Token Ring and FDDI technologies have significantly more complex implementation than the same Ethernet technology on a shared data environment. Much of this complexity was due to the fact that the developers have tried to improve the effectiveness of the technology: to increase fault tolerance, make the environment predictable design priority service specific data packets, such as voice traffic, which is sensitive to delays. In this they succeeded, for example, a more advanced version of Token Ring, FDDI namely for a long time been used as a campus backbone networks of enterprises.
Data transfer rate in Token Ring networks
Initially, the data rate is 4 Mbit/s when it was first developed by the company in the IBM, but was later increased to 16 Mbit/s. Addressing in local networks was the same as in Ethernet technology, that is MAC-addresses are the same size.
Network access method
In Token Ring networks to determine the sequence of nodes that have access to the transmission medium used by Special frame - a token, or a token. Token Ring technology has a ring topology and the token is passed from node to node in one direction. The node that owns the token, has the right to broadcast information in a shared data environment. Thus there is no question of conflicts in the shared data environment, which are present in the Ethernet technology. There is also a time limit on the ownership of the token to the node could not monopolistically fully capture all network resources. When the time of possession node transmits the token neighbor node.
Transfer token in the Token Ring network between nodes
The problem of transmission of delay-sensitive traffic is solved by prioritizing training. The transmitting node sets the priority of each frame. Also, the token itself is always a certain level of priority at any given time.
FDDI technology
Technologists FDDI is an advanced version of Token Ring technology. It also uses a ring topology and the transfer of the token from node to node. A difference is that FDDI operates at high speed and has a better mechanism to ensure fault tolerance. Also this is the first technology that started using an optical fiber. Optical fiber is used as the start time with the 70-ies of the last century.
Transfer token in FDDI
This technology has two rings for transmission of data. The primary ring is the main and it all traffic is transmitted and the secondary reserve. In case of failure of one of the nodes. Nearby sites wrapped traffic to a second ring and a ring topology for reduced backup path. Formed flat ring topology.
Local area 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 run on local network nodes, but the functions they perform are not related 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.
The Token-Ring network was proposed by IBM. Token-Ring was intended to network all types of computers produced by IBM (from personal computers to large ones).
Token Ring - local technology computer network(LAN) rings with "token access" - a local network protocol that is located at the data link layer (DLL) of the OSI model. It uses a special three-byte frame called a token that moves around the ring. Possession of a token gives its owner the right to transmit information on the medium. Token ring frames travel in a loop.
Stations on a Token Ring local area network (LAN) are logically organized in a ring topology with data transferred sequentially from one ring station to another with a control token circulating around the control access ring. This token passing mechanism is shared by ARCNET, the token bus, and FDDI, and has theoretical advantages over stochastic CSMA/CD Ethernet. The maximum payload unit (MTU) size is 4464 bytes.
Token Ring and IEEE 802.5 are prime examples of token-passing networks. Token passing networks move a small block of data called a token around the network. Possession of this token guarantees the right to transfer. If the node receiving the token does not have information to send, it simply forwards the token to the next endpoint. Each station can hold a marker for a certain maximum time (default is 10ms).
This technology offers a solution to the problem of collisions that arise when operating a local network. In Ethernet technology, such collisions occur when information is simultaneously transmitted by several workstations located within the same segment, that is, using a common physical data channel.
If the station that owns the token has information to transmit, it grabs the token, changes one bit of it (resulting in the token becoming a "beginning of data block" sequence), completes it with the information it wants to transmit, and sends that information to the next one. ring network stations. When a block of information circulates around the ring, there is no token on the network (unless the ring provides early token release), so other stations wishing to transmit information are forced to wait. Therefore, in Token networks Ring cannot have collisions. If early token release is ensured, then a new token can be released after the data block transmission is completed.
The information block circulates around the ring until it reaches the intended destination station, which copies the information for further processing. The information block continues to circulate around the ring; it is permanently deleted after reaching the station that sent the block. The sending station can check the returned block to ensure that it was viewed and then copied by the destination station.
