Two examples of token ring networks: a) Using a single MAU b) Using several MAUs connected to each other

Token ring network

An IBM 8228 MAU. A Media Access Unit ( MAU, also called Multistation Access Unit, MSAU) is a device to attach multiple network stations in a Star topology in a

Token ring local area network (LAN) technology was conceived in the late 1960s by Olof Söderblom, then working for IBM [1]). Olof Söderblom, born 1940, is the creator of Token ring networking and grandson of Nathan Söderblom, recipient of the 1930 Nobel Peace Prize US Patents were awarded in 1981^[1] and Token-Ring was developed and promoted by IBM in the early 1980s and standardized as IEEE 802. International Business Machines Corporation abbreviated IBM and nicknamed "Big Blue", is a multinational Computer Technology 5 by the Institute of Electrical and Electronics Engineers. The Institute of Electrical and Electronics Engineers or IEEE (read eye-triple-e) is an international Non-profit, professional organization Initially very successful, it went into steep decline after the introduction of 10BASE-T for Ethernet and the EIA/TIA 568 cabling standard in the early 1990s. Ethernet is a family of frame -based Computer networking technologies for Local area networks (LANs TIA/EIA-568-B is a set of three Telecommunications standards from the Telecommunications Industry Association, a 1988 Offshoot of the EIA. A fierce marketing effort led by IBM sought to claim better performance and reliability compared with Ethernet for critical applications due to its deterministic access method, but the advent of reasonably-priced Ethernet switching led to its demise. In Computer science, a deterministic algorithm is an Algorithm which in informal terms behaves predictably In Telecommunications and Computer networks, a channel access method or multiple access method allows several terminals connected to the same A IBM no longer uses or promotes token ring.

Introduction

Stations on a token ring LAN are logically organized in a ring topology with data being transmitted sequentially from one ring station to the next with a control token circulating around the ring controlling access. A ring network is a Network topology in which each node connects to exactly two other nodes forming a circular pathway This token passing mechanism is shared by ARCNET, token bus, and FDDI, and has theoretical advantages over the stochastic CSMA/CD of Ethernet. In Telecommunication, token passing is a Channel access method where a signal called a token is passed around between nodes that authorizes the node to ARCNET (also CamelCased as ARCnet, an Acronym from Attached Resource Computer NETwork is a Local area network ( LAN) protocol Token bus is a network implementing the Token ring protocol over a "virtual ring" on a Coaxial cable. Fiber distributed data interface ( FDDI) provides a standard for Data transmission in a Local area network Stochastic (from the Greek "Στόχος" for "aim" or "guess" means Random. Carrier Sense Multiple Access With Collision Detection ( CSMA/CD) in Computer networking is a network control protocol in which a

Physically, a token ring network is wired as a star, with 'hubs' and arms out to each station and the loop going out-and-back through each. Star networks are one of the most common Computer network topologies.

IBM hermaphroditic connector with locking clip

Cabling is generally IBM "Type-1" shielded twisted pair, with unique hermaphroditic connectors. Twisted pair Cabling is a form of wiring in which two conductors (two halves of a single circuit) are wound together for the purposes of canceling out In Electrical and mechanical trades and manufacturing each of a pair of mating connectors or Fasteners is conventionally assigned the designation male The connectors had the disadvantage of being quite bulky, requiring at least 3 x 3 cm panel space, and, composed of many complex plastic pieces, being quite fragile.

Initially (in 1985) token ring ran at 4 Mbit/s, but in 1989 IBM introduced the first 16 Mbit/s token ring products and the 802. In telecommunications Bit rate or Data transfer rate is the average number of Bits characters or blocks per unit time passing between equipment in a data transmission 5 standard was extended to support this. In 1981, Apollo Computer introduced their proprietary 12 Mbit/s Apollo token ring (ATR) and Proteon introduced their 10 Mbit/s ProNet-10 token ring network in 1984. However, IBM token ring was not compatible with ATR or ProNet-10.

More technically, token ring is a local area network protocol which resides at the data link layer (DLL) of the OSI model. In computing, a protocol is a convention or standard that controls or enables the connection Communication, and Data transfer between two computing The Data Link Layer is Layer 2 of the seven-layer OSI model. It responds to service requests from the Network Layer and issues service requests to the The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer Network protocol It uses a special three-byte frame called a token that travels around the ring. Token ring frames travel completely around the loop.

