IEEE 802.11 Fundamentals – Riding the Wave

jonathan CCIE Wireless Written Leave a Comment

Let’s take a look at how radio waves are used to transport data between two devices, bearing in mind that I’m only taking into consideration the 802.11 standards that are covered within the CCIE Wireless exam. The process used to transport data is called modulation which essentially creates a pattern that is imprinted onto a wave by the transceiver and then is decoded by the receiver. Modulation creates this pattern by adjusting one or more of the wave form’s properties such as amplitude, frequency and phase. The pattern is then sent over some medium or channel called the Radio Frequency (RF) carrier. The RF carrier within a channel utilises a range of frequencies and the overall frequency spectrum of these frequencies is known as the RF bandwidth. So what modulations are employed by 802.11 standards? There are two main methods for RF modulation and I’ll discuss them in two separate posts.
DSSS Circuit 802.11b utilises Direct-Sequence Spread Spectrum (DSSS) modulation. DSSS divides the information to be sent into smaller pieces, each of which is spread across the RF bandwidth. This spread is achieved as shown in the diagram by multiplying the data signal with the pseudo-random number (PN) chipping code, in this case the Barker or CCK code. The product of this multiplication is a longer data bit sequence with redundant information in the RF signal. The RF signal itself is generated by the modulator (BPSK or QPSK) which takes the data sequence and modulates it with the RF carrier wave. Performing the modulation along with the spreading of the data streams provides improved resistance to interference. The resultant spectrum waveform resembles the spectrum diagram representing an unfiltered SinX/X function with one main lobe and a number of side lobes covering 22Mhz of RF bandwidth.
DSSS Spectrum The main lobe contain the data transmission while the side lobes or spectral product are created due to spectral leakage and is an undesired effect of the waveform. Consider side lobes to be similar to that of a boat’s wake or ripples in a pond. The IEEE specified that a spectrum mask must be applied to the signal to ensure that it attenuates to a certain level at specific frequency off-sets. For DSSS the first side lobe must be filtered to -30dBr from the channel centre frequency, while all other side lobes must be filtered to -50dBr. This mask prevents interference between non-overlapping channels which are generally channels 1, 6 and 11 due to their availability across all of the regulatory domains. These channels are 25Mhz apart from their centre frequency.

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