Token Ring- this is another architecture local networks, standardized by the IEEE. It shares many properties with Ethernet and other networking technologies whose specifications are described by the IEEE 802 family of standards. As a result, Token Ring networks can interoperate with other architectures using conversion bridges. 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 802.5 standard in 1985. Token Ring networks operate at two bit rates - 4 and 16 Mbit/s. Today, there are recommendations that suggest increasing the transmission speed of Token Ring signals to 100, 128 Mbit/s, and in the future up to 1 Gbit/s.
In its "canonical" form, Token Ring technology is a well-defined and efficient local area network architecture. It got its name thanks to the “carousel” scheme of access to the environment. Unlike Ethernet technology, which allows chaotic and unordered multiple access to the medium, Token Ring allows transmission in certain moment time for only one device. Therefore, conflicts cannot arise in principle. Access to the environment is provided to everyone network devices in order of priority by transmission marker(token). Only one token can circulate in the network, which is given the form of a data frame header by the sending device. Without a token, the device cannot construct the data frame header and cannot transmit it. The frame data is copied to the receiving device's buffer, after which some bits of the frame header are inverted, thereby confirming that the data has been received. The frame then continues its journey around the ring. When it returns to the sending device, it removes the frame from the network and removes the intended recipient's address and the actual payload from it. If the same device is going to transmit some more data, it has the right to form a frame again and place it in the ring. Otherwise, the header is converted back into a token, placed on the transmission medium, and sent to the next device.
To ensure that no one station “monopolizes” the entire frequency band, the so-called marker capture timer(Token Holding Timer) monitors and regulates the maximum period of time during which a station has the exclusive right to transmit data. Typically the default token hold time is 10ms. The maximum frame size in the 802.5 standard is not defined. For 4 Mbps networks it is usually 4 KB, and for 16 Mbps networks it is usually 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.
16 Mbps Token Ring networks also use a slightly different ring access algorithm, called the early marker release(Early Token Release). 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 throughput The ring is used more efficiently, since frames from several stations simultaneously move along the ring.
For various types messages transmitted to frames can be assigned different priorities: from 0 (lowest) to 7 (highest). The decision on the priority of a particular frame is made by the transmitting station. 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. Responsible for the presence of a token, and its only copy, on the network active monitor- one of the stations selected for this role during ring initialization as the station with the maximum MAC address value.
The IBM Token Ring standard initially provided for building connections in the network using hubs called MAU (Multistation Access Unit) (Fig. 22). In general, the Token Ring network has a combined star-ring configuration. End nodes are connected to a hub (MAU) in a star topology, and the MAUs themselves are combined through special Ring In (RI) and Ring Out (RO) ports to form a backbone physical ring. A Token Ring hub can be active or passive. A passive hub simply connects ports internal connections so that the stations connected to these ports form a ring. The passive MAU does not perform signal amplification or resynchronization. An active hub performs signal regeneration functions and is therefore sometimes called a repeater, as in the Ethernet standard.
Rice. 22. Physical configuration of the Token Ring network
All stations in the ring must operate at the same speed - either 4 Mbit/s or 16 Mbit/s. The cables connecting the station to the hub are called subscriber, and the cables connecting the hubs are main. Token Ring technology can be used to connect end stations and hubs Various types cables: STP Type 1, UTP Type 3, UTP Type 5, as well as fiber optic cable. When using shielded twisted pair STP Type 1 from the IBM cable system range, it is possible to combine up to 260 stations into a ring with a subscriber cable length of up to 100 meters, and when using unshielded twisted pair, the maximum number of stations is reduced to 72 with a subscriber cable length of up to 45 meters. The distance between passive MAUs can be up to 100 m when using STP Type 1 cable and 45 m when using UTP Type 3 cable. Between active MAUs, the maximum distance increases respectively to 730 m or 365 m depending on the cable type. The maximum length of a Token Ring is 4000 m, although this limitation is not as strict as in Ethernet technology.
IBM recently introduced a new variant of Token Ring technology called High-Speed Token Ring, HSTR. This technology supports bit rates of 100 and 155 Mbps, while maintaining the core features of 16 Mbps Token Ring technology. Token Ring technology supports the following frame types:
· marker frame;
· data frame;
· LLC data frame;
· personnel MAC control;
· transmission interrupt frame.
IEEE 802.5 Token Ring technology uses a special bit sequence structure known as a token to control access to the transmission medium.
Marker frame consists of three fields, each one byte long:
· initial limiter(Starting Delimiter) appears at the beginning of the marker, as well as at the beginning of any frame passing through the network;
· access control field(Access Control) consists of four subfields: PPP - priority bits, T - marker bit, M - monitor bit, RRR - reserved priority bits;
· final limiter(Ending Delimiter) - the last field of the marker.
