ERP Physical Layer Convergence (PLCP)

The ERP PHY's convergence layer is quite complicated. Due to the plethora of operational modes, there are a number of different ways that frames will be bundled up for transmission by the radio interface. Each of the modes described at the start of the chapter has its own physical layer framing.

ERP-OFDM Framing

This mode is the meat of the specification. All stations are required to implement it, and it is what is colloquially meant by "802.11g support" because most stations default to using it. In practice, understanding ERP-OFDM physical framing will provide you with the understanding of most 802.11g transmissions in the air.

The format of the ERP-OFDM frame at the physical layer, shown in Figure 14-3, is nearly identical to 802.11a. ERP-OFDM uses an identical logical protocol data unit, shown as the top line of the figure. In fact, the only major difference from 802.11a is that the frame is followed by a six microsecond idle time called the signal extension, which is shown in the second line of the figure, where the logical construction is assembled for transmission. The reason for the extra 6 ms is to make timing calculations and frame rates identical to 802.11a. 802.11a uses a 16 ms SIFS time, which is used in part to finish decoding the previous frame. For backwards compatibility, 802.11g uses the 10 ms idle time used by 802.11b. To provide enough time for the decoding process to finish, 802.11g adds 6 ms idle time to the end of the frame, leaving a 16 ms gap for the hardware to run its decode process. Of course, the NAV is set so that the virtual carrier sense mechanism reports the medium idle after the signal extension period completes. In the lower line, the OFDM transmissions are shown. It is identical to 802.11a.

Single-Carrier Framing with 802.11g

Although the OFDM modulation discussed in the previous section is by far the most common, it is also possible to use 802.11b-compatible framing directly around the higher speed body. Traditional framing is used with both the packet binary convolution coding and the DSSS-OFDM layer. Because older 802.11b stations can read the frame header, they can avoid using the medium during transmission and do not require protection. Figure 14-4 shows the use of traditional single-carrier framing in 802.11g.

Preamble

The Preamble is identical to the 802.11b preamble discussed in Chapter 12, and consists of a synchronization field followed by a start of frame delimiter. It may be either the long preamble of 144 bits or the short preamble of 72 bits. In either case, the Preamble is transmitted at 1 Mbps using DBPSK modulation. Before modulation, the data is scrambled just as it is in 802.11b.

Figure 14-3. ERP-OFDM PLCP frame format

Figure 14-4. Long preamble ERP PLCP frame format

PLCP Header

The PLCP header is identical to the PLCP header discussed in Chapter 12. It consists of the Signal field, Service field, a Length field, and a PLCP-layer CRC check. The Length and CRC fields are identical to the 802.11b interpretation.

Signal field

This field is used to show the rate at which the PLCP payload (the MAC frame) is modulated. Initially, it was defined as the multiplier of 100 kbps that would result in the encoding rate. With the field defined as only eight bits, it would limit the encoding speed to 25.5 Mbps. To accommodate increased speeds, the Signal field consists of a label that describes the speed, as shown in Table 14-1. The signal field is not needed to tell the receiver what the encoding rate of a DSSS-OFDM frame is because there is a separate OFDM header to perform that task.

Service field

The service field, shown in Figure 14-5, contains control bits to help the receiver decode the frame. As discussed previously, bits 0, 1, and 4 are reserved and must be set to zero. In all 802.11g stations, the transmit and symbol clocks are locked, so bit 2 is always set to 1. Bit 3 is set when the frame body is modulated with PBCC, and set to zero for DSSS, CCK, and DSSS-OFDM modulations. The last three length extension bits are used to assist receivers in determining the frame length in bytes from the Length field, which is expressed in terms of the number of microseconds required for transmission. The standards have a complicated set of rules for when the bits must be set, but they are beyond the level of detail required for this book, especially since the single-carrier framing technique is uncommon.

Figure 14-5. 802.11g SERVICE field

Frame Body

The final component of the PLCP frame is its payload, which is the MAC frame, modulated either by PBCC or OFDM. Details of the frame body modulation will be discussed in subsequent sections.

PBCC coding

To transmit a frame using PBCC, the frame data is first run through a convolutional code. The frame is broken into 2-bit elements, and are used as input into a convolutional code that outputs three bits. Each 3-bit block is mapped on to a symbol using 8PSK. Receivers will reverse the process, translating a single phase shift into one of eight symbols. After translating the symbol to a three-bit sequence, the convolutional code is used to remove the redundant bit and recover the original data. See Figure 14-6.

Figure 14-6. PBCC processing

Achieving a 22 Mbps data rate with this physical encoding is straightforward. The symbol clock continues to run at 11 MHz, just as in 802.11b, but each symbol is now capable of transmitting two bits. To run at 33 Mbps, the symbol rate for the data portion of the frame must be increased to 16.5 MHz. At two bits per symbol, the overall data rate is 33 Mbps. Clock speed switching must occur between the PLCP preamble and its payload.

DSSS-OFDM framing

DSSS-OFDM is a hybrid framing technique. The higher-layer packet is encoded with OFDM, and the OFDM-modulated packet is framed with a traditional-single carrier header, as shown in Figure 14-7. Headers are transmitted in accord with 802.11b (including the data scrambler used by 802.11b), but the frame body is modulated using OFDM, in a process identical to the encoding used in 802.11 and described in Chapter 13. Although similar to the encoding in 802.11a, the DSSS-OFDM framing eliminates the initial short training sequences. It also adds a 6 ms signal extension field to allow the convolution decode extra time to finish. The transition from direct sequence modulation to OFDM that occurs at the end of the PLCP header is somewhat complex from a radio engineering point of view, but is beyond the scope of this book.