CWNA Guide to Wireless LANs, Second Edition Chapter Four IEEE 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 b, g, and a networks

Introduction OSI data flow

Introduction (continued) OSI layers and functions

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 WLANs

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 standards do not use narrowband transmissions

Narrowband Transmission (continued) Narrowband transmission

Spread Spectrum Transmission Spread spectrum transmission

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

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

Frequency Hopping Spread Spectrum (continued) FHSS error correction

Direct Sequence Spread Spectrum (DSSS) Uses expanded redundant code to transmit data bits Chipping code: Bit pattern substituted for original transmission bits –Advantages of using DSSS with a chipping code: Error correction Less interference on other systems Shared frequency bandwidth –Co-location: Each device assigned unique chipping code Security

Direct Sequence Spread Spectrum (continued) Direct sequence spread spectrum (DSSS) transmission

Orthogonal Frequency Division Multiplexing (OFDM) With multipath distortion, receiving device must wait until all reflections received before transmitting –Puts ceiling limit on overall speed of WLAN OFDM: Send multiple signals at same time –Split high-speed digital signal into several slower signals running in parallel OFDM increases throughput by sending data more slowly Avoids problems caused by multipath distortion Used in a networks

Orthogonal Frequency Division Multiplexing (continued) Multiple channels

Orthogonal Frequency Division Multiplexing (continued) Orthogonal frequency division multiplexing (OFDM) vs. single- channel transmissions

Comparison of Wireless Modulation Schemes FHSS transmissions less prone to interference from outside signals than DSSS WLAN systems that use FHSS have potential for higher number of co-location units than DSSS DSSS has potential for greater transmission speeds over FHSS Throughput much greater for DSSS than FHSS –Amount of data a channel can send and receive

Comparison of Wireless Modulation Schemes (continued) DSSS preferred over FHSS for b WLANs OFDM is currently most popular modulation scheme –High throughput –Supports speeds over 100 Mbps for a WLANs –Supports speeds over 54 Mbps for g WLANs

IEEE Physical Layer Standards IEEE wireless standards follow OSI model, with some modifications Data Link layer divided into two sublayers: –Logical Link Control (LLC) sublayer: Provides common interface, reliability, and flow control –Media Access Control (MAC) sublayer: Appends physical addresses to frames

IEEE Physical Layer Standards (continued) Physical layer divided into two sublayers: –Physical Medium Dependent (PMD) sublayer: Makes up standards for characteristics of wireless medium (such as DSSS or FHSS) and defines method for transmitting and receiving data –Physical Layer Convergence Procedure (PLCP) sublayer: Performs two basic functions Reformats data received from MAC layer into frame that PMD sublayer can transmit “Listens” to determine when data can be sent

IEEE Physical Layer Standards (continued) Data Link sublayers

IEEE Physical Layer Standards (continued) PLCP sublayer reformats MAC data

IEEE Physical Layer Standards (continued) IEEE LANs share the same LLC

Legacy WLANs Two “obsolete” WLAN standards: –Original IEEE : FHSS or DSSS could be used for RF transmissions But not both on same WLAN –HomeRF: Based on Shared Wireless Access Protocol (SWAP) Defines set of specifications for wireless data and voice communications around the home Slow Never gained popularity

IEEE b Physical Layer Standards Physical Layer Convergence Procedure Standards: Based on DSSS –PLCP must reformat data received from MAC layer into a frame that the PMD sublayer can transmit b PLCP frame

IEEE b Physical Layer Standards (continued) PLCP frame made up of three parts: –Preamble: prepares receiving device for rest of frame –Header: Provides information about frame –Data: Info being transmitted Synchronization field Start frame delimiter field Signal data rate field Service field Length field Header error check field Data field

IEEE b Physical Layer Standards (continued) Physical Medium Dependent Standards: PMD translates binary 1’s and 0’s of frame into radio signals for transmission –Can transmit at 11, 5.5, 2, or 1 Mbps –802.11b uses ISM band 14 frequencies can be used –Two types of modulation can be used Differential binary phase shift keying (DBPSK): For transmissions at 1 Mbps Differential quadrature phase shift keying (DQPSK): For transmissions at 2, 5.5, and 11 Mbps

