Fddi provides increased fault tolerance in networks. Characteristics of the FDDI technology. Synchronous and asynchronous transmission
FDDI network.10 Mbps is not sufficient for many modern network applications. Therefore, technologies and specific implementations of high-speed LANs are being developed.
FDDI (Fiber Distributed Data Interface) is a ring-shaped LAN using fiber-optic communication lines and a specific version of the token access method.
In the basic version of the network, a double ring is used on the FOCL. The information rate is 100 Mbit / s. The distance between extreme nodes is up to 200 km, between neighboring stations - no more than 2 km. The maximum number of nodes is 500. FOCL uses 1300 nm wavelengths.
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 specific node allows this node to have a total bandwidth of 200 Mbps.Another possible use of the second ring is bypassing the damaged area with it (Fig. 4.5).
Figure: 4.5. FOCL rings in the FDDI network
FDDI uses the original code and access method. A code like NRZ (no return to zero) is used, in which the polarity change in the next time clock is perceived as 1, the absence of polarity change as 0. To make the code self-synchronizing, the transmitter generates a synchronizing edge after every four bits.
This special Manchester coding is called 4b / 5b. Record 4b / 5b means a code in which for self-synchronization when transmitting 4 bits of a binary code, 5 bits are used so that there cannot be more than two zeros in a row, or after 4 bits, another mandatory edge is added, which is used in FDDI.
With such a code, the coding and decoding units are somewhat complicated, but the transmission rate through the communication line increases, since the maximum switching frequency is almost halved in comparison with the Manchester code.
In accordance with the FDDI method, a packet consisting of a marker and information frames circulates around the ring. Any station ready for transmission, recognizing a packet passing through it, writes its frame at the end of the packet. She also eliminates it after the frame returns to her after a revolution around the ring and provided that it was perceived by the recipient. If the exchange proceeds without failures, then the frame returning to the sending station is already the first in the packet, since all previous frames must be discarded earlier.
The FDDI network is commonly used to network many separate LAN subnets. For example, when organizing an information system of a large enterprise, it is advisable to have a LAN of the Ethernet or Token Ring type in the premises of individual project departments, and communication between departments should be carried out through the FDDI network.
Fiber Distribution Data Interface or FDDI was created in the mid-1980s specifically to connect the most important sections of the network. Although the 10 Mbps data transfer rate was excellent for a workstation, it was clearly insufficient for inter-server communication. Based on these needs, FDDI was designed for communication between servers and other critical parts of the network, and was designed to be manageable and highly reliable. 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 one another. A packet can travel through more than 100 nodes before reaching its destination. But don't confuse FDDI with Token Ring! Token Ring uses only one token that is passed from one machine to another. FDDI uses a different idea - the so-called timestamp. Each machine sends data to the next one within 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 don't have to wait for media to become free, the packet size can be up to 20,000 bytes, although most use 4500 bytes, only three times the size of an Ethernet packet. However, if the packet is intended for a workstation connected to the ring via Ethernet, then its size will not exceed 1516 bytes.
One of the greatest advantages of FDDI is its high reliability. It usually has 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 at both ends of the break begin to act as a plug and the system continues to function as one ring that goes through each device twice. Since each specific path is unidirectional and devices transmit data at a specified time, such a scheme completely eliminates collisions. This allows FDDI to achieve near its full theoretical throughput, which is in fact 99% of the theoretically possible data rate. The high reliability of the double ring, given all of the above, makes consumers continue to buy FDDI equipment.
