Distributed Distributed Priority Scheduling Priority Scheduling and Medium Access in and Medium Access in Ad Hoc Networks Ad Hoc Networks Vikram Kanodia Vikram Kanodia E.C.E Rice Univ Houston TX E.C.E Rice Univ Houston TX Chengzhi LI Chengzhi LI C.S Univ of Virginia C.S Univ of Virginia Asutosh Sabharwal,Bahareh Sadegi,Edward Asutosh Sabharwal,Bahareh Sadegi,Edward Knighty Knighty E.C.E Rice Univ Houston E.C.E Rice Univ Houston Presented by Abhijit Pandey
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Distributed Priority Scheduling and Medium Access in Ad Hoc Networks Distributed Priority Scheduling and Medium Access in Ad Hoc Networks Vikram Kanodia.
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Distributed Priority Distributed Priority Scheduling and Scheduling and
Medium Access in Ad Medium Access in Ad Hoc NetworksHoc Networks
• 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
• 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
• 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