IR Physical Medium Dependent (PMD) Sublayer

IR Physical Medium Dependent (PMD) Sublayer
The operation of the PMD translates the binary representation of the PPDUs into an infrared
light signal suitable for transmission. The 802.11 infrared Physical layer operates using nondirected
transmission, as shown in Figure 5.21, eliminating the need for line-of-sight operation.
Most radio LAN suppliers refer to this type of transmission as diffused infrared. Because of this form of transmission, the infrared Physical layer is intended only for indoor
operation where a ceiling is present to reflect the signals. Windows can significantly attenuate
the infrared signals, so be sure to test the operation of infrared devices in the facility before
installing the entire system. Because of the use of a ceiling as a reflection point, 802.11
infrared devices are limited in transmission range. A typical range is 30 to 60 feet (10 to 20
meters), depending on ceiling height.
The infrared Physical layer transmits its signals in the nearly visible 850–950 nanometers
range at a maximum transmit power level of 2 watts peak optical power. Because of the relatively
high transmission frequency, there are no frequency regulatory restrictions for infraredbased
systems. In fact, the only regulatory standards that apply to the 802.11 infrared-based
system are safety regulations, namely IEC 60825-1 and ANSI Z136.1. The infrared PMD transmits the binary data at either 1Mbps (basic access rate) or 2Mbps
(enhanced access rate) using a specific modulation type for each, depending on which data rate
is chosen. For 1Mbps operation, the infrared PMD uses 16-pulse position modulation (PPM).
The concept of pulse position modulation is to vary the position of a pulse to represent different
binary symbols. Thus, changes in pulse positions maintain the information content of the
signal.
Noise usually affects the amplitude of the signal, not the phase. As a result, the use of pulse
position modulation reduces potential interference. As shown in Table 5.7, 16-PPM maps each
possible group of four bits in the PPDU to one of 16 symbols. The 1 bit in the 16-PPM symbol
illustrates the position of a pulse representing a particular group of four PPDU data bits. The
transmission order is from left to right. A 0 (zero) represents no pulse. An infrared wireless LAN offers excellent noise immunity and more security than spread spectrum
radio implementations; however, the lack of products forces you to use proprietary
devices. Thus, be sure to consider 802.11 spread spectrum radio Physical layers before deciding
which one to implement.
As discussed in this chapter, there are several Physical layers to consider when implementing a
wireless LAN. Chapter 8, “Implementing a Wireless LAN,” will explain how to choose the
most appropriate standard based on wireless LAN requirements