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Channel allocation problem

Short answer

The channel allocation problem is dividing a shared broadcast channel among competing users fairly with minimal collisions. Static assignment (FDM/TDM) wastes capacity when traffic is bursty; dynamic allocation (e.g., CSMA/CD) gives channel on demand.

Explain the channel allocation problem with example.

The channel allocation problem is how to divide a single broadcast channel among competing users so that each gets a fair share with the least collisions and highest efficiency. It is solved in the MAC sublayer.

Static allocation

The channel is divided into fixed portions by FDM or TDM and each of N users gets 1/N of the capacity. Example: a 100 Mbps channel is divided among 10 users, each gets 10 Mbps. If only 2 users have data, the other 8 portions are idle, but the 2 active users still get only 10 Mbps each. Delay also becomes N times larger: T_FDM = N·T. So static allocation wastes bandwidth with bursty traffic.

Dynamic allocation

The channel is given on demand, based on these assumptions:

  1. Station model: N independent stations, each generates frames, then waits until sent.
  2. Single channel: all stations use one shared channel.
  3. Collision: if two frames overlap in time, they are destroyed and must be resent.
  4. Time: continuous (frames start at any time) or slotted (only at slot boundaries).
  5. Carrier sense: stations can (CSMA) or cannot (ALOHA) sense whether the channel is busy before sending.

Examples of dynamic protocols: ALOHA, CSMA, CSMA/CD, token passing and reservation.

For instance, in 10 offices sharing one Ethernet cable, CSMA/CD lets any office use the full bandwidth whenever it is free instead of a fixed 1/10 share.

✓ Answer checked by Claude Opus 5.5 (high effort)

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