Priority Scheduling inWireless 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 802.11 DCF

• Modify backoff stage of 802.11

• Modify inter-frame space stage of 802.11

Contention Using Blackburst

• Station transmitting longer burst wins

• Longer bursts for high priority packets

A B C

Contention Using Blackburst

• Hidden terminals may not sense the blackburst

A B C

Variations on IEEE 802.11

Figure source: IEEE 802.11 standard spec.

Modifying Backoff Stage of 802.11

• 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

0 1 2 3

High priority flow Low 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

0 1 2 3

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

Del

iver

y R

atio

of

Hig

h P

rior

ity P

acke

ts

PMAC BTPS

IEEE 802.11

Aggregate Throughput (Kbps)

Number of high priority flows

Agg

rega

te T

hrou

ghpu

t (K

bps)

PMAC

BTPS

IEEE 802.11

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 packetsRandom Topology

Number of nodes

Del

iver

y R

atio

of

Hig

h P

rior

ity P

acke

ts

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