Adaptive Link Layer Protocols for Shared Wireless Links

The error prone nature of wireless links often necessitates the use of a link layer protocol to ensure acceptable application performance. While traditional link layers assume that they fully control the link, in most emerging wireless networks many sessions may dynamically share the link due to the presence of multiple contending users and/or applications. Such networks require link layers that can automatically adapt to bandwidth variations, offering good performance regardless of contention. To this end, we discuss two adaptive protocols, an Adaptive Selective Repeat (ASR) protocol that dynamically modifies its retransmission timeouts, and the Radio Link Control (RLC) protocol used by UMTS, an advanced protocol without retransmission timers. To assess the applicability of each approach, we measure the throughput achieved by File Transfer and Web Browsing over both protocols, with or without contention from a Media Distribution application, as well as the delay induced by these protocols to the contending application. Our results indicate that the complexity of RLC is not justified by its performance, as ASR nearly always outperforms it.