Does the IEEE 802.11 MAC Protocol Work Well in Multihop Wireless Ad Hoc Networks?

Shugong XuTark Saadawi

June, 2001IEEE Communications Magazine

Prolog The IEEE 802.11 MAC protocol is the st

andard for wireless LANs; it is widely used in testbeds and simulations for wireless multihop ad hoc networks research.

Although it can support some ad hoc network architectures, it is not intended to support the wireless mobile ad hoc networks

Prolog In wireless mobile ad hoc networks, m

ultihop connectivity is one of the most prominent features.

So, we will ask:Can the IEEE 802.11 MAC Prot

ocol Function Well in Multihop Networks?

Prolog By presenting some problems encountered i

n an IEEE 802.11-based multihop networks, the author concludes that:The current version of this wireless

LAN protocol does not function well in multihop ad hoc networks.

Outline

Introduction The TCP Instability Problem and

Analysis The Serious Unfairness and Analysis Conclusion

Does 802.11 Work Well in Multihop Wireless Ad Hoc Networks?

Introduction

Intro. IEEE 802.11 MAC Protocol

CSMA / CA Combined network allocation vector

(NAV) with clear channel assessment (CCA) to indicate the busy state of the medium.

Using RTS/CTS scheme to reduce the probability of two stations colliding due to not hearing each other.

Intro. Hidden terminal problem:

This will cause collision on data transmission.

A CB

Intro. Exposed terminal problem:

If these terminals are not minimized, the available bandwidth is underutilized.

A CB D

Intro. The serious problems encountered in

an IEEE 802.11-based multihop ad hoc networks: The TCP Instability Problem. The Serious Unfairness Problem.

Does 802.11 Work Well in Multihop Wireless Ad Hoc Networks?

The TCP Instability Problem and

Analysis

TCP Instability Problem The network topology:

1 2 3 4 5 6

Interfering Range of node 4

100 m

250 m250 m

TCP Instability Problem The TCP session is the only traffic

in the network with four hops from source 1 to destination 5.

1 2 3 4 5 6

Source

Destination

TCP Instability Problem If the network condition does not

vary, the TCP throughput should stay stable within some range.

TCP Instability Problem

TCP Instability Problem

TCP Instability Problem Collision

Collision will occur in node 2 when nodes 1 and 4 is sending at the same time.

Exposed Station Node 2 has to defer when node 4 is

sending.1 2 3 4 5 6

Source

Destination

TCP Instability Problem

CollisionCollision Exposed Terminal

Exposed Terminal

TCP Instability Problem Collision

Node2 cannot receive RTS when node4 is sending (hidden terminal problem)

Exposed Station Problem Node2 cannot send back CTS even if it receives

the RTS from node1 correctly.

After failing to receive CTS from node2 seven times, a route failure event occurs.

1 2 3 4 5 6

Source

Destination

TCP Instability Problem By adjusting the Window size:

TCP Instability Problem Discussion:

The maximum number for possible back-to-back sending is four, greatly reduces possible that other nodes might fail to access the channel in seven tries.

1 2 3 4 5 6

Source

Destination

Does 802.11 Work Well in Multihop Wireless Ad Hoc Networks?

Serious Unfairness and Analysis

Serious Unfairness 2 TCP Connections

First session starts at 10.0s ( 6 4 ) Second session starts 20.0s later ( 2

3 )

1 2 3 4 5 6

Source

Destination

Destination

Source

Serious Unfairness

First session start

Second session start

Serious Unfairness The throughput of the first session

is zero in most of its lifetime after the second session starts.

There is not even a chance for it to restart.

The loser session is completely shutdown even if it starts much earlier.

Serious Unfairness

Serious Unfairness

Serious Unfairness Discussion:

Node5 cannot reach node4 when Node2 is sending (collision) Node3 is sending ACK (defer)

1 2 3 4 5 6

Source

Destination

Destination

Source

Serious Unfairness Discussion:

TCP Instability / Unfairness Problem. These MAC layer problem appear

when the traffic load becomes large enough, even if the traffic is not from TCP.

1 2 3 4 5 6

Does 802.11 Work Well in Multihop Wireless Ad Hoc Networks?

Conclusion

Conclusion The hidden terminal problem still exists in

multihop networks. The exposed terminal problem will be more

harmful in a multihop network and there is no scheme in IEEE 802.11 standard to deal with this problem.

The binary exponential backoff scheme always favors the latest successful node. It will course unfairness.