Carrier sense multiple access with col-
Simplified Algorithm of CSMA/CD
Carrier Sense Multiple Access With
Collision Detection (CSMA/CD), in com-
puter networking, is a network control pro-
tocol in which
• a carrier sensing scheme is used.
• a transmitting data station that detects
another signal while transmitting a frame,
stops transmitting that frame, transmits a
jam signal, and then waits for a random
time interval (known as "backoff delay"
and determined using the truncated
binary exponential backoff algorithm)
before trying to send that frame again.
CSMA/CD is a modification of pure Carrier
Sense Multiple Access (CSMA).
Collision detection is used to improve
CSMA performance by terminating transmis-
sion as soon as a collision is detected, and re-
ducing the probability of a second collision
Methods for collision detection are media
dependent, but on an electrical bus such as
Ethernet, collisions can be detected by com-
paring transmitted data with received data.If
they differ, another transmitter is overlaying
the first transmitter’s signal (a collision), and
transmission terminates immediately. A jam
signal is sent which will cause all transmit-
ters to back off by random intervals, reducing
the probability of a collision when the first
retry is attempted. CSMA/CD is a layer 2 pro-
tocol in the OSI model.
Even when it has nothing to transmit, the
CSMA/CD MAC sublayer monitors the physic-
al medium for traffic by watching the carrier-
Sense signal provided by the PLS (Physical
layer signals to MAC layer). Whenever the
medium is busy,the CSMA/CD MAC defers to
the passing frame by delaying any pending
transmission of its own. After the last bit of
the passing frame (that is, when carrierSense
changes from true to false), the CSMA/CD
MAC continues to proper transmission.
Collisions are detected by monitoring the
collisionDetect signal provided by the Physic-
al Layer. When a collision is detected during
a frame transmission, the transmission is not
terminated immediately. Instead,