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Bridges and CSMA/CD Networks

Suppose two LAN segments A and B connected to each other by a bridge. In the steady state (when bridge has learned the locations of the computers on both segments) the bridge will forward the frames from one segment to another segment if necessary. There is concept of parallelism, it means after bridge learns the locations of all the computers the two segments can be used simultaneously.

Suppose on segment A there is a communication going on among segment A’s computers and segment A is not idle, at the same time a computer on segment B senses the medium and found that the medium is idle and sends a frame to another computer which is on segment A. Now what happens? Bridge will receive the frame and now if bridge forwards this frame on segment A then obviously this results collision, Question arises is, Does bridge will store this frame in its memory and senses the segment A to become idle so that the frame can be transmitted?.

Second question, suppose a computer comp1 on segment A sends data to another computer comp2 on segment B, Comp1 senses the medium and finds that the medium is idle and sends data but at the same time comp2 senses the medium and finds that the medium is idle and sends data to comp1. Frame of comp1 and frame of comp2 both reaches the bridge at the same time, what the bridge will do? Is both the frames collide inside the bridge? Or bridge will store both the frames, which frame will be stored first? and if it stores which frame will be forwarded first ?

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2 Answers 2

Any bridge created in the last 10 years or so is smart enough to only retransmit packets when it is safe to do so. So yes, it does buffer the packet from Segment A for transmission onto Segment B. The size of this buffer varies by device.

Q1: The packet from Segment B will be stored until such time as it is safe to transmit on Segment A.

Q2: The bridge will relay the packets in near parallel. Both packets are received in the NIC buffer of each physical interface, and is then relayed up to the bridging layer. There is no collision. The worst case is a buffer overrun where a packet gets dropped due to lack of buffer space, which is a major fault for a bridge so the systems are seriously engineered to not do that. If they're part of the same TCP session, this may result in an out-of-order event, but that's beyond the scope of your question.

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Read about 802.1D . When a packet is received on a bridge port that is going to be forwarded, it follows the standard rules for doing so. That means it will not transmit the packet to the forwarded port until the medium on that port is idle.

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