Priority Scheduling in Wireless Ad Hoc Networks Xue Yang and NitinVaidya University of Illinois at Urbana-Champaign

Problem Definition High priority & low priority packets High priority packets should be transmitted before low priority packets Packets may reside at different nodes in ad hoc network

Related Work Reservation Based Schemes –Reserve channel for high priority in advance Contention Based Schemes [Aad01] –High priority source stations occupy channel with blackburst while contending. –Simple variations of IEEE DCF Modify backoff stage of Modify inter-frame space stage of

Contention Using Blackburst Station transmitting longer burst wins Longer bursts for high priority packets ABC

Contention Using Blackburst Hidden terminals may not sense the blackburst ABC

Variations on IEEE Figure source: IEEE standard spec.

Modifying Backoff Stage of Backoff intervals for low priority packets biased to be larger than high priority packets Priority reversal problem

Priority Reversal Station 1 (H) Station 2 (H) Station 3 (L) Station 4 (L) High priority backoff in [0-15] Low priority backoff in [16-31]

Modifying Inter-Frame Space Low priority source stations use larger inter-frame space (IFS) than high priority stations PMAC [Aad01] –CWh: maximum contention window for high priority –LIFS (HIFS): IFS of low (high) priority stations. –LIFS = HIFS + CWh Low priority packets wait too long when no high priority packets

Summary of Issues

Priority Scheduling in Multi-hop Networks 0123 High priority flowLow priority flow Impact of hidden terminals on priority scheduling Station 0 is hidden from 2  need some mechanism to make station 2 aware of high priority packets at station 0

Priority Scheduling in Multi-hop Networks High priority flow Low priority flow 4 Collisions among high priority packets increases priority reversals

Trade-Off Some existing schemes trade priority scheduling with throughput for low priority flows –Example: LIFS = DIFS + CWh Can we improve on this?

A Plausible Solution

Busy Tone Priority Scheduling (BTPS) Allows two priority classes Two busy tone used to inform hidden terminals BT1 and BT2

Busy Tone Priority Scheduling (BTPS) High priority source stations send BT1 pulse every M backoff slots

Busy Tone Priority Scheduling Stations sensing BT1 –Low priority stations defer transmission for a short duration –Transmit BT2 pulse  propagate BT1 to hidden terminals Busy Medium

Busy Tone Priority Scheduling The IFS of low priority source stations DIFS + M slots.  access priority to high priority

Busy Tone Priority Scheduling On receiving BT2, Low priority stations defer transmission for a short duration

Busy Tone Priority Scheduling (BTPS) Busy tones inform hidden terminals of pending high priority packets Low priority stations defer transmission after BT1 and BT2 for a short duration (= DIFS + M slots)  allows high priority packets to be transmitted first

Simulations

Fixed Simulation Topology Number of high priority flows is increased from 1 to 6. Bit rate 2 Mbps.

Conflict Graph Flows 5 and 8 have the highest contention degree, while flows 1, 3, 10 and 12 have the lowest contention degree

Delivery Ratio of High Priority Packets Number of high priority flows Delivery Ratio of High Priority Packets PMAC BTPS IEEE

Aggregate Throughput (Kbps) Number of high priority flows Aggregate Throughput (Kbps) PMAC BTPS IEEE

Delivery Ratio of individual flows with six high priority flows (4,5,6,7,8,9) Note flows 5 and 8 which have the highest contention degree.

Random Topology Number of nodes between 10 to 80, randomly located in a 1000m x 1000m rectangle The flows are randomly picked between two nodes within transmission range. Number of flows between 7 to 73. –Number of high priority flows between 4 to 37.

Delivery ratio of high priority packets Random Topology Number of nodes Delivery Ratio of High Priority Packets BTPS PMAC

Aggregate Throughput (Kbps) Random Topology Number of nodes BTPS PMAC

Conclusion Advantages –BTPS provides channel access priority for high priority packets –In absence of high priority packets, BTPS does not degrade throughput for low priority unnecessarily Disadvantages –Protocol more complicated than desired –Uses busy tones