Most WLANs employ spread spectrum techniques for security. The two methods used are frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS). Frequency hopping employs a carrier that changes frequency in a pattern that is determined by a random signal generator. The pattern is known only to the transmitter and the intended receiver. The direct sequence scheme generates a redundant bit pattern for each bit (called a chip) to be transmitted. The longer the chip, the greater the probability that the original data can be recovered.
The Federal Communications Commission (FCC) has approved specific frequency bands for WLANs. These are 902 to 928 MHz, 2.4 to 2.483, 5.15 to 5.35 and 5.725 to 5.875 GHz. WLANs are governed by certain standards. The Institute of Electrical and Electronic Engineering (IEEE) has a specific standard, which is 802.11. There are different levels of this standard such as 802.11 (data up to 2 Mbps in the 2.4 GHz band), 802.11a (data up to 54 Mbps in the 5 GHz band), 802.11b (data up to 11 Mbps in the 2.4 GHz band) and 802.11g (data up to 54 Mbps in the 2.4 GHz band). 802.11n improves on 802.11g in the amount of bandwidth supported by utilizing multiple wireless signals and antennas, i.e. multiple input-multiple output (MIMO) technology. The newest generation, 802.11ac, utilizes dual-band wireless technology, supporting simultaneous connections on both the 2.4 and 5 GHz Wi-Fi bands. 802.11ac offers backward compatibility to 802.11b/g/n and bandwidth rated up to 1300 Mbps on the 5 GHz band plus up to 450 Mbps on 2.4 GHz.
Another standard that is widely used is Bluetooth. This was named after Harald Bluetooth who was king of Denmark in the late 10th century. He united Denmark and part of Norway into a single kingdom and then introduced Christianity into Denmark. The choice of this name for a wireless standard indicates how important companies from the Baltic region are to the communications industry. Bluetooth operates in the 2.4 GHz band, which has been set aside by international agreement for use in industrial, scientific and medical (ISM) areas. In the U.S. and Europe, the frequency range is 2.400 to 2.483.5 GHz with 79 1 MHz channels (The range of 2.402 to 2.480 GHz is actually used). In Japan, the frequency range of 2.472 to 2.479 GHz with 23 1 MHz channels used. It provides an approach which enables various devices to communicate with each other within approximately a 10 m range. Its primary purpose is to unify connections within small work areas so that the electronic devices in close proximity can communicate effectively with one another.
Bluetooth provides a universal short-range wireless network that is available worldwide for unlicensed low-power use. If, for example, two Bluetooth devices are within 10 m of one another, they can share up to 720 Kbps of capacity. Bluetooth can transmit data, audio, graphics and video. Applications include headsets, cordless telephones, home stereos and digital MP3 players. Some of the things that are possible or currently operational are making calls from wireless headsets to cell phones, the elimination of cables between computers and peripheral equipment, the connection of MP3 equipment to other machines for music download, monitoring systems for home appliances and the control of many home appliances. A piconet is a small network with eight devices communicating within this network. A Bluetooth radio can have 10 piconets working together within the same range of coverage.
Wireless LAN Technology
Current wireless technology falls into three categories: infrared (IR), at optical wavelengths extending from 770 nm and upward, spread spectrum systems and narrowband microwave systems. IR applications are rather limited in range since they involve infrared light that cannot penetrate opaque walls. IR communication is therefore practical only within a single room. Spread spectrum systems operate solely within the ISM bands. Within these bands, there is no FCC license required. Narrowband microwave WLAN does not use spread spectrum techniques and when used in bands other than ISM bands, it requires an FCC license. Applications for narrowband microwave in addition to WLAN include land mobile radio and wireless backhaul.
Infrared technology parameters and characteristics are shown in Table 1. The parameters are shown for both diffused infrared (which uses a source, such as a light emitting diode (LED), which spreads the light transmission over an area) and direct beam infrared (which uses a more focused beam of light). Spread spectrum is broken down further into frequency hopping and direct sequence (see Table 2). Narrowband microwave is shown in Table 3.
|Data Rate (Mbps)||1 to 4||1 to 10|
|Range (m)||15 to 60||25|
|Wavelength (nm)||800 to 900||800 to 900|
|Parameter||Frequency Hopping||Direct Sequence|
|Data Rate (Mbps)||1 to 3||2 to 20|
|Range (m)||30 to 100||30 to 250|
|Frequency||—||902 to 928 MHz|
|—||2.4 to 2.435 GHz|
|—||5.725 to 5.85 GHz|
|Radiated Power||< 1 W||< 1 W|
|Data Rata (Mbps)||10 to 20|
|Range (m)||10 to 40|
|Frequency||902 to 928 MHz|
|5.2 to 5.775 GHz|
|18.825 to 19.205 GHz|
|Radiated Power||25 mw|