A study of cross-layer optimization for TCP over wireless networks

Abstract

TCP is an end-to-end reliable protocol designed initially to work over wired networks where the packet losses happen mostly because of the network nodes' congestion. While in the new widely used wired-cum-wireless networks, a new reason for packet losses emerges, which is the bad wireless link's nature that tortures the TCP packets. TCP reacts adversely to this new type of packet loss; it decreases the transmission rate to very low values as a result of wrong interpretation of this type of packet losses. Hence, this thesis tries to contribute to the solution of the above problem by proposing an approach that enhances TCP traffic over wired-cum-wireless networks. The adopted approach is a cross-layer approach where the link layer and transport layer are cooperating to minimize the losses over the wireless channel and eventually enhancing the overall throughput of the network. Basically, the proposed approach works by adaptively choosing the Forward Error Correction (FEC) parameters. Two locations have been proposed to implement the proposed scheme; either over the end-to-end connection (i.e., server to the mobile node) or point-to-point over the wireless link only (i.e., base station to mobile node). The goal of the second implementation is to save the congested wired bandwidth by removing the extra load of redundancy on the backhaul network. Using NS2 simulator, we study the effects of combining the proposed end-to-end/point-to-point FEC with five TCP flavors; Tahoe, Reno, Newreno, Sack and Westwood. The simulation results have shown significant improvement in TCP goodput under both proposals. However, considering congested network, the approach that is implemented over wireless link only shows very outstanding behavior by maintaining high throughput over a wide range of frame loss probabilities and propagation delay.