When was fddi technology standardized? fddi technology. Physical connection structure

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Course work

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 most 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 also as a means of interconnection high-speed computers, located in the 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 others widely famous 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 - 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 changes in the access method only affect asynchronous traffic, which is not critical to small delays in frame transmission. For synchronous traffic, token hold time still 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 protocol physical level 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 allowable 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. Typically, 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 folds 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 computing machines , 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 platform LLC level. 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 represented 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 multiple modes of light propagating along an optical cable can travel different distances(depending on the angle of entry), and therefore reach the destination at different times (a phenomenon called modal dispersion), a single-mode fiber is capable of providing greater bandwidth and carrying cables over 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 stream, a failure is recorded. physical connection and reconfiguring the internal path of the hub or station, 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 it is correct this connection. 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 in accordance with the TP-PMD standard is 10 m. The maximum total length of the FDDI ring is one hundred kilometers, maximum number stations with double connection in the ring - 500. PHY sublayer: The PHY sublayer defines coding and modulation methods, as well as rules for isolating an inoperative station, which we will consider next. 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 in FDDI technology following functions: 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 against 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 indicate an LLC (user data) frame or oo to indicate 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 management 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 has no meaning 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 standard method CRC-32, adopted for other IEEE 802 protocols. The control 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 hardware manufacturers to add their own indicators after the three required ones. A token consists essentially of one significant field- control fields, which in this case contains i in field C and oooo in field ZZZZ. 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 others upper levels communication protocol stack. The station stops transmitting frames in two cases: either when the TNT token holding 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 copying flag b is also set. An error sign is also set if it was detected by the check sequence check. 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 symbols 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 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 on 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 running the Claim Token ring initiation procedure and the 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 is in a permanent Stuck Beacon 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. Her general purpose is that the station must autonomously check the transmission of symbols and frames between all its internal nodes to ensure that it is not the cause of the ring failure. If the internal path Path Test is successful, the SMT process sends a PC_Start signal to the configuration management units, upon which they 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 stations exchange: Neighborhood Information Frames (NIF) allow a station to find out the addresses of its predecessor and successor 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 equipment manufacturers and software management. 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, 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 real time going around the ring with a marker. 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. When the input signal power increases by 2 dBm, this probability should decrease to 10-12.

Maximum permissible level The standard defines signal loss in a cable as equal to and 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 you to work in either 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.

Currently the majority network technologies support fiber optic cables as an option for the physical layer, but FDDI remains the most mature high-speed technology, the standards for which are time-tested and established, so the equipment various 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.

...

History of the creation of the FDDI standard

Technology Fiber Distributed Data Interface- the first local network technology that used fiber optic cable as a data transmission medium.

Attempts to use light as a medium that carries information have been made for a long time - back in 1880, Alexander Bell patented a device that transmitted speech over a distance of up to 200 meters using a mirror that vibrated synchronously with sound waves and modulated the reflected light.

Work on the use of light to transmit information intensified in the 1960s due to the invention of the laser, which could provide light modulation at very high frequencies, that is, to create a broadband channel for transmitting a large amount of information from high speed. Around the same time, optical fibers emerged that could carry light in cable systems, much like copper wires carry electrical signals in traditional cables. However, the light loss in these fibers was too great for them to be used as an alternative to copper cores. Inexpensive optical fibers providing low light signal power loss and wide bandwidth (up to several GHz) appeared only in the 1970s. In the early 1980s, industrial installation and operation of fiber optic communication channels for territorial telecommunications systems began.

In the 1980s, work also began to create standard technologies and devices for using fiber optic channels in local networks. Work on summarizing experience and developing the first fiber optic standard for local networks was concentrated at the American National Standards Institute - ANSI, within the framework of the X3T9.5 committee created for this purpose.

The initial versions of the various components of the FDDI standard were developed by the X3T9.5 committee in 1986 - 1988, and at the same time the first equipment appeared - network adapters, hubs, bridges and routers that support this standard.

