Modulation

Modulation, which is a Physical Layer function, is a process in which the radio transceiver prepares the digital signal within the NIC for transmission over the airwaves. Modulation is the process of adding data to a carrier by altering the amplitude, frequency, or phase of the carrier in a controlled manner. Knowing the many different kinds of modulations used with wireless LANs is helpful when trying to build a compatible network piece-by-piece.


Figure 8.9 shows the details of modulation and spreading code types used with Frequency Hopping and Direct Sequence wireless LANs in the 2.4 GHz ISM band. Differential Binary Phase Shift Keying (DBPSK), Differential Quadrature Phase Shift Keying (DQPSK), and Gaussian Frequency Shift Keying (GFSK) are the types of modulation used by 802.11 and 802.11b products on the market today. Barker Code and Complimentary Code Keying (CCK) are the types of spreading codes used in 802.11 and 802.11b wireless LANs.

As higher transmission speeds are specified (such as when a system is using DRS), modulation techniques change in order to provide more data throughput. For example, 802.11g and 802.11a compliant wireless LAN equipment specify use of orthogonal frequency division multiplexing (OFDM), allowing speeds of up to 54 Mbps, which is a significant improvement over the 11 Mbps specified by 802.11b. Figure 8.10 shows the modulation types used for 802.11a networks. The 802.11g standard provides backwards compatibility by supporting CCK coding and even supports packet binary convolution coding (PBCC) as an option. Bluetooth and HomeRF are both FHSS technologies that use GFSK modulation technology in the 2.4 GHz ISM band.


Orthogonal frequency division multiplexing (OFDM) is a communications technique that divides a communications channel into a number of equally spaced frequency bands. A subcarrier carrying a portion of the user information is transmitted in each band. Each subcarrier is orthogonal (independent of each other) with every other subcarrier, differentiating OFDM from the commonly used frequency division multiplexing (FDM).