17
edits
More actions
no edit summary
No edit summary |
No edit summary |
||
|
Carrier Sense Multiple Access With Collision Detection (CSMA/CD)
-An adapter may begin to transmit at any time
-An adapter never transmits a frame when it senses that some other adapter is transmitting
-A transmitting adapter aborts its transmission as soon as it detects that another adapter is also transmitting
-Before attempting a retransmission, an adapter waits a random time that is typically small compared with the time to transmit a frame
How CSMA/CD works in a specific adapter
1. The adapter obtains a network-layer datagram from its parent node
-prepares an ethernet frame
-puts the frame in an adapter buffer
2. If the adapter senses that the channel is idle it starts to transmit the frame
-If it senses the channel is busy then it waits until it senses no signal energy and then starts to transmit
3. While transmitting the adapter monitors for the presence of signal energy coming from other adapters
-If the adapter transmits the entire frame without detecting signal energy from other adapters, the adapter is finished with the frame
4. If the adapter detects signal energy from other adapters while transmitting it stops transmitting its frame and instead transmits a 48-bit jam signal
5. After aborting the adapter enters an exponential backoff phase.
-After experiencing the nth collision in a row for this frame the adapter chooses a value for K at random from {0,1,2,...,2^m-1}
-where m=min(n,10)
-The adapter then waits K*512 bit times then returns to step 2.
Purpose of the jam signal is to ensure that all the other adapters are aware of the collision.
At 10Mbps a bit time is 0.1 microseconds.
The Ethernet standard imposes limits on the distance between any two nodes. These limits ensure that if adapter A chooses a lower value of K than all the other adapters involved in a collision, then adapter A will be able to transmit its frames without experiencing a new collision
Taking Turns Protocols: Most popular is Token Ring.
| |||