Currently, most network technologies support fiber optic cables as a physical layer option, but FDDI remains the most mature high-speed technology, the standards for which have been tested and established over time, so that equipment from different manufacturers shows a good degree of compatibility

FDDI Technology Basics

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 take advantage of it must be connected to both rings. In normal network operation, data passes through all nodes and all cable sections of the Primary ring, which is why this mode is called the Thru- “end-to-end” or “transit”. The Secondary ring is not used in this mode.

In the event of some type of failure where part of the primary ring cannot transmit data (for example, a cable break or node failure), the primary ring is combined with the secondary ring (Figure 2.1), forming a single ring again. This mode of network operation is called Wrap, that is, the "folding" or "folding" of 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.

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. 2.1. Reconfiguration of FDDI rings upon 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 accessor method Token networks Ring and is also called the token ring method (Figure 2.2, 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 (Figure 2.2, b). It can then transmit its frames, if it has any, for a period of time called the token hold time - 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.

Rice. 2.2. Frame processing by FDDI ring stations

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. This case is shown in the figure (Figure 2.2, 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), transmits its data field for subsequent processing to the protocol of the layer above FDDI (for example, IP), and then transmits the original frame over the network of the subsequent station (Figure 2.2, 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 (Figure 2.2, 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.

Figure 2.3 shows the structure of the FDDI technology protocols 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. 2.3. Structure of FDDI technology protocols

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

PMD level provides the necessary means for transmitting 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 of 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.

PHY level 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.

MAC level responsible for managing network access and receiving and processing data frames. It defines the following parameters:

Token transfer protocol;
Rules for capturing and relaying a token;
Frame formation;
Rules for generating and recognizing addresses;
Rules for calculating and verifying a 32-bit checksum.

SMT level performs all functions for 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. 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: with the help of the PHY layer, network failures due to physical reasons, for example, a broken cable, are eliminated, and with the help of the MAC layer, logical network failures are eliminated, for example, the loss of the required internal token transmission path and data frames between hub ports.

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

Characteristic	FDDI	EthernetToken Ring
Bit rate	100 Mb/s	10 Mb/s16 Mb/s
Topology	Double ring trees	Tire/starStar/ring
Access method	Share of time token turnover	CSMA/CDPriority reservation system
Transmission medium data	Multimode optical fiber, unshielded twisted pair	Thick coaxial thin coaxial, twisted pair, optical fiber Shielded and unshielded twisted pair, optical fiber
Maximum network length (without bridges)	200 km (100 km per ring)	2500 m1000 m
Maximum distance between nodes	2 km (-11 dB loss between nodes)	2500 m 100 m
Maximum number of nodes	500 (1000 connections)	1024260 for shielded twisted pair, 72 for unshielded twisted couples
Clocking and failure recovery	Distributed implementation of clocking and recovery after failures	Not definedActive monitor

Types of nodes and rules for connecting them to the network

All stations in the FDDI network are divided into several types according to the following criteria:

end stations or hubs;
according to the option of connecting to the primary and secondary rings;
by the number of MAC nodes and, accordingly, MAC addresses per station.

Single and double connection to the network

If the station is connected only to the primary ring, then this option is called a single connection - Single Attachment, S.A.(Figure 2.4, a). If the station is connected to both the primary and secondary rings, then this option is called double connection - Dual Attachment, DA(Figure 2.4, b).

Rice. 2.4. Single (SA) and double (DA) connection of stations

Obviously, a station can only take advantage of the fault tolerance properties provided by having two FDDI rings when it is connected twice.

Rice. 2.5. Reconfiguration of dual-connection stations in the event of a cable break

As can be seen from Figure 2.5, the reaction of stations to a cable break is to change the internal paths for transmitting information between the individual components of the station.

Number of MAC nodes at the station

In order to be able to transmit its own data into the ring (and not just relay data from neighboring stations), the station must have at least one MAC node that has its own unique MAC address. Stations may not have a single MAC node, and, therefore, participate only in relaying other people's frames. But usually all stations on an FDDI network, even hubs, have at least one MAC. Hubs use the MAC node to capture and generate service frames, such as ring initialization frames, ring troubleshooting frames, etc.

