Token Ring

Token Ring is a 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.
Token Ring was a successful technology, particularly in corporate environments, but was gradually eclipsed by the later versions of Ethernet.
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 10 Mbit/s ProNet-10 Token Ring network in 1981the 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. Proteon later evolved a 16 Mbit/s 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 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. Shortly thereafter it became the basis for the (ANSI)/IEEE standard 802.5.
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 16 Mbit/s Token Ring was standardized by the 802.5 working group, 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, and while a 1000 Mbit/s 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.
Ethernet and Token Ring have some notable differences:
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 (802.12) and FDDI, and they have theoretical advantages over the CSMA/CD of early Ethernet.
A Token Ring network can be modeled as a polling system where a single server provides service to queues in a cyclic order.
Access control.
The data transmission process goes as follows:
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, commonly referred to as IBM Data Connectors in formal writing or colloquially as Boy George connectors were used. The connectors have the disadvantage of being quite bulky, requiring at least 3 × 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 8P8C. Connectors at the computer were usually DE-9 female.
In later implementations of Token Ring, Cat 4 cabling was also supported, so 8P8C ("RJ45") 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 3 bytes in length and consist of a start delimiter, an access control byte, and an end delimiter.
Abort frame.
Used to abort transmission by the sending station.
Data.
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.
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
A = 1, Address recognized
C = 1, Frame copied
Active and standby monitors.
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:
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 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. 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 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.
Optional priority scheme.
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.
In the Token Ring priority MAC, eight 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.
Here are the following eight access priority and traffic types for devices that support 802.1Q and 802.1p:
Interconnection with Ethernet.
Bridging solutions for Token Ring and Ethernet networks included the AT&T StarWAN 10:4 Bridge, the IBM 8209 LAN Bridge and the Microcom LAN Bridge. 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.