ch "raw" data packet passed to the MAC service access point (MAC SAP) will acquire a MAC header and a message integrity code and additional security related header information before being passed to the PHY layer for transmission. The headline data rate, for example 54 Mbps for 802.11a or 11g networks, measures the transmission rate of this extended data stream at the PHY layer. The effective data rate is the rate at which the underlying user data is being transmitted if all the transmitted bits relating to headers, integrity checking and other overheads are ignored. For example, on average, every 6 bits of raw data passed to the MAC SAP of an 802.11b WLAN will gain an extra 5 bits of overhead before transmission, reducing a PHY layer peak data rate of 11 Mbps to an effective rate of 6 Mbps. Table 6-10 shows the PHY and MAC SAP data rates for 802.11 WLANs. For 802.11g networks the MAC SAP data rate depends on the presence of 802.11b stations, as a result of the mixed-mode media access control mechanisms described in the previous section. 802.11 Enhancements In the following sections some of the key enhancements to 802.11 network capabilities and performance will be described. The security enhancements covered by the 802.11i update are described separately in Chapter 8, which is devoted to WLAN security. Wireless LAN Standards Table 6-10: PHY and MAC SAP Throughput Comparison for 802.11a, b and g Networks Network standard and configuration PHY data Effective Effective rate (Mbps) MAC SAP Throughput throughput versus (Mbps) 802.11b (%) 802.11b network 802.11g network with 802.11b stations (CTS/RTS) 802.11g network with 802.11b stations (CTS-to-self) 802.11g network with no 802.11b stations 802.11a Quality of Service (802.11e specification) The 802.11e specification provides a number of enhancements to the 802.11 MAC to improve the quality of service for time sensitive applications, such as streaming media and voice over wireless IP (VoWIP), and was approved for publication by the IEEE Standards Board in September 2