IEEE 802.11g “Wi-Fi” Ravi Teja Kundeti KU ID:2303778 24 th April 2008

University of Kansas | School of Engineering

Department of Electrical Engineering and Computer Science

IEEE 802.11g“Wi-Fi”

Ravi Teja Kundeti

KU ID:2303778

24th April 2008



Outline

• History and Background

• Overview and basic features of 802.11

• 802.11a and 802.11b

• 802.11g

• Differences between 802.11g and 802.11b

• Summary

• References

• Latest Developments

2



History and Background – 802.11 suite

• Since 802.11g shares the same basic protocols and architecture, this presentation explains the 802.11 and 802.11b systems in some detail. Then the differences between 802.11b and 802.11g are explored to understand why some decisions are taken.

• 802.11, popularly known as “Wi-FiTM”, is a suite for specifications for wireless Ethernet or wireless local area network.

• It operates in 5GHz or 2.4 GHz public spectrum bands.

• All of the specifications use the same basic protocols.

• Security was originally purposefully weak.

• Mainly for the corporate LANs inside a building.

3



History and Background – Highlights of 802.11 specifications

4

Protocol Release Date

Op. Frequency

(GHz)

Data Rate Max –

Typical (Mbit/s)

Modulation Technique

Range (Indoor –Outdoor) meters

Legacy 1997 2.4 2 – 0.9 DSSS 20 -100

802.11a 1999 5 54 - 23 OFDM 35 -120

802.11b 1999 2.4 11 – 4.3 DSSS 38 – 140

802.11g 2003 2.4 54 - 19 OFDM 38 – 140

802.11n 2009 (est)

2.4 and 5 248 -74 70 – 250

802.11y June 08

(est)

3.7 54 - 23 50 – 5000



802.11 legacy

• Covers MAC and Physical layers. One single Mac with three Physical Layers.

• Based on cellular architecture. Cells called the Basic Service Set (BSS), controlled by Access Point (AP).

• Normally APs are connected by Distribution System, usually Ethernet, could be wireless.

• Whole set is seen as a single 802 network called Extended Service Set.

• Adhoc networks (IBSS) possible without AP with reduced features.

• Mac Layer uses two access methods a) Distributed Coordination Function (DCF) –mostly usedb) Point Coordination Function (PCF).

5



802.11 legacy – Typical Configuration

6

http://sss-mag.com/pdf/802_11tut.pdf



802.11 legacy (Cont)

• DCF is basically a CSMA/CA with exponential backoff.

• Waits for Distributed Inter Frame Space (DIFS) –medium free time before transmitted its packet.

• Receiver gives an ack – success, else retransmit.

• Uses virtual carrier sense to avoid problem of indirect collision.

• PCF – optional, used for time-bounded services.

• Uses higher priority that AP may gain by PIFS asAP issues polling requests thus controlling access.

• Must leave enough time for distributed access.

7



802.11 legacy – DCF working

http://sss-mag.com/pdf/802_11tut.pdfSIFS – Short Inter Frame Space, separate transmissions belonging to a

single dialogue and is minimum Inter Frame size <DIFS and thus will have priority.

Slot Time

8



802.11 legacy (Cont)

• Allows for fragmentation and reassembly as shorter frames are beneficial.

• Synchronization through periodic “Beacon Frames”.

• To join an existing BSS

a) Passive Scanning – wait for Beacons

b) Active Scanning – send Probe Request Frames.

• Then Authentication and Association.

• Roaming similar to cellular but with differences.

9



802.11 legacy (Cont) Frame Structures

Fragmentation in 802.11http://sss-mag.com/pdf/802_11tut.pdf

MSDU – MAC Service Data Unit

10



802.11 legacy (Cont) Frame Structures

Three types of Frames

Data - used for data

Control - used to control access to medium (RTS, CTS,ACK)

Management - Frames transmitted the same way as data frames to exchange management info in the same layer.

11

Frame in 802.11


All frames in 802.11 follow the above structure.

Preamble – 96 bits – 80 bits of synch + 16 bits of SFD

PLCP Header – PLCP_PDU Length word + PLCP signalling field + Header Error Check Field (16 Bit CRC)



802.11 legacy (Cont) Frame StructuresMAC Data


12

Frame Control - Protocol Version + type of packet + whether from AP + Power Management + more

Duration/ID - normally used for NAV calculation /station ID in poll messages

Address fields – 1-recepient, 2-transmitter, 3- original source/destination, 4- special case, when (AP to AP)

Sequence Control – order of different fragments



802.11a

• An amendment to the IEEE 802.11 specification that added a higher throughput of up to 54 Mbit/s by using the 5 GHz band, usually mid-20

• Uses 52 OFDM subcarriers, 48 are for data and 4 are pilot subcarriers with a carrier separation of 0.3125 MHz (20 MHz/64).

• OFDM advantage in a multipath environment.

• Not a over crowded frequency but has weak Penetration of walls by frequency compared to 2.4 GHz.

• Had initial regulation issues and also timing and compatibility problems.

• Not reverse compatible with 802.11 or 802.11b except for dual-band.

13



802.11b

• 802.11b has a maximum raw data rate of 11 Mbit/s, typically 4.5 Mbit/s.

• Uses the same CSMA/CA access method.

• Uses exclusively DSSS (Direct-sequence spread spectrum) using CCK (Complementary code keying) or PBCC (packet binary convolutional coding) algorithm modulation scheme.

• slowest maximum speed; home appliances may interfere on unregulated frequency band but signal range is good and not easily obstructed

• Introduced optional support to Short PLCP PPDU format

• Made some changes to Long PLCP PPDU format

14



802.11b –Long PLCP PPDU format

15

http://standards.ieee.org/getieee802/download/802.11b-1999.pdf

• Changed the speed of signal rate

• Changed some uses of service field (Basically the same as 802.11)



802.11b –Short PLCP PPDU format (Optional)

16

http://standards.ieee.org/getieee802/download/802.11b-1999.pdf

• Observe that the preamble has been reduced to half



Motivation for 802.11g

• 802.11b has a maximum raw data rate of 11 Mbit/s, typically 4.5 Mbit/s, while 802.11a can provide up to 54Mbit/s.

• As days progressed 11Mbit/s was too small

• Want the same speed at 2.4MHz

• Be backward compatible to 802.11b

• Wish to take advantage of OFDM modulation scheme of 802.11a

• In short, need for a convergence of 802.11a and 802.11b at frequency range of 2.4MHz

17



802.11g

• 802.11g has a maximum raw data rate of 54 Mbit/s, typically 19 Mbit/s.

• Operates at 2.4MHz and is backward compatible to 802.11b.

• Can take advantage of OFDM modulation scheme.

• Observe typical of 802.11a is 23Mbit/s – the difference is due to legacy overhead for backward compatibility.

• Problem : The presence of even one 802.11b element in an other wise 802.11g network can drastically reduce performance.

• Similar to 802.11b, not compatible with 802.11a unless dual band.

• Today many of the products are dual-band/triple mode for compatibility.

18



Differences between 802.11g and 802.11b

The major differences are• The provision of four different physical layers

• The mandatory support of the short preamble type

• The ERP network attribute

• Newly defined protection mechanisms that deal with interoperability aspects

• The CTS-to-self mechanism

19



802.11g – Four Physical Layers

• ERP-DSSS/CCK (Mandatory): Old physical layer used by IEEE 802.11b. DSSS technology is used with CCK modulation. The data rates provided are those of IEEE 802.11b.

• ERP-OFDM (Mandatory): New physical layer, introduced by IEEE 802.11g. OFDM is used to provide IEEE 802.11a data rates at the 2.4 GHz band.

• ERP-DSSS/PBCC (Optional): Introduced as an option in IEEE 802.11b and provided the same data rates as the DSSS/CCK physical layer. IEEE 802.11g extended the set of data rates by adding 22 and 33 Mb/s (earlier 2,5.5 ,11 Mb/s).

• DSSS-OFDM (Optional): This is a new physical layer that uses a hybrid combination of DSSS and OFDM. The packet physical header is transmitted using DSSS, while the packet payload is transmitted using OFDM. The scope of this hybrid approach is to cover interoperability aspects.

20



802.11g – Four Physical Layers

21

Parameters of the different IEEE 802.11g physical layers.http://ieeexplore.ieee.org/iel5/65/31204/01453395.pdf?arnumber=1453395



802.11g – Mandatory support of Short Preamble

• It was clear even for 802.11b that the long preamble was too big, so they had introduced the short preamble. 802.11g makes it mandatory.

• When the preamble and header are transmitted using DSSS (this happens at all physical layers except the ERP-OFDM), short and long types of preamble and header are defined.

• For the ERP-OFDM physical layer there is only one type of preamble

and header, the format of which is almost identical to that of the IEEE 802.11a standard.

22



802.11g – The ERP network attribute

• Slot time =20 micro seconds, min contention window =31 slots in 802.11b. These values are good for data rates of 802.11b.

• For backward compatibility, 802.11g adapted them. However these values are too big for 6-54Mb/s, especially OFDM with only 20 micro seconds for preamble. The best values are from 802.11a which are 9 micro seconds and 15 slots.

• 802.11g has dynamic adjustments of these values using a flag “ERP network attribute”, sent via a beacon frame.

• For BSS, if ERP attribute enabled, the slot time = 9 micro, mcw=15 and all frame exchanges use ERP-OFDM data rates.

23



802.11g – Interoperability and Protection Mechanisms

• Choice of 14 data rates, four physical rates and then

• Different stations ERP stations – basically 802.11gnon-ERP supporting short preamble – newer 802.11bnon-ERP without short preamble – older 802.11b

• Non-ERP stations do not detect ERP-OFDM from ERP. Solution1: Use of DSSS-OFDM, where every one can detect the PLCP preambleSolution2: Use of RTS/CTS frames to protect the OFDM packets and use of only ERP-DSSS physical layer for those.

24



802.11g – CTS to Self Mechanism

Problem of hidden node.http://ieeexplore.ieee.org/iel5/65/31204/01453395.pdf?arnumber=1453395

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Conclusions

27

• OFDM has been adopted as the mandatory high rate waveform in the 2.4 GHz band, so as to speeds up to 54Mb/s.

• Backward compatibility with 802.11b was assured.

• mandatory use of OFDM for data rates >20 Mbps, there are two optional waveforms: CCK/OFDM and PBCC.

• the case of the optional PBCC waveform, the peak data rate is 33 Mbps as compared to 54 Mbps for OFDM, i.e. the optional PBCC waveform is actually slower than the peak data rates for the mandatory OFDM waveform.

• OFDM already implemented for 802.11a, so for dual-band, it is very easy support for 802.11g.



References

i) http://www.ieee802.org/11/ - site from which I showed the location of IEEE 802.11g standard document.

ii) http://standards.ieee.org/getieee802/download/802.11g-2003.pdf

the standard document

iii)http//ieeexplore.ieee.org/iel5/65/31204/01453395.pdf?arnumber=1453395

The IEEE 802.11g Standard for High Data Rate WLANs

iv) http://easy.intranet.gr/paper_10.pdf

v) http://focus.ti.com/lit/wp/sply012/sply012.pdf IEEE 802.11g

New Draft Standard Clarifies Future of Wireless LAN

vi) http://www.networkworld.com/news/tech/2001/0129tech.html

vii)http://forskningsnett.uninett.no/wlan/download/WP_IEEE802gExpla_12_06.pdf

IEEE 802.11g Explained

27



References

viii) http://www.javvin.com/protocolWLAN.html

ix) http://sss-mag.com/pdf/802_11tut.pdf

A technical tutorial on IEEE 802.11 protocol by Pablo Brenner.

x) http://www.wi-fiplanet.com/tutorials/article.php/2109881

xi) http://www.linksysinfo.org/forums/showthread.php?p=274613

xii) http://networkdictionary.com/protocols/wlan.php

xiii) http://en.wikipedia.org/wiki/802.11 and other wiki pages

xiv) http://grouper.ieee.org/groups/802/11/Reports/802.11_Timelines.htm

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Other Developments in 802.11

27

• As originally the security in 802.11 was low, it was improved by 802.11i.

• In July 2007, a new release of the standard that includes amendments a, b, d, e, g, h, i & j was made called the IEEE 802.11-2007 or 802.11ma.

• 802.11n is trying to improve the data rate up to 300 Mb/s using MIMO antennas, expected to finalize June 09.

• On the other hand, keeping the data rate constant at 54Mb/s but increasing the distance to 5000 m, is 802.11y, using contention based protocol and a “lite licensing” scheme from FCC. This is expected this June.



802.11a

27

• The major problem in 802.11a was delay spread. With the then technology, the ceiling was around 20Mbps.

• It uses a modulation technique known as COFDM (coded OFDM). COFDM sends data in a massively parallel fashion, and slows the symbol rate down so each symbol transmission is much longer than the typical delay spread.

• A guard interval is inserted at the beginning of the symbol transmission to let all delayed signals "settle" before the baseband processor demodulates the data.

• COFDM slows the symbol rate while packing many bits in each symbol transmission, making the symbol rate substantially slower than the data bit rate.



802.11a

27

• It maps the data signal to be transmitted into several lower-speed signals, or subcarriers, which then are modulated individually and transmitted in parallel.

• IEEE 802.11a uses only the PLCP (physical layer convergence protocol) preamble which contains 10 short and 2 long symbols



802.11a Frame

PLCP preamble

Section1 for synchronization

Section2 for channel estimation.

Documents

IEEE 802.11g “Wi-Fi” Ravi Teja Kundeti KU ID:2303778 24 th April 2008