Characteristics of the FH PHY

Table 11-4 shows the values of a number of parameters in the FH PHY. In addition to the parameters in the table, which are standardized, the FH PHY has a number of parameters that can be adjusted to balance delays through various parts of an 802.11 frequency-hopping 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. One other item of note is that the total aggregate throughput of all frequency-hopping networks in an area can be quite high. The total aggregate throughput is a function of the hop set size. All sequences in a hop set are orthogonal and noninterfering. In North America and most of Europe, 26 frequency-hopping networks can be deployed in an area at once. If each network is run at the optional 2 Mbps rate and half the airtime is able to carry user payload data, the area can have a total of 26 Mbps throughput provided that the ISM band is relatively free of interference.

Table 11-4. FH PHY parameters

Parameter	Value	Notes
Slot time	50ms
SIFS time	28ms	The SIFS is used to derive the value of the other interframe spaces (DIFS, PIFS, and EIFS).
Contention window size	15-1,023 slots
Preamble duration	96ms	Preamble symbols are transmitted at 1 MHz, so a symbol takes 1 ms to transmit; 96 bits require 96 symbol times.
PLCP header duration	32ms	The PLCP header is 32 bits, so it requires 32 symbol times.
Maximum MAC frame	4,095 bytes	802.11 recommends a maximum of 400 symbols (400 bytes at 1 Mbps, 800 bytes at 2 Mbps) to retain performance across different types of environments.
Minimum sensitivity	-80 dBm