AIRMAIL: A link-layer protocol for wireless networks

This paper describes the design and performance of a link-layer protocol for indoor and outdoor wireless networks. The protocol is asymmetric to reduce the processing load at the mobile, reliability is established by a combination of automatic repeat request and forward error correction, and link-layer packets are transferred appropriately during handoffs. The protocol is namedAIRMAIL (AsymmetrIc Reliable Mobile Access In Link-layer). The asymmetry is needed in the design because the mobile terminals have limited power and smaller processing capability than the base stations. The key ideas in the asymmetric protocol design consist of placing bulk of the intelligence in the base station as opposed to placing it symmetrically, in requiring the mobile terminal to combine several acknowledgments into a single acknowledgment to conserve power, and in designing the base stations to send periodic status messages, while making the acknowledgment from the mobile terminal eventdriven. The asymmetry in the protocol design results in a one-third reduction of compiled code. The forward error correction technique incorporates three levels of channel coding which interact adaptively. The motivation for using a combination of forward error correction and link-layer retransmissions is to obtain better performance in terms of end-to-end throughput and latency by correcting errors in an unreliable wireless channel in addition to end-to-end correction rather than by correcting errors only by end-to-end retransmissions. The coding overhead is changed adaptively so that bandwidth expansion due to forward error correction is minimized. Integrity of the link during handoffs (in the face of mobility) is handled by window management and state transfer. The protocol has been implemented. Experimental performance results based on the implementation are presented.