802.11g/a PHY operation

802.11g/a PHY operation
In this section we will discuss how the 802.11b specification differs from 802.11g/a.
In our earlier section we discussed the 802.11g specification which prescribes the  extended rate of the HR/DSSS method. The 802.11g/a specifications discuss the
extended rate of the PHY (ERP) layer and divulges compatibility with DSSS, CCK and
PBCC modulations schemes which are supported by their respective data rates of 1,
2, 5.5 and 11Mbit/s. The ERP additionally increases the data rate to a maximum of
54Mbit/s and affords a graceful degradation through 48, 36, 24, 18, 12, 9, 6Mbit/s
subject to environmental conditions. The data rates apply to both 802.11g and
802.11a in addition to the matching modulation scheme, namely OFDM.
Both 802.11g and 802.11a specifications share a similar PPDU format, as
shown in Figure 13.24. The preamble and the header are appended to the PSDU, but
the field make-up slightly differs, as shown. The ERP-OFDM PPDU, as illustrated,
comprises a preamble, a Signal and a Data field. The Signal field comprises a further
five fields, although the Reserved field will be used for future use and the Parity field
will remain even for the bits zero to sixteen, as shown in Figure 13.25. Table 13.4
illustrates the possible combination of bit values that are used to determine the rate
used for the physical interface (802.11g and 802.11a).
The Length field indicates the number of octets that need to be transmitted
whilst the Tail field comprises 6-bits that are set to zero. If we refer back to Figure 13-24,
we will continue to break down the respective fields. At the end of the Tail field
(signal) we start to breakdown the units that comprise the Data field, which are the
Service, PSDU, Tail and Padding fields. The Service field comprises one field, namely the Scramble field (bits zero to six),
which is used to synchronize the descrambler in the receiver, as shown in Figure 13.26;
the remaining bits within the field are reserved for future use and are all initialized to
zero. The Tail field comprises six bits (all set to zero) which, in turn, are used to initialize
the convolutional encoder. The final Padding field uses six bits, which are used
to form the total number of bits that make-up the Data field as a multiple of the total
coded bits in an OFDM symbol.
The PHY layer undertakes a number of operations to facilitate in the transmission
and reception of data on the wireless medium. Typically, there is a transmit procedure,
which is invoked when the MAC layer informs the PHY layer it has some data
to transmit. Alongside the transmit procedure, an additional procedure is employed
to determine if it is viable for the data to be transmitted. The Carrier Sense (CS) and
Clear Channel Assessment (CCA) procedures are executed prior to the payload being
sent to ensure that a channel is clear and that there is no incoming payload; this is very
similar to the Collision Detect (CD) mechanism used with Ethernet data transmissions
to ensure that there are no collisions on the physical medium. The receive procedure
uses the same CS/CCA mechanisms to detect the preamble (sync field) followed by
the SFD. You may recall that the sync field is used to notify the receiver of an incoming
payload and, as such, must synchronize using the series of ones and zeros prior to
receiving the SFD.