Sep 12, 2002 Wireless Networking Seminar 1

Does IEEE 802.11 Work Well in Multi-hop Wireless Network?

Author: Shugong Xu, Tarek Saadawi

City University of New York

Speaker: Weisheng Si

Sep 12, 2002 Wireless Networking Seminar 2

Outline

• Overview of The Paper• Review of TCP• Simulation Environment• Instability Problem• Unfairness Problem• Summary of The Paper• Comments on The Paper

Sep 12, 2002 Wireless Networking Seminar 3

Overview of The Paper

• Conclusion: Current 802.11 Protocol does not function well in multi-hop networks.

• Experiment Methods:– A Static String Topology

– TCP as Transport Layer Protocol

• Evidences: – Instability Problem– Unfairness Problem

0 1 2 3 4 5 6 7

Sep 12, 2002 Wireless Networking Seminar 4

Review of TCP -- Sliding Window

• Goal: reliable data transmission

• Basic idea: – Using acknowledgement/retransmission scheme– Identifying data with sequence numbers

• Sliding Window: the maximum range of data sent but not acknowledged

Sep 12, 2002 Wireless Networking Seminar 5

Review of TCP -- Sliding Window

[ 0 1 2 3 ] 4 5 6 7 8 9 0 1 2 3

ACK 1

0 [ 1 2 3 4 ] 5 6 7 8 9 4

0 [ 1 2 3 4 ] 5 6 7 8 9 1 2 3 4

An Example: Sliding Window Size = 4 bytes

Timeout

Sep 12, 2002 Wireless Networking Seminar 6

Review of TCP—Flow Control

• Goal: prevent the buffer at the receiver from being overloaded.

• Basic Idea:– The receiver advertises its available buffer size to the

sender in each TCP acknowledgment.

– The sender maintains a variable called receiver window which specifies the size of the sliding window. Whenever it receives an acknowledgement from the receiver, it set the receiver window to the available buffer size advertised by the receiver.

Sep 12, 2002 Wireless Networking Seminar 7

Review of TCP – Flow Control

[ 0 1 2 3 ] 4 5 6 7 8 9 0 1 2 3

ACK 1, WIN=4

0 [ 1 2 3 4 ] 5 6 7 8 9 4

ACK 5, WIN=3

0 1 2 3 4 [ 5 6 7 ] 8 9 5 6 7

An Example: Initial Sliding Window Size = 4 bytes

Sep 12, 2002 Wireless Networking Seminar 8

Review of TCP—Congestion Control

• Goal: prevent the network from being overloaded.

• Basic Ideas:– Using timeout as the indication of network congestion.– Additive Increase and Multiplicative Decrease.

• Two Additional Variables– Congestion Window– Slow Start Threshold

Sep 12, 2002 Wireless Networking Seminar 9

Review of TCP—Congestion Control

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Number of Transmissions

Co

ng

es

tio

n W

ind

ow

Slow Start Threshold

Slow Start Threshold

Slow StartSlow Start

Congestion Avoidance

Congestion Avoidance

TCP Tahoe, Reno

Sep 12, 2002 Wireless Networking Seminar 10

Review of TCP – Congestion Control

[ 0 1 2 3 ] 4 5 6 7 8 9 0 1 2 3

ACK 2, WIN=4

0 1 [ 2 3 4 5 ] 6 7 8 9 4 5

0 1 [ 2 ] 3 4 5 6 7 8 9 2

An Example: Sliding Window Size = 4 bytes

Timeout

Sep 12, 2002 Wireless Networking Seminar 11

Review of TCP—Summary

• The minimum value of receiver window and congestion window is used as the size of the sliding window.

• If the network is in good condition, the TCP throughput should be in a steady level governed by the receiver window.

• TCP enters phase of Slow Start upon timeouts. If timeout frequently happens, TCP throughput will be dramatically cut down.

Sep 12, 2002 Wireless Networking Seminar 12

Simulation Environment

• Simulator: ns-2 with the wireless extension implemented by CMU.

• MAC Layer: IEEE 802.11 MAC Distributed Coordination function(DCF).

