802.11b Operating in a Mobile Channel: Performance and Challenges
802.11b; Wireless; WLAN; Mobile
In the past, the worlds of wireless voice and data transmission have been largely disjoint. Voice traffic has been carried over circuit-switched cellular links, and data has been largely restricted to packet-switched wireless LANs. Now, as consumers demand higher bandwidth connections without sacrificing mobility and traffic transitions from primarily voice to data, service providers must produce what is essentially a ubiquitous wireless LAN. To this end, we have studied the effects of a mobile channel on current generation 802.11 A, B, and G wireless LAN cards to see how readily they can be applied to more challenging environments. Not surprisingly, current WLAN technology suffers from significantly degraded performance when subjected to the rigors of a mobile channel. We created emulated bi-directional peer-to-peer links in which we were able to manipulate individual channel parameters. By isolating individual propagation effects and testing several different implementations of the standards, we have discovered which channel parameters have the most significant impact on performance. For instance, the large delay spreads typical of an outdoor channel seem to produce the most deleterious effect on throughput in 802.11b. We use our observations to evaluate the viability of direct-sequence spread-spectrum systems (similar to 802.11b) versus that of OFDM systems (like 802.11a and 802.11g). Then we offer suggestions for how future systems should be adapted in order to manage these effects, and we project the ultimate limitations and possibilities for subsequent 802.11-like systems.