Distributed Priority Distributed Priority Scheduling and Scheduling and

Medium Access in Ad Medium Access in Ad Hoc NetworksHoc Networks

Vikram Kanodia Vikram Kanodia E.C.E Rice Univ Houston TXE.C.E Rice Univ Houston TX

Chengzhi LIChengzhi LIC.S Univ of VirginiaC.S Univ of Virginia

Asutosh Sabharwal,Bahareh Asutosh Sabharwal,Bahareh Sadegi,Edward KnightySadegi,Edward Knighty

E.C.E Rice Univ HoustonE.C.E Rice Univ Houston

Presented by Abhijit Pandey

OutlineOutline• Introduction

• Distributed Priority Scheduling

• Multi Hop Co-ordination

• Related Work and Conclusion

Key insightKey insight• To Utilize the broadcast nature of the

medium

• Store and Forward nature of Multi-hop network

• Communication and co-ordination of priority information among nodes

• Priority Backoff schemes to approximate the idealized schedule.

• Packet to satisfy end to end quality of service.

Distributed Priority SchedulingDistributed Priority Scheduling

• A technique that piggybacks the priority tag of a node’s head of Line packet onto handshake and data packets. RTS/Data

• By monitoring transmitted packets each node maintains a scheduling table into existing 802.11

• Scheduling Table is estimate of its relative priority into medium access control

MethodologyMethodology• Each node issues a Request to Send(RTS), it

piggybacks the priority index of its current packet

• A CTS granted contains priority index of its head of line of the data packet.

• This is inserted into the table of overhearing nodes.

• Each node assess the priority index of its own head of line packet, and with prioritized backoff schemes a distributed priority schedule is obtained

Improvement over 802.11Improvement over 802.11• Distributed Priority Scheduling

With probability q=60% of nodes overhearing, the mean delay is reduced from 2.86sec (802.11)to .6 sec

• Co-ordinated Multi hop schedulingCo-ordination decreases the average delay by 60% as compared to 802.11 and 25% as compared to distributed priority scheduling without co-ordination.

Scheduling AlgorithmScheduling Algorithm

In Ad-hoc networks to satisfy packet’s quality of service becomes increasingly difficult

Earliest deadline First• Packet has a priority index given by arrival

time plus its delay bound.• This priority can be maintained by base

stations.

Distributed Priority Distributed Priority SchedulingScheduling

• Packets are serviced in increasing order of priority index.

• In EDF a packet arriving at time t and having delay bound d has priority index t+d.

• A packet with size L with service rate r has a priority index of L/r.

MechanismMechanism

Due to distributed nature of ad hoc wireless networks

• Each node is equipped with its own buffer state and partial information about other nodes.

• The scheduler is distributed with incomplete system information

I.E.E.E distributed coordination I.E.E.E distributed coordination functionfunction

Distributed Co-ordination Distributed Co-ordination functionfunction

• If the channel is sensed idle for a duration of DIFS the node generates a random back off interval before transmitting the packet.

• The RTS/CTS have information regarding the destination node and the length of the data packet to be transmitted.

• Any other node which hears either the RTS or CTS can use the data packet length to update its network allocation vector containing the information of the period the network will remain busy

Backoff Timer Contention Backoff Timer Contention WindowWindow

• The backoff timer is chosen uniformly from the range[0, w-1] W is the contention window.

• At the first retransmission attempt w is set to CWmin

• After each unsuccessful transmission the value of w is doubled upto the max value CWmax= 2^mCWmin

Piggybacking on IEEE 802.11 four-way Piggybacking on IEEE 802.11 four-way handshake, and the updating of scheduling handshake, and the updating of scheduling

tables.tables.

Priority BroadcastPriority Broadcast• Hidden nodes which are unable to hear the RTS add

an entry in their scheduling table upon hearing the CTS

• The receiving node appends the priority in the CTS frame.

• Each node after hearing data packet adds another entry in its scheduling table.

• Upon successful transmission and Ack, each node removes the current packet from the scheduling table

Simulation ExperimentsSimulation Experiments• A single broadcast region with link

capacity 2Mb/s and data rate of 1.6 Mb/s

• Each node carries variable rate traffic according to exponential on-off model.

• Upon receiving a piggybacked RTS, a node enters the priority index into its local scheduling table with probability q.

Delay versus available Delay versus available informationinformation

No of collisions versus No of collisions versus available informationavailable information

Probability of correct scheduling vs. number of nodes for different values of q.

Increase in probability of correct scheduling as

q increases

Significant gain even for lower values of q

q=0

q=.6

q=.4

q=.2

q=.8

q=1

Multi Hop Co-ordinationMulti Hop Co-ordination• Downstream node can increase a

packet’s relative priority to make up for delays upstream

• Analytical model to study the probability of overhearing another packets priority index.

Multi Hop Co-ordinationMulti Hop Co-ordination• All nodes co-operate to provide end to end

service.• Priority expressed recursively.

• The index of each packet at its downstream node depends on its priority index at its upstream node.

• If a packet arrives early downstream node will reduce the priority of the packet and vice versa.

Priority Index assignment Priority Index assignment schemesschemes

• Time to Live allocation– Priority of packet increases with time spent in the

network– Flows can be differentiated by assigning different TTL’s

• Fixed Per node allocation– Each node has a certain fixed increment of priority

index.

• Uniform delay budget allocation– The increment of Priority index is D/KWhere D =end to end delay target K= no of hops from routing table.

Probability of satisfying end-to-end delay target Probability of satisfying end-to-end delay target under different priority schemesunder different priority schemes

Multi-hop coordination

IEEE 802.11

Single hop scheduling

Simulated delay performance Simulated delay performance of multi-hop coordination.of multi-hop coordination.

ConclusionConclusion• A scheme where priority index of head of line

packets is piggybacked onto existing messages.

• Downstream can make up for latencies upstream by multi hop co-ordination.

• Co-ordination an important ingredient for targeting end to End QOS.

• Moderate fraction of piggybacked message overhead

Important Aspects of this Important Aspects of this paperpaper

• This paper addresses three fundamental issues of providing Quality of Service in Ad-hoc networks

• 1 Distributed priority scheduling• 2 Priority based medium access.• 3 Multi-hop priority management.