History and main characteristics
Token Ring networks, like Ethernet networks, are characterized by a shared data transmission medium, which consists of cable segments connecting all computers on the network into a ring. To access the ring, it is not a random algorithm, as in Ethernet networks, but a deterministic one, based on transferring the right to use the ring to stations. This right is conveyed using a special format frame called a token.
Token Ring technology was developed by IBM in 1984 and then submitted as a draft standard to the IEEE 802 committee, which based it on it adopted the IEEE 802.5 standard in 1985. IBM uses Token Ring technology as its main network technology to build local networks based on computers of various classes - mainframes, minicomputers and PCs. Currently, IBM is the main trendsetter in Token Ring technology, producing about 60% network adapters this technology.
Token Ring networks operate at two speeds – 4 and 16 Mbit/s. Mixing stations operating at different speeds in one ring is not allowed.
Networks operating at 16 Mbit/s have some improvements in the access algorithm compared to the 4 Mbit/s standard.
Token Ring technology is a more complex technology than Ethernet. It has fault tolerance properties. The Token Ring network defines network control procedures that use feedback ring-shaped structure - the sent frame always returns to the sender. In some cases, detected errors in the network operation are eliminated automatically, for example, a lost token can be restored. In other cases, errors are only recorded, and their elimination is carried out manually by maintenance personnel.
To monitor the network, one of the stations plays the role of a so-called active monitor. The active monitor is selected during ring initialization as the station with maximum MAC address value. If the active monitor fails, the ring initialization procedure is repeated and a new active monitor is selected. In order for the network to detect the failure of an active monitor, the latter, in a working state, generates a special frame of its presence every 3 seconds. If this frame does not appear on the network for more than 7 seconds, then the remaining stations on the network begin the procedure for selecting a new active monitor.
Token Ring parameters:
Ø network topology – ring,
Ø network cable– twisted pair,
Ø transmission speed – 4 or 16 Mbit/s,
Ø maximum cable length – 100 m (STP) or 45 m (UTP),
Ø maximum ring length – 4000 m,
Ø maximum number of nodes – 260 (STP) or 72 UTP),
Ø access method – marker.
Token medium access method
In networks with a token access method, which, in addition to Token Ring networks, include networks FDDI, Arc Net and industrial networks MAP, the right to access the medium is transferred cyclically from station to station along a logical ring.
To ensure access of stations to the physical environment, a frame of a special format and purpose circulates around the ring - marker(token). In a Token Ring network, any station always receives data only from the station that is the previous one in the ring. Such a station called the nearest active neighbor located upstream of the data stream. The station always transmits data to its nearest neighbor downstream.
Having received the marker, the station analyzes it and, if it does not have data to transmit, ensures its advancement to the next station. A station that has data to transmit, upon receiving the token, removes it from the ring, which gives it the right to access the physical medium and transmit its data. This station then issues a data frame of the established format into the ring bit sequential. The transmitted data always passes along the ring in one direction from one station to another. The frame is provided with a destination address and a source address.
All stations on the ring relay the frame bit by bit, like repeaters. If the frame passes through the destination station, then, having recognized its address, this station copies the frame to its internal buffer and inserts an acknowledgment sign into the frame. The station that issued the data frame to the ring, upon receiving it back with confirmation of receipt, removes this frame from the ring and transmits a new token to the network to enable other network stations to transmit data. This access algorithm is used in Token Ring networks with an operating speed of 4 Mbit/s, described in the 802.5 standard.
