to endorse different types of changes. A Case Study: Voice over WLAN In order to meet the demands of supporting voice applications, a wireless LAN needs to fulfil stringent performance requirements, particularly if the provision of voice services was not a consideration during the initial design. Close attention will need to be paid to wireless coverage, quality of service and seamless roaming, recognising that the roaming area for voice coverage is likely to be different from the operating area for other roaming coverage such as laptop connections. VoWLAN Bandwidth Requirements Voice usage puts high demands on WLAN bandwidth, and an estimate of the number of concurrent calls that need to be supported will be a key factor in determining either the required design or the suitability of an existing WLAN design. Although a single voice session requires around 64 kbps of bandwidth, or as little as 10 kbps with compression, IP and 802.11 MAC protocol overheads increase the required bandwidth to around 200 kbps per session. Collisions on the shared wireless medium will further limit the number of concurrent voice sessions. The actual number of concurrent calls that can be supported by a single access point, while providing voice quality comparable to a toll call, will depend on the wireless standard in use, as shown in Table 7-16. The concurrent call limits noted here are indicative only, and apply to a network carrying voice traffic alone. If the network is carrying data Chapter Seven Table 7-16: VoWLAN Capacity of 802.11 WLANs Standard PHY data rate (Mbps) MAC SAP data rate (Mbps) Maximum concurrent voice calls 802.11b 802.11b + g 802.11g ca. 20 802.11a ca. 25 as well as voice traffic, call quality will be affected and the maximum number of concurrent calls will drop. As these capacity limits are approached, load balancing will be required to ensure that access points do not become overloaded. Access points alone will not have the overview of the available infrastructure or total traffic to perform this function, but