402 GHZTIMESALESFHSS USE IN IEEE 8011 WIRELESS NETWORKING IS CONSI...

2.402 GHzTimeSalesFHSS use in IEEE 802.11 wireless networking is considered legacy and is rarely supported. Therefore, if a customer wishes to purchase any FHSS wireless LAN equipment, they should be directed to the proper upgrade path for a more current and supported solution. Technical SupportThere is still a number of legacy IEEE 802.11 wireless networking FHSS devices in operation in various industries today. For whatever reason that these are still in use, it is important to

Direct-Sequence Spread Spectrum (DSSS)

Direct-sequence spread spectrum (DSSS) is a spread spectrum technology used with wireless

LANs and defined by the original IEEE 802.11 standard. Like FHSS, DSSS supports data

rates of 1 and 2 Mbps and is considered slow by today’s computer networking requirements.

DSSS uses special techniques to transmit digital data across the air using radio fre-

quency. This is accomplished by modulating or modifying the radio frequency char-

acteristics such as phase, amplitude, and frequency (see Chapter 4, “Radio Frequency

Fundamentals for Wireless LAN Technology”).

In addition to modulation, DSSS uses technology known as a spreading code to provide

redundancy of the digital data as it traverses through the air. The spreading code transmits

information on multiple subcarriers, and the redundancy helps the receiver detect transmis-

sion errors due to interference. This spreading of information across the 22 MHz–wide

channel is what helps makes DSSS resilient to interference. The spreading code technology

allows the receiver to determine if a bit of digital data received is a binary 0 or binary 1.

Depending on the data rate, the transmitter and receiver understand the spreading code in

use and therefore are able to communicate.

An example of a coding technique is Barker code. Barker code is used as the spreading

code for DSSS at the data rates of 1 and 2 Mbps.

DSSS operates within a range of RF frequency also known as a channel. Unlike narrow-

band communication, which operates on a single frequency, a DSSS channel is 22 MHz

wide and is one of 14 channels in the 2.4 GHz to 2.5 GHz ISM band. The country and

location of the device will determine which of the 14 channels are available for use.

Figure 5.7 shows that channel 6 is 22 MHz wide in the ISM unlicensed RF band.

FHSS and DSSS both operate in the same frequency range. If devices that use both technologies are occupying the same physical area, the devices may encounter some interference.

High Rate/Direct-Sequence Spread Spectrum (HR/DSSS)

High rate/direct-sequence spread spectrum (HR/DSSS) is defined in the IEEE 802.11b

amendment to the 802.11 standard. HR/DSSS introduced higher data rates of 5.5 and

11 Mbps. At the time this amendment was released, because of the higher data rates,

this technology helped fuel the acceleration of IEEE standards based on wireless LAN

technology.

Like DSSS, HR/DSSS uses one of 14 channels 22 MHz wide to transmit and receive data.

The main difference between these two technologies is that HR/DSSS supports higher

data rates.

HR/DSSS (802.11b) uses a different spreading code or an encoding technique than

Spread Spectrum Technology 141f i g u r e 5 . 7 DSSS is limited to a 22 MHz–wide channel in the 2.4 GHz ISM band.Power (mW)