802.11 Wireless LANs

The original 802.11 standard defined two WLAN PHY methods:

• 2.4 GHz frequency hopping spread spectrum (FHSS)

• 2.4 GHz direct sequence spread spectrum (DSSS)

Frequency Hopping WLANs

FHSS WLANs support 1 Mbps and 2 Mbps data rates. As the name implies, a FHSS device changes or "hops" frequencies with a predetermined hopping pattern and a set rate, as depicted in Figure 3-8. FHSS devices split the available spectrum into 79 nonoverlapping channels (for North America and most of Europe) across the 2.402 to 2.480 GHz frequency range. Each of the 79 channels is 1 MHz wide, so FHSS WLANs use a relatively fast 1 MHz symbol rate and hop among the 79 channels at a much slower rate.


The hopping sequence must hop at a minimum rate of 2.5 times per second and must contain a minimum of six channels (6 MHz). To minimize the collisions between overlapping coverage areas, the possible hopping sequences can be broken down into three sets of length, 26 for use in North America and most of Europe. Tables 3-1 through 3-4 show the minimum overlap hopping patterns for different countries, including the U.S., Japan, Spain, and France.



In essence, the hopping patterns provide a slow path through the possible channels in such a way that each hop covers at least 6 MHz and, when considering a multicell deployment, minimizes the probability of a collision. The reduced set length for countries such as Japan, Spain, and France results from the smaller ISM band frequency allocation at 2.4 GHz.


FHSS PLCP

After the MAC layer passes a MAC frame, also known as a PLCP service data unit (PSDU) in FHSS WLANs, to the PLCP sublayer, the PLCP adds two fields to the beginning of the frame to form a PPDU frame. Figure 3-9 shows the FHSS PLCP frame format.


Direct Sequence Spread Spectrum WLANs

DSSS is another physical layer for the 802.11 specifications. As defined in the 1997 802.11 standard, DSSS supports data rates of 1 and 2 Mbps. In 1999, the 802.11 Working Group ratified the 802.11b standard to support data rates of 5.5 and 11 Mbps. The 802.11b DSSS physical layer is compatible with existing 802.11 DSSS WLANs. The PLCP for 802.11b DSSS is the same as that for 802.11 DSSS, with the addition of an optional short preamble and short header.



802.11 DSSS

Similar to the PLCP sublayer for FHSS, the PLCP for 802.11 DSSS adds two fields to the MAC frame to form the PPDU: the PLCP preamble and PLCP header. The frame format appears in Figure 3-14.


DSSS Basics

Spread-spectrum techniques take a modulation approach that uses a much higher than necessary spectrum bandwidth to communicate information at a much lower rate. Each bit is replaced or spread by a wideband spreading code. Much like coding, because the information is spread into many more information bits, it has the ability to operate in low signal-to-noise ratio (SNR) conditions, either because of interference or low transmitter power. With DSSS, the transmitted signal is directly multiplied by a spreading sequence, shared by the transmitter and receiver.