Spreading the Spectrum

FHSS divides the 2.4 GHz band into 75 1-MHz channels. The transmitter and receiver rapidly hop channels at a mutually-agreed interval to avoid collision with other stations using the same band. FHSS operates at a maximum rate of 2 Mbps.

DSSS divides the same band into 14 partially-overlapping 22-MHz channels. Instead of frequency-hopping, DSSS uses a technique called "chipping." Chipping spreads modulated data across the spectrum in a fashion that makes it possible to tolerate some signal loss. A DSSS radio chipping with the 1997 standard Barker code generates a carrier wave, modulated with Binary or Quadrature Phase Shift Keying. Modulating with BPSK yields 1 Mbps, while modulating the direct sequence with QPSK 2 Mbps.

The signal to be encoded by this Physical Medium Dependent (PMD) sublayer is defined by the Physical Layer Convergence Protocol (PLCP), which carries a 24-byte preamble, 6-byte header, and packet payload. This lengthy preamble—three times that of Ethernet—combined with other factors tend to make PLCP a relatively pricey protocol.

When you have just 1 or 2 Mbps to spend, the impact of overhead on effective throughput can impact viability of the system. Fortunately, in 1999, 802.11b High Rate replaced the Barker code with a more efficient chipping technique known as Complementary Code Keying (CCK), boosting throughput to 11 Mbps. 802.11b employs DSSS exclusively. It also adds dynamic rate shifting from 11 to 5.5 and 2 to 1 Mbps when required to adjust for distance, interference, and other changes in signal strength. An 802.11b radio can interoperate with an 802.11 DSSS radio at 1 to 2 Mbps, but not with an 802.11 FHSS radio.

It's important to realize that these rates represent maximum raw bandwidth. In practice, applications operating over WLANs don't come close to achieving this throughput. Furthermore, range and throughput vary inversely. An evaluation conducted at Cornell University in April, 2000 found that 802.11b NICs averaged 5 Mbps within 20-feet of an AP, but dropped to 1 Mbps when operating 60-feet from that AP. WLANs are getting faster and reaching farther, but actual results depend on deployment.

Each 802.11 station delays packets for a random allotment of time before transmitting, then its waits to receive acknowledgment from the AP. If no ACK is received, a collision is assumed, and the entire process is repeated. The 802.11 MACs also provide Cyclic Redundancy Checking (CRC) for errors and packet fragmentation functions not present in 802.3 standard. The objective is simple—compensate for a less reliable medium with higher retransmission costs.

Another challenge faced by wireless LANs is referred to as the "hidden station" problem. On a shared Ethernet segment, every station can hear every other station. However, in a wireless cell, it is possible for two stations to see the same AP, but not hear each other. So 802.11 stations may invoke a Request-to-Send/Clear-to-Send (RTS/CTS) exchange before transmitting data. RTS/CTS adds overhead, but reduces retransmission costs due to collision between hidden stations. It is most useful for transporting large packets that have a greater probability of collision and higher retransmission costs.

Wireless network architecture
The 802.11 protocol can be deployed in ad hoc mode for peer-to-peer communication between wireless stations. More often, 802.11 is deployed in "infrastructure" mode enabling communication between stations and APs. A Basic Service Set (BSS) is a group of stations communicating with a single AP. An Extended Service Set (ESS) is a collection of BSSs forming a layer 3 subnet.

Wireless stations select an AP based on signal strength and channel utilization. To join a BSS, the station sends an associate request to the AP. If this request is accepted, the station tunes its radio to the APs channel, then contends for the channel with all other stations in the BSS. The station periodically monitors other channels, and may reassociate with another AP due to changes in distance, interference, or channel utilization.

To create a coverage area, APs are deployed throughout the premises when constructed for use in a home, hotel floor, office building, or campus. Each AP is tuned to a different channel—the area surrounding it is a "cell." Channels are assigned to adjacent cells in a fashion that minimizes cross-talk

It is possible to distribute several APs, tuned to the same channel, across a coverage area to increase range. Alternatively, up to three APs can be deployed at the same location, tuned to different channels for increased bandwidth. If each AP sustains 5 Mbps, then the aggregate throughput provided by an AP trio is roughly 15 Mbps—comparable to wired Ethernet LANs. Three collocated APs is the limit because of the number of non-overlapping channels in the 2.4 GHz band.

The 802.11 standard does not cover AP hand-off when a station roams from one BSS to another. Some vendors use proprietary inter-AP protocols to coordinate station hand-off within an ESS.

When a station roams to a new ESS, it may require a new IP address appropriate for that subnet. Dynamic Host Configuration Protocols (DHCP) can be used to renew the station's IP address. Alternatively, Mobile IP can be used to enable roaming within the same IP address.

—End

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