Static channel assignment and CSMA/CD

Why static channel assignment is not efficient

In static assignment (FDM/TDM) the channel is split into N fixed parts, one per user.

• When a user is idle, its share is wasted and no other user can use it.
• Computer traffic is bursty (long idle periods with short peaks), so most reserved capacity is unused.
• Delay increases N times (T_FDM = N·T) compared with one user having the whole channel.
• It does not adapt when the number of users changes. Dynamic allocation protocols such as CSMA/CD avoid this waste.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

Used in classic Ethernet (IEEE 802.3).

Operation

1. Carrier sense: before sending, a station listens to the channel.
2. If idle, it transmits (1-persistent: it sends immediately when idle). If busy, it waits until the channel becomes idle.
3. Collision detection: while transmitting, the station keeps listening. If the signal on the line differs from what it sends, a collision has occurred.
4. On collision, the station stops and sends a short jam signal so everybody knows.
5. Binary exponential backoff: after the nth collision, it waits a random number of slot times chosen from 0 to 2ⁿ − 1 (limit 2¹⁰) and tries again. After 16 attempts it gives up.

 Ready --> Sense --busy--> wait
             |idle
          Transmit --collision--> Jam --> Backoff --> Sense
             |no collision
           Done

Minimum frame size: the frame must be long enough that the sender is still transmitting when a collision signal returns, i.e. transmission time ≥ 2 × propagation delay. Ethernet uses a 64-byte minimum frame.

CSMA/CD works well for low and medium load, but has performance loss at heavy load due to many collisions.