Lecture9 - MobileCommunications...

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9.1 Prof. Dr.-Ing. Jochen H. Schillerwww.jochenschiller.deMC - 2008 Mobile Communications  Chapter 9: Mobile Transport Layer Motivation, TCP-mechanisms Classical approaches (Indirect TCP, Snooping TCP, Mobile  TCP) Additional optimizations (Fast retransmit/recovery,  Transmission freezing, Selective retransmission, Transaction  oriented TCP)
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Prof. Dr.-Ing. Jochen H. Schillerwww.jochenschiller.deMC - 2008 9.2 Transport Layer Goal is to provide efficient and  reliable service to its users (typically  application layer processes) E.g. 90% of Internet traffic uses TCP:  HTTP (used by web services)  typically uses TCP Reliable transport between client and  server required TCP Network friendly: time-out   congestion   slow down transmission Well known – TCP guesses quite  often wrong in wireless and mobile  networks Packet loss due to transmission  errors Packet loss due to change of network Result Severe performance degradation Client Server Connection setup Data transmission Connection release TCP SYN TCP SYN/ACK TCP ACK HTTP request HTTP response GPRS: 500ms! >15 s no data
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Prof. Dr.-Ing. Jochen H. Schillerwww.jochenschiller.deMC - 2008 9.3 Motivation I Transport protocols typically designed for Fixed end-systems Fixed, wired networks Research activities Performance Congestion control Efficient retransmissions TCP congestion control packet loss in fixed networks typically due to (temporary) overload  situations  router have to discard packets as soon as the buffers are full  TCP recognizes congestion only indirect via missing  acknowledgements,  retransmissions unwise, they would only  contribute to the congestion  slow-start algorithm as reaction
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Prof. Dr.-Ing. Jochen H. Schillerwww.jochenschiller.deMC - 2008 9.4 TCP congestion control: AIMD - additive  increase, multiplicative decrease 8 K b y t e s 1 6 K b y t e s 2 4 K b y t e s t i m e c o n g e s t i o n w i n d o w Approach:   increase transmission rate (window size), probing for  usable bandwidth, until loss occurs additive increase:  increase   CongWin  by 1 MSS every RTT until  loss detected multiplicative decrease :  cut  CongWin  in half after loss  time congestion window size Saw tooth behavior: probing for bandwidth From: J.F Kurose and K.W. Ross,  Computer Networking a Top Down Approach
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Prof. Dr.-Ing. Jochen H. Schillerwww.jochenschiller.deMC - 2008 9.5 TCP Congestion Control: details Roughly, CongWin  is dynamic, function of  perceived network congestion How does  sender perceive  congestion? loss event = 
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This note was uploaded on 11/03/2009 for the course COMPUTERS CS537 taught by Professor Salman during the Spring '09 term at Texas A&M University–Commerce.

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Lecture9 - MobileCommunications...

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