Environment ownership time on the Token Ring network limited by marker holding time, after which the station must stop transmitting its own data and pass the token further along the ring. A station may have time to transmit one or more frames during the marker holding time, depending on their size and the marker holding time. Typically, the default token hold time is 10 ms, and the maximum frame size is undefined in the 802.5 standard. For networks 4 Mbit/s it is usually equal 4 KB, and for networks 16 Mbit/s - 16 KB. This is due to the fact that during the time the marker is held, the station must have time to transmit at least one frame. At a speed of 4 Mbit/s, 5000 bytes can be transferred in 10 ms, and at a speed of 16 Mbit/s – 20,000 bytes. The maximum frame sizes were chosen with some reserve.
In Token Ring networks 16 Mbit/s a slightly different ring access algorithm is used, called early token release algorithm. In accordance with it, a station transmits an access token to the next station immediately after the end of transmission of the last bit of the frame, without waiting for the return of this frame along the ring with an acknowledgment bit. In this case, the ring capacity is used more efficiently, since frames from several stations move along the ring simultaneously. However, only one station can generate its frames at any given time - the one in this moment owns the access token. At this time, the remaining stations only repeat other people's frames, so that the principle of dividing the ring in time is preserved, only the procedure for transferring ownership of the ring is accelerated.
For various types messages transmitted to frames can be assigned different priorities: from 0 to 7. The decision on the priority of a particular frame is made by the transmitting station (the Token Ring protocol receives this parameter through cross-layer interfaces from the protocols top level, for example applied). A token also always has some level of current priority. A station has the right to seize a token transmitted to it only if the priority of the frame it wants to transmit is higher than (or equal to) the priority of the token. Otherwise, the station must pass the token to the next station in the ring.
Token Ring Frame Formats
There are three in Token Ring various formats frames:
Ø marker,
Ø data frame,
Ø interrupting sequence.
Marker
A token frame consists of three fields, each one byte long:
Ø initial limiter appears at the beginning of the marker, as well as at the beginning of any frame passing through the network. The field is a unique sequence of Manchester code characters – JKOJKOOO, therefore it cannot be confused with any sequence within the frame.
Ø access control consists of four subfields: RRR, T, M And RRR, where РРР - priority bits, T - marker bit, M - monitor bit, RRR- reserved priority bits. Bit T, installed in 1 , indicates that this frame is an access token. Bit M installed in 1 active monitor and 0 any other station transmitting the marker or frame. If an active monitor sees a marker or frame containing a monitor bit with a value of 1, then it knows that this frame or marker has already walked around the ring once and has not been processed by stations. If it is a frame, then it is removed from the ring. If it is a token, then the active monitor passes it further along the ring. We'll look at using priority fields later.
Ø final limiter last marker field. Like the start delimiter field, this field contains a unique Manchester code sequence JK1JK1, as well as two one-bit signs I And E. Sign I indicates whether the frame is the last in a series of frames ( I=0) or intermediate ( I=1). Sign E– a sign of an error. It is installed in 0 the sending station, and any ring station through which the frame passes must set this flag to 1 , if she finds an error in checksum or other incorrect frame.
This link layer technology was developed by IBM in the early 1980s and later standardized by IEEE in Project 802 as the IEEE 802.5 specification. Token Ring networks refer to networks with a token access control method in which there is no competition for access to the transmission medium. Logically the Token Ring network is a ring, but physically it is a star. Token Ring networks operate at two bit rates - 4 and 16 Mbit/s. Mixing stations operating at different speeds in one ring is not allowed.
To connect computers in Token Ring networks, hubs are used - so-called. multiple access devices (MSAU,MultiStationAccessUnit). Workstations are connected to MSAU using separate cables using a star topology. Token Ring technology allows you to use shielded or unshielded twisted pair cable for connection.
The maximum segment length when using unshielded twisted pair (UTP) is 150 m (when operating at a speed of 4 Mbit/s) or 60 m (when operating at a speed of 16 Mbit/s), when using shielded twisted pair (STP) – the transmission distance increases up to 300 m (for 4 Mbit/s) or 100 m (for 16 Mbit/s).
A ring based on unshielded cables can operate no more than 72 stations, and a ring based on shielded cables can operate up to a maximum of 260 stations.