Each station passes or repeats the special token frame around the ring to its nearest downstream neighbor. This token-passing process is used to arbitrate access to the shared ring media. Stations that have data frames to transmit must first acquire the token before they can transmit them. Token ring LANs normally use differential Manchester encoding of bits on the LAN media. Differential Manchester encoding (also known as CDP Conditioned Diphase encoding is a method of encoding data in which Data and Clock signals are combined

IBM popularized the use of token ring LANs in the mid 1980s when it released its IBM token ring architecture based on active multi-station access units (MSAUs or MAUs) and the IBM Structured Cabling System. The Institute of Electrical and Electronics Engineers (IEEE) later standardized a token ring LAN system as IEEE 802. The Institute of Electrical and Electronics Engineers or IEEE (read eye-triple-e) is an international Non-profit, professional organization 5. [2]

Token ring LAN speeds of 4 Mbit/s and 16 Mbit/s have been standardized by the IEEE 802. 5 working group. An increase to 100 Mbit/s was standardized and marketed during the wane of token ring's existence while a 1000 Mbit/s speed was actually approved in 2001, but no products were ever brought to market. ^[2]

When token ring LANs were first introduced, there were widely circulated claims that they were superior to Ethernet. Ethernet is a family of frame -based Computer networking technologies for Local area networks (LANs ^[3] These claims did not hold up when tested. ^[4]

With the development of switched Ethernet and faster variants of Ethernet, token ring architectures lagged badly behind Ethernet in both performance and reliability. A The higher sales of Ethernet allowed economies of scale which drove down prices further, and added a compelling price advantage to its other advantages over token ring. Currently, more businesses use Ethernet networks than token ring networks.

Token ring networks have since declined in usage and the standards activity has since come to a standstill as switched Ethernet has dominated the LAN/layer 2 networking market.

Token frame

When no station is transmitting a data frame, a special token frame circles the loop. This special token frame is repeated from station to station until arriving at a station that needs to transmit data. When a station needs to transmit data, it converts the token frame into a data frame for transmission. Once the sending station receives its own data frame, it converts the frame back into a token. If a transmission error occurs and no token frame, or more than one, is present, a special station referred to as the Active Monitor detects the problem and removes and/or reinserts tokens as necessary (see Active and standby monitors). Token ring Local area network (LAN technology is a local area network protocol which resides at the Data link layer On 4 Mbit/s Token Ring, only one token may circulate; on 16 Mbit/s Token Ring, there may be multiple tokens.

The special token frame consists of three bytes as described below (J and K are special non-data characters, referred to as code violations).

Token priority

Token ring specifies an optional medium access scheme allowing a station with a high-priority transmission to request priority access to the token. 8 priority levels, 0-7, are used. When the station wishing to transmit receives a token or data frame with a priority less than or equal to the station's requested priority, it sets the priority bits to its desired priority. The station does not immediately transmit; the token circulates around the medium until it returns to the station. Upon sending and receiving its own data frame, the station downgrades the token priority back to the original priority.

Token ring frame format

A data token ring frame is an expanded version of the token frame that is used by stations to transmit media access control (MAC) management frames or data frames from upper layer protocols and applications. The Media Access Control (MAC Data communication protocol sub-layer also known as the Medium Access Control is a sublayer of the Data Link Layer specified in the

Token Ring and IEEE 802. 5 support two basic frame types: tokens and data/command frames. Tokens are 3 bytes in length and consist of a start delimiter, an access control byte, and an end delimiter. Data/command frames vary in size, depending on the size of the Information field. Data frames carry information for upper-layer protocols, while command frames contain control information and have no data for upper-layer protocols. Token ring can be connected to physical rings via equipment such as 100Base-TX equipment and CAT5e UTP cable.

Data/Command Frame

Token Frame

Abort Frame

Starting Delimiter 

consists of a special bit pattern denoting the beginning of the frame. The bits from most significant to least significant are J,K,0,J,K,0,0,0. J and K are code violations. Since Manchester encoding is self clocking, and has a transition for every encoded bit 0 or 1, the J and K codings violate this, and will be detected by the hardware. In Telecommunication, Manchester code (also known as Phase Encoding, or PE) is a Line code in which the encoding of each data Bit

Access Control 

this byte field consists of the following bits from most significant to least significant bit order: P,P,P,T,M,R,R,R. The P bits are priority bits, T is the token bit which when set specifies that this is a token frame, M is the monitor bit which is set by the Active Monitor (AM) station when it sees this frame, and R bits are reserved bits.

Frame Control 

a one byte field that contains bits describing the data portion of the frame contents. Indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information.

Frame type - 01 indicates LLC frame IEEE 802. 2 (data) and ignore control bits 00 indicates MAC frame and control bits indicate the type of MAC control frame

Destination address 

a six byte field used to specify the destination(s) physical address . The Media Access Control (MAC Data communication protocol sub-layer also known as the Medium Access Control is a sublayer of the Data Link Layer specified in the

Source address 

Contains physical addressa of sending station . It is six byte field that is either the local assigned address (LAA) or universally assigned address (UAA) of the sending station adapter.

Data 

a variable length field of 0 or more bytes, the maximum allowable size depending on ring speed containing MAC management data or upper layer information. Maximum length of 4500 bytes

Frame Check Sequence 

a four byte field used to store the calculation of a CRC for frame integrity verification by the receiver.

