802.11 Wireless Networks: The Definitive Guide, 2nd Edition

Chapter 10. Physical Layer Overview

Any girl can be glamorous.

All you have to do is stand still and look stupid.

— Hedy Lamarr

Protocol layering allows for research, experimentation, and improvement on different parts of the protocol stack. The second major component of the 802.11 architecture is the physical layer, which is often abbreviated PHY. This chapter introduces the common themes and techniques that appear in each of the radio-based physical layers and describes the problems common to all radio-based physical layers; it is followed by more detailed explanations of each of the physical layers that are standardized for 802.11.

Physical-Layer Architecture

The physical layer is divided into two sublayers: the Physical Layer Convergence Procedure (PLCP) sublayer and the Physical Medium Dependent (PMD) sublayer. The PLCP (Figure 10-1) is the glue between the frames of the MAC and the radio transmissions in the air. It adds its own header. Normally, frames include a preamble to help synchronize incoming transmissions. The requirements of the preamble may depend on the modulation method, however, so the PLCP adds its own header to any transmitted frames. The PMD is responsible for transmitting any bits it receives from the PLCP into the air using the antenna. The physical layer also incorporates a clear channel assessment (CCA) function to indicate to the MAC when a signal is detected.

Figure 10-1. Physical layer logical architecture

The Radio Link

Three physical layers were standardized ...