Preemptive Routing in Ad Hoc Networks

1
Preemptive Routing in Ad Hoc Networks

• Tom Goff
• MOBICOM01
• Presented by Se-Hoon Kim

2
Contents

• Introduction
• On-Demand Method
• Preemptive Routing
• Preemptive Region
• Relating the Preemptive Region to Signal Power
• Experimental Study
• Concluding Remarks

3
Introduction

• Pro-active routing (Table-driven)
• Configure the routes toward any node before
  packet transmission
• Traditional method for Wired Network
• DSDV, etc..
• On-Demand routing
• Find the route just before packet transmission
• Due to the high cost of wireless bandwidth
• DSR, AODV, etc...
• Trade-off btw Bandwidth and Packet Latency

4
On-Demand Method

• Route discovery
• By a node which has some packet to send
• Route reply
• By the destination node or an intermediate node
  which knows the route from itself to the
  destination node
• Route maintenance
• If link-error is detected, re-initiate the route
  discovery process

5
Preemptive Routing

• Main idea
• Pure on-demand routing protocols re-initiate
  route discovery only after detecting the link
  break
• This make worse to both average delay and delay
  jitter
• Detect the "likelihood of link break and
  re-initiate route discovery without waiting the
  link break
• Find a better path and switch to it
• How to detect
• Signal power
• Other ways may be used

6
Preemptive Routing

• Detecting by signal power
• Received signal power
• P0 transmitted power
• r distance between transmitter and receiver
• n2 near transmitter
• n4 near range limit
• Preemptive warning is generated when the signal
  power of a received packet drops below a
  preemptive threshold

7
Preemptive Region
8
Preemptive Region

• Choosing preemptive region
• Recovery time Trecover
• Time btw link break and new route finding
• Depends on the size and topology of the network
• Can be estimated, for example, by keeping simple
  average
• Warning interval Tw
• Time taken for a node to go across the preemptive
  region (with its maximum relative velocity)
• In ideal case, Tw Trecover
• Typical value
• Maximum velocity (of a node) 20 m/s
• Recovery time 0.1 sec
• Preemptive region Maximum range - 4 m

9
Relating the Preemptive Region to Signal Power

• We select n 4 cause preemptive region is near
  the maximum range
• Minimum signal power received at the maximum
  transmission range
• 3.6510-10 Watts for WaveLAN card
• rpreemptive range - vmax Tw

10
Relating the Preemptive Region to Signal Power

• Preemptive ratio
• For WaveLAN card, with range 250m and w4meter
• 1.07 and Ptreshold 3.9 10-10 Watts
• Mitigation of channel fading and multipath
  effects
• Maintaining an exponential average of signal
  power
• Sending a warning packet and receiving warning
  Ack (ping-pong)

11
Experimental Study

• Experimental assumption
• PDSR
• Preemptive Routing DSR
• Some modification to packet header and route
  discovery process
• Disable Route Cache
• Environment
• NS-2 simulator
• 35 nodes in 700 700 m2
• 10 sources transmit to 10 destinations with CBR 5
  packets/sec
• High Mobility 20 m/s
• Low Mobility 10 m/s
• 1 ? 2

12
Simulations
13
Simulations
14
Simulations
15
Simulations
16
Simulations
17
Concluding Remarks

• Present algorithms that initiates proactive path
  switches when the quality of path in use becomes
  suspect
• Eliminate the costs of detecting the failure and
  recovering from it

18
Appendix DSR

• Dynamic Source Routing
• Dynamic On-demand
• Source Routing
• Sender tells the entire route of its packet
  explicitly
• Properties
• On-demand No periodic announcement
• No routing table
• Sender must find the route

19
Appendix DSR

• Route Discovery (Simple)
• A host initiating a route discovery broadcasts a
  Route Request packet. Then it waits for a Route
  Reply packet
• If a host receives a Route Request packet, it
  conducts as below
• If the packet have been received already, just
  drop and do nothing
• O/w, if the address of itself is listed in the
  Route Record, just drop and do nothing
• O/w, if the target of the packet is itself, send
  a Route Reply packet with a route of Route Record
• O/w, append its address to the Route Record in
  the Route Request packet and re-broadcast the
  request
• When the original sender receives a Route Reply
  packet, it updates its Route Cache

20
Appendix DSR

• Route Request packet contains
• ltinitiator address, request idgt pair
• Destination address
• Route Record
• some more?
• Each node maintains
• Recent ltinitiator address, request idgt pairs
• Request id
• Route Cache
• some more?

21
Appendix DSR

• Route Discovery and Reply

E
D
C
B
A
G
F
22
Appendix DSR

• Route Maintenance
• Without periodic messages, Route information must
  become out-of-time
• If a host on the route detect a transmission
  problem for which it cannot recover, it send a
  Route Error packet to the original sender
• Data link level error reporting
• Passive acknowledgement
• Transport / Application level Ack
• Route Error packet contains the addresses of the
  hosts at both ends of the hop in error
• When a Route Error packet is received, the hop in
  error is removed from the original senders route
  cache and resend the Route Request packet

23
Appendix DSR

• Optimizations
• Full use of the Route Cache
• The intermediate nodes add Route Cache
• Use Route Cache instead of re-broadcasting the
  Route Request packet
• Use maximal number of hops for Route Request
  packet to propagate
• At first, set the limit one (i.e nonpropagating)
• Then set the limit maximal value (currently 10)
• Piggybacking on Route Discoveries
• Piggyback (small) amount of data on Route Request
  packet
• Especially useful for non-bidirectional link
• Reflecting Shorter Route
• Improved Handling of Errors
• Exponential backoff to limit the rate of new
  route discoveries
• Use Promiscuous mode to hear the errors and
  update the Route Cache