LinkLayer Salvaging for Making Routing Progress in Mobile Ad Hoc Networks

1
Link-Layer Salvaging for Making Routing Progress
in Mobile Ad Hoc Networks

• Authors Chansu Yu
• Kang G. Shin
• Lubo Song
• Adapted by Ting-Yu Lin

2
Overview

• Introduction
• System model problem statement
• Throughput analysis
• Proposed solution (MASA)
• Simulation
• Conclusions future work

3
Introduction

• Tradeoff between Spatial reuse and interference
• More spatial reuse means more concurrent
  communications
• But, it causes more interference
• MASA (Multiple Access with Salvation Army)
• Increases spatial reusability by encouraging more
  exposed terminals to attempt concurrent
  communications
• Alleviates interference influence by salvaging
  collided packets

4
Related Work

• Packet salvaging at network layer
• Packet salvaging in DSR
• Local repair in AODV
• Packet salvaging at MAC layer
• Extremely Opportunistic Routing (ExOR)
• Implicit Geographic Forwarding (IGF)

Salvaging might be less efficient
Use either link-state flooding or location
information

• MASA is a MAC layer solution with less overhead

5
Radio Propagation Model

• Two-ray ground propagation model
• Reception Model

CPj
CSj
TRj
i
j
6
Radio Propagation Model

• Two-ray ground propagation model
• Reception Model

CPj
CSj
TRj
i
j
Nodes in this area do not cause collisions
7
DCF of IEEE 802.11

• Carrier sense mechanism
• Hidden terminal problem
• Vulnerable Space (VS)
• Nodes within VS can not sense ongoing
  communication but can cause collision to
  receiver.
• Exposed terminal problem
• Wasted Space (WS)
• Nodes within WS can sense ongoing communication
  but will not cause collision to receiver.

8
DCF of IEEE 802.11

• Carrier sense mechanism
• Hidden terminal problem
• Vulnerable Space (VS)
• Nodes within VS can not sense ongoing
  communication but can cause collision to
  receiver.
• Exposed terminal problem
• Wasted Space (WS)
• Nodes within WS can sense ongoing communication
  but will not cause collision to receiver.

9
An Example of VS and WS

• 915 MHz WaveLAN radio hardware
• Transmission range
• 250m
• Carrier sense range
• 550m
• Communication distance
• 200m
• Capture zone
• 356m (when z010dB)

10
Problem Statement

• Carrier sense mechanism in this case not only
  reduces interference but also solves the hidden
  terminal problem (VS becomes very small).
• But it introduces another serious problem, the
  exposed terminal problem (large WS), which
  greatly reduces the spatial reusability.
• gt How does CS range affect the network capacity
  (maximum throughput)?

11
Throughput Analysis

• Distances from 6 interferers are
• D-d, D-d, D-d/2, D, Dd/2, Dd
• Distance between two adjacent senders satisfies

• Assuming the 6 interferers are all D apart from
  the receiver j, then

12

• Given an L? L square network
• If Dmin is the minimum D that satisfies the
  previous equation

• The max. number of concurrent successful data
  transfers in this square area is
• The average distance between a communicating pair
  in a square network is
• Then the average hop count will be
• Therefore, (b is the wireless channel bandwidth)
• or

13
Throughput Analysis (contd)

• Maximum Throughput in an L? L square network
• dcs lt Dmin, the senders must be separated by Dmin
  to reach the maximum overall network throughput.
  That is
• dcs Dmin the senders must be separated by dcs.
  The throughput is

14
Throughput Analysis (contd)

• How d and dcs affect Maximum Throughput?
• (b1Mbps, L10km, z010dB, ?4.0)

Total maximum end-to-end throughput (Mbps)
With short-distance communication, smaller CS
range achieves a higher throughput.
15
Proposed Solution MASA

• MASA (Multiple Access with Salvation Army)
• Adopts low CS range to reduce WS
• Encourages more concurrent communications
• More interferences and collisions
• Salvages collided packets
• Automatically breaks a long-distance link into
  two short-distance links, which are more robust
  to interference

16
MASA Algorithm
No collision
Collision at receiver
Salvation army
17
(No Transcript)
18
Benefits and Issues in MASA

• Benefits
• Makes progress toward the receiver
• Offloads senders pending packets as quickly as
  possible and thus, reduces queue size and packet
  delay
• Reduces false alarms for live link and thus,
  reduces routing overhead
• Issue
• Who to become the salvager?

19
Whore Eligible to Salvage?

• Must be a neighbor of sender
• Receive senders DATA packet
• Must be a neighbor of receiver
• Recognize collision at the receiver
• Should make progress
• Salvager is nearer to receiver than sender
• Should have no pending packets
• Too busy to help
• Balancing network traffic

20
Who is Elected among Candidates?

• Based on salvaging backoff
• A candidate gives up its salvaging activity when
  the packet is salvaged by another candidate
• Otherwise, it becomes the salvager and sends SACK

21
Simulation Environment
Simulator ns2 2.27 with modification for
cumulative interference and signal capture
22
Simulation with A Single Interferer

• Simulation scenario
• Simulation parameters
• Simulation time 180s
• Transmission range 250m
• Propagation model two-ray ground propagation
  channel
• Packets 512 bytes CBR or TCP packets
• Data rate 2Mbps

23
Simulation Results with CBR Traffic
(a) Direct scenario
(b) Salvaging scenario
Instantaneous throughput (kbps)
Instantaneous throughput ( kbps)
Packet salvaging is helpful to alleviate the
unfairness problem and to improve the performance.
24
Simulation with Multiple Interferers
25
Routing Overhead and Packet Queue Size
26
Simulation Results with Traffic Intensity and
Node Density
27
Conclusions and Future Work

• Conclusions
• MASA improves network performance regardless of
  mobility, scalability (traffic intensity and node
  density) and routing algorithms.
• In particular, it reduces packet delay
  significantly.
• MASA is more useful in dense networks and in
  delay-sensitive applications.
• Future Work
• Use deterministic algorithm to elect salvager.
• Apply MASA algorithm in sensor networks.

28

• THANK YOU !

29
Extremely Opportunistic Routing (ExOR)2
30
State-free Implicit Forwarding (SIF)3
31
Simulation Results with TCP Traffic
(a) Direct scenario
(b) Salvaging scenario
Instantaneous throughput ( kbps)
Instantaneous throughput (kbps)
32
Duplicate Packet Filtering

• If more than one node salvages the same DATA
  packet, how to suppress the duplicates?
• Exploit the existing packet filtering mechanism
  in 802.11 standard

DATA packet
SDATA packet
33
Simulation Results with DSR and Shadowing
Propagation Model