The wireless physical layer is split into two parts, called the PLCP (Physical Layer Convergence Protocol) and the PMD (Physical Medium Dependent) sub layer. The PMD takes care of the wireless encoding. The PLCP presents a common interface for higher-level drivers to write to and provides carrier sense and CCA (Clear Channel Assessment), which is the signal that the MAC (Media Access Control) layer needs so it can determine whether the medium is currently in use.The wireless physical layer is split into two parts, called the PLCP (Physical Layer Convergence Protocol) and the PMD (Physical Medium Dependent) sub layer. The PMD takes care of the wireless encoding. The PLCP presents a common interface for higher-level drivers to write to and provides carrier sense and CCA (Clear Channel Assessment), which is the signal that the MAC (Media Access Control) layer needs so it can determine whether the medium is currently in use. The PLCP consists of a 144 bits preamble that is used for synchronization to determine radio gain and to establish CCA. The preamble comprises 128 bits of synchronization, followed by a 16 bits field consisting of the pattern 1111001110100000. This sequence is used to mark the start of every frame and is called the SFD (Start Frame Delimiter).
The next 48 bits are collectively known as the PLCP header. The header contains four fields: signal, service, length and HEC (header error check). The signal field indicates how fast the payload will be transmitted (1, 2, 5.5 or 11 Mbps). The service field is reserved for future use. The length field indicates the length of the ensuing payload, and the HEC is a 16bits CRC of the 48 bits header.In a wireless environment, the PLCP is always transmitted at 1 Mbps. Thus, 24 bytes of each packet are sent at 1 Mbps. The PLCP introduces 24 bytes of overhead into each wireless Ethernet packet before we even start talking about where the packet is going. Ethernet introduces only 8 bytes of data. Because the 192 bits header payload is transmitted at 1Mbps, 802.11b is at best only 85 percent efficient at the physical layer.
The IEEE 802.11b is a Direct Sequence Spread Spectrum (DSSS) system very similar in concept to the CDMA Wireless, using a spread spectrum chip sequence. In the 802.11b the transmission medium is wireless and the operating frequency band is 2.4 GHz. 802.11b provides 5.5 and 11 Mbps payload data rates in addition to the 1 and 2 Mbps rates provided by 802.11. To provide the higher rates, 8 chip Complementary Code Keying (CCK) is employed as the modulation scheme. The CCK uses 6 bits to encode the code sent, this increase the speed of the 802.11 by 6.Thechipping rate is 11 MHz, which is the same as the DSSS system as described in 802.11, thus providing the same occupied channel bandwidth. 802.11b describes an optional mode replacing the CCK modulation with packet binary convolutional coding (HR/DSSS/PBCC).
Another optional mode of 802.11b allows data throughput at the higher rates (2, 5.5, and 11 Mbps) to be significantly increased by using a shorter PLCP preamble. This mode is called HR/DSSS/short. This Short Preamble mode can coexist with DSSS, HR/DSSS under limited circumstances, such as on different channels or with appropriate CCA mechanisms. The High Rate PHY contains three functional entities: the PMD function, the physical layer convergence function, and the layer management function. For the purposes of MAC and MAC Management when channel agility is both present and enabled, the High Rate PHY shall be interpreted to be both a High Rate and a frequency hopping physical layer. The High Rate PHY service shall be provided to the MAC through the PHY service primitives. To allow the MAC to operate with minimum dependence on the PMD sub layer, a physical layer convergence procedure (PLCP)sub layer is defined. This function simplifies the PHY service interface to the MAC services. The PMD sub layer provides a means and method of transmitting and receiving data through a wireless medium (WM) between two or more STAs each using the High Rate system. The PLME performs management of the local PHY functions in conjunction with the MAC management entity.