Link-Layer Protocols, MAC, and Wireless Access
Local Access and Shared Media
The link layer problem
The link layer must frame packets, detect errors, arbitrate medium access, and often perform retransmission. In wireless environments, contention, interference, fading, and hidden-node effects make performance far less predictable than in wired Ethernet.
Access methods
| Method | Characteristic |
|---|---|
| CSMA/CD | Classic wired contention method with collision detection |
| CSMA/CA | Collision avoidance used in wireless systems |
| TDMA | Time is divided into scheduled slots |
| FDMA | Frequency resources are partitioned |
| OFDMA | Orthogonal subcarriers are shared among users |
Wireless-specific challenges
Which access method is most associated with wireless contention avoidance?
CSMA/CA is designed to reduce collisions in wireless environments where collision detection is difficult.
Correct answer: CSMA/CA
Why is collision detection less practical in wireless than in wired Ethernet?
This makes collision avoidance techniques more suitable in wireless systems.
Correct answer: Because a transmitter generally cannot reliably listen for collisions while transmitting, and received power from its own transmission overwhelms other signals.
Wired versus wireless links
Wired
- More stable channel conditions
- Typically lower bit error rates
- Collision detection can be feasible
Wireless
- Time-varying channel quality
- More interference and fading
- Medium access and rate adaptation are more complex
Designing a robust link-layer system
-
1
Characterize the channel impairments.
-
2
Choose framing and error-detection methods.
-
3
Select medium access and scheduling strategy.
-
4
Decide when to retransmit or adapt rate.
-
5
Measure interaction with higher-layer congestion control.
Cross-layer effect
Aggressive link-layer retransmissions can hide loss from transport protocols, but they may also inflate delay and confuse congestion inference.
What is the main purpose of rate adaptation in wireless links?
Rate adaptation adjusts modulation and coding to achieve a good reliability-throughput tradeoff under changing conditions.
Correct answer: To match transmission parameters to channel conditions