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. The use of fiber optics as a propagation medium allows to significantly expand the cable bandwidth and increase the distance between network devices. Let's compare the throughput of FDDI and Ethernet networks with multiuser access. The allowable utilization level of the Ethernet network is within 35% (3.5 Mbps) of the maximum throughput (10 Mbps), otherwise the probability of collisions becomes not too high and the cable throughput will decrease dramatically. For FDDI networks, the allowable 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 during packet transmission over the network and the ability to provide guaranteed bandwidth for each station) makes it ideal for use in real-time network control systems and in applications that are critical to the time of information transfer (for example, for transmission video and audio information). FDDI inherits many of its key features from Token Ring (IEEE 802.5 standard) networks. First of all, it is a ring topology and a token medium access method. A marker is a special signal rotating around a ring. The station that received 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 layer has been changed. Token Ring uses a Manchester coding scheme that requires doubling the bandwidth of the transmitted signal relative to the transmitted data. The FDDI implements the "five out of four" coding algorithm - 4B / 5B, which provides the transmission of four information bits with five transmitted bits. When transmitting 100 Mbps of information per second, 125 Mbps is physically transmitted to the network, instead of 200 Mbps, which would be required if Manchester coding was used. Medium Access Control (VAC) is also optimized. In Token Ring it is bit based, and in FDDI it is parallel processing of a group of four or eight transmitted bits. This reduces the hardware performance requirements. Physically, the FDDI ring is formed by a fiber optic cable with two light-guiding fibers. One of them forms the primary ring, is the main one and is used for the circulation of data markers. The second fiber forms a secondary ring, is redundant and is not used in normal mode. The stations connected to the FDDI network fall 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) 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. M (Master) and S (Slave) ports transmit and receive data from the same ring. The M port is used on the hub to connect the Single Attached Station through 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 it is mainly determined by the parameters of the fiber and transmitting equipment (can reach 60 km or more). Topology. The flow control mechanisms used in the construction of LANs 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 has nothing to do with the topology of the distribution medium and is based on completely different principles. Each N_port, when connected to the Fiber Channel lattice, goes through a registration procedure (log-in) and receives information about the address space and capabilities of all other nodes, on the basis of which it becomes clear which of them it will be able to 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 is behind 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 gets address 0, and the other gets address 1. In essence, such a scheme can be considered as 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 arbitration access The classic scheme of connecting up to 126 ports, from which everything began, judging by the abbreviation FC-AL. Any two ports in the ring can communicate over a full duplex connection in the same way as with point-to-point. At the same time, all the others play the role of 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 receiver, so the address header of the SSA frame contains a hop count. Each port on the path of the frame decreases the content of this counter by one and then generates the CRC again, thereby significantly increasing the transmission delay between ports. To avoid this undesirable effect, the developers of FC-AL preferred to use absolute addressing, which ultimately made it possible to relay the frame unchanged and with minimal latency. The word ARB transmitted for the purpose of arbitration is not understood or used by ordinary N_ports, therefore, in this topology, additional node properties are denoted as NL_port. The main advantage of an arbitrated loop is its low cost per the number of connected devices, therefore it is most often used to combine a large number of hard drives with a disk controller. Unfortunately, the failure of any NL_port or connecting cable opens the loop and makes it inoperative, which is why such a circuit in its pure form is no longer considered a trans ...
FDDI technology is largely based on Token Ring technology, developing and improving its basic ideas. The developers of the FDDI technology set the following goals as the highest priority:
Increase the bit rate of data transfer up to 100 Mb / s.
Increase the fault tolerance of the network through standard procedures for recovering it after failures of various kinds - cable damage, incorrect work node, hub, occurrence high level line noise, etc.
Make the most of the 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 the network nodes. The use of two rings is the main way to improve resiliency in an FDDI network, and nodes that want to use it must be connected to both rings. In normal network operation, data passes through all nodes and all cable sections of the Primary ring, therefore this mode is called Thru mode - "through" 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. 31), forming again a single ring. This mode of operation of the network is called Wrap, that is, "folding" or "folding" rings. The folding operation is performed by the concentrators and / or FDDI network adapters. To simplify this procedure, data on the primary ring is always transmitted counterclockwise, and on the secondary ring, 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 makes it possible to correctly transmit and receive information by neighboring stations.
In the FDDI standards, a lot of attention is paid to various procedures that allow you to determine the presence of a network failure and then make the necessary reconfiguration. FDDI network can fully restore its operability in the event of single failures of its elements. When multiple failures the network splits into several unconnected networks.
Figure: 31. Reconfiguring FDDI rings on failure
Rings in FDDI networks are considered as a common shared data transmission medium, therefore 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 method of token (or token) ring - token ring (Fig. 32, a).
A station can start transmitting its own data frames only if it received a special frame from the previous station - an access token (Fig. 32, b). After that, she can transfer her frames, if she has them, during a time called the token holding time - Token Holding Time (THT). After the THT time expires, the station must complete the transmission of its next frame and transmit the access token to the next station. If, at the moment of accepting the token, the station has no 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 next neighbor (Fig. 32, c). It should be noted that if a station grabs a token and transmits its own frames, then during this period of time it does not broadcast incoming frames, but removes them from the network.
