CSMA/CD
The original Ethernet specifications expected collisions to occur on the LAN. The media was
shared, creating a literal electrical bus. Any electrical signal induced onto the wire could collide
with a signal induced by another device. When two or more Ethernet frames overlap on the
transmission medium at the same instant in time, a collision occurs; the collision results in bit
errors and lost frames.
The original Ethernet specifications defined the Carrier Sense Multiple Access with Collision
Detection (CSMA/CD) algorithm to deal with the inevitable collisions. CSMA/CD minimizes the
number of collisions, but when they occur, CSMA/CD defines how the sending stations can
recognize the collisions and retransmit the frame. The following list outlines the steps in the
CSMA/CD process:
1. A device with a frame to send listens until the Ethernet is not busy (in other words, the device
cannot sense a carrier signal on the Ethernet segment).
2. When the Ethernet is not busy, the sender begins sending the frame.
3. The sender listens to make sure that no collision occurred.
4. If there was a collision, all stations that sent a frame send a jamming signal to ensure that all
stations recognize the collision.
5. After the jamming is complete, each sender of one of the original collided frames randomizes
a timer and waits that long before resending. (Other stations that did not create the collision
do not have to wait to send.)
6. After all timers expire, the original senders can begin again with Step 1.