CMPE 257: Wireless and Mobile Networking SET 3f:

1
CMPE 257 Wireless and Mobile NetworkingSET 3f

• Medium Access Control Protocols

2
MAC Protocol Topics

• Fair medium access and fair scheduling (queueing)
• MACAW
• Topology independent fair queuing

3
The Fairness Problem

• Unequal opportunity to access the channel
• Severe throughput degradation
• Causes
• Binary exponential backoff (BEB)
• Location dependent contention

Node B
Node C
Node D
Node A
Example Flow CD will capture the channel.
4
Prior Work on Fairness

• Max-min fairness
• Reduce the ratio between max throughput and min
  throughput of flows
• Backoff or dynamic adjustment of channel access.
• Fair queuing (FQ) approach
• Adapt wireline FQ disciplines to ad hoc networks.
• Flow contention graph as a useful abstraction
• Flows are tagged as either leading or lagging,
  and then backoff window is adjusted accordingly.

5
MACAW (BDSZ94)

• One of the earliest work on throughput and
  fairness enhancement
• Note Packet sensing (not carrier sensing) is
  used!
• Per-node and per-stream fairness
• Maintain backoff windows for different streams
  (also appears in IEEE 802.11e)
• Backoff copy and MILD backoff
• Multiplicative increase, linear decrease to
  address the fairness problem

6
Topology Independent Fair Queuing Design Goals
LL00, LL05

• QoS Support for Advanced Applications in Ad Hoc
  Wireless Networks

• Fair Allocation of Channel Bandwidth

• Maximal Channel Utilization under Fairness
  Constraint
• Communication Intensive Applications
• Limited-bandwidth Wireless Channel

• Distributed Packet Scheduling Design

7
Outline

• Issues in Ad Hoc Wireless Fair Queueing
• Fair Queueing Model for Packet Scheduling
• An Idealized Centralized Algorithm
• A Distributed Implementation
• Performance Evaluation
• Conclusion and Future Work

8
Network Model

• A Single Shared Physical Channel
• Collision Receiver in Transmission Range of
  More than One Transmitting Node
• Flow Stream of Packets from Source to
  Destination ltSender, Receiver, Flow_IDgt
• CSMA/CA MAC Framework

9
Design Issues
Spatial Reuse

• Location Dependent Contention Channel Reuse

10
Design Issues
F3
F1
F2
No Spatial Contention
No Spatial Reuse
11
Flow Contending Graph
F3
F4
F1
F2
12
Design Issues (contd)

• Inherent Conflict between Achieving Fairness
  Maximizing Channel Utilization

• To Maximize Channel Utilization

13
Design Issues (contd)

• Distributed Nature of Packet Scheduling in Ad Hoc
  Wireless Networks
• Unlike Wireline or Packet Cellular Networks. NO
  Single Logical Entity for Scheduling
• NO Direct Access to All Contending Flow Info
• Provide QoS at Finest Time Scale (Packet Level)

14
Solution Space

• Maximize Channel Utilization Always
• Schedule Largest Number of Non-conflicting Flows
• Starvation of Certain Flows
• Ensure Fairness
• Maximize Spatial Channel Reuse Subject to
  Fairness Constraint

15
Enable Spatial Channel Reuse
F1.1
F2.2
F2.1
F3.1
F4.1
F1.2
F3.2
F4.2
16
Look Ahead Window
F1
F2
FQ Tagging
F1.1
F2.2
F2.1
F3.1
F4.1
F1.2
F3.2
F4.2
1
2
3
4
5
6
7
0
Wireline FQ
F4
F3
F1.1
F2.2
F2.1
F3.1
F4.1
F1.2
F3.2
F4.2
FQ with Lookahead
F2.1 F4.1
F1.2 F3.2
F2.2 F4.2
17
Minimum Graph Coloring

• Maximizing Spatial Reuse within a Look Ahead
  Window is a Minimum Coloring Problem

18
Dynamic Graph Coloring

• Look Ahead Window Moves Forward
• Balance Two Design Goals
• Transmit Current Window of Bits ASAP
• Move Window Ahead AFAP

19
An Adaptive Algorithm

• Two Key Components
• A Basic Scheduling Loop
• Adaptive Dynamic Coloring

20
Basic Scheduling Loop

• WFQ to Assign Flow Tags
• V(t) Smallest Finish Tag
• A Lookahead Window of Bits V(t),V(t)
• Partition Flows into Disjoint Sets
• Schedule the Set with Least Tagged Flow
• Move Window Forward to Next Least Tagged Flow

