CWNA Guide to Wireless LANs, Second Edition Chapter Four IEEE 802.11 Physical Layer Standards Objectives • List and describe the wireless modulation schemes used in IEEE WLANs • Tell the difference between frequency hopping spread spectrum and direct sequence spread spectrum • Explain how orthogonal frequency division multiplexing is used to increase network throughput • List the characteristics of the Physical layer standards in 802.11b, 802.11g, and 802.11a networks CWNA Guide to Wireless LANs, Second Edit Introduction Figure 4-2: OSI data flow CWNA Guide to Wireless LANs, Second Edit Introduction (continued) Table 4-1: OSI layers and functions CWNA Guide to Wireless LANs, Second Edit Wireless Modulation Schemes • Four primary wireless modulation schemes: – – – – Narrowband transmission Frequency hopping spread spectrum Direct sequence spread spectrum Orthogonal frequency division multiplexing • Narrowband transmission used primarily by radio stations • Other three used in IEEE 802.11 WLANs CWNA Guide to Wireless LANs, Second Edit Narrowband Transmission • Radio signals by nature transmit on only one radio frequency or a narrow portion of frequencies • Require more power for the signal to be transmitted – Signal must exceed noise level • Total amount of outside interference • Vulnerable to interference from another radio signal at or near same frequency • IEEE 802.11 standards not use narrowband transmissions CWNA Guide to Wireless LANs, Second Edit Narrowband Transmission (continued) Figure 4-3: Narrowband transmission CWNA Guide to Wireless LANs, Second Edit Spread Spectrum Transmission Figure 4-4: Spread spectrum transmission CWNA Guide to Wireless LANs, Second Edit Spread Spectrum Transmission (continued) • Advantages over narrowband: – – – – – – – Resistance to narrowband interference Resistance to spread spectrum interference Lower power requirements Less interference on other systems More information transmitted Increased security Resistance to multipath distortion CWNA Guide to Wireless LANs, Second Edit Frequency Hopping Spread Spectrum (FHSS) • Uses range of frequencies – Change during transmission • Hopping code: Sequence of changing frequencies – If interference encountered on particular frequency then that part of signal will be retransmitted on next frequency of hopping code • FCC has established restrictions on FHSS to reduce interference • Due to speed limitations FHSS not widely implemented in today’s WLAN systems – Bluetooth does use FHSS CWNA Guide to Wireless LANs, Second Edit 10 Channel Allocation (continued) Figure 4-17: 802.11b vs 802.11a channel coverage CWNA Guide to Wireless LANs, Second Edit 35 Error Correction • 802.11a has fewer errors than 802.11b – Transmissions sent over parallel subchannels – Interference tends to only affect one subchannel • Forward Error Correction (FEC): Transmits secondary copy along with primary information – of 52 channels used for FEC – Secondary copy used to recover lost data • Reduces need for retransmission CWNA Guide to Wireless LANs, Second Edit 36 Physical Layer Standards • PLCP for 802.11a based on OFDM • Three basic frame components: Preamble, header, and data Figure 4-18: 802.11a PLCP frame CWNA Guide to Wireless LANs, Second Edit 37 Physical Layer Standards (continued) Table 4-6: 802.11a Rate field values CWNA Guide to Wireless LANs, Second Edit 38 Physical Layer Standards (continued) • Modulation techniques used to encode 802.11a data vary depending upon speed • Speeds higher than 54 Mbps may be achieved using 2X modes Table 4-7: 802.11a characteristics CWNA Guide to Wireless LANs, Second Edit 39 Physical Layer Standards (continued) Figure 4-19: Phase shift keying (PSK) CWNA Guide to Wireless LANs, Second Edit 40 Physical Layer Standards (continued) Figure 4-20: Quadrature phase shift keying (QPSK) CWNA Guide to Wireless LANs, Second Edit 41 Physical Layer Standards (continued) Figure 4-21: 16-level quadrature amplitude modulation (16-QAM) CWNA Guide to Wireless LANs, Second Edit 42 Physical Layer Standards (continued) Figure 4-22: 64-level quadrature amplitude modulation (64-QAM) CWNA Guide to Wireless LANs, Second Edit 43 IEEE 802.11g Physical Layer Standards • 802.11g combines best features of 802.11a and 802.11b • Operates entirely in 2.4 GHz ISM frequency • Two mandatory modes and one optional mode – CCK mode used at 11 and 5.5 Mbps (mandatory) – OFDM used at 54 Mbps (mandatory) – PBCC-22 (Packet Binary Convolution Coding): Optional mode • Can transmit between and 54 Mbps CWNA Guide to Wireless LANs, Second Edit 44 IEEE 802.11g Physical Layer Standards (continued) Table 4-8: IEEE 802.11g Physical layer standards CWNA Guide to Wireless LANs, Second Edit 45 IEEE 802.11g Physical Layer Standards (continued) • Characteristics of 802.11g standard: – – – – – Greater throughput than 802.11b networks Covers broader area than 802.11a networks Backward compatible Only three channels If 802.11b and 802.11g devices transmitting in same environment, 802.11g devices drop to 11 Mbps speeds – Vendors can implement proprietary higher speed • Channel bonding and Dynamic turbo CWNA Guide to Wireless LANs, Second Edit 46 Summary • Three modulation schemes are used in IEEE 802.11 wireless LANs: frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), and orthogonal frequency division multiplexing (OFDM) • Spread spectrum is a technique that takes a narrow, weaker signal and spreads it over a broader portion of the radio frequency band • Spread spectrum transmission uses two different methods to spread the signal over a wider area: FHSS and DSSS CWNA Guide to Wireless LANs, Second Edit 47 Summary (continued) • OFDM splits a single high-speed digital signal into several slower signals running in parallel • IEEE has divided the OSI model Data Link layer into two sublayers: the LLC and MAC sublayers • The Physical layer is subdivided into the PMD sublayer and the PLCP sublayer • The Physical Layer Convergence Procedure Standards (PLCP) for 802.11b are based on DSSS CWNA Guide to Wireless LANs, Second Edit 48 Summary (continued) • IEEE 802.11a networks operate at speeds up to 54 Mbps with an optional 108 Mbps • The 802.11g standard specifies that it operates entirely in the 2.4 GHz ISM frequency and not the U-NII band used by 802.11a CWNA Guide to Wireless LANs, Second Edit 49 ... flow CWNA Guide to Wireless LANs, Second Edit Introduction (continued) Table 4-1: OSI layers and functions CWNA Guide to Wireless LANs, Second Edit Wireless Modulation Schemes • Four primary wireless. .. data CWNA Guide to Wireless LANs, Second Edit 23 IEEE 802.11 Physical Layer Standards (continued) Figure 4-13: IEEE LANs share the same LLC CWNA Guide to Wireless LANs, Second Edit 24 Legacy WLANs... Security CWNA Guide to Wireless LANs, Second Edit 12 Direct Sequence Spread Spectrum (continued) Figure 4-7: Direct sequence spread spectrum (DSSS) transmission CWNA Guide to Wireless LANs, Second