Token Ring is a physical and data link layer 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. Following its introduction, Token Ring technology became widely adopted, particularly in corporate environments, but was gradually eclipsed by newer iterations of Ethernet. The last formalized Token Ring standard that was completed was Gigabit Token Ring (IEEE 802.5z), published on May 4, 2001.
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 1981 – the 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 developed an upgraded 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, 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 dominated the local area networking market.
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Comparison with Ethernet Early Ethernet and Token Ring both used a transmission medium shared among the network's devices. However, they differed in their channel access methods. These differences have become immaterial to modern Ethernet networks, which consist of switches and point-to-point links operating in full-duplex mode. Token Ring and legacy Ethernet have some notable differences:
Token Ring access is more deterministic, compared to Ethernet's contention-based CSMA/CD. Ethernet supports a direct cable connection between two network interface cards by the use of a crossover 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. Token Ring eliminates collision by the use of a single-use token and early token release to alleviate the down time. Legacy Ethernet alleviates collision by carrier-sense multiple access and by the use of an intelligent switch; primitive Ethernet devices like hubs could precipitate collisions due to repeating traffic blindly. Token Ring network interface cards contain all of the intelligence required for speed autodetection and 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 did not have a provision for an access priority system as all nodes have equal access to the transmission medium. Multiple identical MAC addresses are supported on Token Ring (a feature used by S/390 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 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 with 10BASE-T (Cat 3) and 100BASE-TX (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 compared to 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 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.
Token passing The data transmission process operates as follows:
An empty "token frame" is continuously circulated on the ring. When a computer has a message to send, it marks the token frame as seized and appends its transmission before continuing to circulate 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 marks in the frame that it was received and copied. Eventually, the frame loops back to its originator. The originator handles the sending of the packet as a success or error depending on whether or not the frame was marked as received and/or copied. The originator removes the message from the frame and marks it as ready to be seized by another computer, and the frame continues to circulate as an empty token frame.
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