Frequency hopping | Spread-Spectrum LANs

Frequency hopping entails the transmitter jumping from one frequency to the next at a specific hopping rate in accordance with a pseudo-random code sequence. The order of frequencies selected by the transmitter is taken from a predetermined set as dictated by the code sequence. For example, the transmitter may have a hopping pattern of going from channel 6 to channel 1 to channel 5 to channel 9 to channel 3 and so on. The receiver tracks these changes. Since only the intended receiver is aware of the transmitter's hopping pattern, then only that receiver can make sense of the data being transmitted.

Other frequency hopping transmitters will be using different hopping patterns that usually will be on noninterfering frequencies. Should different transmitters coincidentally attempt to use the same frequency and the data of one or both become garbled at that point, retransmission of the affected data packets is required. Those data packets will be sent again on the next hopping frequency of each transmitter. Most LAN protocols have an integral error detection capability. When the protocol's error checking mechanism recognizes incoming packets that are bad or determines that there are missing packets, the receiving station requests a retransmission of only those packets. When the new packets arrive to rendezvous with those held in queue, the protocol's sequencing capability puts them in the correct order.

Some wireless LAN vendors use algorithms for dynamic data rate switching in conjunction with frequency hopping spread spectrum. The algorithms dynamically determine without user intervention whether the wireless signal is uncorrupted and whether the number of retransmissions is sufficiently low to allow data to be transmitted at a higher speed. The transmitting device can then select the maximum reliable data rate on the fly.

As noted, the FCC has authorized the use of spread-spectrum transmitters without requiring individual user licenses. However, only direct sequence or frequency hopping techniques may be used—no other spreading techniques are permitted, per FCC Rule 15.247. Users may define their own channel width up to a maximum of 500 kHz in the 900-MHz band or 1 MHz in the 2.4-GHz band. Frequency hopped systems must not spend more than 0.4 seconds on any one channel each 20 seconds, or 30 seconds in the 2.4-GHz band. Furthermore, they must hop through at least 50 channels in the 900-MHz band, or 75 channels in the 2.4-GHz band. These rules reduce the chance of repeated packet collisions in areas with multiple transmitters.

Direct sequence spread spectrum offers better performance, but frequency-hopping spread spectrum is more resistant to interference and is preferable in environments where many other devices generate electromechanical noise. Direct sequence is more expensive than frequency hopping and uses more power.

What hardware the user chooses will largely be dictated by applications. Users concerned with good performance in environments where interference is not a problem, will generally opt for the direct sequence solution. Users who need a small, inexpensive portable wireless adapter for their notebook or PDA, generally go for frequency-hopping spread spectrum.

Spread-spectrum technology is also used in bridges and routers for LAN extension. Although line-of-sight connections are required, such devices allow users to extend the reach of wireless LANs (Ethernet and token ring) by as much as 25 miles. For example, a bridge can be used to connect to a central server to access e-mail, groupware, and client/server applications without any modifications. Bridges and routers can also be used to link wireless and wireline LANs together into a seamless enterprise network.