isions can be detected, but is not possible for a radio transceiver since the transmitted signal would overload any attempt to receive at the same time. In wireless networks such as 802.11, where collision detection is not possible, a variant of CSMA/CD known as CSMA/CA is used, where the CA stands for Collision Avoidance. Apart from the fact that collisions are not detected by the transmitting device, CSMA/CA has some similarities with CSMA/CD. Devices sense the medium before transmitting and wait if the medium is busy. The duration field in each transmitted frame (see preceding Table 2-6) enables a waiting device to predict how long the medium will be busy. Once the medium is sensed as being idle, waiting devices compute a random time period, called the contention period, and attempt to transmit after the contention period has expired. This is a similar mechanism to the back-off in CSMA/CD, except that here it is designed to avoid collisions between stations that are waiting for the end of another station's transmitted frame rather than being a mechanism to recover after a detected collision. CSMA/CA is further described in the Section "The 802.11 MAC Layer, p. 144", where the 802.11 MAC is discussed in more detail, and variations on CSMA/CA used in other types of wireless network will be described as they are encountered. Physical Layer Technologies When the MPDU is passed down to the PHY layer, it is processed by the PHY Layer Convergence Procedure (PLCP) and receives a preamble and header, which depend on the specific type of PHY layer in use. The PLCP Chapter Two preamble contains a string of bits that enables a receiver to synchronise its demodulator to the incoming signal timing. The preamble is terminated by a specific bit sequence that identifies the start of the header, which in turn informs the receiver of the type of modulation and coding scheme to be used to decode the upcoming data unit. The assembled PLCP Protocol Data Unit (PPDU) is passed to the Physical Medium Dependent (PMD) sublayer, which tr