1 Transport Control Protocol for Wireless Connections ElAarag and Bassiouni Vehicle Technology Conference 1999

2 I. Introduction Technical problems of the wireless mobile networks  Lossy (higher bit error rate)  Mobile disconnections (e.g. hand-off) This paper studies  The effect of bit error rate and mobile disconnection on the performance of different TCP implementations (Tahoe, Reno, New Reno, SACK) This paper shows  The performance of TCP is more sensitive to the link up period than the error rate and the disconnection probability  SACK outperforms Reno in wired networks, but not in the wireless case.

3 II. Simulation Model Network Simulator (NS) by Lawrence Berkeley Labs  Three nodes A fixed host (FH), a base station (BS), and a mobile host (MH)  Good state: the wireless link suffers no loss exponentially distributed with means α g Bad state: the wireless link suffers from packet loss exponentially distributed with means α b  MH:  Link-up state: exponentially distributed with mean link-up time m 1  Link-down state: exponentially distributed with mean link-down time m 2  Disconnection probability p = m 2 / (m 1 +m 2 ) FHBSMH 1.5 Mbps 10 ms 0.8 Mbps 100 ms Drop-tail Queue

4 TCP Parameters Windows size = 50 ssthresh = 32 Packet size = 1024 ACK size = 40 TCP clock granularity = 100 ms Queue size = 50 Focus on:  loss of packets due to mobile link error and mobile disconnections

5 Performance Metrics Four parameter metrics  Throughput  How much data the user can receive per second  Goodput  The ratio between the amount of useful data transmitted by the source and the mount of data transmitted by the source  Packet delay  The average delay incurred by the packet from the time it is deposited into the sender’s windows until it is successfully acknowledged  Transfer time  The time it takes for the receiver to receive a fixed number of packets, e.g packets Better quality of service means…

6 III. Effect of Mobile Disconnection Probability Experiment:  on Tahoe, Reno, New-Reno, and SACK TCP  Two scenarios:  Scenario 1: p = 9% (m 1 =1.0s, m 2 =0.1s)  Scenario 2: p = 28.6% (m 1 =5.0s, m 2 =2.0s) Observation:  TCP Performance depends on  disconnection probability???  Link up time!!! ∵ The sender has enough time to open up a large window before it disconnects ⊕ ⊕ ⊕

7 IV. Effect of Link Up Period The larger the mean link up time, the better the throughout, the good put, and the transfer time p=28.6% m1m1

8 The larger the mean link up time, the better the throughout, the goodput, and the transfer time p=28.6% m1m1

9 The larger the mean link up time, the better the throughout, the goodput, and the transfer time m1m1 P=28.6%

10 V. Performance of Tahoe, Reno, New-Reno, and SACK TCP in Wireless Networks Ideal case (no loss): Lossy wireless link: (Fig.4~6)  BER =  Data (BS → MH) α g =100 & α b =8  ACKs (BS ← MH) α g =1000 & α b =3  m 1 =2.0 & m 2 =0.3

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13 ⊕ Reno Tahoe New Reno SACK

14 Slow start whenever a packet is lost

15 cwnd Tahoe has the worst performance?  the least average cwnd  the largest number of occurrences of cwnd=1 Reno has the best performance?  Not the largest average cwnd  The largest number of timeouts, the most sensitive in changing cwnd, thus the most adaptive to the network need TahoeRenoNew RenoSACK Average cwnd No. of Timeouts less

16 VI. Conclusion The mobile link up period has dominant effect on the performance of TCP than the disconnection probability of the mobile and even the BER of the wireless link. Comparing Tahoe, Reno, New-Reno, and SACK TCP in the presence of wireless link errors and mobile disconnections:  Reno had the most throughput, least average delay, and least transfer time.