If the address of the frame coincides with the address of the station, then it copies the frame into its internal buffer, checks its correctness (mainly by the checksum), transfers its data field for subsequent processing to the protocol lying above the FDDI level (for example, IP), and then transmits the original frame over the network to the next 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 that, the frame continues to travel over 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, having completed a full turn, reaches it again (Fig. 32, e). In this case, the source station checks the signs of the frame, whether it reached the destination station and whether it was damaged. The process of restoring data frames is not the responsibility of the FDDI protocol, it should be done by the higher layer protocols.
Figure: 32. Frame processing by stations of the FDDI ring
Figure 33 shows the FDDI protocol structure compared to the seven-layer OSI model. FDDI defines a physical layer protocol and a link layer media access (MAC) sublayer protocol. Like many other LAN technologies, FDDI uses the 802.2 Data Link Control (LLC) 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 — connectionless and without recovering lost or corrupted frames.
Figure: 33. The structure of FDDI technology protocols
The physical layer is divided into two sublayers: the PHY (Physical) sublayer, and the Physical Media Dependent (PMD) sublayer. The operation of all levels is controlled by the station management protocol SMT (Station Management).
The PMD layer provides the necessary means to transfer data from one station to another over fiber. Its specification defines:
Optical power requirements and 62.5 / 125μm multimode fiber.
Requirements for optical bypass switches and optical transceivers.
Parameters of optical connectors MIC (Media Interface Connector), their marking.
The 1300 nanometer wavelength at which the transceivers operate.
Representation of signals in optical fibers according to the NRZI method.
The TP-PMD specification defines the ability to transfer data between stations over twisted pair in accordance with the MLT-3 method. The PMD and TP-PMD layer specifications have already been discussed in the Fast Ethernet sections.
The PHY layer encodes and decodes the data circulating between the MAC layer and the PMD layer, and also provides timing of information signals. Its specification defines:
information coding in accordance with the 4B / 5B scheme;
signal timing rules;
stability requirements clock frequency 125 MHz;
rules for converting information from parallel to serial form.
The MAC layer is responsible for network access control and for receiving and processing data frames. It defines the following parameters:
Token transfer protocol.
Token capture and relaying rules.
Frame formation.
Rules for generating and recognizing addresses.
Rules for calculating and verifying a 32-bit checksum.
The SMT layer performs all management and monitoring functions for all other layers of the FDDI protocol stack. Each node in the FDDI network takes part in ring management. Therefore, all nodes exchange special SMT frames for network management. The SMT specification defines the following:
Algorithms for error detection and recovery after failures.
Rules for monitoring the operation of the ring and stations.
Ring control.
Ring initialization routines.
FDDI networks resiliency is ensured by managing the SMT layer by other layers: the PHY layer eliminates network failures due to physical reasons, for example, due to a cable break, 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 |
Bus / star |
Star / ring |
|
Access method |
Share of token turnover time |
Priority reservation system |
|
|
Data transmission medium |
Unshielded twisted pair multimode fiber |
Thick coax, thin coax, twisted pair, fiber optic |
Shielded and unshielded twisted pair, fiber optic |
|
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 |
|
|
Clocking and failover |
Distributed implementation of clocking and failover |
Not defined |
Active monitor |
Chapter # 2: "Basic technologies of local networks"
3.5. FDDI technology
2.5. FDDI technology
Technology FDDI (Fiber Distributed Data Interface)- fiber-optic distributed data interface is the first local area 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, soon after the start of industrial operation of such channels in local networks. The ANSI CST9.5 problem group developed between 1986 and 1988. initial versions of the FDDI standard, which provides frames at a rate of 100 Mbps over a dual fiber optic ring up to 100 km in length.
2.5.1. The main characteristics of the technology
FDDI technology is largely based on Token Ring technology, developing and improving its basic ideas. The developers of the FDDI technology set the following goals as the highest priority:
increase the bit rate of data transmission up to 100 Mbps;
to increase the fault tolerance of the network due to the standard procedures for recovering it after failures of various kinds - cable damage, incorrect operation of a node, a hub, a high level of noise on the line, etc .;
make the most of the potential network bandwidth for both asynchronous and synchronous (delay-sensitive) traffic.
