Distributed Priority Scheduling and Medium Access in Ad Hoc Networks

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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
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Outline

• Introduction
• Distributed Priority Scheduling
• Multi Hop Co-ordination
• Related Work and Conclusion

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Key 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

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• Priority Backoff schemes to approximate the
  idealized schedule.
• Packet to satisfy end to end quality of service.

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Distributed Priority Scheduling

• A technique that piggybacks the priority tag of a
  nodes 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

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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

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Improvement over 802.11

• Distributed Priority Scheduling
• With probability q60 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.

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Scheduling Algorithm

• In Ad-hoc networks to satisfy packets 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.

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

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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

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I.E.E.E distributed coordination function
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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

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Backoff Timer Contention Window

• The backoff timer is chosen uniformly from the
  range0, 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 2mCWmin

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

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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.

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Delay versus available information
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No of collisions versus available information
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Probability of correct scheduling vs. number of
nodes for different values of q.
q1
q.8
q.6
q.4
q.2
q0
Increase in probability of correct scheduling as
q increases Significant gain even for lower
values of q
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Multi Hop Co-ordination

• Downstream node can increase a packets relative
  priority to make up for delays upstream
• Analytical model to study the probability of
  overhearing another packets priority index.

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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.

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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 TTLs
• 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.

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Probability of satisfying end-to-end delay target
under different priority schemes
Multi-hop coordination
IEEE 802.11
Single hop scheduling
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Simulated delay performance of multi-hop
coordination.
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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

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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.