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Token Ring network IBM hermaphroditic connector with locking clip. Screen contacts are prominently visible, gold-plated signal contacts less so. Token Ring is a
computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...
ing technology used to build
local area network A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office building. By contrast, a wide area network (WAN) not only covers a large ...
s. It was introduced by IBM in 1984, and standardized in 1989 as
IEEE The Institute of Electrical and Electronics Engineers (IEEE) is a 501(c)(3) professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operati ...
802.5. It uses a special three-byte frame called a ''token'' that is passed around a logical ''ring'' of workstations or
server Server may refer to: Computing *Server (computing), a computer program or a device that provides functionality for other programs or devices, called clients Role * Waiting staff, those who work at a restaurant or a bar attending customers and su ...
s. This
token passing On a local area network, token passing is a channel access method where a packet called a ''token'' is passed between nodes to authorize that node to communicate. In contrast to polling access methods, there is no pre-defined "master" node. The mos ...
is a
channel access method In telecommunications and computer networks, a channel access method or multiple access method allows more than two terminals connected to the same transmission medium to transmit over it and to share its capacity. Examples of shared physical m ...
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 Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1 ...
.


History

A wide range of different
local area network A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office building. By contrast, a wide area network (WAN) not only covers a large ...
technologies were developed in the early 1970s, of which one, the
Cambridge Ring The term Cambridge Ring could refer to: * The Cambridge Ring (computer network) technology developed at the university of Cambridge, England * The Cambridge Five espionage ring. * The inner ring-road of Cambridge Cambridge ( ) is a universi ...
, had demonstrated the potential of a
token passing On a local area network, token passing is a channel access method where a packet called a ''token'' is passed between nodes to authorize that node to communicate. In contrast to polling access methods, there is no pre-defined "master" node. The mos ...
ring topology, and many teams worldwide began working on their own implementations. At the
IBM Zurich Research Laboratory IBM Research is the research and development division for IBM, an American multinational information technology company headquartered in Armonk, New York, with operations in over 170 countries. IBM Research is the largest industrial research org ...
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 A workstation is a special computer designed for technical or scientific applications. Intended primarily to be used by a single user, they are commonly connected to a local area network and run multi-user operating systems. The term ''workst ...
vendor
Apollo Computer Apollo Computer Inc., founded in 1980 in Chelmsford, Massachusetts, by William Poduska (a founder of Prime Computer) and others, developed and produced Apollo/Domain workstations in the 1980s. Along with Symbolics and Sun Microsystems, Apollo ...
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 IEEE 802.5 standard. During this time, IBM argued that Token Ring LANs were superior to
Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1 ...
, 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. An increase to 100 Mbit/s was standardized and marketed during the wane of Token Ring's existence and was never widely used. 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 In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use ...
dominated the local area networking market.


Gallery

100 megabit token ring adapter.JPG, 100 Mbit/s IBM Token Ring Management Adapter with Wake On LAN. Both UTP (RJ45) and STP (IBM Data Connector) interfaces are present. Full length token ring cards.jpg, Assorted full-length Micro Channel Token Ring cards, including a LANStreamer which features multiple RJ45 ports for usage in a Token Ring network. Token ring cards.JPG, Token Ring Network Interface Cards (NICs) with varying interfaces from: ISA, PCI and MicroChannel ISA TokenRing NIC.jpg, Madge 4/16Mbit/s TokenRing ISA NIC Token ring cards tiled.jpg, A series of multiple 16/4 early Micro Channel Token Ring cards which would have prominently been installed in many Personal System/2 machines.


