- Fact:
- Collision will
render all (colliding) frame
to be lost:
- Collision will
render all (colliding) frame
to be lost:
- What to
do when a node
detects a
collision:
- Back off
(= wait) for
a random period
- 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 !!!
- Back off
(= wait) for
a random period
- Flow chart for CSMA/CD
protocol:
- Compare that to the
flow chart for CSMA:
- 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
- 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:
- 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:
- A (any) node
on the LAN
starts
transmitting
at time t:
- All nodes on the
LAN will hear
a node's transmission
after τ sec
after the start of
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...
- 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 !!!
- If 2 different nodes
picks
different random integers:
- Updated
(improved)
flow chart for
CSMA/CD:
