Understanding the network.pdf - Back to Home

A network node continuously listens to the network cable segment. The cable
segment shared by all of the nodes when CSMA/CD is employed is called a collision
domain. When the node is ready to send data, it checks to see if there is any activity.
If the cable is clear, it sends its data. If two stations do this at the same time, a
"collision" occurs. When this happens, the participating stations begin the Collision
Consensus Enforcement Procedure (CCEP), under which a "jam" pattern is
broadcast to all of the workstations on the network. This period lasts as long as it
takes to reach the ends of the network. If the collision domain is quite large, it may
take some time. After the CCEP is over, one of the affected nodes retransmits its
frame. If it's successful and needs to transmit more data, the "listening" process
starts over again.
However, if a collection of nodes collide, they stop transmitting and try again after
a retransmission delay. Each node computes its own delay time using the truncated
binary exponential backoff algorithm. This algorithm basically takes the duration of
the collision event, doubles its value, and uses it as a wait time. This time is usually
longer then the collision domain's round trip delay, which should be no more than
51.2 microseconds and varies depending on the transmission protocol medium used.
For example, according to the IEEE 10-BaseT specification, the round-trip delay
should be no more than 5.76 microseconds. The theory is that if all the stations are
transmitting at random intervals (with random delay periods after collisions),
collisions will be minimized.
The major problem with CSMA/CD is that all transmissions are half-duplex (the
node can either send or receive at one time), and it only allows one source-todestination
transmission at a time because the bandwidth is shared. That means as
the domain adds more nodes or the collision domain becomes larger, the chance of
a collision increases. As a result, overall performance decreases because more time
is spent detecting collisions and performing collision recovery than actually
transmitting data. To maximize the efficiency of a CSMA/CD collision domain, it is
normal to engineer the domain to operate under a sustained load of no more then
30 percent of available capacity.
Non-Contention-Based Access
Token passing is the non-contention-based access method used with transmission
protocols that utilize ring and bus topologies. Token passing works by having the
connected nodes pass a "token" from one station to another. When a station needs
to send data, it takes the token and transmits its data. Only the station that has
possession of the token can send data. The station that has the token can only
transmit one packet; then, it must release the token. The token handling process is
different depending upon the transmission medium.