The FDDI network is built on the basis of two fiber-optic rings, which form the main and backup data transmission paths between the network nodes. Having two rings is the primary way to improve resiliency 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, data passes through all nodes and all cable sections of only the Primary ring, this mode is called Thru - "through" 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 a node failure), the primary ring is combined with the secondary (Fig. 2.16), again forming a single ring. This network mode is called Wrap, that is, "folding" or "folding" the rings. The coagulation operation is performed by means of concentrators and / or network adapters FDDI. To simplify this procedure, data on the primary ring is always transmitted in one direction (in the diagrams this direction is shown counterclockwise), and along the secondary - in the opposite direction (shown in clockwise 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 makes it possible to correctly transmit and receive information by neighboring stations.
Figure: 2.16. FDDI Ring Reconfiguration on Failure
In the FDDI standards, a lot of attention is paid to various procedures that allow you to determine the presence of a network failure and then make the necessary reconfiguration. FDDI network can fully restore its operability in the event of single failures of its elements. In the event of multiple failures, the network splits into several unconnected networks. FDDI supplements Token Ring failure detection mechanisms with network reconfiguration mechanisms based on redundant links provided by the second ring.
Rings in FDDI networks are considered as a common shared data transmission medium, therefore a special access method is defined for it. This method is very close to the Token Ring access method and is also called the token ring method.
The difference in the access method is that the retention time of the token in the FDDI network is not constant, as in the Token Ring network. This time depends on the loading of the ring - with a small load it increases, and with large overloads it can decrease to zero. These changes in the access method apply only to asynchronous traffic, which is not critical to small frame transmission delays. For synchronous traffic, the token retention time is still a fixed value. The frame priority mechanism, similar to that used in Token Ring technology, is absent in FDDI technology. The technology developers decided that dividing traffic into 8 priority levels is redundant and it is enough to divide traffic into two classes - asynchronous and synchronous, the last of which is always served, even when the ring is overloaded.
Otherwise, the transfer of frames between stations of the ring at the MAC level is fully consistent with the Token Ring technology. FDDI stations use an early token deallocation algorithm similar to 16 Mbps Token Ring networks.
MAC layer addresses are in a standard IEEE 802 format. The FDDI frame format is close to the Token Ring frame format, the main differences are in the absence of priority fields. Address recognition, frame copying, and error features preserve the existing Token Ring procedures for processing frames by the sending station, intermediate stations, and receiving station.
In fig. 2.17 shows the correspondence of the structure of protocols to the FDDI technology with the seven-layer OSI model. FDDI defines a physical layer protocol and a medium access sublayer (MAC) protocol link layer... Like many other LAN technologies, FDDI 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.
Figure: 2.17. FDDI technology protocol structure
A distinctive feature of the FDDI technology is the station control level - Station Management (SMT). It is the SMT layer that performs all the functions of managing and monitoring all other layers of the FDDI protocol stack. Each node in the FDDI network takes part in ring management. Therefore, all nodes exchange special SMT frames for network management.
Resiliency of FDDI networks is provided by protocols and other layers: using the physical layer, network failures due to physical reasons, for example, due to a cable break, are eliminated, and using the MAC layer, logical network failures, for example, the loss of the required internal path of the token and data frames between the ports of the hub ...
2.5.2. Features of the FDDI access method
To transmit synchronous frames, a station always has the right to capture a token when it arrives. In this case, the retention time of the marker has a predetermined fixed value.
If the station of the FDDI ring needs to transmit an asynchronous frame (the frame type is determined by the protocols upper levels), then to clarify the possibility capture of the marker when it is next arrival, the station must measure the time interval that has passed since the previous arrival of the marker. This interval is called token Rotation Time (TRT)... The TRT interval is compared with another value - maximum permissible turnover time of the marker along the ring T_0rg... If in Token Ring technology the maximum allowable token turnover time is a fixed value (2.6 s based on 260 stations in the ring), then in FDDI technology the stations agree on the value of T_0rg during ring initialization. Each station can offer its own value T_0rg, as a result, the minimum of the times suggested by the stations is set for the ring. This allows for the needs of station applications. Typically, synchronous applications (real-time applications) need to send data to the network more often in small chunks, while asynchronous applications are better off accessing the network less frequently but in large chunks. Preference is given to stations transmitting synchronous traffic.
Thus, with the next arrival of the token for the transmission of an asynchronous frame, the actual time of the TRT marker revolution is compared with the maximum possible T_0rg. If the ring is not overloaded, then the marker arrives earlier than the interval T_0rg expires, that is, TRT< Т_0рг. В этом случае станции разрешается захватить маркер и передать свой кадр (или кадры) в кольцо. Время удержания маркера ТНТ равно разности T_0pr - TRT, и в течение этого времени станция передает в кольцо столько асинхронных кадров, сколько успеет.
