Efficient Internet Traffic Delivery over Wireless Networks Sandhya Sumathy

Introduction High Demand for Wireless Internet Connectivity Internet applications usually generate elastic traffic. e.g. Web, FTP, . Main objective for elastic traffic is minimization of the total file transmission time. Randomness is the basic characteristics of wireless communications

Overview of Internet Stack Architecture

Link Layer Optimizations for Elastic Traffic Packet Transmission Scheduling-increase throughput by exploiting the temporal fluctuations in Channel qualities. At any one time some users will enjoy better channel conditions. In order to increase user’s channel qualities packets are separated into buffers thus allowing a Scheduler to select an optimal user to transmit at any time. A scheduler will decide on the next queue to be serviced based on : Signal to noise ratio, Mean channel rate, Priority, Queue size.

Packet Transmission Scheduling continued.. Scheduling can be made optimal if buffers has packets queued and awaiting transmission. Scheduling can be made optimal if buffers has packets queued and awaiting transmission. Emptying a users packet at the base station leads to suboptimal scheduling. TCP flow control mechanism should not cause a buffer to drain and hold packets at the source.

Link Layer Rate Adaptation Incremental Redundancy (IR) Techniques vary the code rate on a transmission, tracking fluctuations in the channel quality. Link Adaptations (LA) Techniques measure the average channel quality and choose an appropriate modulation or coding scheme. The net effect : Time series of successful packet transmission time,as perceived by TCP flow control scheme is random and non stationary.

Transport Layer Objectives and adverse effects TCP is a window-based flow control algorithm. Its window size is minimum of congestion window (CWND) which is set by sender and the advertised window (AWND) set by receiver. Two dominant algorithms :Slow start and Congestion avoidance. In a wireless network the link rate is a random quantity which varies on distinct timescales.

Transport Layer Objectives and adverse effects continued… In order to keep the link buffer needs to be very large. TCP’s congestion control algorithm will attempt to fill this buffer. If the link rate drops suddenly excessively large latencies result. Small buffer will cause the link to be frequently starve for packets. This problem has been observed in recent studies of TCP’s performance over UMTS.

Proposed TCP Enhancements Recent proposals Sender-side modifications –changes to Sender-side modifications –changes to Internet Hosts. Internet Hosts. Performance enhancing proxies – affects Performance enhancing proxies – affects IP layer security, scalability IP layer security, scalability Receiver-side modifications –only require Receiver-side modifications –only require changes to wireless hosts. changes to wireless hosts. Explicit Window Adaptation –control TCP sender from receiving host by controlling the AWND feature. This is referred as receiver-side explicit window adaptation

Proposed TCP Enhancements continued.. An Algorithm CLAMP is recently proposed for receiver-side explicit window adaptation. An Algorithm CLAMP is recently proposed for receiver-side explicit window adaptation. This algorithm calculates a new AWND value taking into account the proximity factor, current rate of transfer etc. This algorithm tries to reduce the time that the link is left idle because of TCP holding packets at source.

Conclusion A flow control algorithm over wireless links should allow control over the trade-off between latency and utilization. Receiver-side flow control can be used to control existing TCP sources to reach desired objectives and increase the performance of wireless access to Internet. An algorithm such as CLAMP supplements TCP’s flow control mechanism.This can be used to ensure that lower-layer scheduling mechanisms can perform as intended. used to ensure that lower-layer scheduling mechanisms can perform as intended.

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