A Measurement Study of Path
Capacity in 802.11b based Wireless
Networks
Tony Sun, Guang Yang, Ling-Jyh Chen,
M. Y. Sanadidi, Mario Gerla
Outline
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
Motivation
Background






What are we trying to measure?
Effective Capacity
Previous Method
AdHoc Probe
Experiment Results
Conclusion
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Motivation

Capacity estimation extensively studied in wired
network but not wireless

Evaluation and measurement of of wireless path
capacity is of realistic interest


(i.e. Capacity planning, protocol design, performance
analysis, system deployment)
Need a new tool that can monitor and measure
wireless path capacity well
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What do we want to measure?

The effective end-to-end rate is defined as
the maximum achievable data rate in the
absence of any cross traffic connection.

It is smaller than the raw data rate at the
physical layer due to packet O/H and channel
access coordination to handle multiple,
pipelined packets on the path.
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Effective Capacity of 802.11b
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In 802.11b, a RTS packet is 40 bytes, CTS and ACK
packets are 39 bytes, and the MAC header of a data
packet is 47 bytes, the effective capacity of a onehop link is:
C

S
 CP
S  40  39  47
For instance, when the data packet size is 1500
bytes and the data rate of the wireless link is 2Mbps,
the effective capacity is at most
1500
 2  1.8Mbps
1500  40  39  47
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Effective capacity of a multihop link

If N nodes are within the same interference domain,
C’=C/N
1.
2.
Dr=Di=250m =>
C’=C/3
Dr=250m, Di=500m =>
C’=C/4
The solid-line circle: effective transmission range (Dr)
The dotted-line circle: interference range (Di)
Distance between nodes: 200m
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Previous Work (Morris et al)
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
Dr=250m, Di=500m
Use UDP flows to probe the maximum achievable throughput
(brute force method)
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Outline


Motivation
Background






What are we trying to measure?
Effective Capacity
Previous Method
AdHoc Probe
Experiment Results
Conclusion
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AdHoc Probe
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A recently proposed path capacity estimation tool specially
designed for the multi-hop ad hoc wireless environment.


One-way estimation technique, based on CapProbe concepts
Aimed to simplify the path capacity estimation process

AdHoc Probe measures end-to-end effective capacity in wireless
ad hoc networks.

AdHoc Probe is simple, fast and less intrusive compared to other
schemes.
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Packet Pair Dispersion
T1
T3
Narrowest Link
T2
20Mbps
10Mbps
T3
5MbpsT3
10Mbps
T3
20Mbps
8Mbps
Capacity = (Packet Size) / (Dispersion)
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Issues: Compression and Expansion
• Queueing delay on the first packet => compression
• Queueing delay on the second packet => expansion
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CapProbe concept

Key insight: a packet pair that gets through with zero queueing
delay yields the exact estimate
Capacity
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Estimating Capacity in Ad Hoc
Network
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Path capacity in wireless ad hoc net depends
on bottleneck capacity, topology, interference,
and other environment parameters.
Data rate can be fixed or auto.
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Auto Rate Schemes
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Rate adaptation
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Sender based: ARF, AARF
Receiver based: RBAR
Opportunistic scheduling
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OAR, PAC (MAD)
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Implementation issues
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System time synchronization
S i '  (Trecv1,i  Tsend ,i )  (Trecv 2,i  Tsend ,i )
S i  (Trecv1,i  Tsend ,i   )  (Trecv 2,i  Tsend ,i   )
 S i '2
If S k

 min ( S i ) for k  [1..n],
i 1.. n
then
S k '  min ( S i ' ),
i i .. n
and vice versa.
Clock skew
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Outline


Motivation
Background






What are we trying to measure?
Effective Capacity
Previous Method
AdHoc Probe
Experiment Results
Conclusion
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Multihop path simulation
back to back
packets
dispersion 1
wired
sender Internet
AP
1 hop
dispersion 2
wireless multihop
2 hop 3 hop
4 hop
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5 hop
6 hop
7 hop
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Experiment Results (1)
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Fixed rate, multihops
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Experiment Results (2)
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Auto Rate, w/ different distance
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Experiment Results (3)
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Auto Rate, w/ Bluetooth interference
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Experiment Results (4)
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Probing from the Internet
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Summary of AdHoc Probe
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AdHoc Probe estimates e2e path capacity in
ad hoc net.
AdHoc Probe estimates correctly with
multihop, interference, and auto rate.
AdHoc Probe can be performed from Internet
to the “opportunistic” ad hoc net.
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Thanks!
CapProbe: http://nrl.cs.ucla.edu/CapProbe/
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