If the ring is overloaded and the marker is late, then the TRT interval will be greater than T_0rg. In this case, the station is not allowed to capture the token for the asynchronous frame. If all stations in the network want to transmit only asynchronous frames, and the marker has rotated around the ring too slowly, then all stations skip the marker in repeat mode, the marker quickly makes the next turn, and at the next cycle of operation, the stations already have the right to capture the marker and transmit their frames.
The FDDI access method for asynchronous traffic is adaptive and handles temporary network congestion well.
2.5.3. Fault tolerance of FDDI technology
To ensure fault tolerance, the FDDI standard provides for the creation of two fiber-optic rings - a primary and a secondary. The FDDI standard allows two types of connection of stations to the network. Simultaneous connection to the primary and secondary rings is called Dual Attachment, DA. Connecting to only the primary ring is called Single Attachment, SA.
The FDDI standard provides for the presence in the network of end nodes - stations (Station), as well as concentrators (Concentrator). For stations and hubs, any type of connection to the network is allowed - both single and double. Accordingly, such devices have the corresponding names: SAS (Single Attachment Station), DAS (Dual Attachment Station), SAC (Single Attachment Concentrator) and DAC (Dual Attachment Concentrator).
Typically, hubs are dual wired and stations are single wired, as shown in Fig. 2.18, although not required. To make it easier to correctly connect the devices to the network, their connectors are marked. Connectors of type A and B must be for devices with double connection, connector M (Master) is available for a hub for single connection of a station, in which the mating connector must be of type S (Slave).
Figure: 2.18.Connecting nodes to FDDI rings
In the event of a single cable break between devices with a dual connection, the FDDI network will be able to continue normal operation by automatically reconfiguring the internal frame transmission paths between the hub ports (Figure 2.19). A double cable break will result in two isolated FDDI networks. If a cable breaks to a station with a single connection, it becomes cut off from the network, and the ring continues to work due to reconfiguration of the internal path in the hub - port M, to which this station was connected, will be excluded from the general path.
Figure: 2.19. FDDI network reconfiguration in case of wire break
To keep the network operational during a power outage in stations with dual connections, i.e. DAS stations, the latter must be equipped with Optical Bypass Switches, which create a bypass path for the light flux in the event of a power failure that they receive from the station.
Finally, DAS stations or DACs can be connected to two M ports of one or two hubs, creating a tree structure with primary and secondary links. By default, port B supports primary communications and port A provides backup. This configuration is called Dual Homing
Fault tolerance is maintained by constantly monitoring the SMT layer of hubs and stations for the time intervals of the circulation of the token and frames, as well as for the presence of a physical connection between adjacent ports in the network. The FDDI network does not have a dedicated active monitor - all stations and hubs are equal, and if abnormalities are detected, they begin the process of re-initializing 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.
2.5.4. Physical layer of FDDI technology
The FDDI technology for the transmission of light signals over optical fibers implements logical 4B / 5B coding in combination with physical NRZI coding. This scheme results in the transmission of signals on the communication line with 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, then from the remaining 16 several codes were selected, which are used as service ones. The most important service symbols include the Idle symbol - a simple one that is constantly transmitted between ports during the 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. In the absence of a stream of Idle symbols, a physical communication failure is detected and the internal path of the concentrator or station is reconfigured, if possible.
When two nodes are initially cabled, their ports first perform a physical connection procedure. In this procedure, sequences of 4B / 5B code service characters are used, with the help of which some physical layer command language is created. These commands allow the ports to tell each other the port types (A, B, M, or S) and decide if the correct this connection (eg, s-S connection is invalid, etc.). If the connection is correct, then a channel quality test is performed when transmitting symbols of 4B / 5B codes, and then the operability of the MAC level 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 the physical connection is controlled by the station control protocol SMT.
The physical layer is divided into two sublayers: the PHY (Physical) sublayer and the Physical Media Dependent (PMD) sublayer (see Figure 2.17).
FDDI technology currently supports two PMD sublayers: for fiber-optic cable and for unshielded twisted pair of category 5. The latter standard appeared later than optical and is called TP-PMD.