IEEE b Physical Layer Standards (continued) b ISM channels

IEEE b Physical Layer Standards (continued) IEEE b Physical layer standards

IEEE a Physical Layer Standards IEEE a achieves increase in speed and flexibility over b primarily through OFDM –Use higher frequency –Accesses more transmission channels –More efficient error-correction scheme

U-NII Frequency Band U-NII characteristics ISM and U-NII WLAN characteristics

U-NII Frequency Band (continued) Total bandwidth available for IEEE a WLANs using U-NII is almost four times that available for b networks using ISM band Disadvantages: –In some countries outside U.S., 5 GHz bands allocated to users and technologies other than WLANs –Interference from other devices is growing Interference from other devices one of primary sources of problems for b and a WLANs

Channel Allocation a channels

Channel Allocation (continued) b vs a channel coverage

35 Channel Setup There are two critical steps for a good WLAN deployment: 1. Determine placement of access points or bridges – This includes determining where they should be placed and deciding how many are required for the desired coverage. Very few gaps in the coverage should be left. These gaps are essentially dead air and the client will lack connectivity in these locations. As discussed before, bandwidth requirements have an impact on the coverage areas. 2. Map out the channel assignments – There should be as little overlap as possible between channels that use the same frequency.

36 Multiple Overlapping Networks Coverage Current Thinking: 1.Only three of the 11 channels used by wireless hubs in the US can be allocated simultaneously. 2.Reason: 1. In North America, the b spectrum ranges form 2411 MHz to 2473 MHz, and is divided up into 11 channels. Channels are spaced 5 MHz apart from the center. 2.However, each channel is 22 MHz wide, so there is a great overlap

37 An entire 22MHz is not simply swallowed up in a rectangular pattern with power on the vertical axis and frequency on the horizontal – instead it's more of a parabola, centered around the midpoint of the frequency. Thus, as you get further away from the center, the power drops off substantially. According to Burton's analysis, when three channels separate b frequencies, there is only about 4% of interference. This is the case between frequencies 1 and 4, and 8 and 11. Between 4 and 8, the interference is substantially less than 1%. Burton’s Analysis

Error Correction a has fewer errors than b –Transmissions sent over parallel subchannels –Interference tends to only affect one subchannel Forward Error Correction (FEC): Transmits secondary copy along with primary information –4 of 52 channels used for FEC –Secondary copy used to recover lost data Reduces need for retransmission

Physical Layer Standards PLCP for a based on OFDM Three basic frame components: Preamble, header, and data a PLCP frame

Physical Layer Standards (continued) a Rate field values

Physical Layer Standards (continued) Modulation techniques used to encode a data vary depending upon speed Speeds higher than 54 Mbps may be achieved using 2X modes a characteristics

Physical Layer Standards (continued) Phase shift keying (PSK)

Physical Layer Standards (continued) Quadrature phase shift keying (QPSK)

Physical Layer Standards (continued) Figure 4-21: 16-level quadrature amplitude modulation (16-QAM)

Physical Layer Standards (continued) Figure 4-22: 64-level quadrature amplitude modulation (64-QAM)

IEEE g Physical Layer Standards g combines best features of a and b 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 6 and 54 Mbps

IEEE g Physical Layer Standards (continued) Table 4-8: IEEE g Physical layer standards

IEEE g Physical Layer Standards (continued) Characteristics of g standard: –Greater throughput than b networks –Covers broader area than a networks –Backward compatible –Only three channels –If b and g devices transmitting in same environment, g devices drop to 11 Mbps speeds –Vendors can implement proprietary higher speed Channel bonding and Dynamic turbo

Summary Three modulation schemes are used in IEEE 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

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 b are based on DSSS

Summary (continued) IEEE a networks operate at speeds up to 54 Mbps with an optional 108 Mbps The g standard specifies that it operates entirely in the 2.4 GHz ISM frequency and not the U-NII band used by a