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Performance of DS-CDMA

Protocols in Wireless LANS

M.Parikh, P.Sharma, R.Garg, K. Chandra, C. Thompson

Center for Advanced Computation and Telecommunications

University of Massachusetts Lowell, Lowell, MA 01854

http://morse.uml.edu

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OUTLINE

Description of IEEE802.11b/DSSS

Measurements of TCP Packet Transmission

using 802.11b Products

Computational Model for MultipathEstimation

Statistical Analysis of Location Dependenteffects on TCP Performance

Conclusions

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IEEE 802.11b

Direct Sequence Spread Spectrum IEEE 802.11b specifies MAC and PHY layer specifications for DSSS

PHY layer ± FH, DS, IR

Synchronization(128 bits) SFD(16) Signal(8) Service(8) Length(16) CRC(16) Payload(variable)

PLCP Preamble PLCP header

IEEE 802.11 PHY frame using DSSS

PHY

PLCP

-provides carrier sense and CCA

PMD

- BPSK/QPSK modulation

- Wireless encoding

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Transmisson Specifics Packets retransmitted from PHY layer if receiver

ACK not received within SIFS duration : Assumescollision

Packets retransmitted from higher layers (TCP)when

± channel access fails

± MAC/PHY layer buffers overflow

Protocol parameters fixed

Our O bjective: Examine performance in spatiallyvarying multipath channels

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TCP Performance Measurements Generate TCP flows between server on wired

network & mobile client

± Server Access Point (AP) -> Mobile Terminal (MT) ± AP : Apple AirportTM

± MT: Apple Ibook with AirportTM cardWIN/XP & Linux m/cs with Lucent Orinoco Gold Card

Capture TCP headers at Server, AP & MT using

tcpdump utility

Jointly Analyze TCP logs to determineretransmission and collision statistics

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Channel Impulse Response Computation

Channel impulse response computed using method of images

- Walls of enclosure replaced by point sources of varyingstrength and location

- Image source amplitude chosen by continuity of transverse

electric and magnetic fields across reflecting boundary

- Incorporates influence of varying dielectric constant for sixwalls

- Rectangular room geometry; Empty Room;

- Model details described in paper Generates a sampled approximation of bandpass impulse

response

- 2.4 GHz center frequency, 40 MHz bandwidth

Multipath impulse responses computed for locations A, B and C

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Signal & Noise Statistics

Local and Remote Signal and Noise power distribution averaged across 6 measurementensembles

- noise range fairly stationary

± -98 : -81 dBm : Local

± -102 : -93 dBm: Remote

Distinct bi-modal noise distribution at sourcesuggests : multipath noise may be distinct from

environmental noise - signal range :

-48 - 43 dBm: Local

-43 - 37 dBm: Remote

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TCP Statistics Average TCP throughput stationary across

all locations : 4.2 ± 5 Mbps

TCP retransmission & packet collisionstatistics highly location dependent

- Retransmission detected from server log

- Collision assumed when single occurrence of sequence number in server log, but >1 inMobile TCP log: Assume ACK from MT islost/delayed

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Position dependent

Retransmission & CollisionStatisticsLOC TCP Retransmissions µ iat (Rx) secs

PHY Collisions µ iat (Col) secs

A None - None -

B 0.22 2.18 0.24 0.46

C 0.01 15 0.21 1.34

Mean interarrival time for the mobile terminal ~ 2ms

Retransmissions from the PHY layer for location B occur at afaster rate than location C

Mean interarrival time for retransmissions for loc B is

deterministic

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TCP time-sequence plots

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CO NCLUSI

O NS

Performance degradation in 802.11b products can be

attributed to:

- Retransmissions for the PHY layer due to a lost ack - TCP retransmissions

- Protocol overhead

- Multipath effect

Effective throughput decreases to 50 % due to thecombined effects of the degradation attributes

Future work: Improve the protocol performance due to the

multipath effect

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