Stations that have one MAC node are called SM (Single MAC) stations, and stations that have two MAC nodes are called DM (Dual MAC) stations.

The following combinations of attachment types and number of MAC nodes are possible:

SM/SA	A station has one MAC node and joins only the primary ring. A station cannot take part in the formation of a common ring of two.
SM/DA	The station has one MAC node and is connected directly to the primary and secondary rings. In normal mode, it can only receive data on the primary ring, using the second for fault-tolerant operation.
DM/DA	The station has two MAC nodes and is attached to two rings. It can (potentially) receive data simultaneously on two rings (full duplex mode), and in case of failures, participate in the reconfiguration of rings.
DM/SA	The station has two MAC nodes, but is connected only to the primary ring. Prohibited combination for the end station, a special case of hub operation.

Depending on whether the station is a hub or an end station, the following designations are adopted depending on the type of their connection:

SAS (Single Attachment Station)- end station with single connection,

DAS (Dual Attachment Station)- end station with dual connection,

SAC (Single Attachment Concentrator)- hub with single connection,

DAC (Dual Attachment Concentrator)- dual connection hub.

Types of ports of FDDI stations and hubs and rules for their connection

The FDDI standard describes four types of ports, which differ in their purpose and ability to connect with each other to form correct network configurations.

Port type	Connection	Purpose
A	PI/SO - (Primary In/Secondary Out) Primary ring input/Secondary ring output	rings
B	PO/SI - (Primary Out/Secondary In) Primary Ring Out/Secondary Ring In	Connects devices with dual connection with the main rings
M	Master - PI/PO	The hub port that connects it to devices with single connection; uses only the primary ring
S	Slave - PI/PO Primary Ring In/Primary Ring Out	Connects a device to a single connecting to a hub; uses only the primary ring

Figure 2.6 shows typical port usage different types for connecting SAS and DAS stations to a DAC hub.

Rice. 2.6. Using different types of ports

An S - S port connection is valid because it creates an isolated primary ring connecting only two stations, but is usually unused.

The connection between ports M - M is prohibited, and connections A-A, B-B, A-S, S-A, B-S, S-B - undesirable, as they create ineffective ring combinations.

Dual Homing connection

Connections type A-M and B-M correspond to the so-called case Dual Homing Connections, when a device with dual connectivity, that is, ports A and B, uses them to make two connections to the primary ring through the M ports of another device.

Such a connection is shown in Figure 2.7.

It has two hubs, DAC4 and DAC5, connected to hubs DAC1, DAC2 and DAC3 in a Dual Homing scheme.

Hubs DAC1, DAC2 and DAC3 are connected in the usual way to both rings, forming the root backbone of the FDDI network. Usually such concentrators are called in English literature rooted concentrators .

Hubs DAC4 and DAC5 are connected in a tree diagram. It could also be formed using concentrators SAC4 and SAC5, which in this case would be connected to the M-port of the root concentrators using the S port.

Connecting DAC concentrators according to a tree diagram, but using Dual Homing, allows you to increase network fault tolerance and maintain the advantages of a multi-level tree structure.

Rice. 2.7. Dual Homing connection

The DAC4 hub is connected according to the classic Dual Homing scheme. This scheme is designed for such a hub to have only one MAC node. When you connect the A and B ports of DAC4 to the M ports of DAC1, a physical connection is established between these ports, which is constantly monitored by the PHY layer. However, only port B is switched to the active state in relation to the flow of frames over the network, and port A remains in a standby logical state. The default preference for Port B is defined in the FDDI standard.

If the physical connection on port B does not work correctly, the DAC4 concentrator puts it in a standby state, and port A becomes active. After this, port B constantly checks the physical state of its communication line, and if it is restored, it becomes active again.

