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
