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collision 2
- Thread starter shay1
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2
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For Ethernet there are three types of collisions that will occur.
1) Preamble collision. Device A begins transmitting the preamble (alternating pattern of 1's and 0's, 64 bits in length) prior to sending the first bit of the destination address. Device B has not yet received A's preamble, and B begins transmitting. When A detects B's preamble, A asserts the collision signal at the AUI and transmits a Jam signal. A then backs off based on the number of collisions and tries retransmitting.
2) Early Frame Collision. This type of collision occurs during the first 512 bits of the frame, after the preamble has been transmitted. The transmitting device acts just like A in the example above. The device will retransmit the frame withing a millisecond or two.
3) Late Frame Collision. This is known as a Late Collision and occurs after the first 512 bits of the frame have been transmitted. The transmitting device detects the collision, asserts the collision signal on the AUI and aborts transmission of the frame. The aborted frame will have a bad CRC in the FCS field. The Data Link layer of the transmitting device will not retransmit the frame. Instead, an upper layer such as TCP must detect the frame was lost and retransmit. This can result in a delay of 200ms to 3 seconds. These can be the result of a full duplex device being plugged into a hub.
Hope this helps.
Mike
1) Preamble collision. Device A begins transmitting the preamble (alternating pattern of 1's and 0's, 64 bits in length) prior to sending the first bit of the destination address. Device B has not yet received A's preamble, and B begins transmitting. When A detects B's preamble, A asserts the collision signal at the AUI and transmits a Jam signal. A then backs off based on the number of collisions and tries retransmitting.
2) Early Frame Collision. This type of collision occurs during the first 512 bits of the frame, after the preamble has been transmitted. The transmitting device acts just like A in the example above. The device will retransmit the frame withing a millisecond or two.
3) Late Frame Collision. This is known as a Late Collision and occurs after the first 512 bits of the frame have been transmitted. The transmitting device detects the collision, asserts the collision signal on the AUI and aborts transmission of the frame. The aborted frame will have a bad CRC in the FCS field. The Data Link layer of the transmitting device will not retransmit the frame. Instead, an upper layer such as TCP must detect the frame was lost and retransmit. This can result in a delay of 200ms to 3 seconds. These can be the result of a full duplex device being plugged into a hub.
Hope this helps.
Mike
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- #4
Thanks,mpennac
but according to your answered the collision can occur on the last 4 bytes of the frame .
If the station transmitted 64 bytes ( the minimum )
and the station gets a collision on the last 4 bytes ( CRC)
the station must re transmit the frame again am I right ?
it is not late collision !!!!
but according to your answered the collision can occur on the last 4 bytes of the frame .
If the station transmitted 64 bytes ( the minimum )
and the station gets a collision on the last 4 bytes ( CRC)
the station must re transmit the frame again am I right ?
it is not late collision !!!!
The last 4 bytes of the minimum frame is the CRC/FCS field. So, if the station is transmitting the CRC/FCS field (Bytes 61 - 64) and receives a collision, it should retransmit the frame at a Data Link layer, with only a minimal delay. This would be a fragment, a collision after we have started transmitting data. As opposed to a preamble collision.
If the last bit of the 64th byte is transmitted by sender and the collision occurs within a hub along the path, the sender would not know. As a result, the upper layer would have to wait the retranmission timeout before resending the frame.
Here is one way I figure out which one it is.
If the Data Link layer retransmits, both the collision frame and the retransmitted frame will have the same IP identification number in the IP header. If the sender didn't know the frame suffered a collision, the retransmitted frame will have a different IP identification number.
You are correct in your statement. If the collision occurs during the last 4 bytes of a 64 byte frame and the sending host detects the collision, it is not a late collision.
This is why I am thankful for switches and full duplex, so we don't have to deal with these stinking collisions!
Sorry about the long winded answers....
Mike
If the last bit of the 64th byte is transmitted by sender and the collision occurs within a hub along the path, the sender would not know. As a result, the upper layer would have to wait the retranmission timeout before resending the frame.
Here is one way I figure out which one it is.
If the Data Link layer retransmits, both the collision frame and the retransmitted frame will have the same IP identification number in the IP header. If the sender didn't know the frame suffered a collision, the retransmitted frame will have a different IP identification number.
You are correct in your statement. If the collision occurs during the last 4 bytes of a 64 byte frame and the sending host detects the collision, it is not a late collision.
This is why I am thankful for switches and full duplex, so we don't have to deal with these stinking collisions!
Sorry about the long winded answers....
Mike
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