1. 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

2. OUTLINE Description of IEEE802.11b/DSSS Measurements of TCP Packet Transmission using 802.11b Products Computational Model for Multipath Estimation Statistical Analysis of Location Dependent effects on TCP Performance Conclusions

3. 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

4. Transmission Specifics CSMA/CA DCF : Distributed Control Function : Contention based channel access Interval durations: DIFS= 50 µs SIFS= 10 µs Slot Time= 20 µs Defer Access Slot time Select slot and decrement backoff Backoff windowNext Frame Contention Window DIFS PIFS SIFS Busy Medium DIFS Immediate access if medium is free >= DIFS

5. Transmisson Specifics Packets retransmitted from PHY layer if receiver ACK not received within SIFS duration : Assumes collision Packets retransmitted from higher layers (TCP) when –channel access fails –MAC/PHY layer buffers overflow Protocol parameters fixed Our Objective: Examine performance in spatially varying multipath channels

6. TCP Performance Measurements Generate TCP flows between server on wired network & mobile client –Server  Access Point (AP) -> Mobile Terminal (MT) –AP : Apple Airport TM –MT: Apple Ibook with Airport TM card WIN/XP & Linux m/cs with Lucent Orinoco Gold Card Capture TCP headers at Server, AP & MT using tcpdump utility Jointly Analyze TCP logs to determine retransmission and collision statistics

8. Channel Impulse Response Computation Channel impulse response computed using method of images - Walls of enclosure replaced by point sources of varying strength and location - Image source amplitude chosen by continuity of transverse electric and magnetic fields across reflecting boundary - Incorporates influence of varying dielectric constant for six walls - 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

9. CHANNEL IMPULSE RESPONSE

11. Signal & Noise Statistics Local and Remote Signal and Noise power distribution averaged across 6 measurement ensembles - noise range fairly stationary –-98 : -81 dBm : Local –-102 : -93 dBm: Remote Distinct bi-modal noise distribution at source suggests : multipath noise may be distinct from environmental noise - signal range : -48 - 43 dBm: Local -43 - 37 dBm: Remote

12. TCP Statistics Average TCP throughput stationary across all locations : 4.2 – 5 Mbps TCP retransmission & packet collision statistics highly location dependent -Retransmission detected from server log -Collision assumed when single occurrence of sequence number in server log, but >1 in Mobile TCP log: Assume ACK from MT is lost/delayed

13. Position dependent Retransmission & Collision Statistics LOC TCP Retransmissions µ iat (Rx) secs PHY Collisions µ iat (Col) secs ANone- - B0.222.180.240.46 C0.01150.211.34 Mean interarrival time for the mobile terminal ~ 2ms Retransmissions from the PHY layer for location B occur at a faster rate than location C Mean interarrival time for retransmissions for loc B is deterministic

14. TCP time-sequence plots

15. CONCLUSIONS 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 the combined effects of the degradation attributes Future work: Improve the protocol performance due to the multipath effect