IEEE 802.11g

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IEEE 802.11g Wi-Fi Ravi Teja Kundeti KU
ID2303778 24th April 2008
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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

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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.

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History and Background Highlights of 802.11
specifications
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
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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 used
• b) Point Coordination Function (PCF).

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802.11 legacy Typical Configuration
http//sss-mag.com/pdf/802_11tut.pdf
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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.

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802.11 legacy DCF working

• http//sss-mag.com/pdf/802_11tut.pdf
• SIFS Short Inter Frame Space, separate
  transmissions belonging to a single dialogue and
  is minimum Inter Frame size ltDIFS and thus will
  have priority.
• Slot Time

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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.

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802.11 legacy (Cont) Frame Structures

• Fragmentation in 802.11
• http//sss-mag.com/pdf/802_11tut.pdf
• MSDU MAC Service Data Unit

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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.

Frame in 802.11 http//sss-mag.com/pdf/802_11tut.p
df 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)
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802.11 legacy (Cont) Frame Structures

• MAC Data
• http//sss-mag.com/pdf/802_11tut.pdf

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
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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.

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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

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802.11b Long PLCP PPDU format

• 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)

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802.11b Short PLCP PPDU format (Optional)

• http//standards.ieee.org/getieee802/download/802.
  11b-1999.pdf
• Observe that the preamble has been reduced to half

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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

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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.

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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

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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.

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802.11g Four Physical Layers
Parameters of the different IEEE 802.11g
physical layers. http//ieeexplore.ieee.org/iel5/6
5/31204/01453395.pdf?arnumber1453395
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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.

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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, mcw15 and all frame exchanges use
  ERP-OFDM data rates.

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802.11g Interoperability and Protection
Mechanisms

• Choice of 14 data rates, four physical rates and
  then
• Different stations
• ERP stations basically 802.11g
• non-ERP supporting short preamble newer
  802.11b
• non-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 preamble
• Solution2 Use of RTS/CTS frames to protect the
  OFDM packets and use of only ERP-DSSS physical
  layer for those.

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802.11g CTS to Self Mechanism

• Problem of hidden node.
• http//ieeexplore.ieee.org/iel5/65/31204/01453395.
  pdf?arnumber1453395

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Conclusions

• 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 gt20 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.

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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/iel5/65/31204/014533
  95.pdf?arnumber1453395
• 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/2001/012
  9tech.html
• vii)http//forskningsnett.uninett.no/wlan/download
  /WP_IEEE802gExpla_12_06.pdf
• IEEE 802.11g Explained

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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.ph
  p/2109881
• xi) http//www.linksysinfo.org/forums/showthread.p
  hp?p274613
• xii) http//networkdictionary.com/protocols/wlan.p
  hp
• 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

• 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.

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802.11a

• 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.

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802.11a

• 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

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802.11a Frame

• PLCP preamble
• Section1 for synchronization
• Section2 for channel estimation.