Distributed Priority
Scheduling and
Medium Access in Ad
Hoc Networks
Vikram Kanodia
E.C.E Rice Univ Houston TX
Chengzhi LI
C.S Univ of Virginia
Asutosh Sabharwal,Bahareh
Sadegi,Edward Knighty
E.C.E Rice Univ Houston
Presented by Abhijit Pandey
Outline
• Introduction
• Distributed Priority Scheduling
• Multi Hop Co-ordination
• Related Work and Conclusion
Key insight
• To Utilize the broadcast nature of the
medium
• Store and Forward nature of Multihop 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 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
Methodology
• 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 802.11
• 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 scheduling
Co-ordination decreases the average delay
by 60% as compared to 802.11 and 25%
as compared to distributed priority
scheduling without co-ordination.
Scheduling 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
Scheduling
• 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.
Mechanism
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
function
Distributed Co-ordination
function
• 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
Window
• 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
handshake, and the updating of scheduling
tables.
Priority 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 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
information
No of collisions versus
available information
Probability of correct scheduling vs. number of
nodes for different values of q.
q=1
q=.8
q=.6
q=.4
q=.2
q=0
Increase in probability of correct scheduling as
q increases
Significant gain even for lower values of q
Multi 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-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
schemes
• 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/K
Where D =end to end delay target
K= no of hops from routing table.
Probability of satisfying end-to-end delay target
under different priority schemes
Multi-hop coordination
IEEE 802.11
Single hop
scheduling
Simulated delay performance
of multi-hop coordination.
Conclusion
• 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
paper
• 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.