The priority field is used to identify the importance of the token. The value of this field can range from 000 to 111. The marker bit is the bit that must be inverted to turn the marker into the start of frame sequence. The token bit is set to 1 in order to inform other stations that the token is now integral part frame. The request priority field allows stations to service data requests from higher priority stations that have urgent data first. Stations can communicate the priority of their data by setting the appropriate bits in the request priority field.
Minimum length data frame Token Ring is 21 octets. The maximum length of a data frame is determined by the ring signaling rate. The data frame contains three marker frame fields, each one octet in length. Six more fields and subfields are added to this basic structure.
The first field is reserved for initial limiter, defining the beginning of the frame. Then it is located medium access field and eight-bit frame control field. This field stores the "typical" bits that define the transport protocol. In addition, the same field is used to separate data frames from control frames. The next two six-octet fields contain MAC addresses of the intended recipient and sender of the frame. Data field Token Ring networks have an arbitrary size, determined by the speed of signal transmission along the ring. Networks with a capacity of 4 Mbit/s allow the transmission of data fields from 0 to 4332 octets in length. Networks with a capacity of 16 Mbit/s allow the transmission of data fields from 0 to 17832 octets in length. The last three fields in the data frame are 32-bit frame check sequence(Frame Check Sequence - FCS), 8-bit final limiter(Ending Delimiter) and 8-bit frame status field. The frame check sequence contains checksum- a value that is calculated based on the length and contents of the frame. The last two octets, which include the end delimiter field and the frame status field, are considered the End of Frame Sequence.
MAC Control Frames differ from data frames only in the information field and sometimes in the frame control field. MAC frames perform purely ring maintenance and management functions. They never carry data to higher layers and are never transmitted to other collision areas by bridges, switches, or routers. Each MAC frame performs a clearly defined network management function:
· subscriber cable control;
· initialization of the ring;
· cleaning the ring;
· creation (declaration) of a marker;
· active monitoring functions.
Considering the fairly large number of different types of MAC frames (more than 25 types), there is no point in considering each of them separately. Suffice it to say that these MAC frames are used to collect network performance characteristics that can be obtained from standards-compliant network management applications.
Transmission Interrupt Frame consists only of the start and end delimiter fields. Although this structure may seem meaningless due to the lack of content and addressing block, such frames are used to immediately stop the transmission.
TokenRing networks, like Ethernet networks, are characterized by a shared data transmission medium, which in this case consists of cable sections connecting all network stations into a ring. The ring is considered as a common shared resource, and access to it requires not a random algorithm, as in Ethernet networks, but a deterministic one, based on transferring the right to use the ring to stations in a certain order. This right is conveyed using a special format frame called marker or token (token). TokenRing 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 802.5 standard in 1985. TokenRing networks operate at two bit rates - 4 and 16 Mbit/s. Mixing stations operating at different speeds in one ring is not allowed. TokenRing networks operating at 16Mbps have some improvements in the access algorithm compared to the 4Mbps standard. Token Ring technology is a more complex technology than Ethernet. It has fault tolerance properties. The TokenRing network defines network control procedures that use feedback ring-shaped structure - the sent frame always returns to the sending station. 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 control the network, one of the stations acts as a so-called active monitor. The active monitor is selected during ring initialization as the station with the maximum MAC address value. If the active monitor fails, the ring initialization procedure is repeated and a new active monitor is selected. So that the network can 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.
In networks with token access method(and these, in addition to TokenRing networks, include FDDI networks, as well as networks close to the 802.4 standard, -ArcNet, industrial networks MAP) the right to access the medium is transferred cyclically from station to station along a logical ring.
In the TokenRing network, a ring is formed by cable segments connecting neighboring stations. Thus, each station is connected to its predecessor and successor station and can only directly exchange data with them. To provide stations with access to the physical environment, a frame of a special format and purpose - a token - circulates around the ring. In the TokenRing network, any station always directly receives data from only one station - the one that is the previous one in the ring. This station is called nearestactive upstream neighbor(data) - Nearest Active UpstreamNeighbor, NAUN. The station always transmits data to its nearest neighbor down the data stream. Having received the marker, the station analyzes it and, if it does not have data to transmit, ensures its progress 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. Then this station issues a data frame of the established format into the ring sequentially by bit. 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 TokenRing networks with an operating speed of 4 Mbit/s, described in the 802.5 standard.
