Definition of spread spectrum in The Network Encyclopedia.
How It Works
Unlike other forms of wireless communication, spread spectrum technologies take advantage of a large portion of the electromagnetic spectrum, making it difficult for distrusted users to “listen in” on private conversations. Two basic mechanisms can be used to implement spread spectrum wireless communication: direct sequence technology and frequency-hopping technology.
Direct sequence technology takes an individual binary bit from the transmission signal and converts it to a binary string. This string is then transmitted as a single wideband signal over an adjacent set of frequencies, with each bit in the string transmitted at a different frequency. The receiving station examines the bit pattern of the binary string and determines which single bit was originally transmitted by the sending station. This technology has built-in fault tolerance because electromagnetic interference (EMI) might degrade a portion of the binary string, but if the receiving station can recognize a different portion of the string, communication is assured. A typical example of direct sequencing technology might be to assign the string 10011011 to bit 1 and its inverse 01100100 to bit 0. Transmission of the bit sequence 110 would then consist of three transmitted strings: 10011011, 10011011, and 01100100.
Frequency-hopping technology uses a continually changing carrier frequency. The pattern by which the carrier frequency is changed is programmed according to an algorithm known to both the sending and receiving stations. For communication to take place, the two stations must remain synchronized throughout the session. One station is designated the master station and the other the slave station. If particular frequencies within the spread spectrum communication band contain interference from other radio sources, frequency-hopping technology can avoid these frequencies by using adaptive techniques. To further enhance security, either station can also dynamically change the pattern of frequency hopping.
Spread spectrum technologies can have a variety of uses in networking, including point-to-point links between networks, wireless local area networks (LANs), and cellular-based roving network communication. One common use in networking environments is for connecting stations to a LAN when it is impractical or impossible to lay cabling. Communication is currently limited to speeds of about 2 Mbps. Spread spectrum networking systems generally use very low power signals in the high radio or low microwave portion of the electromagnetic spectrum.
Spread spectrum communication can take place in three portions of the electromagnetic spectrum allocated by the Federal Communications Commission (FCC) without special licensing for wireless devices:
- Industrial band:902 to 928 MHz
- Scientific band:2400 to 2483.5 MHz
- Medical band:5725 to 5850 MHz
You can also use spread spectrum wireless bridge technologies to establish point-to-point or multipoint communication between buildings on a campus. These devices usually support line-of-sight connections that function to distances of 30 kilometers or more, with speed decreasing as the distance increases. Spread spectrum devices for wireless LAN stations generally have a much shorter range, usually no more than about 200 meters.