Efficient Internet Traffic Delivery over Wireless Networks

1. Efficient Internet Traffic Delivery over Wireless Networks Sandhya Sumathy

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

3. Overview of Internet Stack Architecture

4. 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.

5. Packet Transmission Scheduling continued.. • 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.

6. 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.

7. 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.

8. 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.

9. Proposed TCP Enhancements • Recent proposals • Sender-side modifications –changes to Internet Hosts. • Performance enhancing proxies – affects IP layer security, scalability • Receiver-side modifications –only require 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

10. Proposed TCP Enhancements continued.. • 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.

11. 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.

12. Questions??

13. Thank You.