Download Does the IEEE 802.11 MAC Protocol Work Well in Multihop Wireless

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 Protocol
The TCP Instability Problem and Analysis
The Serious Unfairness and Analysis
Conclusion
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
B
C
IEEE 802.11 MAC Protocol – (3)
Exposed terminal problem:
If these terminals are not minimized, the
available bandwidth is underutilized.
B
A C
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:
Interfering Range of node 4
250 m
250 m
1
2
3
4
100 m
5
6
TCP Instability Problem – (2)
The TCP session is the only traffic in the
network with four hops from source 1 to
destination 5.
1
Source
2
3
4
5
Destination
6
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
Source
2
3
4
5
Destination
6
TCP Instability Problem – (7)
Exposed
Terminal
Collision
Exposed
Terminal
Collision
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
Source
2
3
4
5
Destination
6
TCP Instability Problem – (9)
By adjusting the Window size:
TCP Instability Problem – (10)
Discussion:
The maximum number for possible back-toback sending is four, greatly reduces possible
that other nodes might fail to access the
channel in seven tries.
1
Source
2
3
4
5
Destination
6
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
Source Destination Destination
5
6
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
Source Destination Destination
5
6
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.