• Transport Layer: TCP connections carrying very large files.• Network Environment

– A Static String Network Topology– Interfering range is a little more than two times of the

communication range

0 1 2 3 4 5 6 7

Interfering Range Communication Range

Sep 12, 2002 Wireless Networking Seminar 13

Instability Problem—Experiment Setup

1 2 3 4 5Source Destination

• A single TCP connection, with node 1 as the source and node 5 as the destination.

• Three sets of experiments with Maximum Window Size(window_) 32, 8, and 4 respectively.

Sep 12, 2002 Wireless Networking Seminar 14

Instability Problem—Experiment Result

• When window_=32 or 8, serious oscillation of throughput is observed.

• When window_4, throughput is stable.

Sep 12, 2002 Wireless Networking Seminar 15

Instability Problem—Trace Analysis(1)

1 2 3 4 5Data

Ack

RTS

CTS

Interfering Range of Node 2

Sep 12, 2002 Wireless Networking Seminar 16

Instability Problem—Summary

• Collision and exposed terminal problem prevent node 2 from receiving RTS from or sending CTS to node 1.

• The random back-off, big data packet, and sending back-to-back packets worsen the above problems.

• When window_ = 4, the chance to send back a CTS is greatly increased, so the throughput becomes stable.

• After node 1 fails seven times to receive CTS, node 1 believes there is a route failure and starts a route discovery.

• Before a route is available, node 1 can not send out a data packet. This period usually is long enough to cause a timeout at the TCP sender.

• For TCP, timeout triggers Slow Start, which significantly reduces the throughput.

Sep 12, 2002 Wireless Networking Seminar 17

Unfairness Problem—Experiment Setup

2 3 4 5 6

Source Destination

Source

First SessionSecond Session

• In the first session, data flow from 6 to 4. In the second session, data flow from 2 to 3.

• The first session starts at 10.0s. The second session starts at 30.0s.

Sep 12, 2002 Wireless Networking Seminar 18

Unfairness Problem—Experiment Result(1)

• The first session has a throughput of about 450kbps from 10s to 30s, and 0kbps after 30s.

• The second session has a throughput of about 900kbps from 30s to 130s.

Sep 12, 2002 Wireless Networking Seminar 19

Unfairness Problem—Experiment Result(2)

• The first session never succeeds to send out packet with sequence number 2164.

Sep 12, 2002 Wireless Networking Seminar 20

Unfairness Problem—Trace Analysis(1)

2 3 4 5 6RTSData

CTS

Interfering Range of Node 5

Ack

Interfering Range of Node 4

Data

No Route

Sep 12, 2002 Wireless Networking Seminar 21

Unfairness Problem—Trace Analysis(2)

2 3 4 5 6RTSData

CTS

Interfering Range of Node 5

Ack

Interfering Range of Node 4

Data

No Route

Sep 12, 2002 Wireless Networking Seminar 22

Unfairness Problem—Summary

• In one-hop TCP connections, the interval between packet transmission is larger than that of the multi-hop TCP connections, which gives the one-hop connection more chances to transmit data.

• Random back-off is actually advantageous to the last succeeding host.

• The authors refer to this kind of unfairness problem “One-hop unfairness problem” and argue that since one-hop connection is common in a wireless network, one-hop unfairness problem can not be ignored.

Sep 12, 2002 Wireless Networking Seminar 23

Summary of The Paper

• Problems Shown:– Instability Problem– Unfairness problem

• Conclusions:– IEEE 802.11 does not work well in multi-hop wireless

networks.– It may be inappropriate to take IEEE 802.11 as the MAC

layer to simulate routing or transport protocols for multi-hop wireless networks.

Sep 12, 2002 Wireless Networking Seminar 24

Comments on The Paper

• Rooted in IEEE 802.11 MAC?– TCP is not designed with wireless networking in mind.– Timeout Slow Start

• Instability problem can also be reduced to unfairness problem– They have almost the same network traces.– If we break down the network topology in the first

problem, we obtain the topology in the second problem.

• Interfering range and communication range– If interfering range is the same as the communication

range, the two problems presented in this paper will disappear.

– Is the configuration of the interfering range simply an engineering issue?