Why Medium Access Control (MAC) Techniques Are Required

A LAN such as Ethernet or Wi-Fi is a broadcast channel: one shared medium carries the transmission of every station. If two stations transmit at the same time their signals collide and both frames are destroyed. Because there is no separate control channel to arbitrate use, the Medium Access Control (MAC) sublayer — the lower half of the data-link layer (OSI Layer 2) — must decide which station may transmit, and when.

The MAC sublayer is required to:

1. Arbitrate access and handle collisions so that overlapping transmissions do not waste the shared bandwidth.
2. Address frames on the shared medium — attach source and destination addresses so a frame is accepted only by the intended station.
3. Make the use of the channel fair and orderly — every station gets a chance to transmit and none can monopolise the medium.
4. Deal with the consequences of sharing — acknowledgements, retransmissions and inter-frame timing rules.

1. Static channel allocation — TDM and FDM

• The channel is divided in advance among the $N$ stations: in FDM each station gets a private frequency band; in TDM each station gets a private time slot (one slot carries one frame of that station).
• Advantage: simple, collision-free, constant delay.
• Disadvantage: an idle station still holds its slot/band, so bandwidth is wasted. LAN traffic is bursty (stations are idle most of the time), which makes static allocation inefficient; it suits constant-rate traffic such as telephony and FDM radio.

2. Token passing — controlled access

• A small frame called the token circulates logically; only the station currently holding the token may transmit. After sending, it passes the token to its neighbour (early or late release). Examples: IEEE 802.5 Token Ring, FDDI.
• Advantages: no collisions, deterministic worst-case delay (good for real-time traffic), fair access, and nearly 100 % efficiency under heavy load.
• Disadvantages: complex (token monitoring, insertion/removal, error recovery), slow under light load because the token must still travel, and costlier hardware.

3. CSMA/CD — contention (random access)

• A station with data first senses the channel (carrier sense); if it is idle the station transmits and keeps monitoring while transmitting. On detecting a collision it stops, sends a jam signal, waits a random backoff time (binary exponential backoff) and retries. This is the MAC protocol of classic Ethernet — IEEE 802.3.
• Advantages: very efficient for bursty traffic — no time is wasted when the channel is idle and there is no token overhead.
• Disadvantages: collisions still occur and efficiency falls as the load rises; the protocol also needs the minimum-frame/slot-time rule. It cannot be used in WLANs because a radio transmitter cannot listen while it is transmitting — hence 802.11 uses CSMA/CA.

Comparison

Technique	Example	Collisions	Efficiency at low load	Complexity
Static (TDM/FDM)	TDM telephony, FDM radio	None	Low — idle slots wasted	Simple
Token passing	802.5 Token Ring	None	Low — token circulation delay	High
Contention (CSMA/CD)	802.3 Ethernet	Possible, resolved by random backoff	Very high	Moderate