Page 255 - Sensors and Control Systems in Manufacturing

Networking of Sensors and Contr ol Systems in Manufacturing
than using no guidelines at all. However, because RS-232 does not 215
completely define all of the relationships that must exist in communi-
cation links, it falls far short of being a true standard or protocol.

4.9.2 Ethernet
As illustrated in Fig. 4.11, one approach to local area networks (LANs)
is to define a protocol for the first two layers of a communication
strategy and then allow individual users to define the upper layers.
This approach has been widely applied using a method referred to
as Ethernet [Metcalfe and Boggs (1976), Shock and Hupp (1980),
Tanenbaum (1988)], which was created by Xerox Corp.
In every computer communication system, there must be a means
of scheduling for each node to transmit onto the network and listen
to receive messages. This may be done on a statistical basis. For
example, when a unit needs to transmit over the network, it makes an
effort to transmit. If another node tries to transmit at the same time,
both nodes become aware of the conflict, wait for a random length of
time, and try again. It might seem that this would be an inefficient
means of controlling a network, since the various nodes are randomly
trying to claim the network for their own use, and many collisions
may occur. As it turns out, for lower communication volumes, this
method works very well. As the number of nodes on the system and
the number of messages being exchanged increases, however, the
number of collisions between active nodes goes up and reduces the
effectiveness of the system (Fig. 4.12).

FIGURE 4.12
Delay and
throughput versus
Throughput
network load
(number of users).

Delay and throughput

Delay

Network load (number of users)