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Lecture 6

Lecture 6. 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

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Lecture 6

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  1. Lecture 6 IEEE 802.11 Physical Layer Standards

  2. 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

  3. Introduction Figure 4-2: OSI data flow

  4. IEEE 802.11 Physical Layer Standards (continued) Figure 4-10: Data Link sublayers

  5. IEEE 802.11 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

  6. IEEE 802.11 Physical Layer Standards (continued) Figure 4-11: PHY sublayers

  7. IEEE 802.11 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

  8. IEEE 802.11 Physical Layer Standards (continued) Figure 4-12: PLCP sublayer reformats MAC data

  9. IEEE 802.11 Physical Layer Standards (continued) Figure 4-13: IEEE LANs share the same LLC

  10. Legacy WLANs • Two “obsolete” WLAN standards: • Original IEEE 802.11: 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

  11. IEEE 802.11b 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 Figure 4-14: 802.11b PLCP frame

  12. IEEE 802.11b 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

  13. IEEE 802.11b Physical Layer Standards (continued) • Physical Medium Dependent Standards: PMD translates binary 1’s and 0’s of frame into radio signals for transmission • It can use many discussed techniques such as DSSS, FHSS, OFDM or others.

  14. Spreading using Barker Sequence • Barker sequences are short codes • (3 to 13 bits) with very good autocorrelation • properties. • Autocorrelation is a mathematical tool for finding repeating patterns, such as the presence of a periodic signal which has been buried under noise. • Maximizes correlation and minimizes cross correlation in other words make it as difficult as possible to mix 0/1

  15. Barker modulation

  16. 1Mbps DSSS with Barker code • >>>>>> Figure 7.5 goes here

  17. 802.11b • 802.11b uses three different types of modulation, depending upon the data rate used: • Binary phase shift keyed (BPSK) — BPSK uses one phase to represent a binary 1 and another to represent a binary 0, for a total of one bit of binary data. This is utilized to transmit data at 1 Mbps.  • Quadrature phase shift keying (QPSK) — With QPSK, the carrier undergoes four changes in phase and can thus represent two binary bits of data. This is utilized to transmit data at 2 Mbps. • Complementary code keying (CCK) — CCK uses a complex set of functions known as complementary codes to send more data. One of the advantages of CCK over similar modulation techniques is that it suffers less from multipath distortion. Multipath distortion will be discussed later. CCK is utilized to transmit data at 5.5 Mbps and 11 Mbps.

  18. 802.11b – High rate DSSS using CCK • Built on top of the 802.11 DSSS (No FHSS/IR) • Change the encoding from Barker to CCK • Allows speedup to 5.5 mbps and 11mbps • The signal rate of 11M is split into 8 bit words (1.375 Mwords/sec) instead of 11 bit words as in Barker • CCK is used to encode up to 6 more bits per word • Barker encodes 1 bit in each word (two using QDPSK) • CCK encodes 2 or 6 bits in each word (+2 by QDPSK) • Therefore the speed is 1.375 * (4 or 8) = 5.5 or 11 mbps

  19. CCK coding explained • CCK encodes bits by choosing an available 8 bit sequence out of the total 256 available ones • For 2 bit CCK this means using 4 of the 256 codes • For 6 bit CCK this means using 62 of the 256 codes • The selection is done to maximize self correlation and minimize cross correlation • In other words to minimize the chance of misinterpreting a code • The code word selections is detailed in the standard and is based on an imaginary number formula • But we will not go into the theory here

  20. Complementary Code Keying (CCK) Formula The complementary codes in 802.11b are defined by a set of 256 8-chip code words. where

  21. IEEE 802.11b Physical Layer Standards (continued) Table 4-3: IEEE 802.11b Physical layer standards

  22. IEEE 802.11a Physical Layer Standards • IEEE 802.11a achieves increase in speed and flexibility over 802.11b primarily through OFDM • Use higher frequency • Accesses more transmission channels • More efficient error-correction scheme

  23. 802.11 DSSS channel settings • 11 Channels (in the US) in the 2.4 – 2.5 GHz are used, (referred to as C-Band Industrial, Scientific, and Medical (ISM)). • Microwave ovens and some cordless phones operate in the same band

  24. U-NII Frequency Band Table 4-4: ISM and U-NII WLAN characteristics Table 4-5: U-NII characteristics

  25. U-NII Frequency Band (continued) • Total bandwidth available for IEEE 802.11a WLANs using U-NII is almost four times that available for 802.11b 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 802.11b and 802.11a WLANs

  26. 802.11 Standard Evolution

  27. Channel Allocation Figure 4-16: 802.11a channels

  28. 802.11a OFDM speeds

  29. IEE 802.11 b

  30. 802.11b Channels—FCC

  31. IEEE 802.11b Physical Layer Standards (continued) Table 4-2: 802.11b ISM channels

  32. Channel Allocation (continued) Figure 4-17: 802.11b vs. 802.11a channel coverage

  33. 802.11 g

  34. Signal distortion - multipath • Multipath is the same signal received in two different paths • Can cause destructive interference and time dispersion (inter symbol interference)

  35. Signal distortion – Fading and noise • A signal will fade as it propagates over the media • It will also pick up interference from other signals in the same media/frequency • As a result the signal to noise ration will get worse and worse

  36. 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 • 4 of 52 channels used for FEC • Secondary copy used to recover lost data • Reduces need for retransmission

  37. 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

  38. Physical Layer Standards (continued) Table 4-6: 802.11a Rate field values

  39. 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

  40. 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 6 and 54 Mbps

  41. Short History of PBCC • PBCC-11 introduced in IEEE 802.11 • 64 State Binary + Signal Scrambler • Introduced in March 1998 meeting • Backward compatible with IEEE 802.11b standard • Adapted as “High Performance” option • Although it provided a more robust solution, +3dB C.G., it was deemed “too complex” • Time to market was a major factor in selection of CCK

  42. IEEE 802.11g Physical Layer Standards (continued) Table 4-8: IEEE 802.11g Physical layer standards

  43. 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 with 802.11b • Only three channels (non overlapping) • 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 up to 108 Mbps

  44. Summary • 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

  45. Summary • 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

  46. Labs • LAB – B (Download from resources area in my web site) • Project 4-2 from the text book (Download netstrumbler from my web site)

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