Token ring network (IEEE 802.5)

A Token Ring Network is a Local Area Network (LAN) in which all computers are connected in a ring or star topology and a bit or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time. The Token Ring protocol is the second most widely used protocol on local area networks after Ethernet.

The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5. Both protocols are used and are very similar. The IEEE 802.5 Token Ring technology provides for data transfer rates of either 4 or 16 megabytes per second.

How Token Ring Networks Works

Empty information frames are continuously circulated on the ring. When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame.

The frame is then examined by each successive workstation. If the workstation sees that it is the destination for the message, it copies the message 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.

The standard for the Token protocol is the Institute of Electrical and Electronics Engineers (IEEE) 802.5.

Fiber Distributed Data Interface (FDDI)

The Fiber Distributed Data Interface (FDDI) specifies a 100 Mbps token passing, dual-ring LAN using fiber optic cable. FDDI is frequently used as high-speed backbone technology because of its support for high bandwidth and greater distances than copper. It should be noted that relatively recently, a related copper specification, called Copper Distributed Data Interface (CDDI), has emerged to provide 100 Mbps service over copper. CDDI is the implementation of FDDI protocols over twisted-pair copper wire.

FDDI uses dual-ring architecture with traffic on each ring flowing in opposite directions called counter-rotating. The dual rings consist of a primary and a secondary ring. During normal operation, the primary ring is used for data transmission, and the secondary ring remains idle. The primary purpose of the dual rings is to provide superior reliability and robustness.

Differences between Token Ring IEEE 802.5 and Token Bus IEEE 802.4

Token Bus (IEEE 802.4), networks use a Bus technology and do not have a centralized active monitor. Toke Bus was designed with large factories in mind where machines and equipment would be moving around under computer control. Network failures could have serious consequences and so had to be avoided at all costs. On the other hand the Token Ring Network standard was designed with office automation in mind, where a failure once in a rare while could be tolerated as the price for simpler system.

Token Bus is not as deterministic as Token Ring but like Token Ring it has excellent throughput and efficiency at high load.

Token Bus uses broadband transmission and cabling which obviously gives it a bandwidth advantage over Token Ring but the downside is the equipment costs (modems, wideband amplifiers etc.). The IEEE 802.4 protocol is also extremely complex compared to IEEE 802.5 and it has a substantial delay at low loads.