In networks with token access method the right to access the medium is transferred cyclically from station to station along a logical ring. The ring is formed by cable segments connecting all workstations and is considered as a shared transmission medium. To ensure access of stations to the physical environment, a frame of a special format and purpose circulates around the ring - marker or token (token).
A token is a specific sequence of bits and can only be used by one workstation or node at a time. Having received the token, the workstation analyzes it, modifies it if necessary, and if it does not have data to transmit, ensures its advancement to the next station. A station that has data to transmit, upon receiving the token, removes it from the ring, which gives it the right to access the physical medium and transmit its data. This station then converts the token into a frame of the established format and begins transmitting it around the ring. The frame is provided with a destination address and a source address (each workstation has a unique 48-bit MAC address).
The transmitted data always passes along the ring in one direction from one station to another, so it is received by all workstations on the network. Each station checks whether the frame is intended for it. If not, the station acts as a relay and transmits the received frame to the next station on the network. When the destination station recognizes the frame, it copies it into its memory, then modifies some bits in the frame format (an acknowledgment signal) and loops it back to the sending station. The latter removes this frame from the ring and checks whether the message is received normally. It then issues a new token to enable other stations on the network to transmit data.
The time of ownership of a shared environment in the Token Ring network is limited token holding time, after which the station must stop transmitting its own data (the current frame is allowed to be completed) and pass the token further along the ring. The station may have time to transmit one or more frames during the marker holding time, depending on the size of the frames and the marker holding time.
Token Ring networks operating at 16 Mbps have a different ring access algorithm than 4 Mbps networks, called the Early Token Release. In accordance with it, the station transmits a token to the next station immediately after the end of transmission of the last bit of the frame, without waiting for the return of this frame along the ring with an acknowledgment bit. In this case, the ring capacity is used more efficiently, since frames from several stations move along the ring simultaneously. However, only one station can generate its frames at any given time - the one that currently owns the marker. At this time, the remaining stations only relay other people's frames, so the principle of dividing the ring in time is preserved, only the procedure for transferring ownership of the ring is accelerated.
Token Ring technology has fault-tolerant properties. To control the operation of the network and handle errors in Token Ring networks, one of the stations plays the role of active monitor, which examines the frames circulating on the network, removes all defective frames, issues a new token and provides correct work networks.
The advantages of Token Ring technology include:
- ease of calculating transmission delay between any two devices, which is especially important in automated systems controls that require real-time processing of processes;
- no collisions.
Flaws:
- high cost, low equipment compatibility;
- low transmission speed.
Introduction
Token Ring networking technology was first introduced by IBM in 1982 and was adopted by the IEEE (Institute for Electrical and Electronic Engineers) as an 802.5 standard in 1985. Token Ring remains IBM's primary local area network (LAN) technology, second only to Ethernet/IEEE 802.3 in popularity among LAN technologies. Token Ring networks operate at two bit rates - 4 Mb/s and 16 Mb/s. The first speed is defined in the 802.5 standard, and the second is a new de facto standard that emerged as a result of the development of Token Ring technology.
The Token Ring is wired in a star configuration, but it functions as a logical ring.
A token (a small frame of a special format, sometimes called a token) circulates in the logical ring; when it reaches the station, it seizes the channel. The marker always circulates in one direction. A node that receives a token from its nearest upstream active neighbor passes it on to its downstream neighbor. Each station in the ring receives data from the busy token and sends it (exactly repeating the token) to the neighboring network node. In this way, data circulates around the ring until it reaches the destination station. In turn, this station stores the data and transmits it to upper-level protocols and transmits the frame further (by changing two bits in it - a sign of receipt). When the token reaches the sending station, it is released, and then the process continues in the same way.
