Token Bus (IEEE 802.4)

Token Bus is described in the IEEE 802.4 specification, and is a Local Area Network (LAN) in which the stations on the bus or tree form a logical ring. Each station is assigned a place in an ordered sequence, with the last station in the sequence being followed by the first, as shown below. Each station knows the address of the station to its "left" and "right" in the sequence.

A Token Bus network

The Token Bus frame format

The Token Bus frame format is shown above.

The Preamble field is used to synchronise the receiver's clock.

The Start Delimeter and End Delimeter fields are used to mark the start and end of the frame, and contain an analogue encoding of symbols other than 0s and 1s that cannot occur accidentally within the frame data. For this reason, a length field is not required.

The Frame Control field identifies the frame as either a data frame or a control frame. 

The Destination and Source address fields contain either a 2-byte or a 6-byte hardware address for the destination and source stations respectively (a given network must use either 2-byte or 6-byte addresses consistently, not a mixture of the two).

If 2-byte addresses are used, the Data Field can be up t0 8,182 bytes. If 6-byte addresses are used, it is limited to 8,174 bytes.

The Checksum is used to detect transmission errors. The various control frames used are shown in the table below.

Token Ring (IEEE 802.5)

Token Ring was developed by IBM in the 1970s and is described in the IEEE 802.5 specification. It is no longer widely used in LANs. Token passing is the method of medium access, with only one token allowed to exist on the network at any one time. Network devices must acquire the token to transmit data, and may only transmit a single frame before releasing the token to the next station on the ring. When a station has data to transmit, it acquires the token at the earliest opportunity, marks it as busy, and attaches the data and control information to the token to create a data frame, which is then transmitted to the next station on the ring. The frame will be relayed around the ring until it reaches the destination station, which reads the data, marks the frame as having been read, and sends it on around the ring. When the sender receives the acknowledged data frame, it generates a new token, marks it as being available for use, and sends it to the next station. 

Two basic frame types are used - tokens, and data/command frames. The token is three bytes long and consists of a start delimiter, an access control byte, and an end delimiter. The format of the token is shown below.

The Token Ring frame format

• Start delimiter - alerts each station of the arrival of a token or frame.
• Access control byte - contains the priority field, the reservation field, the token bit and amonitor bit.
• End delimiter - signals the end of the token or frame, and contains bits that may be used to indicate a damaged frame, and to identify the last frame in a logical sequence.