The time of ownership of a shared environment in the TokenRing network is limited marker retention time (token holding time), after the expiration of which the station must stop transmitting its own data (the current frame is allowed to be completed) and transmit 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. Typically, the default token hold time is 10ms, and the maximum frame size is undefined in the 802.5 standard. For 4Mbps networks it is usually 4Kbytes, and for 16Mbps networks it is usually 16Kbytes. 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 can be transferred, respectively. The maximum frame sizes were chosen with some reserve.
TokenRing 16Mbps networks also use a slightly different ring access algorithm, called the early release token (Early Token Release). 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 that 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 different types of messages, frames transmitted can be assigned different priorities: from 0 (lowest) to 7 (highest). The decision on the priority of a particular frame is made by the transmitting station (the TokenRing protocol receives this parameter via cross-layer interfaces from upper-level protocols, for example, application). 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.
The active monitor is responsible for the presence of a token on the network, and its only copy. If the active monitor does not receive a token for a long time (for example, 2.6s), then it spawns a new token.
The IBM TokenRing standard initially provided for building connections in the network using hubs called MAU (Multistation Access Unit) or MSAU (Multi-Station Access Unit), that is, multiple access devices (Fig. 3.15). The TokenRing network can include up to 260 nodes.
A TokenRing hub can be active or passive. A passive hub simply interconnects ports so that stations connected to those ports form a ring. The passive MSAU does not perform signal amplification or resynchronization. Such a device can be considered a simple crossover unit with one exception - MSAU provides bypass of a port when the computer connected to this port is turned off. This function is necessary to ensure ring connectivity regardless of the state of the connected computers. Typically, port bypass is accomplished using relay circuits that are powered by DC power from the AC adapter, and when the AC adapter is turned off, normally closed relay contacts connect the port's input to its output. An active hub performs signal regeneration functions and is therefore sometimes called a repeater, as in the Ethernet standard.
In general, the TokenRing network has a combined star-ring configuration. End nodes are connected to the MSAU using a star topology, and the MSAUs themselves are connected through special ports RingIn (RI) and RingOut (RO) to form a backbone physical ring. All stations in the ring must operate at the same speed - 4Mbit/s or 16Mbit/s. The cables connecting the station to the hub are called lobecable, and the cables connecting the hubs are called trunkcable. TokenRing technology allows you to use different types of cable to connect end stations and hubs: STPTypeI, UTPType 3, UTPType 6, as well as fiber optic cable. When using shielded twisted pair STPType 1 from the IBM cable system range, it is possible to combine up to 260 stations into a ring with a drop cable length of up to 100 meters, and when using unshielded twisted pair cable, the maximum number of stations is reduced to 72 with a drop cable length of up to 45 meters. The distance between passive MSAUs can reach 100 m when using STPType 1 and 45 m cable when using a UTPType 3 cable. Between active MSAUs, the maximum distance increases respectively to 730m or 365m depending on the cable type. The maximum length of the TokenRing is 4000m.
IBM recently introduced a new variant of TokenRing technology called High-SpeedTokenRing, HSTR. This technology supports bit rates of 100 and 155 Mbit/s, while maintaining the main features of the 16 Mbit/s TokenRing technology.
conclusions
TokenRing technology is developed primarily by IBM and also has IEEE 802.5 status, which reflects the most important improvements being made to IBM technology.
TokenRing networks use a token access method, which guarantees that each station will gain access to the shared ring within the token rotation time. Because of this property, this method is sometimes called deterministic.
The access method is based on priorities: from 0 (lowest) to 7 (highest). The station itself determines the priority of the current frame and can capture the ring only if there are no higher priority frames in the ring.
TokenRing networks operate at two speeds: 4 and 16 Mbit/s and can use shielded twisted pair, unshielded twisted pair, and fiber optic cable as the physical medium. The maximum number of stations in the ring is 260, and the maximum length of the ring is 4 km.
TokenRing technology has elements of fault tolerance. Due to the feedback of the ring, one of the stations - the active monitor - continuously monitors the presence of the marker, as well as the rotation time of the marker and data frames. If the ring does not operate correctly, the procedure for its reinitialization is launched, and if this does not help, then the beaconing procedure is used to localize the faulty section of the cable or the faulty station.
The maximum size of the data field of a TokenRing frame depends on the speed of the ring. For a speed of 4 Mbit/s it is about 5000 bytes, and at a speed of 16 Mbit/s it is about 16 KB. The minimum size of the frame data field is not defined, that is, it can be equal to 0.
In the TokenRing network, stations are connected into a ring using hubs called MSAU. The passive hub MSAU acts as a crossover panel that connects the output of the previous station in the ring with the input of the next one. The maximum distance from the station to MSAU is 100m for STP and 45m for UTP.
