Ethernet Frame

brAun

Hi guys,
i got 1 question about ethernet collision. this is seem a silly question. i really have doubt in this question.

2 hosts transmiting frame at same time, a collision has occurred. what will happen to the frame?
is the frame will be discarded or put in the queque sequences and retransmitting after a period of time?

thanks

EdTheLad

Nice question, the sender is listening for a carrier signal on the line.If no signal is present it sends.If by chance this happens with two machine simultaneously there will be a collision.Since the frame has already been partally or fully transmitted its discarded.
Its up to the transport layer to retransmit the data.
This is where TCP comes in.

milliamp

Is there a gray area here?

I know that an excessive amount of collisions will = packet loss, but I thought in the situation where a collision is detected in CSMA/CD the data link layer will wait a random amount of time and attempt to retransmit the frame?

eg. this is part of CSMA/CD, it will be handled similarly to if a signal was detected on the wire (sent later) rather than just have TCP compensate later on for a dropped/corrupt packet.

EdTheLad

Your right Milliamp,now that i think of it, it doesnt make sense to involve the upper layers on a single frame collision.I would image the sender will make a copy of every frame before it transmits.If a collision is detected it will retransmit its copy.If the collision is late and a couple of frames are transmited this is when the upper layers will get involved.
Since i havent seen this documented anywhere dont take this as bible,it just makes more sense to be implement this way.

johnnynodough

The entire segment will listen for a quiet ethernet, if the ethernet is clear, a frame will be sent. The sender will route a copy of the frame through its own loopback circuitry into its buffer. This way it can detect a collision, by comparing the original frame. Once this occurs the sender will send a Jam Signal on the ethernet segment and all hosts will set a random timer before attempting to retransmit. This is CSMA/CD and is only applicable on shared networks utilizing half duplex operation.

EdTheLad

Hey Johnny,what i've read from somewhere i cant remember where but there is no actual loopback circuitry,i read about this loopback circuitry in an rfc but it was just a concept of the author.What i have read since and makes more sense is that the sender monitors the voltage levels of the line and when the voltage level increases above a threshold due to the extra signal a collision is flaged.If anyone else has some more input regarding this fire away as i think its an interesting topic.

johnnynodough

http://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/csma-cd.html


Thats probably everything you ever wanted to know about CSMA/CD. You know what Ed? Sometime in the past I read about the loopback circuitry being used to detect collisions, but I cant find it now, all I can find is your comment regarding the current level on the line.

But, you cant use full duplex in a shared medium, you can only send or recieve at any given time, because the second transciver is used for the CSMA/CD process. I was alomst positive it was used to loopback the frame for comparison, but I could be wrong.

brAun

so the frame will be discarded after a collision is detected. the host(transmitter) will apply backoff algorithm for period of time and retransmit again?

is that the answer? sry i am just confused with the answer

Webmaster

brAun wrote:

so the frame will be discarded after a collision is detected. the host(transmitter) will apply backoff algorithm for period of time and retransmit again?

is that the answer? sry i am just confused with the answer

Both stations will apply the backoff algorithm. The result is a random time which means the two stations won't collide again when they try to retransmit.

Here's a quote from my Network+ TechNotes:

The access method – how the wire is accessed and signals are places on it – for Ethernet networks is Carrier Sense Multiple Access/Collision Detection (CSMA/CD). In a CSMA/CD network, a stations listen to check if the network is busy transmitting data before starting its own data transmission. If the network is free, the station transmits data. When two stations listen and both determine the network is not busy and start sending the data simultaneously, a collision occurs. When the collision is detected, both stations will retransmit the data after a random wait time created by a backoff algorithm.