Token ring

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Token ring network
IBM hermaphroditic connector with locking clip

Token ring local area network (LAN) technology is a communications protocol for local area networks. It uses a special three-byte frame called a "token" that travels around a logical "ring" of workstations or servers. This token passing is a channel access method providing fair access for all stations, and eliminating the collisions of contention-based access methods.

Introduced by IBM in 1984, it was then standardized with protocol IEEE 802.5 and was fairly successful, particularly in corporate environments, but gradually eclipsed by the later versions of Ethernet.

(The main focus of this article is the IBM/IEEE 802.5 version, but there were several other earlier implementations of token rings).


A wide range of different local area network technologies were developed in the early 1970s, of which one, the Cambridge Ring had demonstrated the potential of a token passing ring topology, and many teams worldwide began working on their own implementations. At the IBM Zurich Research Laboratory Werner Bux and Hans Müller in particular worked on the design and development of IBM's token ring technology,[1] while early work at MIT[2] led to the Proteon 10 Mbit/s ProNet-10 token ring network in 1981 – the same year that workstation vendor Apollo Computer introduced their proprietary 12 Mbit/s Apollo token ring (ATR) network running over 75-ohm RG-6U coaxial cabling. [3] Proteon later evolved a 16 Mbit/s version that ran on unshielded twisted pair cable.

IBM launched their own proprietary token ring product on October 15, 1985.[4] It ran at 4 Mbit/s, and attachment was possible from IBM PCs, midrange computers and mainframes. It used a convenient star-wired physical topology, and ran over shielded twisted-pair cabling, and shortly thereafter became the basis for the (ANSI)/IEEE standard 802.5.[5]

During this time, IBM argued strongly that token ring LANs were superior to Ethernet, especially under load,[6] but these claims were fiercely debated.[7]

In 1988 the faster 16 Mbit/s token ring was standardized by the 802.5 working group,[8] and an increase to 100 Mbit/s was standardized and marketed during the wane of token ring's existence. However it was never widely used, ("IBM does not view high-speed token ring as a requirement for the majority of its customers, and therefore the decision has been made not to provide 100 Mbps high-speed token ring uplinks on its products..."),[9] and while a 1000 Mbit/s was approved in 2001, no products were ever brought to market[10] and standards activity came to a standstill as Fast Ethernet and Gigabit Ethernet dominated the local area networking market.

Comparison with Ethernet[edit]

Ethernet and Token ring have some notable differences:

  • Token Ring access is more deterministic, compared to Ethernet's contention-based CSMA/CD
  • Ethernet supports a direct cable connection between two network interface cards by the use of a crossover cable or through auto-sensing if supported. Token ring does not inherently support this feature and requires additional software and hardware to operate on a direct cable connection setup.[11]
  • Token ring eliminates collision by the use of a single-use token and early token release to alleviate the down time. Ethernet alleviates collision by carrier sense multiple access and by the use of an intelligent switch; primitive Ethernet devices like hubs can precipitate collisions due to repeating traffic blindly.[12]
  • Token ring network interface cards contain all of the intelligence required for speed autodetection, routing and can drive themselves on many Multistation Access Units (MAUs) that operate without power (most MAUs operate in this fashion, only requiring a power supply for LEDs). Ethernet network interface cards can theoretically operate on a passive hub to a degree, but not as a large LAN and the issue of collisions is still present.[13]
  • Token ring employs 'access priority' in which certain nodes can have priority over the token. Unswitched Ethernet does not have provisioning for an access priority system as all nodes have equal contest for traffic.
  • Multiple identical MAC addresses are supported on token ring (a feature used by S/390 mainframes).[9] Switched Ethernet cannot support duplicate MAC addresses without reprimand.[14]
  • Token Ring was more complex than Ethernet, requiring a specialized processor and licensed MAC/LLC firmware for each interface. By contrast, Ethernet included both the (simpler) firmware and the lower licensing cost in the MAC chip. The cost of a token ring interface using the Texas Instruments TMS380C16 MAC and PHY was approximately three times that of an Ethernet interface using the Intel 82586 MAC and PHY.[citation needed]
  • Initially both networks used expensive cable, but once Ethernet was standardized for unshielded twisted pair with 10BASE-T (Cat 3) and 100BASE-TX (Cat 5(e)), it had a distinct advantage and sales of it increased markedly.
  • Even more significant when comparing overall system costs was the much-higher cost of router ports and network cards for token-ring vs Ethernet. The emergence of Ethernet switches may have been the final straw.


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. Similar token passing mechanisms are used by ARCNET, token bus, 100VG-AnyLAN (802.12) and FDDI, and they have theoretical advantages over the CSMA/CD of early Ethernet.[15]

A token ring network can be modeled as a polling system where a single server provides service to queues in a cyclic order.[16]

Access control[edit]

The data transmission process goes as follows:

  • Empty information frames are continuously circulated on the ring.

Multistation Access Units and Controlled Access Units[edit]

Main article: Media Access Unit
The IBM 8228 Multistation Access Unit with accompanying Setup Aid to prime the relays on each port

Physically, a token ring network is wired as a star, with 'MAUs' in the center, 'arms' out to each station, and the loop going out-and-back through each.[17]

A MAU could present in the form of a hub or a switch; since token ring had no collisions many MAUs were manufactured as hubs. Although Token Ring runs on LLC, it includes Source Routing to forward packets beyond the local network. The majority of MAUs are configured in a 'concentration' configuration by default, but later MAUs also supporting a feature to act as splitters and not concentrators exclusively such as on the IBM 8226.[18]

MAUs operating as either concentrators or splitters.

