medium access control

Why is medium access control needed?

If access to medium is not regulated, then multiple stations may transmit simultaneously resulting in collisions and retransmission.

There will be little or no throughput on the network.

Access to the medium could be either in a random or controlled manner.

Classify the various protocols used for medium access.

Random access		Controlled access	Channelization
Aloha		Reservation	FDMA
CSMA		Polling	TDMA
o	CSMA/CA	Token passing	CDMA
o	CSMA/CD

Explain CSMA medium access technique.

In Carrier Sense Multiple Access (CSMA), each station first checks state of the medium using one of the persistence methods before sending.

The possibility of collision still exists because of propagation delay. When a station sends a frame, it takes time for the first bit to reach every station.

1-Persistent

When a station finds the line idle, sends its frame immediately (with probability 1).

This method has the highest chance of collision because two or more stations may find the line idle and send frames immediately.

Non-persistent

When a station senses the line to be idle, it sends immediately.

If the line is not idle, it waits a random amount of time and then senses the line again.

This approach reduces the chance of collision because it is unlikely that two or more stations will wait the same amount of time and retry to send simultaneously.

However, it reduces the efficiency because the medium remains idle when there may be stations with frames to send.

p-Persistent

This method is used if the channel has time slots with slot duration equal to or greater than the maximum propagation time.

It reduces collision and improves efficiency as follows:

1.      With probability p, the station sends its frame.

2.      With probability q = 1 – p, the station waits for the beginning of the next time slot and checks the line again.

a.       If the line is idle, it goes to step1.

b.      If the line is busy, back off procedure is adopted.