16 Mb/s Token Ring networks also use a slightly different ring access algorithm, called the Early Token Release algorithm. . In accordance with it, a station transmits an access token to the next station immediately after the end of transmission of the last bit of the frame, without waiting for the return of this frame along the ring with an acknowledgment bit. In this case, the ring capacity is used more efficiently and approaches 80% of the nominal. When a block of information is circulating around the ring, there is no token on the network (unless the ring provides "early token release"), so other stations wishing to transmit information must wait. Thus, only one packet can be transmitted over the network at a time, therefore, There can be no collisions in Token Ring networks. If early token release is ensured, then a new token can be released after the data block transmission is completed.
Token Ring networks use a complex priority system that allows certain stations with high priority, designated by the user, to use the network more frequently. Token Ring data blocks contain two fields that control priority: the priorities field and the reservation field.
Only stations with a priority that is equal to or higher than the priority value contained in the token can take possession of it. Once a token is captured and modified (resulting in it becoming an information block), only stations whose priority is higher than that of the transmitting station can reserve the token for the next traverse of the network. When the next token is generated, the higher priority of the given reservation station is included in it. Stations that increase the priority level of a token must restore the previous priority level after completing the transmission.
When a ring is established, each station's interface stores the addresses of the preceding station and the subsequent station in the ring. Periodically, the token holder broadcasts one of the SOLICIT_SUCCESSOR frames, inviting new stations to join the ring. This frame contains the address of the sender and the address of the station next to it in the ring. Stations with addresses in this address range can join the ring. This maintains the order (in ascending order) of the addresses in the ring. If no station responds to the SOLICIT_SUCCESSOR frame, then the station holding the token closes the response window and continues to function as normal. If there is exactly one response, then the station that responded is included in the ring and becomes the next one in the ring. If two or more stations respond, a collision is detected. The station holding the token initiates the collision resolution algorithm by sending a RESOLVE_CONTENTION frame. This algorithm is a two-bit modification of the reverse binary counter algorithm.
Each station has two bits in its interface that are set randomly. Their values are 0,1,2 and 3. The value of these bits determines the amount of delay when a station responds to an invitation to connect to the ring. The values of these bits are reset every 50ms.
The procedure for connecting a new station to the ring does not violate the worst-case guaranteed time for transmitting a token around the ring. Each station has a timer that resets when the station receives a token. Before it is reset, its value is compared with some value. If it is greater, then the procedure for connecting the station to the ring does not start. In any case, no more than one station is connected at a time. Theoretically, a station can wait as long as it wants to connect to the ring; in practice, no more than a few seconds. However, from the point of view of real-time applications, this is one of the weakest points of 802.4.
Disconnecting a station from the ring is very simple. Station X with predecessor S and successor P sends a SET_SUCCESSOR frame, which indicates to P that its predecessor is now S. X then stops transmitting.
Ring initialization is a special case of connecting a station to a ring. At the initial moment the station turns on and listens to the channel. If it detects no evidence of transmission, then it generates a CLAIM_TOKEN token.
If no competitors are found, then it generates a marker itself and sets up a ring of one station. It periodically generates SOLICIT_SUCCESSOR frames, inviting other stations to join the ring. If at the initial moment two stations were turned on at once, then the algorithm of a reverse binary counter with two digits is launched.
ISU (Information Symbol Unit) refers to an information transmission unit
a common part
Token Ring networks use Various types frames:
Data/Command Frame, Token, Abort.
Network hardwareToken Ring
When connecting devices in ARCNet, a bus or star topology is used. ARCNet adapters support the Token Bus access method (token bus)
Mixing stations operating at different speeds in one ring is not allowed.
Collisions
Due to transmission errors and equipment failures, problems with token transmission may occur - collisions. The Token Ring standard clearly defines collision resolution methods:
Important for collision resolution is the ability of stations to “listen” after transmission.
If a station transmits a token to a neighboring one, and at that time it turns off (for example, due to a hardware failure), then if no frame or token transmissions follow, the token is sent a second time.
If nothing happens when the token is retransmitted, then the station sends a WHO_FOLLOWS frame, where the non-responding neighbor is indicated. Seeing this frame, the station for which the predecessor station is not responding sends a SET_SUCCESSOR frame, and becomes the new neighbor. In this case, the non-responding station is excluded from the ring.
If not only the next station has stopped, but also the one after it, a new procedure is launched by sending the SOLICIT_SUCCESSOR_2 frame. It involves a conflict resolution procedure. At the same time, anyone who wants to connect to the ring can do so. In effect, the ring is reset.
Another type of problem occurs when the marker holder stops and the marker disappears from the ring. This problem is solved by running the ring initialization procedure. Each station has a timer that resets every time a marker appears. If the value of this timer exceeds some preset value (time out), then the station generates a CLAIM_TOKEN frame. This starts the reverse binary counter algorithm.
If there are two or more tokens on the bus, the station that owns the token, seeing the transfer of the token on the bus, resets its token. This is repeated until exactly one marker remains in the system.
Not all stations in the ring are equal. One of the stations is designated as active monitor, which means additional responsibility for managing the ring. The active monitor controls timeout in the ring, spawns new tokens (if necessary) to maintain the operational state, and generates diagnostic frames under certain circumstances. The active monitor is selected when the ring is initialized, and any station on the network can act as this monitor. The algorithm for determining the active monitor is as follows: when turned on or if some station notices the absence of a monitor, it sends a CLAIM_TOKEN frame. If she is the first to send such a frame, then she becomes the monitor
If a monitor fails for any reason, there is a mechanism by which the other stations (backup monitors) can negotiate which one will be the new active monitor. One of the functions that an active monitor serves is to remove constantly circulating blocks of data from the ring. If the device that sent the data block
failed, then this block can constantly circulate around the ring. This can prevent other stations from transmitting their own data blocks and effectively blocks the network. The active monitor can detect and remove such blocks and generate a new token. An important function of the monitor is to set the delay on the ring; the delay must be sufficient for the 24-bit marker to fit in the ring.
The IBM Token Ring network's star topology also helps improve overall network reliability. Because All Token Ring network information is viewed by active MSAUs, these devices can be programmed to check for problems and selectively remove stations from the ring if necessary.
The Token Ring algorithm, called "signaling" ( beaconing), identifies and attempts to resolve some network faults. If any station detects a serious problem in the network (for example, such as a cable break), it sends a signal data block. The signaling data block specifies the fault domain, which includes the station reporting the fault, its nearest active upstream neighbor (NAUN), and everything in between. The alarm initiates a process called "auto-reconfiguration" ( autoreconfiguration), in which nodes located within the failed domain automatically perform diagnostics in an attempt to reconfigure the network around the failed zone. Physically, MSAU can accomplish this through electrical reconfiguration.
Practical part
Let us have a network of 50 stations operating at a speed of 10 Mbit/s and configured so that 1/3 remains at the substation with priority 6 bandwidth. Then each station is guaranteed to have a speed of at least 67 Kb/s for priority 6. This bandwidth can be used to control devices in real time.
An important issue when creating a ring network is the "physical length" of a bit. Let the data flow at speed R Mbps. This means that every 1/R ms a bit appears on the line. Considering that the signal travels at a speed of 200 m/ms, one bit occupies 200/R meters of the ring. Hence, at a speed of 1 Mbps and a circumference of 1 km, the ring can accommodate no more than 5 bits at a time.
A consequence of the token-ring network design is that the network must be of sufficient length so that the entire token can fit within it even when all stations are waiting. Delays consist of two components - 1 bit delay at the station interface and signal propagation delay. Considering that stations can be switched off, e.g.
at night, it follows that there should be an artificial delay on the ring if the ring is not long enough. When the network load is low, the ring and token will be able to immediately transmit their messages. As the load increases, the stations will have larger queues for transmission and, in accordance with the ring algorithm, they will capture the marker and transmit. The ring load will gradually increase until it reaches 100%.
Marker Format
The token frame consists of three fields, each one byte long.
Start delimiter field appears at the beginning of the marker, as well as at the beginning of any frame passing through the network. The field consists of a unique series of electrical pulses that are different from the pulses that encode the 1s and 0s in the data bytes. Therefore, the initial delimiter cannot be confused with any bit sequence.
Access control field. Divided into four data elements:
PPP T M RRR,
where PPP - priority bits, T - marker bit, M - monitor bit, RRR - reserve bits.
Each frame or marker has a priority, set by the priority bits (value from 0 to 7, 7 being the highest priority). A station can use a token only if it has received a token with a priority less than or equal to its own. The station network adapter, if it fails to acquire the token, places its priority in the reserved bits of the token, but only if the priority written in the reserved bits is lower than its own. This station will have preferential access the next time a token arrives at it.
The scheme for using the priority method of token capture is shown in Figure 13. First, the monitor places the maximum priority value in the current priority field P, and the reserve priority field R is reset to zero (token 7110). The token passes through a ring in which stations have current priorities of 3, 6 and 4. Since these values are less than 7, the stations cannot capture the token, but they write their priority value in the backup priority field if their priority is higher than its current one meanings. As a result, the token is returned to the monitor with a reserve priority value of R = 6. The monitor rewrites this value in the P field, resets the reserve priority value to zero, and sends the token around the ring again. During this rotation, it is captured by a station with priority 6 - the highest priority in the ring at a given time.
The marker bit has the value 0 for a marker and 1 for a frame.
The monitor bit is set to 1 by the active monitor and to 0 by any other station transmitting the token or frame. If the active monitor
sees a marker or frame containing a monitor bit of 1, then the active monitor knows that this frame or marker has already bypassed the ring once and has not been processed by stations. If it is a frame, then it is removed from the ring. If it is a token, then the active monitor rewrites the priority from the reserved bits of the received token into the priority field. Therefore, the next time the marker passes through the ring, it will be captured by the station with the highest priority.
End delimiter field- last marker field. Just like the initial limiter field, this field contains a unique series of electrical pulses that cannot be confused with data. In addition to marking the end of the marker, this field also contains two subfields: the intermediate frame bit and the error bit. These fields relate more to the data frame, which we will consider
--------
The Start delimiter and End delimiter fields are intended to recognize the beginning and end of the frame. They have a special encoding that cannot be encountered by the user. Therefore, the frame length field is not required. The Frame control field separates control fields from data fields. For data frames, the frame priority is specified here. This field is also used by the receiving station to confirm whether the frame was received correctly or incorrectly. Without this field, the recipient would be unable to give confirmations - he does not have a token.
Token ringAndFDDI
Technology Fiber Distributed Data Interface (FDDI) - the first local network technology that used fiber optic cable as a data transmission medium.
FDDI is essentially a high-speed, fiber optic version of Token Ring. Unlike Token Ring, FDDI is implemented without traditional “hubs”. Another difference between FDDI and Token Ring is the ability to transmit data simultaneously, i.e. In FDDI networks, multiple frames can circulate simultaneously.
According to its topology, FDDI consists of two logical rings with markers circulating through them in opposite directions. Rings form the main and backup paths for data transmission between network nodes. Using two rings is the main way to improve fault tolerance in FDDI networks, and nodes that want to use it must be connected to both rings. In normal network operation mode, data passes through all nodes and all cable sections of the Primary ring, which is why this mode is called Thru mode - “end-to-end” or “transit”. The Secondary ring is not used in this mode. In the event of some type of failure where part of the primary ring cannot transmit data (for example, a broken cable or node failure), the primary ring is merged with the secondary
), forming a single ring again. This mode of network operation is called Wrap, that is, “folding” or “folding” of rings. The collapse operation is performed by FDDI hubs and/or network adapters. To simplify this procedure, data is always transmitted counterclockwise on the primary ring, and clockwise on the secondary ring. Therefore, when a common ring of two rings is formed, the transmitters of the stations still remain connected to the receivers of neighboring stations, which allows information to be correctly transmitted and received by neighboring stations.
