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Cha p te r
F o u r

This type of access control for a computer network is referred to
as carrier-sense multiple-access with collision detection (CSMA/CD). Eth-
ernet and similar solutions are widely applied to create CSMA/CD
networks, particularly in settings in which a maximum waiting time
for access to the network does not have to be guaranteed. This type of
network is simple to install, and a wide range of hardware and soft-
ware products are available for support. On the other hand, as indi-
cated in Fig. 4.12, network performance can degrade significantly
under high load; therefore, the utility of an Ethernet-oriented net-
work will depend on the particular configuration and loads expected
for the network.

4.9.3 Transmission Control Protocol
(TCP)/Internet Protocol (IP)
TCP/IP applies to the transport and network layers indicated in
Fig. 4.9. TCP/IP thus provides a means for addressing intermediate
protocol levels, and in fact is often combined with Ethernet in a com-
munication approach that defines both the lower and middle aspects
of the system. TCP/IP functions by dividing any message provided
to these middle layers into packets of 64 kbytes and then sending
packets one at a time to the communication network. TCP/IP must
also reassemble the packets in the correct order at the receiving user.
TCP/IP provides a common strategy to use for networking.
It allows extension of the Ethernet lower layers to a midlayer proto-
col on which the final application and presentation layers may be
constructed.

4.10 Manufacturing Automation Protocol
The manufacturing automation protocol (MAP) is one of the proto-
cols developed for computer communication systems, and was devel-
oped specifically for use in a factory environment. General Motors
Corp. has been the leading advocate of this particular protocol. When
faced with a need for networking many types of equipment in its fac-
tory environment, General Motors decided that a new type of proto-
col was required. Beginning in 1980, General Motors began to develop
a protocol that could accommodate the high data rate expected in its
future factories and provide the necessary noise immunity expected
for this environment. In addition, the intent was to work within a
mature communications technology and to develop a protocol that
could be used for all types of equipment in General Motors factories.
MAP was developed to meet these needs. The General Motors effort
has drawn on a combination of Institute of Electrical and Electronics
Engineers (IEEE) and ISO standards, and is based on the open system
interconnect (OSI) layered model, as illustrated in Fig. 4.10.