Figure 3-5 CSMA/CD Process
The extent of the network segment over which collisions occur is referred to as the collision
domain. The size of the collision domain has an impact on efficiency and therefore on
data throughput. In today’s LANs, switches have replaced hubs. The reason this occurs is
that switches create tiny collision domains containing just one device. This eliminates the
potential for collisions. This process is often called “microsegmentation” of the network.
In the CSMA/CD process, priorities are not assigned to particular stations, so all stations
on the network have equal access. This is the “multiple access” part of the protocol. If
two or more stations attempt a transmission simultaneously, a collision occurs. The stations
are alerted of the collision, and they execute a backoff algorithm that randomly
schedules retransmission of the frame. This scenario prevents the machines from repeatedly
attempting to transmit at the same time. Collisions are normally resolved in microseconds.
This is the “collision detection” part of the protocol.
While collisions are resolved quickly, it is still advantageous to eliminate them entirely
from the network. This allows much more efficient communications. This is accomplished
through the use of switches as described earlier.
Ethernet Frames
Bits that are transmitted over an Ethernet LAN are organized into frames. In Ethernet terminology,
the “container” into which data is placed for transmission is called a frame . The
frame contains header information, trailer information, and the actual data that is being
transmitted