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Token ring local area network (LAN) technology is a protocol which resides at the data link layer (DLL) of the OSI model. It uses a special three-byte frame called a token that travels around the ring. Token-possession grants the possessor permission to transmit on the medium. Token ring frames travel completely around the loop.
Initially used only in IBM computers, it was eventually standardized with protocol IEEE 802.5.
The data transmission process goes as follows:
The token scheme can also be used with bus topology LANs.
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. This token passing mechanism is shared by ARCNET, token bus, 100VG-AnyLAN(802.12) and FDDI, and has theoretical advantages over the stochastic CSMA/CD of Ethernet.
Physically, a token ring network is wired as a star, with 'MAUs' and arms out to each station and the loop going out-and-back through each.
Cabling is generally IBM "Type-1" shielded twisted pair, with unique hermaphroditic connectors, commonly referred to as IBM data connectors in formal writing or colloquially as Boy George connectors. The connectors have the disadvantage of being quite bulky, requiring at least 3 x 3 cm panel space, and being relatively fragile. Connectors at the computer were usually DE-9 female.
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 802.5 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.
Each station passes or repeats the special token frame around the ring to its nearest downstream neighbour. 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.
IBM popularized the use of token ring LANs in the mid 1980s when it released its IBM token ring architecture based on active MAUs (Media Access Unit, not to be confused with Medium Attachment Unit) and the IBM Structured Cabling System. The Institute of Electrical and Electronics Engineers (IEEE) later standardized a token ring LAN system as IEEE 802.5. Although Token Ring runs on LLC, it includes Source Routing  to forward packets beyond the local network.
Token ring LAN speeds of 4 Mbit/s and 16 Mbit/s were 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.
With the development of switched Ethernet and faster variants of Ethernet, token ring architectures lagged behind Ethernet, and the higher sales of Ethernet allowed economies of scale which drove down prices further, and added a compelling price advantage. Token Ring MAC hardware was more complex than Ethernet, requiring a specialized processor and licensed MAC/LLC firmware for each interface. The Ethernet MAC included both the (simpler) firmware and the lower licensing cost in the MAC chip. Token Ring interface parts cost (using a Texas Instruments TMS380C16 MAC and PHY) was approximately 3x the cost of an Ethernet interface using the Intel 82586 MAC and PHY. The lower cost of unshielded twisted pair (CAT3 cable) was also significant, as the 10-BASE-T and 100-BASE-T signalling waveforms were optimized for this media, while the Token Ring waveform with its sharp edges and short risetimes caused EMI issues when used on unshielded cables.
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 receiving 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. 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 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.
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.
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.
|SD||AC||FC||DA||SA||PDU from LLC (IEEE 802.2)||CRC||ED||FS|
|8 bits||8 bits||8 bits||48 bits||48 bits||up to 18200x8 bits||32 bits||8 bits||8 bits|
|1 bit||1 bit||1 bit||1 bit||1 bit||1 bit||1 bit||1 bit|
|+||Bits 0–1||Bits 2–7|
|0||Frame type||Control Bits|
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
|1||1 bit||1 bit||1 bit||1 bit||1 bit||1 bit||1 bit|
|1 bit||1 bit||1 bit||1 bit||1 bit||1 bit||1 bit||1 bit|
A = 1, Address recognized C = 1, Frame copied
|Start Delimiter||Access Control||End Delimiter|
|8 bits||8 bits||8 bits|
|8 bits||8 bits|
Used to abort transmission by the sending station
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
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 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.
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