A Bidirectional Multi-channel MAC
Protocol for Improving TCP
Performance on Multihop Wireless Ad
Hoc Networks
Tianbo Kuang and Carey Williamson
Department of Computer Science University of Calgary
ACM MSWiM 2004
(Modeling, Analysis and Simulation of Wireless and
Mobile Systems)
Outline


Introduction
Bi-MCMAC


Simulations




Extensions
Throughput
Fairness
Transfer delay
Conclusions
Introduction

A well-known problem of multihop ad hoc
wireless networks is the hidden node problem

IEEE 802.11 MAC Protocol attempts to solve
the problem by using RTS/CTS handshake

But RTS collision and Exposed node problem are
not completely solved.
Introduction (cont.)

For a transport-layer protocol working above
RTS/CTS based protocol, the problems
described above will affect the network
performance
Problem description

Hidden node

When data packets travel in the same direction
transmission
RTS collision
1
sender
2
3
4
interference
5
receiver
hidden
Problem description (cont.)

Exposed node

When data packets travel in opposite direction
transmission
1
sender
2
RTS
3
interference
no transmission
4
5
receiver
Problem description (cont.)

Capture effect

Unfairness can occur between different TCP flows
hidden
transmission
RTS collision
1
2
3
4
5
sender
receiver
interference
sender
receiver
Related works

C. Cordeiro, S. Das, and D. Agrawal. COPAS: Dynamic
contention-balancing to enhance the performance of TCP over
multi-hop wireless networks. In Proceedings of ICCCN’02,
pages 382–387. Miami, FL, USA, October 2002.
Goal


Design a multi-channel MAC protocol to
reduce TCP DATA-DATA collision
Use bidirectional RTS/CTS channel
reservations to reduce TCP DATA-ACK
contention
Bi-MCMAC




Static multihop wireless ad hoc networks
One control channel, K-1 data channels
Single transceiver
Extends the RTS/CTS handshake to do the
bidirectional channel reservation

CRN (Channel Reservation Notification) control
frame
Bi-MCMAC (cont.)



Channel state is included in RTS/CTS frames
CRN frame is sent after the sender receives
CTS, containing the channel and reservation
duration information
Subsequent data frame sent by receiver is
indicated as MAC-layer ACK
Bi-MCMAC (cont.)
Announce
channel 7 and
CRN NAV
Channel: 1,5,7,11
Indicated as
MAC-layer ACK
Channel: 2,6,7,11
Choose 7
Extension

Head-of-Line (HOL) blocking



If the first packet in the buffer is not destined to
the sender
Per-neighbor queue
Multi-channel Hidden Terminal Problem

Receiver always selects the channel used for the
last successful transmission
Further work

Heterogeneous channel rates

If all the data channel are 54Mbps and the control
channel is 1Mbps, then the control channel may
become congested
Simulations




100 nodes
300 seconds, 50 repetition
Throughput
Fairness



Strict sense
General sense
Transfer delay



Chain topology
Grid topology
Random topology


Sparse (500*500)
Dense (250*250)
Environmental parameters
Data packet size: Ld
Control packet size: Lc
Maximum number of channels
should not exceed Ld/3Lc
Ex: 1000/3*64
Throughput in chain topology
Collisions in chain topology
Throughput in grid topology
Throughput in sparse random
topology
Throughput in dense random
topology
Fairness

Strict sense


Similar path and competition
General sense

Share the same channels
regardless of their local
contention
Jain’s Fairness Index (FI)
Strict sense fairness
General sense fairness in grid
topology
General sense fairness in sparse
random topology
General sense fairness in dense
random topology
Web transfer time
Web transfer time (cont.)
Conclusions

The Bi-MCMAC protocol is explicitly
designed to improve TCP performance over a
static multihop wireless ad hoc network

Bi-MCMAC extends IEEE 802.11 RTS/CTS
handshake to do bidirectional channel
reservations
Conclusions (cont.)


Subsequent data frame resolves the TCP
DATA-ACK problem
Simulations show that
Throughput improved
 Lower transfer delay
 Good fairness


Its unfairness in the single-cell case is a minor
disadvantage
Thank you !