The fiber optic PMD sublayer provides the necessary means to transfer data from one station to another over an optical fiber. Its specification defines:
use as the main physical medium of a multimode fiber-optic cable 62.5 / 125 μm;
requirements for 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 marking;
use for transmission of light with a wavelength of 1300 nm;
presentation of signals in optical fibers according to the NRZI method.
The TP-PMD sublayer defines the ability to transfer data between stations over twisted pair in accordance with the MLT-3 physical coding method, using two potential levels: + V and -V to represent data in the cable. To obtain a signal spectrum 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 100 kilometers, and the maximum number of double-connected stations in the ring is 500.
2.5.5. Comparison of FDDI with Ethernet and Token Ring Technologies
Table 2.7 shows the results of comparing FDDI technology with Ethernet and Token Ring technologies.
Table 2.7. Characteristics of FDDI, Ethernet, Token Ring technologies
FDDI technology was developed for use in critical areas of networks - on backbones between large networks, such as building networks, as well as for connecting high-performance servers to the network. Therefore, the main thing for the developers was to provide high data transfer speed, fault tolerance at the protocol level and long distances between network nodes. All these goals have been achieved. As a result, the FDDI technology turned out to be of high quality, but very expensive. Even the appearance of a cheaper twisted pair option did not significantly reduce the cost of connecting one node to the FDDI network. Therefore, practice has shown that the main area of \u200b\u200bapplication of FDDI technology has become the backbone of networks consisting of several buildings, as well as a network of a large city scale, that is, of the MAN class. The technology proved to be too expensive to connect client computers and even small servers. And since FDDI equipment has been produced for about 10 years, no significant cost reduction can be expected.
As a result, network specialists from the early 90s began to look for ways to create relatively inexpensive and at the same time high-speed technologies that would work just as successfully on all floors of the corporate network, as they did in the 80s. ethernet technologies and Token Ring.
conclusions
FDDI technology was the first to use fiber-optic cable in local networks, as well as work at a speed of 100 Mbps.
There is significant continuity between Token Ring and FDDI technologies: both are characterized by a ring topology and a token access method.
FDDI technology is the most resilient LAN technology. In case of single failures of the cable system or station, the network, due to the "folding" of the double ring into a single one, remains fully operational.
The FDDI token access method works differently for synchronous and asynchronous frames (the type of frame is determined by the station). To transmit a synchronous frame, the station can always capture the incoming marker for a fixed time. To transmit an asynchronous frame, the station can capture the token only when the token has rotated around the ring fast enough, which indicates that there is no ring congestion. This access method, firstly, prefers synchronous frames, and secondly, it regulates the ring load, slowing down the transmission of non-urgent asynchronous frames.
FDDI uses fiber optic cables and Category 5 UTP (this version of the physical layer is called TP-PMD) as the physical medium.
The maximum number of double connection stations in a ring is 500, the maximum diameter of a double ring is 100 km. The maximum distances between neighboring nodes for multimode cable are 2 km, for twisted pair UPT category 5-100 m, and for single-mode fiber depends on its quality.
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Usually FDDI was used to provide quick access to network servers.
FDDI and token ring access methods are similar in that they use token passing to send data over the network. The difference between FDDI and the standard token ring is that it uses a synchronous token passing access method. The FDDI marker travels along the network ring from node to node. If some node has no data to transmit, it takes the token and forwards it to the next node. If the node that owns the token needs to transmit data, it can send any desired number of frames within a fixed amount of time called the target token round trip time (TTRT). Since the FDDI standard uses synchronous method transmission of a token, several frames from several nodes can be in the network at any given time, which ensures a high data transfer rate.
After a node has transmitted a frame, the latter moves to the next node on the network ring. Each of the nodes determines if the frame is for the current node and if there are errors in this frame. If the node is a data sink, it marks the frame as read. If any node detects an error, it sets the status bit of the frame to indicate an error. When the frame returns to the sending node, it determines whether the target node received the given frame and whether there were any errors. In case of errors, the frame is re-transmitted. If there are no errors, the sending node removes the frame from the ring.
The FDDI standard allows two ways to transfer packets: synchronous and asynchronous. Synchronous data transmission is used to send information that is continuous in time: voice, video or multimedia. Asynchronous transmission is used for normal network traffic that does not need to be sent in continuous chunks. For a particular network, TTRT is equal to the total time required for synchronous data transmission from a node plus the time it takes for a maximum frame length to travel around the ring.
