Token ring

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Two examples of token ring networks: a) Using a single MAU b) Using several MAUs connected to each other
Token ring network
IBM hermaphroditic connector with locking clip

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:

  • 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.

The token scheme can also be used with bus topology LANs.

Description[edit]

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 CSMA/CD of Ethernet[citation needed].

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' and arms out to each station and the loop going out-and-back through each.

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, or Multistation Access Unit as referred to by IBM.[1] Not to be confused with Medium Attachment Unit) and the IBM Structured Cabling System. 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. The Institute of Electrical and Electronics Engineers (IEEE) later standardized a token ring LAN system as IEEE 802.5.[2] Although Token Ring runs on LLC, it includes Source Routing [3] 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.[4]

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

Token ring interfaces[edit]

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.[6] 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 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.[1]

Token ring speeds[edit]

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.

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.[7] IBM slowly adopted 100 Mbit/s as the reasoning was that the avoidance of collisions allowed higher link utilization.[8]

When token ring LANs were first introduced at 4 Mbit/s, there were widely circulated claims that they were superior to Ethernet,[9] but these claims were fiercely debated.[10] On a single wire, it is known that CSMA/CD performance can break down (due to "packet storms") at 1/e link utilization (approximately 36% utilization, or 3.6 Mbit/s channel utilization on a 10 Mbit/s coaxial link.) However, these problems in Ethernet were soon overcome by switching each Ethernet station into a central hub, so that the full 10 Mbit/s capacity between 2 or more station-pairs could be achieved using low-cost 10-BASE-T cabling.

Token Ring MAC architecture 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) approximately 3x more than an Ethernet interface using the Intel 82586 MAC and PHY. Initially both networks (802.3, 802.5) used Thickwire Coax (CAT1) cable which was cost-prohibitive and slowed adoption of both Ethernet and Token Ring. However, a few years later unshielded twisted pair (CAT3 cable) was standardized for Ethernet and the 10-BASE-T and 100-BASE-T signalling waveforms were optimized for this media. The 10-BASE-T and 100-BASE-T used simple phone connectors that were suitable for IBM PC Clones and adoption by home users. With the development of switched Ethernet and faster variants of Ethernet, and cheaper cabling, token ring architectures lagged behind Ethernet, and the higher sales of Ethernet allowed economies of scale which drove down prices further, resulting in a compelling price advantage.

Standards activity has since come to a standstill as Fast Ethernet and Gigabit Ethernet have come to dominate the local area networking market.

Token frame[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.

Token access priority[edit]

Token ring specifies an optional medium access scheme allowing a station with a high-priority transmission to request priority access to the token. This makes Token ring superior to Ethernet which does not have a method of finding the highest priority station, however, the Controller Area Network Bus solves the same problem in an faster and simpler way, and is widely used for its realtime performance in automotive applications.

In the Token ring priority MAC, 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.

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 sensitivty (i.e. VoIP)
x'111' Station management

Token ring frame format[edit]

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.

Data/command frame[edit]

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
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. Both the Starting Delimiter and Ending Delimiter fields are used to mark frame boundaries.
J K 0 J K 0 0 0
1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit
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.
+ Bits 0–2 3 4 5–7
0 Priority Token Monitor Reservation
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.
+ 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

Destination address 
A six byte field used to specify the destination(s) physical address.
Source address 
Contains physical address of sending station . It is 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. 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.
J K 1 J K 1 I E
1 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit
Frame status 
A one byte field used as a primitive acknowledgement scheme on whether the frame was recognized and copied by its intended receiver.
A C 0 0 A C 0 0
1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit

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

Token frame[edit]

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

Abort frame[edit]

SD ED
8 bits 8 bits

Used to abort transmission by the sending station

Active and standby monitors[edit]

An active monitor is the device that performs network-management duties, such as keeping track of tokens and weeding out frames that would otherwise circulate indefinitely. The device that has the highest MAC address in the token ring (the first device online) is automatically selected as the active monitor.

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

  • a loss of signal on the ring is detected.
  • an active monitor station is not detected by other stations on the ring.
  • 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.

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 insertion process[edit]

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 Recognized (AR) and Frame Copied (FC) bits in the frame-status are set to 0 (indicating that no other station currently on the ring uses that address), 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 Neighbour (NAUN) and makes its address known to its nearest downstream neighbour, leading to the creation of the ring map. Station waits until it receives an AMP or SMP frame with the AR and FC bits set to 0. When it does, the station flips both bits (AR and FC) 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 the new station needs to know.

Differences between Token ring and Ethernet[edit]

While Ethernet and Token ring have been regarded as being equally efficient, there are a few notable differences between the two technologies:

  • Ethernet supports a direct cable connection between two network interface cards by the use of a passthrough 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 agitate a collision 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 MAUs that operate without power (most MAUs operate in this fashion, with only requiring a power supply for LEDs). Ethernet network interface cards can operate on a passive hub to a degree, but not as a large LAN and the issue of collisions is still present.[12]
  • Token ring employs 'access priority' in which certain nodes can have priority over the token. 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).[8] Ethernet cannot support duplicate MAC addresses without reprimand.[13]

Modeling[edit]

A token ring network can be modelled as a polling system where a single server provides service to queues in a cyclic order.[14]

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.[15] Other 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.[16]

See also[edit]

References[edit]

  • Castelli, Matthew (2002). Network Consultants Handbook. Cisco Press. ISBN 1-58705-039-0. 
  • Gallo, Michael; Hancock, William M. (2001). Networking Explained. Digital Press. ISBN 1-55558-252-4. 

External links[edit]