Later IBM would release Controlled Access Units that could support multiple MAU modules known as a Lobe Attachment Module. The CAUs supported features such as Dual-Ring Redundancy for alternate routing in the event of a dead port, modular concentration with LAMs, and multiple interfaces like most later MAUs.[19] This offered a more reliable setup and remote management than with an unmanaged MAU hub.

Cabling and interfaces[edit]

Cabling is generally IBM "Type-1", a heavy two-pair 150 Ohm shielded twisted pair cable. This was the basic cable for the "IBM Cabling System", a structured cabling system that IBM hoped would be widely adopted. Unique hermaphroditic connectors, commonly referred to as IBM Data Connectors in formal writing or colloquially as Boy George connectors were used.[20] The connectors have the disadvantage of being quite bulky, requiring at least 3 x 3 cm panel space, and being relatively fragile. The advantages of the connectors being that they are genderless and have superior shielding over standard unshielded RJ45. Connectors at the computer were usually DE-9 female.

In later implementations of Token Ring, Cat 6 cabling was also supported, so RJ45 connectors were used on both of the MAUs, CAUs and NICs; with many of the network cards supporting both RJ45 and DE-9 for backwards compatibility.[17]

Frame types[edit]


When no station is sending a 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 send data.


Start Delimiter Access Control End Delimiter
8-bits 8-bits 8-bits

Abort frame[edit]

Used to abort transmission by the sending station

8 bits 8 bits


Data frames carry information for upper-layer protocols, while command frames contain control information and have no data for upper-layer protocols. Data/command frames vary in size, depending on the size of the Information field.

8 bits 8 bits 8 bits 48 bits 48 bits up to 4500x8 bits 32 bits 8 bits 8 bits
Starting delimiter 
J K 0 J K 0 0 0
Access control 
+ Bits 0–2 3 4 5–7
0 Priority Token Monitor Reservation
Frame control 
+ Bits 0–1 Bits 2–7
0 Frame type Control Bits


Destination address 
Source address 
Frame check sequence 
Ending delimiter 
J K 1 J K 1 I E
Frame status 
A C 0 0 A C 0 0

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

Other technical details[edit]

Active and standby monitors[edit]

Every station in a token ring network is either an active monitor (AM) or standby monitor (SM) station. 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 the following happens:

  • a loss of signal on the ring is detected.



Token ring insertion process[edit]



Optional priority scheme[edit]

In some applications there is an advantage to being able to designate one station having a higher priority. Token ring specifies an optional scheme of this sort, as does the CAN Bus, (widely used in automotive applications) - but Ethernet does not.


Here are the following 8 access priority and traffic types for devices that support 802.1Q and 802.1p:

Priority bits Traffic type
x'000' Normal data traffic
x'001' Not used
x'010' Not used
x'011' Not used
x'100' Normal data traffic (forwarded from other devices)
x'101' Data sent with time sensitivity requirements
x'110' Data with real time sensitivity (i.e. VoIP)
x'111' Station management

Bridging Token ring and Ethernet[edit]

Both Token ring and Ethernet interfaces on the 2210-24M

Bridging solutions for Token ring and Ethernet networks included the AT&T StarWAN 10:4 Bridge, the IBM 9208 LAN Bridge and the Microcom LAN Bridge. [21] Alternative connection solutions incorporated a router that could be configured to dynamically filter traffic, protocols and interfaces, such as the IBM 2210-24M Multiprotocol Router which contained both Ethernet and token ring interfaces.[22]

See also[edit]


  1. ^ "IEEE honors Zurich LAN pioneers", Zurich, Switzerland, 14 April 2003
  2. ^ "Early Token Ring Work at MIT", J. Noel Chiappa,
  3. ^ "A History of Computer Communications 1968-1988", James Pelkey
  4. ^ "Local Area Networks", InfoWorld 24 Mar 1986
  7. ^ David R. Boggs; Jeffrey C. Mogul; Christopher A. Kent (1988). "Measured capacity of an Ethernet: myths and reality" (PDF). ACM SIGCOMM Computer Communication Review. 25 (1): 123–136. doi:10.1145/205447.205460. 
  8. ^ "ETHERNET VS. TOKEN RING IN THE LOCAL AREA NETWORKING BUSINESS", URS VON BURG AND MARTIN KENNY, Industry and Innovation, Volume 10, Number 4, 351–375, December 2003
  9. ^ a b "" (PDF). Retrieved 2016-08-03. 
  10. ^ IEEE 802.5 activities. Retrieved on 2011-10-30.
  11. ^ Louis Ohland. "8228 Multistation Access Unit". Retrieved 2016-08-03. 
  12. ^ "What is the difference between an Ethernet hub and switch?". Retrieved 10 May 2016. 
  13. ^ "A Passive Ethernet Hub". Retrieved 2016-08-03. 
  14. ^ "networking - Duplicate MAC address on the same LAN possible?". Server Fault. 2013-01-03. Retrieved 2016-08-03. 
  15. ^ "Does anyone actually still USE Token Ring?", John Sheesley, April 2, 2008, TechRepublic
  16. ^ Bux, W. (1989). "Token-ring local-area networks and their performance". Proceedings of the IEEE. 77 (2): 238. doi:10.1109/5.18625. 
  17. ^ a b "Whu buy from IBM?" (PDF). Retrieved 2016-08-03. 
  18. ^ Louis Ohland. "8226". Retrieved 2016-08-03. 
  19. ^ "IBM 8230 Controlled access Unit" (PDF). Retrieved 2016-08-03. 
  21. ^ "Network World". 1991-06-03. p. 56. Retrieved 2016-08-03. 
  22. ^ "SG244446" (PDF). Retrieved 2016-08-03. 

External links[edit]