An active monitor also acts as a repeater in the ring - it resynchronizes the signals passing through the ring.
The ring can be built on the basis of an active MSAU hub, which in this case is called a repeater.
The TokenRing network can be built on the basis of several rings separated by bridges that route frames based on the “from the source” principle, for which a special field with the route of the rings is added to the TokenRing frame.
Networks Token standard Ring uses a shared data transmission medium, which consists of cable segments connecting all network stations into a ring. Token Ring networks operate at two bit rates - 4 Mb/s and 16 Mb/s.
The ring is considered as a common shared resource, and to access it, not a random algorithm is used, as in Ethernet networks, but a deterministic one, based on the transfer of the right to use the ring by stations in a certain order. To ensure access of stations to the physical environment, a frame of a special format and purpose circulates around the ring - marker (token).
Having received the token, the station 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 sends a data frame of the established format into the ring bit by bit. The transmitted data always passes along the ring in one direction from one station to another.
When a data frame arrives at one or more stations, these stations copy this frame for themselves and insert an acknowledgment of reception into this 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 issues a new token to enable other stations on the network to transmit data.
16 Mbps Token Ring networks use a slightly different algorithm for accessing the ring, called the early release of the marker. 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.
For different types of messages, the transmitted data can be assigned different priorities.
Each station has mechanisms to detect and correct network faults resulting from transmission errors or transient phenomena (for example, when the station connects and disconnects).
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. 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.
There are three in Token Ring various formats frames:
dataframe;
Interrupting sequence.
The Token Ring access method was developed by IBM and remains one of the main technologies of local area networks, although no longer as popular as Ethernet. The data transfer speed in older versions of marker networks is 4 Mbit/s or 16 Mbit/s, and in new high-speed networks it is 100 Mbit/s. The token ring communication method uses a physical star topology combined with ring topology logic. Even though each node is connected to a central hub, the packet moves from node to node as if there were no starting and ending points. Each node is connected to the others using a Multistation Access Unit (MAU). MAU is a specialized hub that ensures packet transmission through a closed chain of computers. Because packets travel around the ring, there are no terminators on the workstations or MAU.
Marker- a special frame that is continuously transmitted around the ring to determine the moment when a certain node can send a packet. This frame is 24 bits long and consists of three 8-bit fields: the start flag (SD), the access control field (AC) and the end flag (ED). The start sign is a combination of signals distinct from any other signals on the network, which prevents the field from being misinterpreted. It looks like a missing data signal. This unique combination of eight bits can only be recognized as the start of frame (SOF) flag.
The access control field (8-bit) indicates whether a frame containing data is attached to the token, that is, this field determines whether the frame is carrying data or is free for use by some node. The terminator is also a uniquely encoded no-data signal. Its eight bits represent a signal that cannot be confused with a start sign or interpreted as data. This part of the token determines whether the node should still transmit subsequent frames (the last frame ID). It also contains information about errors detected by other stations.
In most implementations, there can be only one token per ring, although IEEE specifications allow two tokens in networks operating at 16 Mbps and above. Before a node starts transmitting, it must intercept the token. Until the active node finishes, no other node can acquire the token and transmit data. The station that has acquired the token creates a frame that has a start flag and an access control field at the beginning of that frame. The terminator is placed at the end of the given frame. The received frame is sent around the ring and transmitted until it reaches the target node. The destination node changes the values of two bits, indicating that the frame has reached its destination and that the data has been read. The target node then places the frame back on the network, where it is passed around the ring until the sending station receives the frame and verifies that it has received it. The sending station then generates the next frame with the token and encapsulated data, or creates a token without the data, returning the token to the ring so that another station can use it.
In Fig. Figure 3.3 shows a marker ring frame with marker fields added to the data fields. The first 16 bits are occupied by the start attribute and access control fields. Next comes the frame control field. This field identifies the frame as a data frame or as a frame intended for network management (for example, as a frame containing codes network errors). The next two fields are 16 or 48 bits long and are used for addressing. The first field contains the destination node address, and the second field contains the source node address. Next is the Routing Information Field (RIF), which is 144 bits or less in length. This field contains initial routing data that can be used on Network level OSI models.
Rice. 3.3. Bitwise representation of the 802.5 Token Ring frame format
The next three fields—the target service access point (DSAP) field, the source service access point (SSAP) field, and the control (CTRL) field—have the same functionality and size as in 802.3 and Ethernet II frames. The DSAP field identifies the destination host SAP, and the SSAP field indicates which access point the frame was sent from, such as Novell or TCP/IP. An 8- or 16-bit control field determines whether the frame contains data or error control information. The data field follows the control field. It contains data or error codes used to manage the network. The data field does not have a predefined size. A 32-bit checksum (FCS) field is used to verify the integrity of the entire frame. Like the Ethernet frame, it uses a coded redundancy check (CRC) algorithm to ensure that the signal is sent and received correctly. The checksum in the received frame must match the sent value.
The last part of the token, the terminator, follows the checksum field of the frame. This field contains information that informs the receiving node that the end of the frame has been reached. The field also indicates whether the next frame will be sent from the source node or whether this frame is the last. In addition, this field may contain information that other stations have detected errors in the frame. If the frame contains an error, it is removed from the network and then resent by the sending node.
The last field in the token ring frame is the 8-bit frame status field. Two bits of this field are especially important to the sending node: the address recognition bit indicates that the target node "saw" its address contained in the frame; The frame copy bit determines whether the target node copied the sent frame or whether there were errors.
In each token ring, one node acts as an activity monitor or dispatcher. Typically, these tasks are performed by the first station discovered after the network is launched. The dispatcher is responsible for synchronizing packets across the network and for generating a new token frame if problems occur. At intervals of several seconds, the dispatcher sends out a broadcast frame to the MAC sublayer, indicating that the dispatcher is operational. A broadcast frame or packet is addressed to all nodes on the network. Other workstation nodes are backup dispatchers. Periodically, they generate broadcast frames, called standby dispatcher presence frames, confirming the health of the nodes and their ability to replace the active dispatcher if it fails.
The broadcast frame is generated at the Link Layer of the OSI model, and its destination field is filled with binary ones. The broadcast packet is generated at the Network layer of the OSI model in networks using the IP protocol. Its destination address is 255.255.255.255. In addition to broadcasts, there are unidirectional packets that are transmitted only to the target node for which a particular packet is intended. In addition, there are multicast packets that the sender sends to several target nodes, with each of these nodes receiving a copy of the packet.
If there are no broadcasts from the active or standby controllers, the ring enters the "beaconing" state. This state begins when some node generates a so-called beacon frame indicating the detection of some error. The ring attempts to automatically resolve the error, for example by assigning a new active manager if the original manager fails. After entering the beacon emitting state, the transmission of data tokens stops until the problem is resolved.
Token rings are a very robust topology and are therefore sometimes used in critical configurations. One of the advantages of token rings over Ethernet networks is that they rarely experience broadcast storms or contention between workstations. A broadcast storm sometimes occurs on Ethernet networks when a large number of computers or devices attempt to transmit data at the same time, or when computers or devices get stuck in a transmission loop. Network conflicts also occur in Ethernet networks when a faulty network adapter continues to transmit broadcast packets, despite the network being busy. Such problems are rare in token networks because only one node can transmit data at a time.
Nizhny Novgorod branch
Course work
Discipline: Computer networks and telecommunications
Topic: Characteristics of the Token Ring network
Student Tarasov Artema Yurievich
Introduction
1. Main part
Conclusion
Glossary
Introduction
Local Area Networks (LAN) are a combination of computers concentrated in a small area, usually within a radius of no more than 1-2 km. In general, a local network is a communication system owned by one organization.
The needs of computer users were growing. They were no longer satisfied with isolated work own computer, they wanted to automatic mode exchange computer data with users of other departments. This is how local networks appeared within enterprises.
At first, non-standard software and hardware were used to connect computers to each other. Various interface devices, using their own method of providing data on communication lines, their own types of cables, etc., could connect only those specific models computers for which, for example, PDP-11 mini-computers with an IBM 360 mainframe or Nairi computers with Dnepr computers were developed.
In the mid-80s, the situation in local networks began to change dramatically. Standard technologies for connecting computers into a network have become established - Ethernet, Arcnet, Token Ring, Token Bus, and somewhat later - FDDI. Personal computers served as a powerful stimulus for their appearance. PCs began to dominate local networks, not only as client computers, but also as data storage and processing centers, that is, network servers, displacing mini-computers and mainframes from these usual roles.
The end of the 90s revealed a clear leader among local network technologies - the Ethernet family, which included the classic Ethernet 10 Mbit/s technology, as well as Fast Ethernet 100 Mbit/s and Gigabit Ethernet 1000 Mbit/s.
Token Ring technology was developed by IBM in 1984, and then transferred as a draft standard to the IEEE 802 committee, which based it on it adopted the 802.5 standard in 1985. IBM uses Token Ring technology as its main network technology for building local area networks based on computers of various classes - mainframes, mini-computers and personal computers. Currently, IBM is the main trendsetter of Token Ring technology, producing about 60% of network adapters for this technology.
