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IEEE 802.11 Physical Layer Standards

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Lecture 6 IEEE 802.11 Physical Layer Standards * * * 1. 8 chip complex code - 16 x 1 vector - 4096 complex 8 bit sequences out of which only 64 of them have good ... – PowerPoint PPT presentation

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Title: IEEE 802.11 Physical Layer Standards


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 1s and 0s 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
  • gtgtgtgtgtgt 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
Data Rate (Mbps) Modulation Coding Rate Coded bits per Coded bits per subcarrier Coded bits per OFDM symbol Data bits per OFDM sybmol
6 BPSK ½ 1 48 24
9 BPSK ¾ 1 48 36
12 QPSK ½ 2 96 48
18 QPSK ¾ 2 96 72
24 16-QAM ½ 4 192 96
36 16-QAM ¾ 4 192 144
48 64-QAM 2/3 6 288 192
54 64-QAM ¾ 6 288 216
29
IEE 802.11 b
30
802.11b ChannelsFCC
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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