Page 79 - Intelligent Communication Systems

62 INTELLIGENT COMMUNICATION SYSTEMS
Once a source node acquires a. free token, it changes the token into a busy token
and issues a frame following the busy token. Fundamentally, the transmission of a
frame is permitted with a busy token. When the transmission is successfully per-
formed, the source node issues a free token into a ring network.
In the single-token method, the node that acquired a busy token issues a frame
following the busy token and finally receives the busy token, which runs through
the ring network. The transmission of the frame is defined as finished. The time
when the node receives a busy token is defined as the time of completion for trans-
mission of a frame. In this method, only one free token or busy token is running
through the ring network.
Figure 7.7 presents an example of the single-token method. In Figure 7.7(a),
a free token is running through the ring network. In Figure 7.7(b), node E acquires
a free token and issues a frame following a busy token. In Figure 7.7(c), node B
receives the frame and releases the frame with the busy token. In Figure 7.7(d),
node E receives the frame with the busy token and releases a free token into me
network. As said before, in this method only one token is running in the network.
In the multitoken method, the time when a node that acquires a free token fin-
ishes issuing a frame is defined as the time of completion for transmission of the
frame. When the transmission is complete, the node releases a free token into the
network. In this method, a busy token and a free token exist at the same time in the
ring network. Generally speaking, one free token and two or more busy tokens exist
in a ring network.

FIGURE 7.7 Single-token method.