Comparison with Ethernet

Ethernet and Token Ring have some notable differences: * Token Ring access is more
deterministic Determinism is a philosophical view, where all events are determined completely by previously existing causes. Deterministic theories throughout the history of philosophy have developed from diverse and sometimes overlapping motives and cons ...
, compared to Ethernet's contention-based
CSMA/CD Carrier-sense multiple access with collision detection (CSMA/CD) is a medium access control (MAC) method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other stati ...
* Ethernet supports a direct cable connection between two network interface cards by the use of a
crossover cable A crossover cable connects two devices of the same type, for example DTE-DTE or DCE-DCE, usually connected asymmetrically (DTE-DCE), by a modified cable called a crosslink. Such a distinction between devices was introduced by IBM. The cros ...
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. Ethernet alleviates collision by carrier sense multiple access and by the use of an intelligent switch; primitive Ethernet devices like hubs can 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 (MAUs) that operate without power (most MAUs operate in this fashion, only requiring a power supply for LEDs). 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 does not have a provision for an access priority system as all nodes have equal access to the
transmission medium A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modulat ...
. * Multiple identical
MAC address A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use is common in most IEEE 802 networking te ...
es are supported on Token Ring (a feature used by
S/390 The IBM System/390 is a discontinued mainframe product family implementing the ESA/390, the fifth generation of the System/360 instruction set architecture. The first computers to use the ESA/390 were the Enterprise System/9000 (ES/9000 ...
mainframes). Switched Ethernet cannot support duplicate MAC addresses without reprimand. * Token Ring was more complex than Ethernet, requiring a specialized processor and licensed MAC/
LLC A limited liability company (LLC for short) is the US-specific form of a private limited company. It is a business structure that can combine the pass-through taxation of a partnership or sole proprietorship with the limited liability of ...
firmware for each interface. By contrast, Ethernet included both the (simpler) 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 Twisted pair cabling is a type of wiring used for communications in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted b ...
with
10BASE-T 1 (one, unit, unity) is a number representing a single or the only entity. 1 is also a numerical digit and represents a single unit of counting or measurement. For example, a line segment of ''unit length'' is a line segment of length 1 ...
( Cat 3) and
100BASE-TX In computer networking, Fast Ethernet physical layers carry traffic at the nominal rate of 100 Mbit/s. The prior Ethernet speed was 10 Mbit/s. Of the Fast Ethernet physical layers, 100BASE-TX is by far the most common. Fast Ether ...
( Cat 5(e)), 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 On a local area network, token passing is a channel access method where a packet called a ''token'' is passed between nodes to authorize that node to communicate. In contrast to polling access methods, there is no pre-defined "master" node. The mos ...
mechanisms are used by ARCNET, token bus, 100VG-AnyLAN (802.12) and FDDI, and they have theoretical advantages over the
CSMA/CD Carrier-sense multiple access with collision detection (CSMA/CD) is a medium access control (MAC) method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other stati ...
of early Ethernet. A Token Ring network can be modeled as a
polling system In queueing theory, a discipline within the mathematical theory of probability, a polling system or polling model is a system where a single server visits a set of queues in some order. The model has applications in computer networks and telecom ...
where a single server provides service to queues in a cyclic order.


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

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 A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
, 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 A limited liability company (LLC for short) is the US-specific form of a private limited company. It is a business structure that can combine the pass-through taxation of a partnership or sole proprietorship with the limited liability of ...
, 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 In telecommunications, structured cabling is building or campus cabling infrastructure that consists of a number of standardized smaller elements (hence structured) called subsystems. Structured cabling components include twisted pair and op ...
system that IBM hoped would be widely adopted. Unique
hermaphroditic connector In electrical and mechanical trades and manufacturing, each half of a pair of mating connectors or fasteners is conventionally assigned the designation male or female. The female connector is generally a receptacle that receives and holds the ...
s, 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 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 The D-subminiature or D-sub is a common type of electrical connector. They are named for their characteristic D-shaped metal shield. When they were introduced, D-subs were among the smallest connectors used on computer systems. Description, no ...
female. In later implementations of Token Ring, Cat 4 cabling was also supported, so
8P8C A modular connector is a type of electrical connector for cords and cables of electronic devices and appliances, such as in computer networking, telecommunication equipment, and audio headsets. Modular connectors were originally developed fo ...
(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. 8228 ring in.JPG, IBM Data Connectors on the IBM 8228 Multistation Access Unit. Token ring media filters.JPG, 8P8C 'Media Filters' that plug into an IBM Data Connector converting it for use with 8P8C connectors.


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. ; 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. ; 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. ; 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. 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 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. 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. 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: * 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 A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use is common in most IEEE 802 networking te ...
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. * 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.


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 A Controller Area Network (CAN bus) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other's applications without a host computer. It is a message-based protocol, designed originally for m ...
, (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 IEEE 802.1Q, often referred to as Dot1q, is the networking standard that supports virtual local area networking (VLANs) on an IEEE 802.3 Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying pro ...
and 802.1p:


Interconnection with Ethernet

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 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.


See also

* IBM PC Network


References


General

* *


External links


IEEE 802.5 Web Site

Troubleshooting Cisco Router Token Ring Interfaces


* ttp://www.networkworld.com/article/2161208/lan-wan/what-if-ethernet-failed-.html What if Ethernet had failed? {{Authority control Network topology Local area networks IEEE 802 IBM PC compatibles IEEE standards Serial buses Link protocols Systems Network Architecture