optional · CSS study notes
Multiple Access Protocols
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OSI Layer: Data Link Layer (L2) — Medium Access Control (MAC) sublayer
Definition
Multiple access protocols govern how multiple hosts sharing a single broadcast communication channel (e.g., a wireless medium, or a classic shared Ethernet segment) decide when each may transmit, so their signals don't destructively collide and the shared medium is used efficiently.
Why Needed
On a broadcast link, if two or more stations transmit simultaneously, their signals interfere and both transmissions are typically lost — a collision. Without a coordination scheme, a shared channel degrades badly as the number of active stations grows.
Three Broad Categories
1. Channelization / Partitioning protocols
Divide the channel's resource in advance — via TDM, FDM, or CDMA — so each station gets a private slice with no possibility of collision, but the resource is wasted if a station has no data during its allotted slice. (See Multiplexing under Physical Layer Functionalities.)
2. Random Access Protocols
- Pure ALOHA — a station transmits whenever it has data, with no coordination; if a collision occurs (no acknowledgment received), it waits a random backoff time and retransmits — simple but inefficient (max ~18% channel utilization)
- Slotted ALOHA — time is divided into discrete slots and a station may only begin transmission at the start of a slot, roughly doubling maximum utilization to ~37% by reducing the "vulnerable period" for collisions
- CSMA (Carrier Sense Multiple Access) — a station first "listens" to check if the channel is idle before transmitting ("sense before send") — much better than ALOHA's blind transmission, though collisions can still occur due to propagation delay
- CSMA/CD (Collision Detection) — used in classic wired Ethernet: a station keeps listening while transmitting, and if it detects a collision, it immediately stops, sends a jam signal, and backs off for a random time before retrying (binary exponential backoff)
- CSMA/CA (Collision Avoidance) — used in wireless LANs (Wi-Fi/802.11), where collision detection is impractical (a station can't reliably listen while transmitting on a wireless medium, and the hidden terminal problem exists); instead it reduces collision probability using carrier sensing, inter-frame spacing and random backoff. Optional RTS/CTS can help with hidden terminals; it is not mandatory for every frame
3. Controlled Access Protocols
- Reservation — stations reserve channel time in advance before actually transmitting
- Polling — a designated primary station polls each secondary station in turn, granting permission to transmit only when polled — avoids collisions entirely but adds polling overhead and a single point of failure at the primary
- Token Passing — a special frame (the token) circulates among stations in a logical ring; only the station currently holding the token may transmit — used in Token Ring/Token Bus LANs — guarantees fairness and a bounded access delay, at the cost of token-management overhead (token loss/duplication must be handled)
Comparison Table
| Protocol | Coordination | Collision Handling | Typical Use |
|---|---|---|---|
| Pure ALOHA | None | Detect after transmit, random backoff | Historical satellite/radio |
| Slotted ALOHA | Time slots | Reduced vulnerable window | Historical satellite systems |
| CSMA | Sense before send | Reduced, not eliminated | General shared media |
| CSMA/CD | Sense + detect while sending | Detect + jam + backoff | Classic wired Ethernet |
| CSMA/CA | Sense, wait and random backoff | Collision probability reduced; optional RTS/CTS | Wi-Fi (802.11) |
| Token Passing | Token ownership | Eliminated (no contention) | Token Ring/Token Bus |
CSS Exam Angle
- A commonly tested topic — expect "differentiate CSMA/CD and CSMA/CA" as a direct short question, since it links wired vs wireless LAN design
- Know why CSMA/CD doesn't work well for wireless (can't detect collisions while transmitting, plus the hidden terminal problem) — this reasoning is what separates strong answers
- Cite Pure vs Slotted ALOHA utilization figures (~18% vs ~37%) as a quick, memorable numeric fact
Example
A modern Wi-Fi router's channel is shared by every connected phone and laptop; because a wireless card cannot reliably "listen while talking" the way a wired Ethernet card can, 802.11 devices use CSMA/CA with sensing, spacing and random backoff; optional RTS/CTS helps with hidden nodes. Collisions can still occur. CSMA/CD applies to legacy shared half-duplex Ethernet, not modern full-duplex switched Ethernet.
Likely Questions
- Explain the need for multiple access protocols on a shared/broadcast channel.
- Differentiate CSMA/CD and CSMA/CA, and explain why wireless LANs cannot use CSMA/CD.
- Compare Pure ALOHA and Slotted ALOHA in terms of channel utilization.
- Explain token passing as a controlled-access protocol, with its advantages and drawbacks.
📜 Past-Paper Questions & Structured Answers
Coverage check: no standalone question on ALOHA, CSMA/CA, TDMA or FDMA as a standalone topic was located in the reviewed 2016–2026 descriptive papers or the available 2024–2026 Paper-II MCQ sheets. This is not proof that it has never appeared. Questions labelled “Likely” above are practice, not verified past-paper questions.
Relevant preparation: The 2017 Ethernet question includes legacy CSMA/CD: see LAN Technologies. The 2019 multiplexing answer in Multiplexing In UDP and TCP explains FDM/TDM/WDM/CDM; distinguish sharing one link’s capacity from deciding which station may transmit.