Modeling Wireless Links

This paper is essentially a survey of the properties of wireless links. The paper does a good job of splitting wireless links into three prevalent categories: WLAN, cellular links, and satellite. It then makes a long list of ways in which wireless links differ from regular links, and describes how this affects each of the three categories.

The first characteristic of wireless links is losses due to errors rather than congestion. The paper points out that this is a relatively rare occurrence, but that when errors do occur, they are somewhat clustered, and affect the link in both directions. Alongside error losses is packet reordering, which the paper claims are currently relatively rare, but have the potential to have an adverse affect on transport protocols if the reordering is severe.

In addition to the above issues, there is also the issue of variation - both in bandwidth and in delay. Variation in delay can be caused by a number of factors, including link-layer recovery and handovers. Variations in bandwidth also exist in wireless systems (due to scheduling); these kinds of variations are likely to confuse transport layer protocols, which assume that neither the bandwidth nor the RTT is fluctuating too wildly. A fluctuating RTT can be the cause of unnecessary retransmits, or unnecessarily long recovery times. Fluctuating bandwidths can cause underuse of the link or sudden congestion.

Other important aspects include bandwidth asymmetry (which is fairly obvious) and handovers, which can cause the loss of all in-flight packets and a temporary but large increase in delay, as well as permanent changes in both bandwidth and RTT.