wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Wireless Local Area Networks 48-3
The a, b, and g encoding protocols of working group 11 of the IEEE 802 wireless LAN standards
committee use the ISM bands (802.11a operates at 5.GHz, while 802.11b/g operates in the unlicensed
2.45.GHz band, and 802.11n at 2.45 and 5.GHz). This implies that 802.11b/g/n, operating in an unregulated
frequency band can incur interference from microwave ovens, cordless phones, and other appliances
using the same 2.4.GHz range. Therefore, coexistence has to be considered in any case, and is of
major importance for the successful deployment of WLANs.
48.3.2 Modulation Techniques
Modulation techniques are techniques typically used in telecommunications to transmit a message.
A high frequency periodic (sinusoid) waveform is used as a carrier signal, with amplitude, phase, and
frequency modulation. The receiving unit performing the inverse operation of modulation is known
as the demodulator. The modem (Modulator-Demodulator) is capable of performing both operations.
Wireless communication protocols use various modulation techniques. While Bluetooth and the
early 802.11 LANs use the Frequency-Hopping Spread Spectrum signaling method (FHSS), 802.11b
and ZigBee 802.15.4 use Direct Sequence Spread Spectrum (DSSS) signaling. The 802.11a uses the
Orthogonal Frequency-Division Multiplexing (OFDM) system.
Frequency hopping was the first step in the evolution to DSSS and other data transmission techniques.
The idea was to transmit via a predefined frequency hopping pattern known to both the transmitter and
the receiver. The 802.11 frequency hopping separates the whole 2.4.GHz ISM band into 1.MHz-spaced
channels. The transmitter has to change channels at least 2.5 times per second (every 400.ms or less).
This allows dealing with high energy interference in a narrow band, as well as the mutual interference
of two FHSS transmitters positioned close to each other.
With Direct Sequence Spread Spectrum (DSSS) signaling, the carrier signals occur over the full
bandwidth (spectrum) of a device’s transmitting frequency. The data signal at the sending station is
combined with a higher data rate bit sequence, or a chipping code that divides the user data according
to a spreading ratio. The chipping code is a redundant bit pattern for each bit that is transmitted, which
increases the signal’s resistance to interference. If one or more bits in the pattern are damaged during
transmission, the original data can be recovered due to the redundancy of the transmission.
Complementary Code Keying (CCK) is used in conjunction with DSSS technology. CCK is a set of
64 8-bit code words used to encode data for 5.5 and 11.Mbps data rates in the 2.4.GHz band of 802.11b
wireless networking. The code words have unique mathematical properties that allow them to be
correctly distinguished from one another by a receiver even in the presence of substantial noise and
multipath interference. CCK applies sophisticated mathematical formulas to DSSS codes, permitting
the codes to represent a greater volume of information per clock cycle (11.Mbps of data rather than the
2.Mbps in the original standard). CCK does not work with FHSS.
Orthogonal frequency-division multiplexing (OFDM) is a Frequency-Division Multiplexing (FDM) scheme
utilized as a digital multicarrier modulation method. OFDM splits the radio signal into multiple smaller subsignals
that are then transmitted simultaneously at different frequencies to the receiver. OFDM, therefore,
reduces the amount of cross talk in signal transmissions. OFDM is used in the 802.11a/g, the European alternative
for the IEEE 80211 High Performance Radio LAN (HIPERLAN/2), and in 802.16 (WiMAX).
48.4 Medium Access Control
According to the original IEEE 802.11 MAC protocol [IEE97], the architecture of the MAC sub-layer
includes a mandatory Distributed Coordination Function (DCF) and an optional Point Coordination
Function (PCF). Moreover, due to the limitations of this architecture for transmitting time-critical traffic
flows, real-time enhancements were introduced. They are addressed by the standard amendment IEEE
802.11e [IEE05e]. It provides advanced QoS capabilities by adding the hybrid coordination function
(HCF), which defines two new access mechanisms. The overall MAC architecture is shown in Figure 48.1.
© 2011 by Taylor and Francis Group, LLC