Topics in Distributed Wireless Medium Access Control Nitin Vaidya © 2010

Outline Service differentiation Rate control Power control Priority scheduling Throughput fairness Rate control Power control Impact of directional antennas

IEEE 802.11 Distributed Coordination Function (DCF) Physical & virtual carrier sensing (RTS-CTS) Contention window (cw) : Backoff chosen uniformly in [0,cw-1] Exponential backoff after a packet loss Contention window reset to CWmin on a success Inter-frame spacing: SIFS & DIFS

IEEE 802.11 DCF

Service Differentiation at MAC Layer Priority scheduling Throughput fairness

Priority Scheduling Priority assigned to packets Goal: When two packets are competing for transmission, packet with higher priority should “win” access first Let us consider two priority levels, high & low Ideas can be extended for more levels

Priority Scheduling Two packets at the same host Transmit low priority packets only if high priority queue empty Low priority High priority

Priority Scheduling Packets at the different hosts How to ensure that high priority packets will be transmitted first? Host A Host B High priority Low priority Low priority High priority

Priority Scheduling: Inter-frame Spacing Differentiation

Inter-frame Spacing Differentiation IFS for new high priority packet : DIFS IFS for new low priority packet: LIFS LIFS = DIFS + Bmax + 1 where Bmax is the largest backoff interval used by a high priority packet

Inter-frame Spacing Differentiation High priority Low backoff data DIFS Bmax LIFS D

Issues Low priority traffic waits for LIFS even when no competing high priority traffic from other hosts Different hosts maysee different channel conditions A host with high priority packet may observe channel busy, while another host may observe it idle  A low priority packet may be transmitted when a competing high priority packet is pending

Priority Scheduling: Backoff Interval Differentiation Different range of backoff intervals for packets of different priority Example: High priority: [0,H] Low priority: [H+1, L]

Priority Reversal May Occur: Different Packet Arrival Times Different hosts may start “counting down” backoffs at different times High priority

Priority Reversals May Occur: Collisions Priority Low High freeze backoff data X Y Z denotes backoff

Priority Reversals May Occur: Collisions Priority Low High freeze backoff data X Y Z Can we improve the situation by changing actions for low priority? denotes backoff

Throughput Fairness

IEEE 802.11 DCF “Symmetric” protocol All things being equal, different single-hop flows should achieve similar throughput In general, performance may not be “fair”

Potential Causes of “Unfairness” Short-lived flows: Random selection of backoffs may cause short-term unfairness Collisions & exponential backoff Spatial variations Number of flows per host Channel variations If host with poor channel transmits at high rat  packet losses If it transmit with low rate  uses up more channel time

What is Fair ? Due to channel variations, and differences in interference levels, it is not always reasonable to requires all flows to achieve same throughput Alternative definitions of fairness

Throughput Fairness Only one host may transmit to C reliably at any time 11 Mbps 2 Mbps

How to achieve (approximate) these rates? Adapt backoff to achieve this?

Service Differentiation: Throughput Fairness To be discussed later

Rate Control

Rate Control Consider node A sending packets to node B Choose rate based on channel conditions Better channel  Higher rate How to estimate channel conditions? Implicit versus explicit feedback

Implicit Feedback Ack received  Channel good enough for currently used transmission rate Ack not received  Chosen rate too high

Rate Adaptation with Implicit Feedback X = 3 , Y = 2

Rate Adaptation with Implicit Feedback initial Xi = 3 , Y = 2 Xi adapted dynamically

Explicit Feedback Receiver can send current SINR estimate along with an Ack Transmitter can choose rate based on SINR estimate Interaction with the NAV (virtual carrier sensing) mechanism

Power Control

Distributed Power Control Transmitter i transmitting to node ri Transmitter i need to achieve SINR

Iterative Algorithm Converges if all transmissions are feasible above desired SINR threshold

Power Control with Interference Margin Dissemination Parameter q A transmitting data to B: Interference margin at B = M Busy tone D power = q/M data B A C E

Power Control with Interference Margin Dissemination Parameter q Interference margin at B = M Busy tone D power = q/M data B A C Transmit power < q / Pb Received busy-tone power = Pb E

Analysis Interference by C at B = (q / Pb) gCB where Pb = (q/M) * gBC = M * gBC / gCB = M if gBC = gCB = Interference margin

Issues gBC and gCB may not be equal: different channels Multiple interferers

Diversity Multi-channel Multiple antennas Directional (beamforming) antennas

Directional Antennas

Issues Impact on virtual carrier sensing Even if C receives CTS from B, may be OK to transmit to D directionally Potential solution: directional NAV

Issues Impact on physical carrier sensing E cannot sense A’s directional transmission to B E might collide at B

Issues Interaction with collision avoidance A transmitting data to B Unaware that A is busy, E attempts to transmit to A, but no response is received E assumes collision, and performs exponential backoff Process may be repeated While E is counting down a long backoff, A may finish transmitting to B, count down a short backoff, and again begins transmitting to B  Short-term unfairness, or packet loss at B

Chapter Summary Service differentiation Rate control Power control Priority Fairness Rate control Power control Directional antennas