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

Shugong Xu

Tark Saadawi

June, 2001

IEEE Communications Magazine(Adapted from mnet.cs.nthu.edu.tw/paper/jbb/010704.pps)

Overview

IEEE 802.11 MAC ProtocolThe TCP Instability Problem and AnalysisThe Serious Unfairness and AnalysisConclusion

IEEE 802.11 MAC Protocol – (1)

IEEE 802.11 MAC Protocol

is the standard for wireless LANs

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.

IEEE 802.11 MAC Protocol – (2)

Hidden terminal problem: This will cause collision on data transmission.

A CB

IEEE 802.11 MAC Protocol – (3)

Exposed terminal problem: If these terminals are not minimized, the

available bandwidth is underutilized.

A CB D

IEEE 802.11 MAC Protocol – (4)

The IEEE 802.11 MAC protocol in wireless mobile ad hoc networks, multihop connectivity is one of the most prominent features.

Can the IEEE 802.11 MAC Protocol Function Well in Multihop Networks?

IEEE 802.11 MAC Protocol – (5)

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

Authors concludes that the current version of this wireless LAN protocol does not function well in multihop ad hoc networks.

TCP Instability Problem – (1)

The network topology:

1 2 3 4 5 6

Interfering Range of node 4

100 m

250 m250 m

TCP Instability Problem – (2)

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 – (3)

If the network condition does not vary, the TCP throughput should stay stable within some range.

TCP Instability Problem – (4)

TCP Instability Problem – (5)

TCP Instability Problem – (6)

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 – (7)

CollisionCollision Exposed Terminal

Exposed Terminal

TCP Instability Problem – (8)

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 – (9)

By adjusting the Window size:

TCP Instability Problem – (10)

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

Serious Unfairness – (1)

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 – (2)

First session start

Second session start

Serious Unfairness – (3)

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 – (4)

Serious Unfairness – (5)

Serious Unfairness – (6)

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 – (7)

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

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 cause unfairness.