Technical data
110 Theoretical and general applications www.westermo.com
Industrial Ethernet
As a communication standard, Ethernet has existed for many years and today forms
the basis of most networks throughout the world. Despite many claims over the years
that Ethernet will be replaced, it continues to be developed and offers the properties
that users have requested. In recent years Ethernet has also won approval in the indus-
trial market.
IEEE 802.3 Ethernet
Access methods
In order for two or more parties to communicate requires a set of rules, this applies to
everything, especially to data communication. How data is transmitted on to a line is
known as the access method, the original method used by Ethernet was called
CSMA/CD, which means: Carrier Sense Multiple Access/Collision Detect. It is impor-
tant to establish that Ethernet uses two access methods, constant access or
CSMA/CD. CSMA/CD is referred to regularly in literature but is not so commonly
used today. It has a historical background and for this reason we will give a brief
description of the parts in CSMA/CD:
… Carrier Sense, which means that a single unit, before it sends, must detect
whether someone is using the network. If so, the unit must wait before it trans-
mits.
… Multiple Access, means that everyone can use the network, but not simultane-
ously.
… Collision Detect, means that when two or more units transmit simultaneously
this should be detected. When a collision is detected, a collision signal is sent
and all those concerned stop sending. All units then wait for a random period
before new attempts are made, this minimises the risk of them starting to send
at the same time. Naturally, collisions have the effect of slowing traffic in the sys-
tem. A network with a high load results in many collisions, which leads to fur-
ther network traffic, which in turn creates more collisions, etc. Some equipment
has LEDs that indicate collisions, in doing so you can easily check the load on
the network. The advantage of a CSMA/CD network is that all equipment can
start transmitting at any time compared with a polled system or token ring
where transmission is strictly controlled.
CSMA/CD
CS
MA/CD
CSMA/
CD
B A C K