- The general CSMA protocol:
Explanation:
- When a node has a message
to transmit:
- The node will sense the (broadcast) transmission channel (= "carrier")
- If the (transmission) channel
is idle:
- The node
transmits
the message
(The message will be considered to be transmitted correctly because error rate is low)
Otherwise:
- The node will back off (= wait some time)
- retry after waiting the back off period
- The node
transmits
the message
- When a node has a message
to transmit:
- CSMA Variants:
- There are 3 different (variants)
of CSMA
(They all operate in a similar manner)
- The different variants of CSMA protocols differents in the back off algorithm used.
- There are 3 different (variants)
of CSMA
- The 3
types of CSMA protocols are:
- Non-persistent CSMA
(the "whimp")
- 1-persistent CSMA (the "stalker")
- p-persistent CSMA (the "gambler")
- Non-persistent CSMA
(the "whimp")
- Why are these protocols
called
Carrier Sense Multiple Access:
- Carrier sense:
- because the CSMA protocol requires nodes to senses the carrier (= transmission channel)
- Multiple Access:
- because the
CSMA protocol must
try multiple times before
it will succeed in
transmitting a message:
(= multiple access)
- because the
CSMA protocol must
try multiple times before
it will succeed in
transmitting a message:
- Carrier sense:
- Fact:
- CSMA uses
slotted transnissions to
improve
performance
(Slotted Aloha has better performance than unslotted Aloha !!!)
- CSMA uses
slotted transnissions to
improve
performance
- The generalized
slotted operation of CSMA:
- A transmission must
begin
at the start of a
(time) slot:
- However:
- A transmission can last longer (or shorter) than one time slot
Example:
- A transmission must
begin
at the start of a
(time) slot: