A Framework and Experimental Study for Discrimination of

Collision and Channel Errorsin Wireless LANs

What is Packet Differentiation?

Wireless Networks share a non deterministic medium:

Possible Problems:• Noise ⇒ discard the packet• Collision ⇒ discard the packet

Noise & Collision

Collision

Noise

Rate AdaptationPredict Channel Quality and choose the “right” rate/coding

Prediction based on Acknowledgments (ACKs):Successful Transmission ⇒ increase rateFailed Transmission ⇒ decrease rate

Why?Noise is the sole reason for transmission failure, because: • 802.11 -> 1997 – 1999• CSMA/CA -> collision avoidance

CSMA/CACarrier Sensing Multiple Access/Collision Avoidance

Avoidance: users perceive the medium differently

Backoff Timer: [0, CW]

“Packet Fair”

Problems:• too many nodes• hidden terminal • capture effect

Wrong Prediction, so what?Lower encoding = lower transmission rate

Lower throughput for the node itself Lower throughput for the SYSTEM

Price of “Fairness” – 802.11 Anomaly

1 Mbps 1 Mbps

54 Mbps

54 & 1 Mbps

How to differentiate?Packet Frame

FCS: Frame Check Sequence

Packet destroyed by Noise:

Packet destroyed by CollisionCollision scenarios:i) Visible Nodes

ii) Hidden Terminal

Packet destroyed by CollisionCollision scenarios in Hidden Terminal:

Capture Effect

• If Power of Packet A ≫ Packet B1. Packet A received correctly2. Packet B lost due to collision

The IdeaA more sophisticated checksum function

Smaller Segments (1500bytes/numOfSegs) ⇒

smaller CRC (16bits instead of 32) ⇒ small overhead in data transmitted and in CPU

usage

Why distributed?

The Idea 1.1Pr[Collision | Noise]: too noisy (P0)Pr[Noise | Collision]: small collision (P1)

Count the erroneous CRCs• big threshold ⇒ Po && P1• small threshold ⇒ Po && P1

ImprovementCount the successive erroneous CRCs

• lower threshold & more accurate prediction

Use the extra InfoThe receiver knows the cause of failure but the sender must be informed in time

Successful Transmission ⇒ ACK ⇒ rate

“Noisy” Transmission ⇒ NAK ⇒ rateCollision ⇒ - ⇒ -

NAK per node and when needed• Hardware limitations ⇒ Implemented in

software• Sent according to SampleRate specs

Why NAK on Noise?

Collision ⇒ at least 1/2 of the headers are destroyed

Receiver keeps per node statistics ⇒ header is needed ⇒ header small size (24bytes) ⇒ small possibility to be destroyed by noise

Testbed

Experimental Results

Congested Networks Hidden Terminal

Experimental Results

Capture Effect Capture Effect with Half-hidden Terminal