CWNA – CCK,DSSS, FHSS & OFDM
CCK – Complementary Code Keying
DSSS – Direct Sequence Spread Spectrum
OFDM – Orthogonal Frequency Divisional Multiplexing
FHSS – Frequency Hopping Spread Spectrum
There are various versions of WLAN standard developed to address different data rate and coverage requirements. IEEE 802.11b supports four data rates viz. 1 Mbps, 2 Mbps, 5.5 Mbps and 11 Mbps.
– DSSS is used to provide support for 1 Mbps and 2 Mbps data rate.
– CCK (to old for CWNA Exam) for 5.5 and 11 Mbps while OFDM is used for higher data rate applications.
OFDM is used in IEEE 802.11a, 11g, 11n, 11ac and 11ad versions. OFDM is employed along with MIMO to increase the data rate further.
CCK is the modulation form used in the 802.11b standard when operating in 5.5 Mbps or 11 Mbps. CCK was chosen because it uses the same approximate bandwidth as MOK and can use the same header and preamble of pre-existing 1 and 2 Mbps wireless networks, thus facilitating interoperability.
FHSS – RF carrier frequency is changed according to the Pseudo-random sequence(PRS or PN sequence). This PN sequence is known to both transmitter and Receiver and hence help demodulate/decode the information. Within one chip duration, RF frequency does not vary. Based on this fact there are two types of FHSS, fast hopped FHSS and slow hopped FHSS. Dwell time usually 400ms, amount of time that a system transmits on a frequency. Hop time is measurement of amount of time taken by transmitter to change from one frequency to another.
DSSS – In DSSS, information bits are spread across both frequency and time planes, hence minimizes effect of interference as well as fading. Hence DSSS system prone to errors but at low level compare to FHSS systems. FHSS produces strong bursty errors. DSSS delivers capacity upto 11 Mbps while FHSS supports upto 3 Mbps. DSSS is very sensitive technology while FHSS is very robust technology. This is observed in harsh environment comprising large coverage, noises, collocated cells, multi-path and presence of bluetooth frequency waves etc. DSSS is ideal for point to point applications while FHSS can be used in point to multipoint deployment with excellent performance.
OFDM – The idea of OFDM is to map complex data on to multiple narrow band subcarriers so that higher data rate can be achieved. The same is shown in the figure. As shown complex modulation scheme such as 16-QAM is first used to map binary data information into complex frequency domain vector form. 16-QAM maps 4 bits on each of the subcarrier. This bunch of subcarriers as per IFFT size are combined and given as input to IFFT block. This block converts frequency domain complex mapper data into time domain data vector. This vector is converted to analog form before being provided as input to RF converter before transmission into the air using antenna. OFDM solves multipath issues.