The DAC5 hub is also included in the Dual Homing scheme, but with more complete functionality connection control of the backup port A. The DAC5 hub has two MAC nodes, so not only is port B active in the primary ring, transmitting frames to the primary MAC node from the DAC3 port M, but also port A is also in the active state, receiving frames from the same primary ring, but from the M port of the DAC2 hub. This allows the secondary MAC node to continuously monitor the logical state of the backup link.

It should be noted that devices supporting Dual Homing mode can be implemented by several different ways, therefore, there may be incompatibility between these modes from different manufacturers.

Attaching a Station to a Roaming MAC Node

When a new station is included in the FDDI network, the network temporarily suspends its operation, going through the ring initialization process, during which the basic parameters of the ring are agreed upon between all stations, the most important of which is the nominal time of rotation of the token on the ring. This procedure can be avoided in some cases. An example of such a case is connecting a new SAS station to the M port of a hub with the so-called "wandering" MAC node (Roving MAC), which is also called local MAC node.

An example of such a connection is shown in Figure 2.8.

Rice. 2.8. Attaching a Station to a Roaming MAC Node

The DM/DAC1 hub has two MAC nodes: one is involved in the normal operation of the primary ring, and the second, local, is attached to the path connecting port M to the SAS3 station. This path forms an isolated ring and is used for local check performance and parameters of the SAS3 station. If it is operational and its parameters do not require reinitialization of the main network, then the SAS3 station is included in the operation of the primary ring “smoothly” (smooth-insertion).

Connecting stations using Optical Bypass Switches

The fact that the power of a station with a single connection is turned off will be immediately noticed by the physical layer means servicing the corresponding M-port of the hub, and this port, at the command of the SMT layer of the hub, will be bypassed along the internal data path through the hub. This fact will not have any impact on the further fault tolerance of the network (Figure 2.9).

Rice. 2.9. Optical Bypass Switch

If you turn off the power at the DAS station or DAC concentrator, then the network, although it will continue to operate, going into the Wrap state, but the fault tolerance margin will be lost, which is undesirable. Therefore, for devices with dual connections, it is recommended to use optical bypass switches - Optical Bypass Switch, which allows you to short-circuit the input and output optical fibers and bypass the station in the event of its shutdown. The optical bypass switch is powered from the station and consists in the simplest case of reflecting mirrors or moving optical fiber. When the power is turned off, such a switch bypasses the station, and when its power is turned on, it connects the inputs of ports A and B to the internal PHY circuits of the station.

Specification of the media-dependent physical sublayer PMD

Physical connection structure

Consider the physical sublayer PMD (Physical Media Dependent layer), defined in the FDDI standard for optical fiber - Fiber PMD.

This specification defines the hardware components to create physical connections between stations: optical transmitters, optical receivers, cable parameters, optical connectors. For each of these elements, design and optical parameters are indicated that allow the stations to interact stably at certain distances.

The physical connection is the basic building block of an FDDI network. A typical physical connection structure is shown in Figure 2.10.

Rice. 2.10. Physical connection of the FDDI network

Each physical connection consists of two physical connections - primary and secondary. These communications are one-way - data is transferred from the transmitter of one PHY device to the receiver of another PHY device.

Optical signal power requirements

The Fiber PMD standard does not explicitly define distance limits between a pair of communicating devices over a single physical connection.

Instead, the standard specifies a maximum level of optical signal power loss between two stations communicating over the same physical link. This level is -11 dB, where

dB = 10 log P 2 /P 1,

and P 1 is the signal power at the transmitting station, and P2- signal power at the input of the receiving station. Since the power decreases as the signal is transmitted through the cable, the attenuation is negative.

Based on the cable attenuation parameters of the Fiber PMD standard and industry-produced connectors, it is believed that to ensure an attenuation of -11 dB, the length of the optical cable between adjacent nodes should not exceed 2 km.

The correctness of the physical connection between nodes can be more accurately calculated by taking into account the exact attenuation characteristics introduced by the cable, connectors, cable solders, as well as transmitter power and receiver sensitivity.

