Token Ring

Token Ring is a physical and data link layer computer networking technology used to build local area networks. It was introduced by IBM in 1984, and standardized in 1989 as IEEE 802.5. It uses a special three-byte frame called a token that is passed 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.
Following its introduction, Token Ring technology became widely adopted, particularly in corporate environments, but was gradually eclipsed by newer iterations of Ethernet. The last formalized Token Ring standard that was completed was Gigabit Token Ring, published on May 4, 2001.

History

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, while early work at MIT led to the Proteon ProNet-10 Token Ring network in 1981 the same year that workstation vendor Apollo Computer introduced their proprietary Apollo Token Ring network running over 75-ohm RG-6U coaxial cabling. Proteon later developed an upgraded version that ran on unshielded twisted pair cable.

1985 IBM launch

IBM launched their own proprietary Token Ring product on October 15, 1985. It ran at, 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. Shortly thereafter it became the basis for the IEEE 802.5 standard.
During this time, IBM argued that Token Ring LANs were superior to Ethernet, especially under load, but these claims were debated.
In 1988, the faster Token Ring was standardized by the 802.5 working group. An increase to was standardized and marketed during the wane of Token Ring's existence and was never widely used. While a standard was approved in 2001, no products were ever brought to market and standards activity came to a standstill as Fast Ethernet and Gigabit Ethernet dominated the local area networking market.

Comparison with Ethernet

Early Ethernet and Token Ring both used a shared transmission medium. They differed in their channel access methods. These differences have become immaterial, as modern Ethernet networks consist of switches and point-to-point links operating in full-duplex mode.
Token Ring and legacy Ethernet 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.
Token Ring eliminates collision by the use of a single-use token and early token release to alleviate the down time. Legacy Ethernet alleviates collision by carrier-sense multiple access and by the use of an intelligent switch; primitive Ethernet devices like hubs could precipitate collisions due to repeating traffic blindly.
Token Ring network interface cards contain all of the intelligence required for speed autodetection, routing and can drive themselves on many Multistation Access Units that operate without power. 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.
Token Ring employs access priority in which certain nodes can have priority over the token. Unswitched Ethernet did not have a provision for an access priority system as all nodes have equal access to the transmission medium.
Multiple identical MAC addresses are supported on Token Ring. Switched Ethernet cannot support duplicate MAC addresses without reprimand.
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 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.
Initially both networks used expensive cable, but once Ethernet was standardized for unshielded twisted pair with 10BASE-T and 100BASE-TX, 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.
Operation

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 and FDDI, and they have theoretical advantages over the CSMA/CD of early Ethernet.

Access control

The data transmission process goes as follows:

Empty information frames are continuously circulated on the ring.
When a computer has a message to send, it seizes the token. The computer will then be able to send the frame.
The frame is then examined by each successive workstation. The workstation that identifies itself to be the destination for the message copies it from the frame and changes the token back to 0.
When the frame gets back to the originator, it sees that the token has been changed to 0 and that the message has been copied and received. It removes the message from the frame.
The frame continues to circulate as an empty frame, ready to be taken by a workstation when it has a message to send.
Multistation Access Units and Controlled Access Units

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.
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.
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. This offered a more reliable setup and remote management than with an unmanaged MAU hub.

Cabling and interfaces

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, referred to as IBM Data Connectors in formal writing were used. The connectors have the disadvantage of being quite bulky, requiring at least panel space, and being relatively fragile. The advantages of the connectors being that they are genderless and have superior shielding over standard unshielded 8P8C. Connectors at the computer were usually DE-9 female. Several other types of cable existed such as type 2, and type 3 cable.
In later implementations of Token Ring, Cat 4 cabling was also supported, so 8P8C connectors were used on both of the MAUs, CAUs and NICs; with many of the network cards supporting both 8P8C and DE-9 for backwards compatibility.

Technical details

Frame types

Token

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.
Tokens are three octets in length and consist of a start delimiter, an access control octet, and an end delimiter.

Abort frame

Used by the sending station to abort transmission.

Data

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

SD	AC	FC	DA	SA	PDU from LLC	CRC	ED	FS
8 bits	8 bits	8 bits	48 bits	48 bits	Up to 4500 × 8 bits	32 bits	8 bits	8 bits

; Starting delimiter: The 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 of Differential Manchester encoding. Differential Manchester encoding has a mid symbol transition for every coded 0 or 1, however the J and K codes do not have a mid symbol transition. Both the Starting Delimiter and Ending Delimiter fields are used to mark frame boundaries.
; 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 station when it sees this frame, and R bits are reservation bits, which indicate that the next token should be issued with that priority.
; Frame control: A one-byte field that contains bits describing the data portion of the frame contents which indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information.
; Destination address: A six-byte field used to specify the destination physical address.
; Source address: Contains physical address of sending station. It is a six-byte field that is either the local assigned address or universally assigned address 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 acknowledgment scheme on whether the frame was recognized and copied by its intended receiver.