CS485 Sylabus

### Action taken after a positive collision detection --- the CSMA/CD protocol

• What happens after a node detects a collision ?

• Fact:

• Collision will render all (colliding) frame to be lost:

• What to do when a node detects a collision:

1. Back off (= wait) for a random period

2. Re-try again

Graphically:

Note:

 The random backoff period will help the nodes to avoid a new collision when they attempt to re-transmit their frames !!!

• The CSMA protocol with collision detection (CD)

• Flow chart for CSMA/CD protocol:

• Compare that to the flow chart for CSMA:

• The time unit used in the Back Off Algorithm in CSMA/CD

• Recall: End-to-end propagation delay

• End-to-end propagation dela = amount of time that it takes for electric sigmals to travel from one-end of the network to the other end of the network:

• Notation:

 τ = end-to-end propagation delay

• Property of the end-to-end propagation delay (τ)

• All nodes on the LAN will hear a node's transmission after τ sec after the start of the transmission

Graphically:

• A (any) node on the LAN starts transmitting at time t:

• At time (t + τ) sec, every node on the LAN will hear (= detect) the transmission:

• Back off time period:

 Back off time duration = r × τ          (r = a random number)

We choose τ to be the "time unit" of the back off algorithm

We see see the reason why next...

• The generic Back Off Algorithm in CSMA/CD

• A generic back off algorithm:

 ``` Pick a random integer r Wait r × τ seconds (Start the transmission (CSMA/CD) protocol from the start) ```

Important fact:

• If 2 different nodes picks different random integers:

 The back off periods used by these nodes will ensure (guarantee) the later starting node will hear (= detect !) the transmission of the earlier starting node when the later starting node re-tries its transmission !!!

This will allow the later starting node avoid colliding with the earlier starting node !!!

• Updated (improved) flow chart for CSMA/CD: