COMP90007 Chap.4 Shared Channels, Wireless and Ethernet
Shared Channels, Wireless and Ethernet
A broadcast channel needs a rule for deciding who transmits. Static allocation divides time or frequency in advance and is predictable, but unused allocations waste capacity when traffic is bursty. Dynamic allocation lets active stations compete or reserve access. Pure ALOHA sends immediately and retries after collision; slotted ALOHA restricts starts to slot boundaries.
Carrier-sense methods listen first, with persistent behaviour controlling the choice between immediate transmission and random deferral. Collision detection shortens a failed attempt by aborting once interference is noticed. Collision-free methods reserve turns, while limited-contention methods change group size so neither idle reservations nor collisions dominate.
Wireless carrier sensing is local, creating hidden and exposed terminal problems; an RTS/CTS exchange shifts attention toward activity around the receiver. Ethernet combines a frame format, addresses, a CRC and access behaviour. Classic shared Ethernet uses collision logic and a minimum frame length so a sender is still transmitting when a worst-case collision returns.
Switched full-duplex Ethernet separates collision domains and removes that contention requirement from each link.
What this chapter covers
- 01
Persistent CSMA
- 02
RTS/CTS
- 03
Switched Ethernet
- 04
Static time and frequency allocation
- 05
ALOHA, slotted ALOHA and carrier sense
- 06
Persistent CSMA and collision detection
- 07
Collision-free and limited-contention access
- 08
Hidden and exposed wireless terminals
- 09
Ethernet frames, collision domains and switches
Diagnose a hidden-terminal exchange
- 1A and C are hidden from each other because carrier sensing at either sender cannot reveal the other transmission.
- 1Their frames overlap at B, which is the location where the collision matters.
- 1An RTS from a sender asks B for permission, and B's CTS can be heard by stations near the receiver.
- 1A station hearing the CTS defers, reducing the hidden-terminal risk; the exchange adds control overhead and is not universal collision prevention.
Key terms
- Persistent CSMA
- A carrier-sense family in which the station's rule specifies when to transmit after finding the channel idle or busy.
- RTS/CTS
- Short request-to-send and clear-to-send control frames used to coordinate access around a wireless receiver.
- Switched Ethernet
- Ethernet in which switch ports form separate collision domains, enabling concurrent traffic and full-duplex links.
Shared Channels, Wireless and Ethernet FAQ
Why is static allocation weak for bursty traffic?
A reserved time or frequency segment cannot automatically be borrowed whenever its assigned station is idle, while active stations may need more than their fixed share. The stability is useful under regular load, but the mismatch wastes capacity when participation and demand vary.
Why does classic Ethernet need a minimum frame size?
Collision detection must occur while the sender is still transmitting. If a very short frame finished before a signal could travel across the network and a collision indication return, the sender might incorrectly treat the transfer as successful. The minimum length preserves the detection interval.
Exam move
For every access protocol, state five assumptions before judging performance: whether time is slotted, whether stations sense the carrier, whether load is light or heavy, whether a controller exists and where collisions are observable. For Ethernet, connect each field or timing rule to the failure it controls instead of memorising an isolated frame diagram.
Compare medium-access rules under a sequence of traffic regimes rather than declaring one universally efficient. Begin with sparse arrivals, then moderate contention and finally saturated demand.
For static time or frequency allocation, mark capacity reserved to idle stations; for Pure and Slotted ALOHA, mark vulnerable periods and successful retransmission opportunities; for carrier sense, distinguish what a station hears from what the intended receiver experiences. State the persistence rule and backoff assumption before predicting collisions.
Add a collision-free reservation method and a limited-contention grouping method to the same table, recording coordination overhead as well as collision cost. For wireless cases, draw sender, receiver and neighbouring stations with sensing and interference ranges.
Use separate traces for hidden and exposed terminals, and explain precisely how an RTS/CTS exchange changes the information available near the receiver without claiming that it solves every wireless loss. Treat Ethernet as an integration case. Label destination and source addresses, type or length, payload and CRC in the frame; then connect classic shared-medium collision detection to propagation time and minimum frame length.
Rework the diagram for a switched full-duplex link and identify which collision-domain assumption disappears while framing and forwarding functions remain. When a scenario changes from hubs to switches or from cable to radio, revise only the mechanisms affected by that change. End with an assumption audit covering offered load, synchronisation, sensing locality, propagation, duplex mode and switch behaviour.
A final recommendation should name the access objective—low delay, stable throughput, fairness or bounded coordination—and the observation that would cause a different rule to be preferred. When comparing throughput claims, retain the same offered load and timing assumptions across protocols. Otherwise an apparent improvement may come from changing the scenario rather than from the access mechanism itself.
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