IEEE 802.4 Token Bus : In token bus Computer network station must have possession of a token before it can transmit on the computer network. The IEEE 802.4 Committee has defined token bus standards as broadband computer networks, as opposed to Ethernet's baseband transmission technique. Physically, the token bus is a linear or tree-shape cable to which the stations are attached
The topology of the computer network can include groups of workstations connected by long trunk cables. Logically, the stations are organized into a ring. These workstations branch from hubs in a star configuration, so the network has both a bus and star topology. Token bus topology is well suited to groups of users that are separated by some distance. IEEE 802.4 token bus networks are constructed with 75-ohm coaxial cable using a bus topology. The broadband characteristics of the 802.4 standard support transmission over several different channels simultaneously.
When the logical ring is initialized, the highest numbered station may send the first frame. The token and frames of data are passed from one station to another following the numeric sequence of the station addresses. Thus, the token follows a logical ring rather than a physical ring. The last station in numeric order passes the token back to the first station. The token does not follow the physical ordering of workstation attachment to the cable. Station 1 might be at one end of the cable and station 2 might be at the other, with station 3 in the middle.
In such a case, there is no collision as only one station possesses a token at any given time. In token bus, each station receives each frame; the station whose address is specified in the frame processes it and the other stations discard the frame.
MAC Sublayer Function
• When the ring is initialized, stations are inserted into it in order of station address, from highest to lowest.
• Token passing is done from high to low address.
• Whenever a station acquires the token, it can transmit frames for a specific amount of time.
• If a station has no data, it passes the token immediately upon receiving it.
• The token bus defines four priority classes, 0, 2, 4, and 6 for traffic, with 0 the lowest and 6 the highest.
• Each station is internally divided into four substations, one at each priority level i.e. 0,2,4 and 6.
• As input comes in to the MAC sublayer from above, the data are checked for priority and routed to one of the four substations.
• Thus each station maintains its own queue of frames to be transmitted.
• When a token comes into the station over the cable, it is passed internally to the priority 6 substation, which can begin transmitting its frames, if it has any.
• When it is done or when its time expires, the token is passed to the priority 4 substation, which can then transmit frames until its timer expires. After this the token is then passed internally to priority 2 substation.
• This process continues until either the priority 0 substation has sent all its frames or its time expires.
• After this the token is passed to the next station in the ring.
Frame format of Token Bus
The various fields present in the frame format are
1. Preamble: This. Field is at least 1 byte long. It is used for bit synchronization.
2. Start Delimiter: This one byte field marks the beginning of frame.
3. Frame Control: This one byte field specifies the type of frame. It distinguishes data frame from control frames. For data frames it carries frame's priority. For control frames, it specifies the frame type. The control frame types include. token passing and various ring maintenance frames, including the mechanism for letting new station enter the ring, the mechanism for allowing stations to leave the ring.
4. Destination address: It specifies 2 to 6 bytes destination address.
5. Source address: It specifies 2 to 6 bytes source address.
6. Data: This field may be upto 8182 bytes long when 2 bytes addresses are used & upto 8174 bytes long when 6 bytes address is used.
7. Checksum: This 4 byte field detects transmission errors.
8. End Delimiter: This one byte field marks the end of frame.
The various control frames used in token bus are: