Wireless communications utilize the radio spectrum as a means of transmission and, depending on the type of wireless technology being used, determine the characteristics of this transmission.
Today's WLANs provide mobility to network users while maintaining the requisite connectivity to corporate resources. WLANs offer organizations greater productivity per employee by providing constant connectivity to portable devices such as laptops to traditional network services.
As laptops become more pervasive in the workplace for their portability in meetings and conferences and during business travel, users are more prone to use them as their primary computing device.
WLANs have existed for many years and provide connectivity to wired infrastructures in which mobility is a requirement to specific working environments. Today several standards exist for WLAN applications, including the following:
802.11 A family of IEEE standards for WLANs
HiperLAN A WLAN protocol developed by ETSI that provides a 23.5 Mbps data rate in the 5 GHz band
HomeRF SWAP A wireless technology from the HomeRF working group
Bluetooth A wireless personal-area network (PAN) technology from the Bluetooth Special Interest Group
This chapter focuses on 802.11 WLANs and their specific technologies and associated components. Standard 802.11-based wireless technologies take advantage of the radio spectrum that is deemed usable by the public and that is unlicensed. This spectrum is known as the Industrial, Scientific, and Medical (ISM) band. The 802.11 standard specifically takes advantage of two of the three frequency bands: the 2.4 GHz to 2.4835 GHz UHF band used for 802.11 and 802.11b networks, and the 5.15 GHz to 5.825 GHz SHF band used for 802.11a-based networks.
The 2.4 GHz ISM band (used by 802.11b) makes use of spread-spectrum technology, which dictates that data transmissions are spread across numerous frequencies.
The 802.11 standard specifies two different types of Layer 1 physical interfaces for radio-based devices:
Frequency hopping The 2.4 GHz ISM band provides for 83.5 MHz of available frequency spectrum. The frequency-hopping architecture makes use of the available frequency range by creating hopping patterns to transmit on one of 79 1-MHz-wide frequencies for no more than 0.4 seconds at a time. This setup allows for an network that tolerates interference.
Direct sequencing Direct sequencing provides 11 overlapping channels of 83 MHz within the 2.4 GHz spectrum. Within the 11 overlapping channels, there are 3 22-MHz-wide nonoverlapping channels.
Finally, an 802.11 WLAN consists of the following components:
Access point An access point operates within a specific frequency spectrum and uses an 802.11 standard specified modulation technique. It also informs the wireless clients of its availability and authenticates and associates wireless clients to the wireless network.
NIC or client adapter A PC or workstation uses a wireless NIC to connect to the wireless network. The NIC scans the available frequency spectrum for connectivity and associates it with an access point or another wireless client.
Bridge Wireless bridges are used to connect multiple LANs (both wired and wireless) at the Media Access Control (MAC) layer level. Used in building-to-building wireless connections, wireless bridges can cover longer distances than access points.
Antenna An antenna radiates the modulated signal through the air so that wireless clients can receive it. Antennas are needed on both the access point and bridge and the clients.