CSMA/CD and Collision Detection

ggauravr · February 2010

Plz someone clear me on this..
The system using CSMA/CD is essentially half-duplex.. but in order to sense collisions and send Jam signal ,the node that's transmitting should be hearing the network ,while transmitting..Doesn't it mean that the node is receiving and transmitting simultaneously ??

and if so how is it a half-duplex system ?? plz help .. thanks in advance ..

mikem2te · February 2010

ggauravr wrote: »

Plz someone clear me on this..
The system using CSMA/CD is essentially half-duplex.. but in order to sense collisions and send Jam signal ,the node that's transmitting should be hearing the network ,while transmitting..Doesn't it mean that the node is receiving and transmitting simultaneously ?? and if so how is it a half-duplex system ?? plz help .. thanks in advance ..

While the NIC will be transmitting and listening at the same time it does not mean it is a full duplex system. From memory a half duplex / CSMA the schematic looks something like-

Host 1        Host 2
TX------<>------TX
RX<-|        |-<RX

The Rx pins in the NIC are connected to the Tx pins. The listeneing(Rx) side of a NIC expects to hear the same signal the transmitting(Tx) side of a NIC actually sends, if the listening side recieves a different signal to what was sent it assumes there is a collision, then the NIC sends a jam signal and backs of. In effect there is only one pair of wires carrying signal so as a result a Cat5 Multi access network can only support one signal (half duplex) at once.

A full duplex system where there is only two nodes on a wire (point to point) there is no risk of collision, the transmitting pins on one NIC are connected to the receiving pins on another NIC and vice versa using two pairs of wires-

Host 1      Host 2
TX>----->----->RX
RX<-----<-----<TX

There is only one transmitting device on each wire inside the cable, hence no risk of collisions so therefore each NIC can both receive and transmit a different signal at the same time.

ggauravr · March 2010

thanks mikem ..but sorry.. im not yet clear bout wat u said .. i couldnt understand the figure.. Could u plz explain a bit more ?? thanks in advance..

ggauravr · March 2010

i just saw the figure in wendell odoms book.. and understood wat u said here..
thanks man..

mikem2te · March 2010

ggauravr wrote: »

i just saw the figure in wendell odoms book.. and understood wat u said here..
thanks man..

Cool. Ascii art is not my strong point.

ggauravr · March 2010

good one .. but thanks a lot ..

super22 · April 2010

who sends the jam signal? and who receives them? and who backsoff for a random time? is the backoff only for the nodes who is reponsible for the collision or is it everyone?

Forsaken_GA · April 2010

it works something like this -

all transmitting stations are monitoring for collision detects, and when it receives one, it continues transmitting for up to jamsize # of bits, this ensures that the collision lasts long enough so that everyone that was transmitting can hear it. the transmitting stations then terminate their transmissions and execute their random backoffs.

non-transmitting stations don't bother to execute the random backoffs, since they weren't transmitting. once the carrier sense on the line is clear (ie, no one is transmitting), they'll go ahead and transmit

CSMA/CD and Collision Detection

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