Title: IEEE 802.11 Physical Layer Standards
1Lecture 6
- IEEE 802.11 Physical Layer Standards
2Objectives
- 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
3Introduction
Figure 4-2 OSI data flow
4IEEE 802.11 Physical Layer Standards (continued)
Figure 4-10 Data Link sublayers
5IEEE 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
6IEEE 802.11 Physical Layer Standards (continued)
Figure 4-11 PHY sublayers
7IEEE 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
8IEEE 802.11 Physical Layer Standards (continued)
Figure 4-12 PLCP sublayer reformats MAC data
9IEEE 802.11 Physical Layer Standards (continued)
Figure 4-13 IEEE LANs share the same LLC
10Legacy 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
11IEEE 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
12IEEE 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
13IEEE 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.
14Spreading 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
15Barker modulation
161Mbps DSSS with Barker code
- gtgtgtgtgtgt Figure 7.5 goes here
17802.11b
- 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.
18802.11b 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
19CCK 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
20Complementary Code Keying (CCK) Formula
The complementary codes in 802.11b are defined by
a set of 256 8-chip code words.
where
21IEEE 802.11b Physical Layer Standards (continued)
Table 4-3 IEEE 802.11b Physical layer standards
22IEEE 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
23802.11 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 -
24U-NII Frequency Band
Table 4-4 ISM and U-NII WLAN characteristics
Table 4-5 U-NII characteristics
25U-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
26802.11 Standard Evolution
27Channel Allocation
Figure 4-16 802.11a channels
28802.11a 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
29IEE 802.11 b
30802.11b ChannelsFCC
31IEEE 802.11b Physical Layer Standards (continued)
Table 4-2 802.11b ISM channels
32Channel Allocation (continued)
Figure 4-17 802.11b vs. 802.11a channel coverage
33802.11 g
34Signal distortion - multipath
- Multipath is the same signal received in two
different paths - Can cause destructive interference and time
dispersion (inter symbol interference)
35Signal 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
36Error 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
37Physical Layer Standards
- PLCP for 802.11a based on OFDM
- Three basic frame components Preamble, header,
and data
Figure 4-18 802.11a PLCP frame
38Physical Layer Standards (continued)
Table 4-6 802.11a Rate field values
39Physical 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
40IEEE 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
41Short 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
42IEEE 802.11g Physical Layer Standards (continued)
Table 4-8 IEEE 802.11g Physical layer standards
43IEEE 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
44Summary
- 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
45Summary
- 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
46Labs
- LAB B (Download from resources area in my web
site) - Project 4-2 from the text book (Download
netstrumbler from my web site)