1. Main part
1.1 General information about Token Ring technology
The Token Ring network was proposed by IBM in 1984 (the first version appeared in 1980). The purpose of Token Ring was to network all types of computers produced by IBM (from personal computers to large ones). The very fact that it is supported by IBM, the largest manufacturer of computer equipment, indicates that it occupies a special place among computer networks. But just as important is what Token Ring is currently international standard IEEE 802.5 This puts this network on the same status level as Ethernet.
IBM has done everything to ensure the widest possible distribution of its network: detailed documentation has been released up to circuit diagrams adapters. As a result, many companies, such as 3COM, Novell, Western Digital, Proteon has started producing adapters. By the way, the NetBIOS concept was developed specifically for this network, as well as for another network, the IBM PC Network. By the way, if in the previously developed PC Network NetBIOS programs were stored in the built-in read-only memory in the adapter, then in the Token Ring network a program emulating NetBIOS was already used, which made it possible to respond more flexibly to the features of specific equipment, while maintaining compatibility with higher-level programs .
Compared to Ethernet equipment, Token Ring equipment turns out to be noticeably more expensive, since it uses more complex exchange control methods, so the Token Ring network is much less widespread. However, its use becomes justified when high exchange rates are required (for example, when communicating with large computers) and limited access time.
Figure 1.1 - Star-ring topology of the Token Ring network
The Token Ring network has a ring topology, although outwardly it looks more like a star. This is due to the fact that individual subscribers (computers) do not connect to the network directly, but through special hubs or multiple access devices (MSAU or MAU - Multistation Access Unit). Therefore, physically the network forms a star-ring topology (Fig. 1.1). In reality, the subscribers are still united in a ring, that is, each of them transmits information to one neighboring subscriber, and receives information from another neighboring subscriber.
Token Ring and IEEE 802.5 networks are generally nearly compatible, although their specifications have relatively minor differences. IBM's Token Ring network stipulates a star connection, as I described above. While IEEE 802.5 does not specify the network topology (although virtually all IEEE 802.5 implementations are also based on a star network). There are other differences, including the media type (IEEE 802.5 does not specify a media type, while IBM's Token Ring networks use twisted pair) and the size of the routing information field.
Unlike CSMA/CD networks (such as Ethernet), token-passing networks are deterministic networks. This means that it is possible to calculate the maximum time that will pass before any end station can transmit. This characteristic, as well as some reliability characteristics, make the Token Ring network ideal for applications where latency must be predictable and network stability is important. Examples of such applications are the environment of automated stations in factories. It is used as a cheaper technology and has become widespread wherever there are critical applications for which it is not so much speed that is important as reliable delivery of information. Currently, Ethernet is not inferior to Token Ring in reliability and is significantly higher in performance.
There are 2 modifications for transfer speeds: 4Mb/s and 16Mb/s. Token Ring 16Mb/s uses early token release technology. The essence of this technology is that the station that has “captured the token”, upon completion of data transmission, generates a free token and launches it into the network. Attempts to introduce 100Mb/s technology were unsuccessful. Token Ring technology is not currently supported.
1.2 Token method of access to shared media
The Token Ring network uses the classic token access method, that is, a token constantly circulates around the ring to which subscribers can attach their data packets. This implies such an important advantage of this network as the absence of conflicts, but this also leads to such disadvantages as the need to monitor the integrity of the token and the dependence of the functioning of the network on each of the subscribers (in the event of a malfunction, the subscriber must be excluded from the ring).
Figure 2.1 - Token Ring network token format
To monitor the integrity of the token, one of the subscribers (the so-called active monitor) is used. His equipment is no different from the others, but his software monitor time relationships in the network and create a new marker if necessary. The active monitor is selected when the network is initialized; it can be any computer on the network. If the active monitor for some reason fails, then a special mechanism is activated, through which other subscribers (spare monitors) decide to assign a new active monitor.
The marker is a control packet containing only three bytes (Fig. 2.1): the initial division byte (SD - Start Delimiter), the access control byte (AC - Access Control) and the end delimiter byte (ED - End Delimiter). The initial separator and the final separator are not just a sequence of zeros and ones, but contain pulses of a special type. This ensures that these delimiters cannot be confused with any other bytes in the packets. Four bits of the separator are zero bits in the accepted encoding, and the other four bits do not correspond to the Manchester-P code: during two bit intervals one signal level is held, and during the remaining two - another level. As a result, such a synchronization failure is easily detected by the receiver.
