OFDM PLCP

Like all the other physical layers, the OFDM PHY includes its own PLCP, which adds physical layer-specific framing parameters.

Framing

The OFDM PHY adds a preamble and a PLCP header. It also adds trailing bits to assist the encoding schemes used. This section divides the PLCP frame logically, but some components span different fields in the protocol unit. Figure 13-14 is the jumping-off point for discussion of the OFDM frame.

Figure 13-14. OFDM PLCP framing format

Figure 13-15 shows the start of a frame, but includes the guard intervals and windowing used by the transmitter. The preamble lasts 16 ms, which is evenly divided between short and long training sequences; the difference between the two is described in the next section. After the preamble, one OFDM symbol carries the Signal field, then a variable number of data symbols carry the end of the PLCP header, the MAC payload, and the trailer. All symbols use a modified cosine window to ensure smooth transitions. After the short preamble, which is used to synchronize frequencies, a guard time protects against multipath fading.

Figure 13-15. Preamble and frame start

Preamble

As with all other common IEEE 802 networks, and certainly all 802.11 physical layers, the OFDM physical protocol unit begins with a preamble. It is composed of 12 OFDM symbols that synchronize various timers between the transmitter and the receiver. The first 10 symbols are a short training sequence, which the receiver uses to lock on to the signal, select an appropriate antenna if the receiver is using multiple antennas, and synchronize the large-scale timing relationships required to begin decoding the following symbols. The short training sequences are transmitted without a guard period. Two long training sequences follow the short training sequences. Long training sequences fine-tune the timing acquisition and are protected by a guard interval.

Header

The PLCP header is transmitted in the Signal field of the physical protocol unit; it incorporates the Service field from the Data field of the physical protocol unit. As shown in Figure 13-16, the Signal field incorporates the Rate field, a Length field, and a Tail field.

Figure 13-16. Signal field of OFDM PLCP frame

Rate (4 bits)

Four bits encode the data rate. Table 13-2 shows the bits used to encode each of the data rates. See the section "OFDM PMD" later in this chapter for details on the encoding and modulation scheme used for each data rate.

Length (12 bits)

12 bits encode the number of bytes in the embedded MAC frame. Like most fields, it is transmitted least-significant bit to most-significant bit. The length is processed by a convolutional code to protect against bit errors.

Parity (1 bit) and Reserved (1 bit)

Bit 4 is reserved for future use and must be set to 0. Bit 17 is an even parity bit for the first 16 Signal bits to protect against data corruption.

Tail (6 bits)

The Signal field ends with six 0 tail bits used to unwind the convolutional code. As such, they must by definition be processed by the convolutional code.

Service (16 bits)

The final field in the PLCP header is the 16-bit Service field. Unlike the other components of the PLCP header, it is transmitted in the Data field of the physical protocol unit at the data rate of the embedded MAC frame. The first eight bits are set to 0. As with the other physical layers, MAC frames are scrambled before transmission; the first six bits are set to 0 to initialize the scrambler. The remaining nine bits are reserved and must set to 0 until they are adopted for future use.

Data

The encoding scheme used for the data depends on the data rate. Before transmission, data is scrambled, as it is with the other physical layers. The Service field of the header is included in the Data field of the physical protocol unit because it initializes the scrambler.

Trailer

The Data field of the physical protocol unit ends with a trailer. (The 802.11a specification does not call the ending fields a trailer, but it is a descriptive term.) It is composed of two fields:

Tail (6 bits)

Like the tail bits in the PLCP header, the tail bits appended to the end of the MAC frame bring the convolutional code smoothly to an end. Six bits are required because the convolutional code has a constraint length of seven.

Pad (variable)

As used by 802.11a, OFDM requires that fixed-size blocks of data bits be transferred. The Data field is padded so that its length is an integer multiple of the block size. The block size depends on the modulation and coding used by the data rate; it is discussed in the next section.