21
Adaptive Dynamic Coloring

• As Lookahead Window Moves from V(t-1),V(t-1)
  to V(t),V(t)
• Retain Disjoint Sets of Unserved Packets in
  V(t-1),V(t-1)
• Merge Newly Joined Packets, Create New Set if No
  Merge Possible
• Retain All Disjoint Sets until t1

22
Properties of Central Algorithms

• Number of Disjoint Sets Non-increase
• Adaptively Reduce Total Number of Disjoint Sets
• Move the Window Forward
• Fairness Guarantee
• Spatial Channel Reuse
• Throughput Delay Bounds

23
Distributed Implementation

• Approximate Central Algorithm A Back-off Based
  Approach
• Within CSMA/CA MAC Framework
• Approximate WFQ with Modified WRR
• Backoff-based Implementation of Largest-degree
  First (LF) Coloring Algorithm
• Backoff-based Implementation of Adaptive
  Algorithms

24
An Example

• Adaptive Dynamic Coloring

• Partition Flows into Disjoint Sets

25
An Example

• Largest Degree First
• Set Backoff Flow_Degree

F0
F3
(3)
F1
(4)
F4
F1
F2
(5)
F3
F5
F0
(2)
(2)
F4
F2
(2)
F5
Time
26
An Example
F0
4
F3
1
F1
F4
F1
F2
4
3
F3
F5
F0
2
F4
F2
4
F5
Time
27
An Example
FQ Tagging
F0.1
F4.1
F3.1
F2.1
F1.2
F4.2
1
2
3
4
5
6
7
0
Wireline FQ
F1.1 F5.1
F0
4
F3
1
F1
F4
F1
F2
4
3
F3
F5
F0
2
F4
F2
4
F5
Time
28
An Example
FQ Tagging
F3.1
F2.1
F1.2
F4.2
1
2
3
4
5
6
7
0
Wireline FQ
F1.1 F5.1
F4.1 F0.1
F0
4
F3
1
F1
F4
F1
F2
4
3
F3
F5
F0
2
F4
F2
4
F5
Time
29
Other Issues

• Detailed MAC Layer Design
• CSMA/CA Paradigm
• Global Flow Information (i.e. Flow Weights)
  Propagation
• Conflict-free Multicast Tree

30
Simulation Example

• 17 flows
• 15 source nodes
• Simulation slots 100,000
• Poisson MMPP Traffic

31
Simulation Example

• Normalized Throughput

Normalized Throughput
1.05
1
0.95
Throughput
0.9
0.85
0.8
0
2
4
6
8
10
12
14
16
Flow ID
32
Average Delay

• Average Delay

Average Delay
4
3.5
3
2.5
Delay
2
1.5
1
0.5
0
0
2
4
6
8
10
12
14
16
Flow ID
33
Convergence of Adaptive Coloring

• Numbers of Disjoint Sets

34
Related Work

• Distributed Fair Queuing
• Adapt Fair Queuing Algorithm to Wireless LAN
• Distributed Fair Scheduling
• Backoff Based on Virtual Time
• No Explicit Consideration of Spatial Channel
  Reuse
• Distributed Fair Scheduling in a Wireless LAN,
  by N. Vaidya, P. Bahl S. Gupta, MOBICOM 2000
• Multihop Packet Scheduling
• Focus on Resolving Conflict between Fairness
  Maximal Channel Utilization
• Per-flow Service is not Fair
• A New Model for Packet Scheduling in Multihop
  Wireless Networks by H. Luo S. Lu, MOBICOM 2000

35
References

• BDSZ94 Bharghavan et al., MACAW Media Access
  Protocol for Wireless LANs, in ACM SIGCOMM 1994.
• LL00 H. Luo and S. Lu, A Topology-Independent
  Fair Queueing Model in Ad Hoc Wireless Networks,
  in IEEE ICNP 2000.
• LL05 H. Luo and S. Lu, A Topology-Independent
  Wireless Fair Queueing Model in Ad Hoc Networks
  2005, in IEEE JSAC 2005. (Extended version of
  LL00).

36
Acknowledgments

• Parts of the presentation are adapted from the
  following sources
• Prof. Luos ICNP 2000 presentation