Unit 4

Token Bus

You will recall that on a CSMA/CD LAN, nodes vie for the ability to transmit messages on a first-come, first-served basis. Ties are broken by nodes waiting a random time interval before trying to retransmit the frame. A problem with this approach is that, under conditions of maximum network activity, a node can get "unlucky" and not be able to transmit for a long period of time. CSMA/CD provides no way to guarantee that the wait time won't exceed a given maximum; it's a matter of probability and network load.

Figure 62. Token Bus

The Token Bus protocol was designed to solve this media access control problem. The basic idea of Token Bus is to treat the bus as if it were a "logical ring." This is accomplished as follows. See the Token Bus diagram above.

Each node knows the network address of its successor in the logical ring.
A token is passed from node to node, each node passing the token to its successor on the ring. As with Token Ring, a token looks like a message, except for a header bit saying, "I'm a token."
There is only one token, and only the node which has the token, called the "token holder," can transmit. All other nodes can only receive.
When a node becomes the token holder by receiving the token, it has a period of time available to it during which it can transmit if it wishes.
If the token holder does not need to transmit, it just passes the token to its successor.
If the token holder needs to transmit, it can do so until its time interval has expired; then it must stop transmitting and send the token onward. This implies a maximum message length, with segmenting of long messages.

As you can see each node is guaranteed the opportunity to transmit within some defined maximum interval. This is not the case with Ethernet bus. This guaranteed access to the network is important for the environment that the Token Bus protocol is aimed — the factory floor. However, this predictability has an associated cost since a significant amount of time that could be used to transmit data messages on a CSMA/CD bus is wasted passing tokens on a Token Bus.

Advantages and Disadvantages of Token Bus

Advantages
 
Uses cable TV cables and parts readily and cheaply available.
Deterministic and able to prioritize traffic.
Short minimum frames.
Excellent performance under conditions of high load.
Broadband can support multiple channels (for example, video and voice).
 
Disadvantages
 
Complex protocol and engineering of equipment.
Expensive; requires modems and repeaters.
Since node must wait for token to come around before transmitting, messages are delayed waiting for token even when network is idle.

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