The Fiber PMD standard defines the following physical interconnect element parameter limits (called FDDI Power Budget):

The absolute power values of optical signals (for the transmitter output and for the receiver input) are measured in decibels relative to a standard power of 1 milliwatt (mW) and are designated as dBm:

dBm = 10 log P/1,

where is the power R also measured in milliwatts.

From the table values it can be seen that the maximum loss between stations of -11 dB corresponds to the worst combination of transmitter (- 20 dBm) and receiver (- 31 dBm) power limits.

Cables and connectors

The main type of cable for the Fiber PMD standard is a multimode cable with a core diameter of 62.5 microns and a reflective shell diameter of 125 microns. The Fiber PMD specification does not specify cable attenuation requirements in dB per km, but only requires a total attenuation requirement of -11 dB between stations connected by cable and connectors. The cable bandwidth must be no worse than 500 MHz per km.

In addition to the basic cable type, the Fiber PMD specification allows the use of multimode cables with core diameters of 50 microns, 85 microns and 100 microns.

The Fiber PMD standard defines optical connectors as connectors. MIC (Media Interface Connector). The MIC connector connects the 2 cable fibers connected to the MIC plug to the 2 station port fibers connected to the MIC socket. Only the design parameters of the MIC receptacle are standardized, and any MIC plugs that fit into standard MIC receptacles are considered acceptable for use.

The Fiber PMD specification does not specify the level of loss in the MIC connector. This level is up to the manufacturer, the main thing is that the permissible loss level of -11 dB is maintained throughout the physical connection.

MIC connectors must be keyed to indicate the port type, which should prevent the connectors from being miswired. Four identified various types key:

MIC A;
MIC B;
MIC M;
MIC S.

The types of key for these types of connectors are shown in Figure 2.11.

Rice. 2.11. MIC jack keys

In addition to MIC connectors, it is possible to use ST and SC connectors produced by industry.

Light emitting diodes (LEDs) or laser diodes with a wavelength of 1.3 microns can be used as a light source.

In addition to multimode cable, it is possible to use higher quality single-mode cable (Single Mode Fiber, SMF) and SMF-MIC connectors for this cable. In this case, the range of physical connection between neighboring nodes can increase to 40 km - 60 km, depending on the quality of the cable, connectors and connections. The requirements defined in the SMF-PMD specification for transmitter output and receiver input power are the same as for single-mode cable.

PMD level detection function

The Fiber PMD specification requires this layer to perform the Signal_Detect function to determine the presence of optical signals at the input of the station's physical connection. This signal is sent to the PHY layer, where it is used by the Line State Detect function (Figure 2.12).

The PMD level generates Signal_Detect for the PHY if the input signal power exceeds -43.5 dBm, and removes it when this power decreases to -45 dBm and below. Thus, there is a hysteresis of 1.5 dBm to prevent frequent changes in line status when the input signal power fluctuates around -45 dBm.

Rice. 2.12. PMD input detection function

Technology FDDI (Fiber Distributed Data Interface) - 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 80s, shortly after the start of industrial operation of such channels in territorial networks. The HZT9.5 problem group of the ANSI Institute developed during 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.

Main characteristics of the technology

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

increase the bit rate of data transfer to 100Mbit/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 level of interference on the line, etc.;

Make the most of potential network bandwidth for both asynchronous and synchronous (latency-sensitive) schedules.

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

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

In the event of some type of failure, when part of the primary ring cannot transmit data (for example, a cable break or node failure), the primary ring is combined with the secondary (Fig. 9.8), again forming a single ring. This mode of network operation is called Wrap, that is, the “folding” or “folding” of the rings. The folding operation is performed by means of FDDI hubs and/or network adapters. To simplify this procedure, data on the primary ring is always transmitted in one direction (in the diagrams this direction is depicted counterclockwise), and on the secondary - in reverse (shown clockwise). Therefore, when a common ring of two rings is formed, the transmitters of the stations still remain connected to the receivers of neighboring stations, which allows information to be correctly transmitted and received by neighboring stations.

In the FDDI standards, much attention is paid to various procedures that make it possible to determine the presence of a failure 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. In case of multiple failures, the network breaks up into several unrelated 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.

Fig.9. 8 Reconfiguration of FDDI rings upon 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.

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 of the ring - with a small load it increases, and with large overloads it can decrease to zero. These changes in access method concerns only the asynchronous schedule, which is not critical to small delays in frame transmission. For a synchronous schedule, the marker holding time is still a fixed value.

MAC level addresses are in a standard format for IEEE 802 technologies.

In Fig. 9.9 shows the correspondence of the protocol structure of FDDI technology to the seven-layer OSI model. FDDI defines the physical layer protocol and the media access sublayer (MAC) protocol of the data link layer. Like many other local area network technologies, FDDI technology uses the LLC data link control sublayer protocol defined in the IEEE 802.2 standard. Thus, although FDDI technology was developed and standardized by ANSI and not by IEEE, it fits entirely within the structure of the 802 standards.

Fig.9. 9 Structure of FDDI technology protocols

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

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

Local computer network(LAN, local network; English Local Area Network, LAN) - computer network, usually covering a relatively small area or a small group of buildings (house, office, company, institute). There are also local networks, the nodes of which are geographically separated over distances of more than 12,500 km (space stations and orbital centers). Despite such distances, such networks are still classified as local.

Local network technologies, as a rule, implement the functions of only the two lower layers of the OSI model - physical and data link. The functionality of these layers is sufficient to deliver frames within the standard topologies that LANs support: star, bus, ring, and tree. However, this does not mean that computers connected to a local network do not support protocols of layers located above the channel one. These protocols are also installed and running on nodes local network, but the functions they perform do not relate to LAN technology.

Local network technology defines all the components that are needed to exchange information. Local network technologies consist of topology, data transmission media, control algorithm and information encoding methods. For each of the listed components there are corresponding standards. These standards are published by the IEEE and are known as IEEE 802.

Ethernet technology is now the most popular in the world. A classic Ethernet network uses a standard coaxial cable two types (thick and thin). However, the version of Ethernet that uses twisted pairs as a transmission medium has become increasingly widespread, since their installation and maintenance are much simpler. Topologies of the “bus” and “passive star” types are used.

The standard defines four main types of transmission media.

· 10BASE5 (thick coaxial cable);

· 10BASE2 (thin coaxial cable);

· 10BASE-T (twisted pair);

· 10BASE-F (fiber optic cable).

Fast Ethernet is a high-speed type of Ethernet network that provides a transmission speed of 100 Mbit/s. Fast Ethernet networks are compatible with networks based on the Ethernet standard. The basic topology of a Fast Ethernet network is passive star.

Gigabit Ethernet is a high-speed type of Ethernet network that provides transmission speeds of 1000 Mbit/s.

Due to the fact that the networks are compatible, it is easy and simple to connect Ethernet, Fast Ethernet and Gigabit Ethernet segments into a single network.

FDDI (Fiber Distributed Data Interface) is a standard for data transmission in a local network stretched over a distance of up to 200 kilometers. The standard is based on the Token Ring protocol. In addition to its large area, the FDDI network is capable of supporting several thousand users.

It is recommended to use fiber optic cable as the data transmission medium for FDDI, but copper cable can also be used, in which case the abbreviation CDDI (Copper Distributed Data Interface) is used. The topology uses a double ring scheme, with data circulating in the rings different directions. One ring is considered the main one; information is transmitted through it in the normal state; the second is auxiliary; data is transmitted through it in the event of a break on the first ring. To control the state of the ring, a network token is used, as in Token Ring technology.

Since such duplication increases the reliability of the system, this standard is successfully used in trunk communication channels.

The standard was developed in the mid-1980s by the American National Standards Institute (ANSI).

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