There are two types of errors tracked in the FDDI network: long periods of inactivity and long periods of token missing. In the first case, it is assumed that the marker has been lost. In the second case, it is assumed that some node is continuously transmitting. For any type of error, the host that detects the error generates a sequence of special frames called claim frames or claim frames. The claim frame contains the suggested time TTRT. The first node stops transmitting, and the next node in the ring compares its TTRT with the value sent by the previous node. After comparing, it passes the lower of the TTRT values \u200b\u200bto the next node, writing this value to its claim frames. By the time the information reaches the last node, the smallest TTRT value will be selected. At this moment, the ring is initialized, for which a token is passed to it and a new TTRT time is set for each node; this state lasts until the last node receives new information.
The FDDI network uses two rings, so that if one ring fails, the data can reach the target node on the other ring. Nodes of two classes are connected to the FDDI network. Class A nodes are connected to both network rings. This class forms networking equipment such as hubs. Class A nodes can reconfigure a ring so that one ring can be used in the event of a network failure. Class B nodes connect to the FDDI network through Class A devices. This class includes servers and workstations.
FDDI (Fiber Distributed Data Interface) technology - fiber-optic distributed data interface is the first local area network technology in which the data transmission medium is a fiber-optic cable.
Work on the creation of technologies and devices for using fiber-optic channels in local networks began in the 80s, soon after the start of industrial operation of such channels in local networks. The ANSI CST9.5 problem group developed between 1986 and 1988. initial versions of the FDDI standard, which provides frames at a rate of 100 Mbps over a dual fiber optic ring up to 100 km in length.
FDDI technology is largely based on Token Ring technology, developing and improving its basic ideas. The developers of the FDDI technology set the following goals as the highest priority:
Increase the bit rate of data transmission up to 100 Mbps;
Increase the fault tolerance of the network due to standard procedures for recovering it after failures of various kinds - cable damage, incorrect operation of a node, a hub, a high level of noise on the line, etc .;
Make the most of your potential bandwidth
network capability for both asynchronous and synchronous (delay-sensitive) traffic.
The FDDI network is built on the basis of two fiber-optic rings, which form the main and backup data transmission paths between the network nodes. Having two rings is the primary way to improve resiliency 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, data passes through all nodes and all sections of the cable only in the Primary ring, this mode is called Thru mode - "through" 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 (see figure), again forming a single ring. This mode of operation of the network is called Wrap, that is, "folding" or "folding" rings. The folding operation is performed by means of hubs and / or FDDI network adapters. To simplify this procedure, data on the primary ring is always transmitted in one direction (in the diagrams this direction is shown counterclockwise), and along the secondary ring in the opposite direction (shown in clockwise 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 makes it possible to correctly transmit and receive information by neighboring stations.
Features of the access method.
To transmit synchronous frames, the station always has the right to capture the token when it arrives. In this case, the retention time of the marker has a predetermined fixed value. If the station of the FDDI ring needs to transmit an asynchronous frame (the frame type is determined by the protocols of the upper layers), then to find out the possibility of capturing a marker when it next appears, the station must measure the time interval that has passed since the previous arrival of the marker. This interval is called the Token Rotation Time (TRT). The TRT interval is compared with another value - the maximum permissible time of the marker revolution along the T_Opr ring. If in Token Ring technology the maximum allowable token turnover time is a fixed value (2.6 s based on 260 stations in a ring), then in FDDI technology the stations agree on the value of T_Opr during ring initialization. Each station can offer its own T_Opr value, as a result, the minimum of the times suggested by the stations is set for the ring.
Fault tolerance technology.
To ensure fault tolerance, the FDDI standard provides for the creation of two fiber-optic rings - a primary and a secondary.
The FDDI standard allows two types of connection of stations to the network:
Simultaneous connection to the primary and secondary rings is called Dual Attachment, DA.
Connecting to only the primary ring is called Single Attachment, SA.
The FDDI standard provides for the presence in the network of end nodes - stations (Station), as well as concentrators (Concentrator). For stations and hubs, any kind of connection to the network is allowed - both single and double. Accordingly, such devices have the corresponding names: SAS (Single Attachment Station), DAS (Dual Attachment Station), SAC (Single Attachment Concentrator) and DAC (Dual Attachment Concentrator).
