Principles and technologies of local area networks - Intelligent Buildings and Building Automation - page 61

may be more than one device waiting to use the medium. If every device starts
sending as soon as the medium becomes free, a collision is highly probable. If
a random wait is included, the devices will not all start sending at the same
time and collisions can be avoided by the devices checking the medium once
more before sending.
Token passing: The principle of token passing can be used on both ring
and bus networks. This simple principle is based on defining a token in the
form of a bit pattern, with 8 bits for example. Tokens circulate between the
nodes if the network is a ring. If the network is a bus, the tokens 'circulate'
in a particular sequence. Using a baton in a relay race as an analogy, only the
one who 'is holding' the baton at a given instant can use the medium. This
ensures that only one recipient uses the medium at a time. The principle has
advantages over the CSMA/CD collision protocol when the network load
is high. Administration of token passing is the same whatever the load, and
the medium can be exploited to the full. On the other hand, the efficiency of
CSMA/CD declines sharply when there are many collisions and the frames
have to be resent.
Generally, token passing offers the ability to prioritize traffic. The principle

Send the request

N

Is medium
free?

Y

Wait for a random time

N

Is medium
free?

Y

Start transmission

Y

Collision?

N

Transmission completed

Figure 4.16 Logic scheme of CSMA/CD in accessing medium.