retransmit the frame unchanged and with minimal latency. The word ARB transmitted for the purpose of arbitration is not understood and not used by ordinary N_ports, therefore, with this topology, additional properties of nodes are designated as NL_port. The main advantage of a loop with arbitrated access is its low cost in terms of the number of connected devices, so it is most often used to combine a large number of hard drives with a disk controller. Unfortunately, failure of any NL_port or connecting cable opens the loop and makes it inoperable, which is why such a circuit in its pure form is no longer considered a...

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 Mb/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 efficient use of potential network bandwidth for both asynchronous and synchronous 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. Using two rings is the primary way to improve fault tolerance in an FDDI network, and nodes that want to use it must be connected to both rings. In normal network operation mode, data passes through all nodes and all cable sections of the Primary ring, which is why this mode is called Thru mode - “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 cable break or node failure), the primary ring is combined with the secondary ring (Fig. 31), forming a single ring again. This mode of network operation is called Wrap, that is, “folding” or “folding” of rings. The collapse operation is performed by FDDI hubs and/or network adapters. To simplify this procedure, data is always transmitted counterclockwise on the primary ring, and clockwise on the secondary ring. 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.

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.

Rice. 31. Reconfiguration of FDDI rings during failure

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 (Fig. 32, a).

A station can start transmitting its own data frames only if it has received a special frame from the previous station - an access token (Fig. 32, b). It can then transmit its frames, if it has any, for a time called the Token Holding Time (THT). After the THT time has expired, the station must complete the transmission of its next frame and transfer the access token to the next station. If, at the moment of accepting the token, the station does not have frames to transmit over the network, then it immediately broadcasts the token to the next station. In an FDDI network, each station has an upstream neighbor and a downstream neighbor, determined by its physical connections and the direction of information transfer.

Each station in the network constantly receives frames transmitted to it by its previous neighbor and analyzes their destination address. If the destination address does not match its own, then it broadcasts the frame to its subsequent neighbor (Fig. 32, c). It should be noted that if a station has captured the token and is transmitting its own frames, then during this period of time it does not broadcast incoming frames, but removes them from the network.

If the frame address coincides with the station address, then it copies the frame to its internal buffer, checks its correctness (mainly by checksum), transfers its data field for subsequent processing to a protocol higher than the FDDI level (for example, IP), and then transmits the original frame over the network of the subsequent station (Fig. 32, d). In the frame transmitted to the network, the destination station notes three signs: recognition of the address, copying of the frame, and the absence or presence of errors in it.

After this, the frame continues to travel through the network, broadcast by each node. The station, which is the source of the frame for the network, is responsible for removing the frame from the network after it has completed a full revolution and reaches it again (Fig. 32, e). In this case, the source station checks the characteristics of the frame to see whether it has reached the destination station and whether it has not been damaged. The process of restoring information frames is not the responsibility of the FDDI protocol; this should be handled by higher-level protocols.

Rice. 32. Frame processing by FDDI ring stations

Figure 33 shows the protocol structure of FDDI technology in comparison with 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 802.2 data link control (LLC) sublayer protocol defined in the IEEE 802.2 and ISO 8802.2 standards. FDDI uses the first type of LLC procedures, in which nodes operate in datagram mode - without establishing connections and without recovering lost or damaged frames.

Rice. 33. Structure of FDDI technology protocols

The physical layer is divided into two sublayers: the media-independent PHY (Physical) sublayer, and the media-dependent PMD (Physical Media Dependent) sublayer. The operation of all levels is controlled by the station management protocol SMT (Station Management).

The PMD layer provides the necessary means to transmit data from one station to another over fiber optics. Its specification defines:

Requirements for optical signal power and 62.5/125 µm multimode fiber optic cable.

Requirements for optical bypass switches and optical transceivers.

Parameters of optical connectors MIC (Media Interface Connector), their markings.

The wavelength is 1300 nanometers at which the transceivers operate.

Representation of signals in optical fibers according to the NRZI method.

The TP-PMD specification defines the ability to transmit data between stations over twisted pair cable in accordance with the MLT-3 method. The specifications of the PMD and TP-PMD levels have already been discussed in the sections devoted to Fast Ethernet technology.

The PHY layer performs encoding and decoding of data circulating between the MAC layer and the PMD layer, and also provides clocking of information signals. Its specification defines:

encoding information in accordance with scheme 4B/5B;

signal timing rules;

requirements for clock frequency stability of 125 MHz;

rules for converting information from parallel to serial form.

The MAC layer is responsible for controlling network access and receiving and processing data frames. It defines the following parameters:

Token transfer protocol.

Rules for capturing and relaying a token.

Formation of the frame.

Rules for generating and recognizing addresses.

Rules for calculating and verifying a 32-bit checksum.

The SMT layer performs all control and monitoring functions of 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. The SMT specification defines the following:

Algorithms for detecting errors and recovering from failures.

Rules for monitoring the operation of the ring and stations.

Ring control.

Ring initialization procedures.

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

The following table compares FDDI technology with Ethernet and Token Ring technologies.

Characteristic		Ethernet	Token Ring
Bit rate
Topology	Double ring of trees	Tire/star	Star/ring
Access Method	Share of token turnover time		Priority reservation system
Data transmission medium	Multimode fiber, unshielded twisted pair	Thick coax, thin coax, twisted pair, fiber optic	Shielded and unshielded twisted pair, optical fiber
Maximum network length (without bridges)	200 km (100 km per ring)
Maximum distance between nodes	2 km (-11 dB loss between nodes)
Maximum number of nodes	500 (1000 connections)		260 for shielded twisted pair, 72 for unshielded twisted pair
Timing and recovery after failures	Distributed implementation of clocking and fault recovery	Not defined	Active monitor