Typically, hubs are dual wired and stations are single wired, as shown in the figure, although this is not required. To make it easier to correctly connect devices to the network, their connectors are marked. Connectors of type A and B must be for devices with double connection, connector M (Master) is available for a hub for single connection of a station, in which the mating connector must be of type S (Slave).
The physical layer is divided into two sublayers: the PHY (Physical) sublayer and the Physical Media Dependent (PMD) sublayer.
13. Structured cabling system / SCS /. Hierarchy in the cabling system. Choice of cable type for different subsystems.
Structured cabling system (SCS) is the physical basis of the information infrastructure of an enterprise, which allows unified system many information services for different purposes: local computing and telephone networks, security systems, video surveillance, etc.
SCS is a hierarchical cabling system of a building or a group of buildings, divided into structural subsystems. It consists of a set of copper and optical cables, cross-panels, patch cords, cable connectors, modular jacks, information outlets and ancillary equipment. All of these elements are integrated into a single system and are operated according to certain rules.
A cabling system is a system whose elements are cables and components that are associated with a cable. Cable components include all passive switching equipment used to connect or physically terminate (terminate) a cable - telecommunication sockets at workplaces, cross-connect and patch panels (jargon: patch panels) in telecommunications rooms, couplings and splices;
Structured. A structure is any set or combination of related and dependent constituent parts. The term "structured" means, on the one hand, the ability of the system to support various telecommunication applications (voice, data and video transmission), on the other hand, the possibility of using various components and products different manufacturers, and thirdly, the ability to implement the so-called multimedia environment, in which several types of transmission media are used - coaxial cable, UTP, STP and optical fiber. The structure of the cable system is determined by the infrastructure of information technology, IT (Information Technology), it is she who dictates the content of a specific project of the cable system in accordance with the requirements end user, regardless of the active equipment that can be used subsequently.
14. Network adapters / CA /. Functions and characteristics of the CA. SA classification. Principle of operation.
Network adapters act as a physical interface between a computer and network cable... They are usually inserted into expansion slots on workstations and servers. To provide a physical connection between the computer and the network, a network cable is connected to the appropriate port on the adapter after the adapter is installed.
Functions and characteristics of network adapters.
Network adapter and its driver in computer network perform the function of the physical layer and the MAC - layer. The network adapter and driver receive and transmit the frame. This operation takes place in several stages. Most often, the interaction of protocols with each other inside the computer occurs through buffers located inside the RAM.
It is known that network adapters implement protocols, and depending on which protocol they work with, adapters are divided into: Ethernet adapters, FDDI adapters, Token Ring adapters, and many others. Most modern Ethernet adapters support two speeds, and therefore also contain the 10/100 prefix in their name.
Before installing a network adapter on a computer, you need to configure it. In the event that the computer operating system and the network adapter itself is Plug-and-Play, the adapter and its driver are automatically configured. If this standard is not supported, then first you need to configure the network adapter, and then apply exactly the same parameters in the driver configuration. In this process, much depends on the manufacturer of the network adapter, as well as on the parameters and capabilities of the bus for which the adapter is intended.
Classification of network adapters.
There have been four generations of Ethernet network adapters. For the manufacture of the first generation of adapters, discrete, logical microcircuits were used, so they did not differ in high reliability. Their buffer memory was designed for only one frame, which already suggests that their performance was very low. In addition, the configuration of a network adapter of this type was done using jumpers, and therefore manually.
3.5.1. Main characteristics of the technology FDDI technology is largely based on Token Ring technology, developing and improving its basic ideas. The developers of the FDDI technology set the following goals as the highest priority:- increase the bit rate of data transmission up to 100 Mbps;
- to increase the fault tolerance of the network due to the standard procedures for recovering it after failures of various kinds - cable damage, incorrect operation of a node, a hub, a high level of noise on the line, etc .;
- make the most of the potential network bandwidth for both asynchronous and synchronous (delay-sensitive) traffic.
Figure: 3.17. The structure of FDDI technology protocols A distinctive feature of FDDI technology is the station control level - Station Management (SMT). It is the SMT layer that performs all the functions of managing and monitoring all other layers of the FDDI protocol stack. Each node in the FDDI network takes part in ring management. Therefore, all nodes exchange special SMT frames for network management. Resiliency of FDDI networks is provided by protocols and other layers: using the physical layer, network failures are eliminated for physical reasons, for example, due to a cable break, and using the MAC layer, logical network failures, for example, the loss of the required internal path of transmission of the token and data frames between the ports of the hub ...
