|
JPL's Wireless Communication Reference WebsiteChapter: Network Concepts and Standards. Section: Wireless computing |
The significant increase in recent activity in the area of wireless networks indicates that mobile hosts along with their wireless links will be an integral part of future networks. Communication over such networks shows quite different characteristics compared to traditional wired networks. Wireless links do not (at present) have bandwidths as high as on wired links, show much higher error rates. Furthermore, since the predominant model for wireless communication is the cellular connection, frequent delays are caused due to handoffs as users move from cell to cell. These handoffs involve transfer of communication state from one base station to another, and typically last for anywhere between a few tens to a few hundreds of milliseconds.
Reliable transport protocols like TCP have been tuned for traditional networks made up of wired links and stationary hosts. TCP performs very well on such networks by adapting to end-to-end delays and packet losses.
TCP provides reliability by maintaining a running average of estimated round-trip delay and mean deviation, and by retransmitting any packet whose acknowledgment isn't received within twice the deviation from the average. Due to the relatively low error rates over wired networks, packet losses are assumed to be as a result of congestion. Unfortunately, in the presence of the high error rates present in wireless links, this assumption causes TCP to suffer a significant degradation in performance in the form of poor throughput and very high interactive delays when used in a wireless environment. This behavior arises due to the fact that errors on the wireless link or delays due to handoffs are incorrectly interpreted by TCP at the sender as congestion. In response to this "congestion", TCP drops its transmission window size before retransmitting packets, initiates congestion control mechanisms and resets the retransmission timer using an exponential backoff computation from the previous value , thereby unnecessarily reducing its bandwidth utilization.
Audio| Hear Randy Katz talk about research challenges in mobile computing. He also discusses how transport layer protocols and applications should anticipate to fading and channel throughput fluctuations. This many be at odds with strictly OSI layered protocol stacks. |