FDDI achieves a bit rate of 100 Mb/s
The FDDI initialization procedure is slightly different from the Token Ring initialization:
To complete the initialization procedure, each network station must be aware of its requirements for the maximum time for a token to circulate around the ring. These requirements are contained in a parameter called "required token rotation time" - TTRT (Target Token Rotation Time).
The TTRT parameter reflects the degree of demand of a station for ring capacity - the shorter the TTRT time, the more often the station wants to receive a token to transmit its frames. The initialization procedure allows stations to learn about the token rotation time requirements of other stations and select the minimum time as a general parameter T_Opr, based on which the ring capacity will be allocated in the future. The TTRT parameter must be between 4 ms and 165 ms and can be changed by the network administrator.
To carry out the initialization procedure, stations exchange MAC-level service frames - Claim frames. These frames have the value 1L00 0011 in the control field, the destination address field contains the source address (DA = SA), and the information field contains the 4-byte value of the requested token turnaround time T_Req.
If any station decides to begin the ring initialization process on its own initiative, then it generates a Claim Token frame with its value of the required TTRT token turnover time, that is, it assigns its TTRT value to the T_Req field. No token capture is required to send a Claim frame. Any other station, having received the Claim Token frame, begins executing the Claim Token process. In this case, stations set the sign that the ring is in an operational state, Ring_Operational, to the False state, which means the cancellation of normal operations for transferring a token and data frames. In this state, stations exchange only service Claim frames.
To perform the initialization procedure, each station maintains a timer of the current token rotation time TRT (Token Rotation Timer), which is also used in the future when the ring operates in normal mode. To simplify the presentation, we will assume that this timer, like other station timers, is initialized with a zero value and then increases its value to a certain value, called the timer expiration threshold. (In a real FDDI ring, all timers operate in two's complement code.)
The TRT timer is started by each station when it detects the start of the Claim Token procedure. The maximum permissible token rotation time is selected as the limit value of the timer, that is, 165 ms. Expiration of the TRT timer before the procedure is completed means that the procedure ended unsuccessfully - the ring could not be initialized. If the Claim Token process fails, the Beacon and Trace processes are launched, with the help of which the ring stations try to identify the incorrectly functioning part of the ring and disconnect it from the network.
During the Claim Token process, each station can first send a Claim frame around the ring with a T_Req value equal to the value of its TTRT parameter. In doing so, it sets the value of T_Opr equal to the value of TTRT. Consider the example of an initialized ring shown in Figure 9.
At some point in time, all stations transmitted their proposals around the ring about the value of the maximum token rotation time: 72 ms, 37 ms, 51 ms and 65 ms. A station, having received a Claim frame from a previous station, must compare the T_Req value specified in the frame with the TTRT value of its proposal.
If another station asks to set the token rotation time less than this (that is, T_Req
The station that is the source of the frame for the network is responsible for removing the frame from the network after it has completed a full rotation and reaches it again.
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.
Block diagrams
Token Ring
Ring Token Ring working with two... technologies Token Ring. Mixing stations operating at different speeds in one ring is not allowed. Networks Token Ring ...Development of local computing networks production cooperative
Coursework >> Computer ScienceTrendsetter technologies Token Ring, producing about 60% of the network adapters of this technologies. Networks Token Ring working with two... Token Ring is more complex technology than Ethernet. It has fault tolerance properties. IN networks Token Ring ...
Technologies frame switching in local networks
Abstract >> Computer Science... (frame switching) in local networks Limitations of traditional technologies(Ethernet, Token Ring), based on shared media... transmissions in networks Token Ring or FDDI (Figure 2.13). The principle of operation of the switch in networks any technologies stayed...
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