Ending Delimiter 

The counterpart to the starting delimiter, this field marks the end of the frame and consists of the following bits from most significant to least significant: J,K,1,J,K,1,I,E. I is the intermediate frame bit and E is the error bit.

Frame Status  

a one byte field used as a primitive acknowledgement scheme on whether the frame was recognized and copied by its intended receiver.

A = 1 , Address recognized C = 1 , Frame copied

Abort Frame 

Used to abort transmission by the sending station

Active and standby monitors

Every station in a token ring network is either an active monitor (AM) or standby monitor (SM) station. However, there can be only one active monitor on a ring at a time. The active monitor is chosen through an election or monitor contention process.

The monitor contention process is initiated when

• a loss of signal on the ring is detected,
• an active monitor station is not detected by other stations on the ring, or
• when a particular timer on an end station expires such as the case when a station hasn't seen a token frame in the past 7 seconds.

When any of the above conditions take place and a station decides that a new monitor is needed, it will transmit a "claim token" frame, announcing that it wants to become the new monitor. If that token returns back to the sender, it is OK for it to become the monitor. If some other station tries to become the monitor at the same time then the station with the highest MAC address will win the election process. In Computer networking a Media Access Control address ( MAC address) or Ethernet Hardware Address ( EHA) hardware address Every other station becomes a standby monitor. All stations must be capable of becoming an active monitor station if necessary.

The active monitor performs a number of ring administration functions. The first function is to operate as the master clock for the ring in order to provide synchronization of the signal for stations on the wire. Another function of the AM is to insert a 24-bit delay into the ring, to ensure that there is always sufficient buffering in the ring for the token to circulate. A third function for the AM is to ensure that exactly one token circulates whenever there is no frame being transmitted, and to detect a broken ring. Lastly, the AM is responsible for removing circulating frames from the ring.

Token ring insertion process

Token ring stations must go through a 5-phase ring insertion process before being allowed to participate in the ring network. If any of these phases fail, the token ring station will not insert into the ring and the token ring driver may report an error.

• Phase 0 (Lobe Check) — A station first performs a lobe media check. A station is wrapped at the MSAU and is able to send 2000 test frames down its transmit pair which will loop back to its receive pair. The station checks to ensure it can receive these frames without error.
• Phase 1 (Physical Insertion) — A station then sends a 5 volt signal to the MSAU to open the relay.
• Phase 2 (Address Verification) — A station then transmits MAC frames with its own MAC address in the destination address field of a token ring frame. When the frame returns and if the address copied , the station must participate in the periodic (every 7 seconds) ring poll process. This is where stations identify themselves on the network as part of the MAC management functions.
• Phase 3 (Participation in ring poll) — A station learns the address of its Nearest Active Upstream Neighbour (NAUN) and makes its address known to its nearest downstream neighbour, leading to the creation of the ring map. Station waits until it receives an AMP or SMP frame with the ARI and FCI bits set to 0. When it does, the station flips both bits (ARI and FCI) to 1, if enough resources are available, and queues an SMP frame for transmission. If no such frames are received within 18 seconds, then the station reports a failure to open and de-inserts from the ring. If the station successfully participates in a ring poll, it proceeds into the final phase of insertion, request initialization.
• Phase 4 (Request Initialization) — Finally a station sends out a special request to a parameter server to obtain configuration information. This frame is sent to a special functional address, typically a token ring bridge, which may hold timer and ring number information with which to tell the new station about.

See also

• Cambridge Ring
• List of device bandwidths

References

1. ^ US4,293,948 (PDF version) (1981-10-06) Olof Söderblom Data transmission system 
2. ^ IEEE 802.5 activities
3. ^ "IEEE 802. The Cambridge Ring was an experimental Local area network architecture developed at the Cambridge University Computer Laboratory in the mid-late 1970s and early 1980s This is a list of device bandwidths: the Net bit rate (or more informally Digital bandwidth) of some computer devices employing methods of data transport is quantified Year 1981 ( MCMLXXXI) was a Common year starting on Thursday (link displays the 1981 Events 105 BC - Battle of Arausio: The Cimbri inflict the heaviest defeat on the Roman army of Gnaeus Mallius Maximus 3 Local Area Network considerations", IBM document GG22-9422-0
4. ^ David R. Boggs, Jeffrey C. Mogul, Christopher A. Kent (1995). "Measured capacity of an Ethernet: myths and reality". ACM SIGCOMM Computer Communication Review 25 (1): 123-136. doi:10.1145/205447.205460. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  

• Castelli, Matthew (2002). Network Consultants Handbook. Cisco Press. ISBN 1-58705-039-0.  
• Gallo, Michael; Hancock, William M. (2001). Networking Explained. Digital Press. ISBN 1-55558-252-4.  

External links

• IEEE 802.5 Web Site
• Get the IEEE 802.5 standard
• Troubleshooting Cisco Router Token Ring Interfaces
• Futureobservatory.org discussion of IBM's failure in token ring technology