Token ring
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Token ring local area network (LAN) technology was conceived in the late 1960s by Olof Söderblom, then working for IBM [1]). US Patents were awarded in 1981 and Token-Ring was developed and promoted by IBM in the early 1980s and standardized as IEEE 802.5 by the Institute of Electrical and Electronics Engineers. Initially very successful, it went into steep decline after the introduction of 10BASE-T for Ethernet and the EIA/TIA 568 cabling standard in the early 1990s. A fierce marketing effort led by IBM sought to claim better performance and reliability over Ethernet for critical applications due to its deterministic access method, but was no more successful than similar battles in the same era over their Micro Channel architecture. IBM no longer uses or promotes token ring. Madge Networks, a one time competitor to IBM, is now considered to be the market leader in token ring.
Contents |
[edit] Overview
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, and FDDI, and has theoretical advantages over the stochastic CSMA/CD of Ethernet.
Physically, a token ring network is wired as a star, with 'hubs' 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.
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 Computers 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.
More technically, token ring is a local area network 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 ring frames travel completely around the loop.
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 multi-station access units (MSAUs or MAUs) 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.[2]
Token ring LAN speeds of 4 Mbit/s, 16 Mbit/s, 100 Mbit/s and 1 Gbit/s have been standardized by the IEEE 802.5 working group.
Token ring networks had significantly superior performance and reliability compared to early shared-media implementations of Ethernet (IEEE 802.3), and were widely adopted as a higher-performance alternative to shared-media Ethernet.
However, with the development of switched Ethernet, token ring architectures lagged badly behind Ethernet in both performance and reliability. The higher sales of Ethernet allowed economies of scale which drove down prices further, and added a compelling price advantage to its other advantages over token ring. Currently, more businesses use Ethernet networks than token ring networks.
Token ring networks have since declined in usage and the standards activity has since come to a standstill as switched Ethernet has dominated the LAN/layer 2 networking market.
[edit] Token frame
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 sending station receives its own data frame, it converts the frame back into a token. If a transmission error occurs and a no token, or more than one token, is present, a special station referred to as the Active Monitor detects the problem and removes and/or reinserts tokens as necessary (see Active and standby monitors). The special token frame consists of three bytes as follows (J and K are special non-data characters, referred to as code violations):
- 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. Since Manchester encoding is self clocking, and has a transition for every encoded bit 0 or 1, the J and K codings violate this, and will be detected by the hardware.
- 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 (AM) station when it sees this frame, and R bits are reserved bits.
- 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.
[edit] Token priority
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.
[edit] Token ring frame format
A data token ring frame is an expanded version of the token frame that is used by stations to transmit medium access control (MAC) management frames or data frames from upper layer protocols and applications.
The token ring frame format is defined as follows:
- Starting Delimiter — as described above.
- Access Control — as described above.
- Frame Control — a one byte field that contains bits describing the data portion of the frame contents.
- Destination address — a six byte field used to specify the destination(s).
- Source address — a six byte field that is either the local assigned address (LAA) or universally assigned address (UAA) 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.
- Frame Check Sequence — a four byte field used to store the calculation of a CRC for frame integrity verification by the receiver.
- Ending Delimiter — as described above.
- Frame Status — a one byte field used as a primitive acknowledgement scheme on whether the frame was recognized and copied by its intended receiver.
[edit] Active and standby monitors
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
- a loss of signal on the ring is detected,
- an active monitor station is not detected by other stations on the ring, or
- when a particular timer on an end station expires such as the case when a station hasn't seen a token frame in the past 7 seconds.
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.
[edit] Token ring 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.
- Phase 0 (Lobe Check) — A station first performs a lobe media check. A station is wrapped at the MSAU and is able to send 2000 test frames down its transmit pair which will loop back to its receive pair. The station checks to ensure it can receive these frames without error.
- Phase 1 (Physical Insertion) — A station then sends a 5 volt signal to the MSAU to open the relay.
- Phase 2 (Address Verification) — A station then transmits MAC frames with its own MAC address in the destination address field of a token ring frame. When the frame returns and if the address copied , the station must participate in the periodic (every 7 seconds) ring poll process. This is where stations identify themselves on the network as part of the MAC management functions.
- Phase 3 (Participation in ring poll) — A station learns the address of its Nearest Active Upstream Neighbor (NAUN) and makes its address known to its nearest downstream neighbor, what leads to creating the ring map. Station waits until it receives an AMP or SMP frame with the ARI and FCI bits set to 0. When it does, the station flips both bits (ARI and FCI) to 1, if enough resources are available, and queues an SMP frame for transmission. If no such frames are received within 18 seconds, then the station reports a failure to open and de-inserts from the ring. If the station successfully participates in a ring poll, it proceeds into the final phase of insertion, request initialization.
- Phase 4 (Request Initialization) — Finally a station sends out a special request to a parameter server to obtain configuration information. This frame is sent to a special functional address, typically a token ring bridge, which may hold timer and ring number information with which to tell the new station about.
[edit] See also
[edit] References
- Network Consultants Handbook by Matthew Castelli, Cisco Press 2002 ISBN 1-58705-039-0
- Networking Explained by Michael Gallo and William M. Hancock, Digital Press 2001 ISBN 1-55558-252-4