Figure 2.2 - Access control byte format
The control byte is divided into four fields (Fig. 2.2): three priority bits, a marker bit, a monitor bit and three reservation bits. The priority bits allow a subscriber to assign priority to its packets or token (priority can be from 0 to 7, with 7 being the highest priority and 0 being the lowest). A subscriber can attach its packet to a token only when its own priority is the same as or higher than the priority of the token. The token bit determines whether a packet is attached to the token (a one corresponds to a token without a packet, a zero to a token with a packet). A monitor bit set to one indicates that this token was sent by the active monitor. Reservation bits allow the subscriber to reserve their right to further take over the network, that is, to take a turn for service, so to speak. If the subscriber's priority is higher than the current value of the reservation field, he can write his priority there instead of the previous one.
In addition to the start and end delimiters and the access control byte, the packet also includes a packet control byte, network addresses receiver and transmitter, data, checksum and status byte, packet.
Figure 2.3 - Token Ring network packet format (field lengths are given in bytes)
The purpose of the packet fields is as follows:
The start separator (SD) is an indication of the beginning of a packet.2. The access control (AC) byte has the same meaning as in the token.3. The packet control byte (FC - Frame Control) determines the type of packet (frame).4. The six-byte addresses of the sender and recipient of the packet have a standard format.5. The data field includes transmission information or exchange control information.6. The checksum field is a 32-bit packet cyclic checksum (CRC).7. The trailing delimiter indicates the end of the packet. In addition, it determines whether a given packet is intermediate or final in the sequence of transmitted packets, and also contains an indication that the packet was erroneous (special bits are allocated for this).8. The packet status byte tells you what happened to the packet: whether it was received and copied to the receiver's memory. Using it, the sender of the packet finds out whether the packet reached its destination and without errors or whether it needs to be transmitted again.
network token ring marker
I note that the larger permissible size of transmitted data in one packet compared to Ethernet network can be a decisive factor in increasing network performance. Theoretically, for a transfer rate of 16 Mb/s, the data field length can even reach 18 KB, which is very important when transferring large amounts of data. But even at a speed of 4 Mbit/s, thanks to the token access method, the Token Ring network often provides a higher actual transmission speed than the faster Ethernet network (10 Mbit/s), especially under heavy loads (over 30 - 40%), when the imperfections of the method noticeably affect CSMA/CD, which in this case spends a lot of time resolving repeated conflicts.
In addition to the token and the regular packet, a special control packet can be transmitted in the Token Ring network, which serves to interrupt the transmission. It can be sent at any time and anywhere in the data stream. This packet consists of only two one-byte fields - the initial and final delimiters of the described format.
Interestingly, the faster version of Token Ring (16Mb/s and higher) uses the so-called Early Token Release method (ETR). It avoids wasted network usage while the data packet is looping back to its sender. The ETR method boils down to the fact that immediately after transmitting its packet attached to the token, any subscriber issues a new free token to the network, that is, all other subscribers can begin transmitting their packets immediately after the end of the previous subscriber’s packet, without waiting for him to complete bypassing the entire network ring.
As mentioned earlier, the Token Ring network has a ring topology. Let me remind you that individual subscribers do not connect to the network directly, but through special hubs or multistation access units (MSAU or MAU - Multistation Access Unit). Therefore, physically the network forms a star-ring topology (Fig. 1.1). In reality, the subscribers are still united in a ring, that is, each of them transmits information to one neighboring subscriber, and receives information from another neighboring subscriber.
The concentrator (MAU) only allows you to centralize configuration settings, disconnecting faulty subscribers, monitoring network operation, etc. (Figure 3.1). To connect the cable to the hub, special connectors are used, which ensure that the ring is always closed even when the subscriber is disconnected from the network. The hub in the network may be the only one; in this case, the ring is closed only by the subscribers connected to it.
Figure 3.1 - Connecting Token Ring network subscribers into a ring using a hub (MAU)
Each cable connecting the adapters and the hub (adapter cables) actually contains two multidirectional communication lines. The same two multidirectional communication lines included in the main cable (path cable) will connect various hubs together into a ring (Figure 3.2), although a single unidirectional communication line can be used for the same purpose (Figure 3.3).
Figure 3.2 Connecting hubs with a bidirectional communication line
Figure 3.3 Connecting hubs with a unidirectional communication line
Structurally, the hub is a self-contained unit with eight connectors for connecting subscribers (computers) using adapter cables and two (outermost) connectors for connecting to other hubs using special trunk cables (Fig. 3.4). There are wall-mounted and desktop versions of the concentrator.
