Installation guide
Network Infrastructure for EtherNet/IP™
Designing the Infrastructure
4-35
information wherever and whenever it is needed by qualified users, such as plant and
maintenance engineers or managers.
4.3 EtherNet/IP Infrastructure Design Issues
4.3.1 Full-Duplex vs. Half-Duplex Implications
Ethernet is based on Carrier Sense Multiple Access/Collision Detect (CSMA/CD) technology. In its
original design, this technology put all nodes on a common circuit, and they could all “talk” at the same
time, which created a problem—collisions of data—because when nodes talked they could not
simultaneously listen. It was analogous to two-way radios with push-to-talk switches. If two radio
operators simultaneously responded to the same caller, the caller heard nothing but garble from the two
radios transmitting at the same time and interfering with one another. Neither operator had any way of
knowing that the other was talking. Thus, Ethernet nodes had to deal with how collisions were handled
and how connected devices could maximize their ability to transmit Ethernet without monopolizing the
time such that other nodes could not transmit.
The issue was that Ethernet was based on
half-duplex (HDX) mode of operation for the connected
devices as shown in Figure 4-2. Thus, a node could only transmit data or receive data. It could not do
both at the same time. Half-duplex mode is similar to the traffic problem created when road construction
causes one lane of a two-lane highway to be closed. Traffic from both directions is trying to use the same
lane. This means that traffic coming from one direction must wait until traffic coming from other
direction stops.
Ethernet Talks and Listens at the Same Time
With advances in switch technology, Ethernet connections have moved toward full-duplex
operation (FDX), enabling connected devices to both send and receive packets of Ethernet data at
the same time.
Figure 4-2 Full-Duplex Operation Eliminates Collisions.
9
6
3
T
X
T
X
Collision
Half-Duplex
Sending simultaneously over one
path leads to collisions.
Full-Duplex
Sending simultaneously over one
path doesn’t lead to collisions.
T
X
Ethernet Half-Duplex (HDX) and Full-Duplex (FDX)