is combination is also used for multicast or broadcast frames; - 11: the block acknowledgment mechanism must be used. 802.11a/g Interfaces 3.1. 802.11a interface 3.1.1. PLCP sub-layer On transmission, the physical layer convergence procedure (PLCP) converts the PLCP service data units (PSDU) from the MAC (Medium Access Control) layer to form the PLCP protocol data units (PPDU), adding a preamble and a header. On reception, the preamble and header facilitates demodulation of the signal and the delivery of the PSDU data units. The PLCP frame ends with tail and padding bits. The PLCP header contains the LENGTH and RATE fields, a reserved bit, an even parity bit and the SERVICE field (Figure 3.1). The LENGTH and RATE fields, the reserved bit and the even parity bit, the SIGNAL set, constitute an OFDM (Orthogonal Frequency-Division Multiplexing) symbol and are transmitted with the most robust modulation and coding scheme: - BPSK (Binary Phase-Shift Keying) modulation; - coding rate of 1/2. The SERVICE field, the PSDU data units, the tail and padding bits, the DATA set, are transmitted at the rate indicated in the RATE field and constitute several OFDM symbols (Table 3.1). Wi-Fi Integration to the 4G Mobile Network, First Edition. André Perez. C ISTE Ltd 2018. Published by ISTE Ltd and John Wiley & Sons, Inc. Wi-Fi Integration to the 4G Mobile Network The first seven bits of the SERVICE field are used to synchronize the descrambler on reception. The other nine bits are reserved for later use. The LENGTH field encodes the number of bytes of the PSDU data unit. The tail bits of the SIGNAL symbol allow the decoding of the RATE and LENGTH fields. The RATE and LENGTH fields allow the mobile to predict the duration of the PLCP frame, even if the bit rate is not supported by the mobile. PLCP header Reserved LENGTH Parity SERVICE 4 bits 1 bit 12 bits 1 bit 6 bits 16 bits 6 bits Preamble SIGNAL Figure 3.1 Format of PLCP frame RATE field Rate (Mbps) Table 3.1. Rates of DATA field 802.11a/g Interfaces 3.1.2. PMD sub-layer 3.1.2.1