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Wireless Network Logical Architecture 35
Device A
Data packet
Time
Device B
attempts to send
Medium busy
Carrier
sensing
Medium free
Collision
Random
backoff
Data packet
Device C
attempts to send
Medium busy Medium free Collision Medium busy Medium free
Carrier
sensing
Random
backoff
Slot time
Carrier
sensing
Figure 2.6 : Ethernet CSMA/CD timing
Data packet
2.3.3.1 Carrier Sense Multiple Access/Collision Detection (CSMA/CD)
The most commonly used MAC method to control device transmission, and the one
specified for Ethernet based networks, is carrier sense multiple access/collision detection
(CSMA/CD) ( Figure 2.6 ). When a device has a data frame to transmit onto a network that
uses this method, it first checks the physical medium (carrier sensing) to see if any other
device is already transmitting. If the device senses another transmitting device it waits
until the transmission has finished. As soon as the carrier is free, it begins to transmit
data, while at the same time continuing to listen for other transmissions.
If it detects another device transmitting at the same time (collision detection), it stops
transmitting and sends a short jam signal to tell other devices that a collision has occurred.
Each of the devices that were trying to transmit then computes a random back-off period
within a range 0 to t max , and tries to transmit again when this period has expired. The
device that by chance waits the shortest time will be the next to gain access to the medium,
and the other devices will sense this transmission and go back into carrier sensing mode.
A very busy medium may result in a device experiencing repeated collisions. When this
happens t max is doubled for each new attempt, up to a maximum of 10 doublings, and if
the transmission is unsuccessful after 16 attempts the frame is dropped and the device
reports an “ excessive collision error. ”
2.3.3.2 Other Wired Network MAC Methods
Another common form of media access control for wired networks, defined by the IEEE
802.5 standard, involves passing an electronic “ token ” between devices on the network in
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