Token ring

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Token Ring in the TCP / IP protocol stack :
application HTTP IMAP SMTP DNS ...
transport TCP UDP
Internet IP ( IPv4 , IPv6 )
Network access Token ring
Token Ring with AppleTalk -
Protocol Stack (TokenTalk)
application AFP ADSP
transport ATP
network DDP
Network access TLAP AARP
Token ring

Token Ring is a networking technology for computer networks , defined in the IEEE 802.5 specification . It defines cable types and signaling for the physical layer , packet formats and protocols for the media access control ( Media Access Control , MAC) / data link layer of the OSI model . It is one of the two forms of implementation of the token passing process .

The token ring technology was practically completely displaced by the various Ethernet variants. It will not be further developed technically and is no longer relevant in network technology.



Structure and basic scheme

The first implementations were Cambridge Ring and StarNET from the English company Procom .

In 1981 Apollo Computer introduced its 12 Mbit / s Apollo Token Ring (ATR) and in 1984 Proteon introduced the 10 Mbit / s ProNet-10 Token Ring network.

Token Ring was further developed at the IBM Zurich Research Laboratory in mid- 1980 and has long been the standard in IBM networks and therefore also in all who use IBM computers. IBM deviates from the IEEE 802.5 standard in some details.

This technology has been considered obsolete since IBM, the main representative of the token ring process, ended the marketing and sale of token rings due to the advent of cheap Ethernet networking technology.

Transmission (technical)

The basic principle is the collision-free transmission of the data packets between the individual stations.

The name Token Ring comes from the fact that the network works with the token-passing access method and corresponds to the structure of a logical ring topology . The type of connection to the medium is thus active (for example, in contrast to passive Ethernet), that is, the network stations continuously participate actively in the transfer of the token (see below) and not only become active when they want to send themselves. Token passing is a layer 2 protocol (data link layer) in the OSI model .

The line code used is the differential Manchester code .

Topology properties

IBM MAU 8228

The logical topology of the Token Ring is a ring . However, so-called MAU s (= Multistation Access Unit , also MSAU , German RLV = ring line distributor) or CAU s (Controlled Access Unit) were used, which enable star- shaped (when using several ring line distributors a star ring) cabling. These distributors are only intelligent to the extent that they loop through unused ports or ports occupied by stations that are switched off in order to maintain the ring. Compared to the Ethernet switch, which requires collision detection , the demands on the distributor are minimal. Switches for transmission speeds of 4, 16 or 100 Mbit / s were also manufactured.

Transmission (logical)

Token Passing Process

In token ring networks, a token circles the ring: the token is always passed from one node to the next. Even when it is idle, the stations continue to pass on the package.

If a computer now wants to send data, it waits until the token has reached it and then appends its user data to it. At the same time, it supplements the token with control signals and also sets the token bit from 0 (for “free token”) to 1, so the free token becomes a data frame.

After the process, the computer places the data frame back on the ring, where it is passed on by the individual nodes, just like the free token. Each computer checks whether the packet is addressed to it, and otherwise puts it back on the ring. If the intended recipient receives the data frame addressed to him, he copies the user data and acknowledges receipt of the data. The sender receives the acknowledgment and sends the token with the next user data or places a free token on the ring. A sender may only use the token for a certain amount of time before it has to release it again. This guarantees every node in a ring that it is allowed to send in a ring after this specified time ("Token Holding Time") has expired.

Structure of the token frame

A free token consists of three bytes with the following structure:

Start delimiter (SD, Start Delimiter): JK 0 JK 0 0 0
Access control (AC, Access Control): PPPTMRRR
End delimiter (ED): JK 1 JK 1 IE
  • J and K indicate code violations in the differential Manchester code , i.e. H. no polarity change occurs in the middle of the cycle.
  • I - interframe bit - 1: further frames follow; 0: last frame
  • E - error bit - 0: at the beginning; 1: If an error was detected
  • P - access priority
  • T - token bit - 0: free token; 1: frame
  • M - monitor bit - 1: monitor 0: producer
  • R - reservation bit - the next token is generated with this bit in the priority bit

Types of Token Ring

  • Single frame: Token release after receipt of the last bit of the own frame .
  • Single Token: Token release after receipt of the first bit of the own frame.
  • Multiple Frame: Token release after sending the last bit of the own frame.

Error situations

Error situations in the Token Ring are usually resolved by a so-called monitor station (AM, Activity Monitor). This AM does not necessarily have to be the server, it is usually rather the adapter that generated the first token. This can also be a completely normal workstation :

  • Loss of the token: After a control time (timeout) has elapsed, the monitor station generates a new free token.
  • Endlessly circling packet: If a sender station fails before it has removed the packet it has sent from the ring, it would circling endlessly without error handling . To prevent this, the monitor station sets the M-Bit in the AC area of ​​the token (see above) when the token passes it. If the station receives the same token again without the recipient taking it from the network, the monitor station destroys the token and generates a new free token.
  • Double token: The sending station terminates as soon as it receives an externally addressed token.
  • Failure of the monitor: If the above-mentioned monitor station fails, the remaining stations negotiate a new monitor with one another.
  • Failure of a network interface: If a ring line distributor is used, this bridges the affected area.

See also


  • Hans-Georg Göhring, Franz-Joachim Kauffels: Token Ring: Basics, Strategies, Perspectives . Datacom 1990, ISBN 3-89238-026-0
  • Carlo, Love, Siegel, Wilson: Understanding Token Ring Protocols and Standards , Artech House 1998, ISBN 0-89006-458-X
  • IBM Token-Ring Network Architecture Reference , SC30-3374
  • IBM Token-Ring Network Introduction and Planning Guide , GA27-3677
  • IBM Token-Ring Network Installation Guide , GA27-3678
  • IBM Token-Ring Network Problem Determination Guide , SX27-3710
  • LAN Technical Reference: 802.2 and NetBIOS Application Programming Interface , SC30-3587

Web links

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