Larger SNR -> easier to extract signal from noise (good)
Wireless BER (bit error rates)
Higher than in wired networks
Errors often occur in bursts
Variers over time of connection
Hidden terminal problem
Imagine you have 3 devices: A, B, and C.
A & B can hear each other, B & C can hear each other
A & C cannot hear eachother, meaning A & C are unaware of their interference at B
Riddle me this: As a mobile node gets farther and farther away from a base station, how can we ensure that loss probability doesn't spike?
1) Increase transmission power
- Lots more battery consumption and increased radiation
2) Reducing transmission rate
- Introducing FEC (decreases good throughput, other physical layer reasons)
Wireless MAC
CDMA (Physical Layer Protocol)
Uses spread spectrum technology, to signal spread over frequencies
Highly resistant to interference
Supports large number of users
FHSS (Frequency hoping spread spectrum)
DSSS (Direct Sequence Spread Spectrum)
CDMA (Code Division Multiple Access)
802.111 CSMA/CA
CA: Backoff + ACK mechanism
No CD, because wireless nodes can't transmit and listen at the same time
Missing ACK is the only indicator of interference
802.111 uses Stop-and-wait
Modern 802.111 bundles multiple frames
Block ACK, bitmap-based, not cumulative, each bit on bitmap indicates success/failure of a specific frame
Sender only retransmits missing frames, and then a BlockAck Request
Receiver sends one Block ACK containing the bitmap