by Dinesh Thakur Category: Computer Network

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.

                          A Token Bus

 Physical Layer of the Token Bus

The conventional 75 ohm coaxial cable used for the cable TV can be used as the physical layer of the token bus. The different modulation schemes are used. They are, phase continuous frequency shift keying, phase coherent frequency shift keying, and the multilevel duo binary amplitude-modulated phase shift keying. Signal speeds in the range 1 Mbps, 5 Mbps, and 10 Mbps are achievable. The physical layer of the token bus is totally incompatible to the IEEE 802.3 standard.

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.

            Frame Format of IEEE 802.4

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:

                          Control Frame in Token Bus

Maintaining the Ring

Control frames Control frames are frames used for controlling and maintaining the logical ring. The format is same as the format specified in Figure, except the presence of an info field. There are different bit patterns used in the frame control field to define different types of control frames. Some important ring-maintaining activities are discussed below.

Adding a new station To add a new station to the logical ring, the station which currently holds the token broadcasts periodically sends a special control frame called a Solicit-successor frame. This frame contains the address of the sending station and its successor. This intimates the stations within the specified range to join the ring. Hence, the ring remains sorted.

Leaving the logical ring A station can simply leave the ring by sending its successor address to its predecessor and asks it to set this as its successor. To perform this, it passes a special control frame called set successor. The frame contains the address of predecessor as destination address and the address of the successor as sending address.

Ring initialization When the network is powered on, initially, all the stations are off. As soon as the first station is initiated, it checks the channel for the presence of any contenders, by sending a special control frame called claim-token. If there is no response, it generates a new token; thereby, creating a new logical ring with a single station. Periodically it transmits solicit successor tokens, hence, new stations are added frequently, making 'the logical ring grow.

Failure of a successor station When a station finishes the transmission, it passes the token to the next station (successor). If the successor station fails to claim the token, then other stations in the network may not be able to claim the token. Allowing stations to watch their successor's activity after passing the token to them, can rectify this problem. If nothing takes place, until a predetermined time, it retransmits the token. If again there is no response, the successor station is disconnected from the logical ring and a new control frame set-successor is sent to the successor of the failed station.

Failure of multiple successors If a station is not able to pass the token to its successor, or if the successor of its successor also failed, then it sends a special control frame called solicit successor-2, to check the availability of other stations on the network. The network is then relatively established, by running the contention algorithm for the second time.

Failure of token holder If the token holder fails, none of the other stations can get the token. Each station in the ring has a timer internally. They wait until the timer expires and transmits a claim-token frame, and the network is reestablished.

Multiple tokens If the token holder detects the transmission of frames from some other station, it discards its token. If multiple tokens are present in the network, the above process is repeated until only one token is present in the network. If all the tokens in the network are discarded, then one or more stations transmit claim-token frame, and restart the network activity.





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Dinesh ThakurDinesh Thakur holds an B.SC (Computer Science), MCSE, MCDBA, CCNA, CCNP, A+, SCJP certifications. Dinesh authors the hugely popular blog. Where he writes how-to guides around Computer fundamental , computer software, Computer programming, and web apps. For any type of query or something that you think is missing, please feel free to Contact us.



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