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)