Characteristics of the OFDM PHY

Parameters specific to the OFDM PHY are listed in Table 13-5. Like the physical layers presented in Chapters 11 and 12, the OFDM PHY also incorporates a number of parameters to adjust for the delay in various processing stages in the electronics. As a final note, the extra radio bandwidth provided offers a great deal of throughput. In the United States, there are currently 12 channels, with the additional space for eight channels recently cleared. 20 co-located channels operating with 50% of the transmission time devoted to user traffic can move over 500 Mbps. This number is far greater than any other physical layer yet standardized.

Table 13-5. OFDM PHY parameters

Parameter	Value	Notes
Maximum MAC frame length	4,095 bytes
Slot time	9 ms
SIFS time	16 ms	The SIFS is used to derive the value of the other interframe spaces (DIFS, PIFS, and EIFS).
Contention window size	15 to 1,023 slots
Preamble duration	20 ms
PLCP header duration	4 ms
Receiver sensitivity	-65 to -82 dBm	Depends on speed of data transmission.

Like the other physical layers, the OFDM PHY has a number of attributes that can be adjusted by a vendor to balance delays in various parts of the system. It includes variables for the latency through the MAC, the PLCP, and the transceiver, as well as variables to account for variations in the transceiver electronics.