Several hubs can be structurally combined into a group, a cluster, within which subscribers are also connected into a single ring. The use of clusters allows you to increase the number of subscribers connected to one center (for example, up to 16 if the cluster includes two hubs).
Figure 3.4 Token Ring Hub (8228 MAU)
Twisted pair cables were first used as a transmission medium in the IBM Token Ring network, but then hardware options for coaxial cable, as well as for fiber optic cable in the FDDI standard. Twisted pair is used both unshielded (UTP) and shielded (STP).
Basic specifications Token Ring networks:
The maximum number of IBM 8228 MAU type hubs is 12.
The maximum number of subscribers in the network is 96.
The maximum cable length between the subscriber and the hub is 45m.
The maximum cable length between hubs is 45m.
The maximum length of the cable connecting all hubs is 120m.
Data transfer speed - 5 Mb/s and 16 Mb/s.
All given characteristics refer to the case of unshielded twisted pair. If you use a different transmission medium, network characteristics may differ. For example, when using shielded twisted pair cable, the number of subscribers can be increased to 260 (instead of 96), the cable length - up to 100 m (instead of 45), the number of hubs - up to 33, and the total length of the ring connecting the hubs - up to 200 m. Fiber optic cable allows you to increase the cable length up to 1 km.
As we can see, the Token Ring network is inferior to the Ethernet network both in terms of the permissible network size and the maximum number of subscribers. In terms of transfer speeds, 100 Mbps and 1000 Mbps versions of Token Ring are currently being developed. IBM is not at all going to abandon its network, considering it as a worthy competitor to Ethernet.
To transfer information to the Token Ring, a variant of the Manchester code is used - P. As in any star topology, no additional measures for electrical matching or external grounding are required.
To connect the cable to network adapter An external 9-pin DIN connector is used. Just like Ethernet adapters, Token Ring adapters have switches or jumpers on their board to configure system bus addresses and interrupts. If an Ethernet network can be built only on adapters and a cable, then for a Token Ring network it is necessary to purchase hubs. This also increases the cost of Token Ring hardware.
At the same time, unlike Ethernet, the Token Ring network holds the load better (more than 30 - 40%) and provides guaranteed access time. This is extremely necessary, for example, in industrial networks, where a delay in the response to an external event can lead to serious accidents.
Conclusion
In this work, I looked at the Token Ring local network, its advantages and disadvantages, and also compared it with the Ethernet network. While working on this course project, I learned that Token Ring networks are based on deterministic algorithms. Token Ring is based on a ring topology. Data transfer is only possible in a ring from one node to the second, from the second to the third, and so on. In the event that no data is transmitted, a frame of a special format - a token - circulates in the network. If the computer needs to transmit a data frame, it waits to receive a token. Having received the token, the computer instead of the token sends a data frame along the ring, which is transmitted to the recipient, and then from the recipient to the sender. Having received the previously sent token, the sender returns the token to the network. The right to transmit the data frame can then be acquired by another computer that intercepts the token. Thus, the right to transfer data alternately passes from one computer to another. The bandwidth of Token Ring networks is 4 and 16 Mbit/s, the number of computers in one logical ring is up to 240.
Token Ring networks are characterized by a shared data transmission medium, which in this case consists of cable sections connecting all network stations into a ring. The ring is considered as a common shared resource, and access to it requires not a random algorithm, as in networks, but a deterministic one, based on transferring the right to use the ring to stations in a certain order. This right is conveyed using a special format frame called a token.
Token Ring technology is a more complex technology than Ethernet. It has fault tolerance properties. The Token Ring network defines network operation control procedures that use feedback of a ring-shaped structure - the sent frame always returns to the sending station. 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 done manually.
The Token Ring network is used mainly in enterprises where high reliability is required. Therefore, the Token Ring network choices are the best solution to organize reliable, uninterrupted network operation.
Glossary
List of sources used
Normative legal acts
1.High performance networks. User Encyclopedia: Trans. from English /Mark A. Sportak and others - K.: Publishing house "DiaSoft", 1998. - 432 p. 2.Guk M. Local network hardware. Encyclopedia - St. Petersburg: Publishing house "Piter", 2000. - 576 pp.: ill. .Computer networks. Principles, technologies, protocols. /V.G. Olifer, N.A. Olifer. - St. Petersburg: Publishing House "Peter", 1999. - 672 pp.: ill. .Computer Networks+. Training course(MSCE 70-058) /Trans. from English - M.: "Russian Edition", 2000. - 552 p. .Kulgin M. Technologies corporate networks. Encyclopedia - St. Petersburg: Publishing house "Piter", 2000. - 704m.: ill.
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