Wireless Course Project

NETW320

Assignment week 4

The basic requirement for a WLAN infrastructure to support voice is dense, pervasive coverage. Density refers to the signal strength and pervasiveness refers to the coverage. Signal strength impacts the transmission rate users receive on the network and hence the number of simultaneous calls that can be supported on an access point. The generally accepted design parameter is a received signal strength floor of -67 dBm, though better designed handsets can often work down to -70 dBm. The goal is to provide signal strength that will result in the most efficient network utilization, the shortest transit delays and the maximum number of calls supported.

With regard to density, there are two important factors that characterize WLANs: shared media and adaptive modulation. Shared media means that all devices associated with an access point take turns using one half duplex channel. As with any contention-based network, the greater the volume of traffic vying for access to the channel, the greater the delay that users will experience. Good signal coverage results in better network efficiency, and that in turn leads to lower transit delays, a key factor in providing high quality voice. For enterprise-grade voice service, the requirement is to provide one-way, end-to-end delay below 150 msec.

Better signal coverage also leads to higher transmission rates. WLAN devices use adaptive modulation, which means the WLAN device reduces its transmission rates as the signal strength decreases and the signal-to-noise ratio degrades; the range of data rates supported on WLANs is summarized in Table 1. Signal strength is primarily a factor of the distance to the access point and any material obstructions in the path. In a shared media network, adaptive modulation means that faster and slower transmitters will be sharing the same channel. It stands to reason that the channel will be used most efficiently if all stations transmit at their highest data...