f data devices. They incorporate no inherent mechanisms for controlling latency or jitter. WLANs are half-duplex (only one part can send at a time), they share media (all users in an area share one radio channel), and the transmission rate depends on the distance from the end station to the WAP and on the effects of any impairments in the environment. The CSMA/CA process used in the 802.11 MAC protocol uses a system of backoff timers to help avoid collisions. All successful transmissions must be acknowledged, and if no acknowledgment is received, the transmitter sets a backoff counter with a random value. That backoff value is increased with every unsuccessful attempt. Because of these processes, the effective throughput of a WLAN is only some 50% to 55% of the raw transmission rate. These factors-chan- nel contention, waiting intervals, acknowledgments, and retransmissions-make it virtually impossible to provide a service that has consistent delay. To work around Chapter 15 WMANs, WLANs, and WPANs this, as with other packet voice systems, a time stamp is placed in the Real-Time Transport Protocol (RTP) header of each voice packet, and a buffer at the receiving end is used to mask the jitter effect. The long-term solution to the problem of capacity and QoS in VoWLANs is an enhanced MAC-layer protocol called Hybrid Coordination Function (HCF) that is developed in the IEEE 802. lle specification. The 802. 11e HCF protocol is a very important development in VoWLAN systems. In order to improve service for voice, the standard includes two operating modes, either of which can be used: Enhanced Digital Control Access (EDCA) and Polled Access. EDCA EDCA, which is mandatory, is an enhanced version of the Distributed Control Function (DCF) defined in the original 802.11 MAC protocol. EDCA defines eight levels of access priority to the shared wireless channel. EDCA access is a contention-based protocol (as is DCF) and relies on a set of waiting intervals and backoff timers designed to avoid collisions. However, there is a d