The 802.11b Modem

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The 802.11b Modem
(((((((
)))))))

• Brian Kessler
• EE194SDR

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Overview

• IEEE 802.11b (Wifi) provides mid-range wireless
  networking between multiple devices.
• The routers role is
• Media Access Control (MAC layer)
• Physical Medium (PHY layer)

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The MAC layer Crash Course

• Recognizing where frames begin and end in the
  bit-stream received from the physical layer
• Delimiting the frames - inserting information
  into the frames being sent
• Detection of transmission errors - checksum
  creation and calculation
• Inserting the source and destination MAC
  addresses into every frame transmitted
• Filtering out the frames intended for the station
  by verifying the destination address in the
  received frames
• The control of access to the physical
  transmission medium

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802.11b MAC

• The MAC in a 802.11b modem provides most of the
  aforementioned services in the same way as its
  Ethernet predecessor.
• A few of these services must be enhanced and
  modified to create a more efficient wireless
  system, they include
• Authentication
• Association
• Encryption
• Collision Detection

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Association/Authentication/Encryption

• The modem uses these three services to Connect,
  verify and protect each user.
• A station first associates with a user,
  determining speeds and modulation techniques
  (next section)
• A wireless station can then determine if the user
  is allowed on the system using MAC address
  filtering, along with a WPA password or WEP key
• After the initial connection, the user is kept
  (relatively) safe by the encryption of the data
  by means of the WPA password or WEP key.

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Collision detectionNear/Far Issue
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Carrier Sense Multiple Access with Collision
Avoidance

• Avoids collisions by using explicit packet
  acknowledgment (ACK).
• A station wishing to transmit senses the air
• If no activity is detected, the station waits an
  additional, randomly selected period of time and
  then transmits if the medium is still free.
• If the packet is received intact, the receiving
  station issues an ACK frame that, once
  successfully received by the sender, completes
  the process.
• If the ACK frame is not detected by the sending
  station, either because the original data packet
  was not received intact or the ACK was not
  received intact, a collision is assumed to have
  occurred and the data packet is transmitted again
  after waiting another random amount of time.

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Hidden Node
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RTS/CTS

• To solve this problem, 802.11 specifies an
  optional Request to Send/Clear to Send (RTS/CTS)
  protocol at the MAC layer.
• A sending station transmits an RTS and waits for
  the access point to reply with a CTS.
• Since all stations in the network can hear the
  access point, the CTS causes them to delay any
  intended transmissions

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Physical Layer
Data Rate Code Length
Modulation Symbol Rate Bits/Symbol
1 Mbps 11(Barker Sequence)
BPSK 1MSps
1
2 Mbps 11 (Barker Sequence)
QPSK 1 MSps 2
5.5 Mbps 8 (CCK) QPSK
1.375 MSps 4
11 Mbps 8 (CCK) QPSK
1.375 MSps 8

• Operating at 2.4 2.4835 GHZ
• Uses High Rate Direct Sequence Spread Spectrum
  (HR-DSSS)
• The DSSS Spectrum is divided into 11 equidistant
  (20MHZ) channels, 802.11b uses the
  non-overlapping channels 1, 6 and 11
• Not compatible with 802.11, 802.11a and
  802.11gs use of FHSS and OFDM

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Dynamic Rate Shifting

• Data rates can be automatically adjusted to
  compensate for the changing nature of the radio
  channel.
• When
• Users move beyond the optimal range for 11 Mbps
  operation,
• Substantial interference is present,
• 802.11b devices will fall back to 5.5, 2, and 1
  Mbps.
• If the device moves back within the range of a
  higher-speed transmission, the connection will
  automatically speed up again.
• Rate shifting is a physical-layer mechanism
  transparent to the user and the upper layers of
  the protocol stack.

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DSSS

• DSSS is a modulation technique that is highly
  resilient to noise and other degrading factors.
• Each data bit is represented by a n-bit spreading
  code
• The spreading code, known to sender and receiver,
  is a pseudo noise (PN) signal.

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DSSS
A B C
0 is high
C A XOR B
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DSSS

• Once the data is received, it is again modulated
  with the noise signal to produce the original
  signal.
• C XOR B
• A XOR B XOR B
• A

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DSSS Transmitter
Spreader
Sd(t)
Digital Data d(t)
S(t)
Modulator MPSK
C1(t)
PN Source

• Let
• Sd(t) Ad(t)cos(2?fct)
• Where
• d(t) 1 if data 1
• d(t) -1 if data 0
• Therefore
• S(t) C1(t)d(t)Acos(2?fct)

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DSSS Receiver
Digital Data d(t)
De-spreader
Sd(t)
S(t)
Demodulator MPSK
C1(t)
PN Source
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Barker Sequence

• A binary sequence (1, -1) where the
  autocorrilation 1
• R(t) 1/N S(BkBk- t) 1
• From K1 to N
• Barker codes maintain this property under
• s(t) gt -s(t)
• s(t) gt (-1)ts(t)
• s(t) gt -s(n-1-t)

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Known Barker Codes
Excluding reverses and inverses

• 802.11b uses the 11 bit Barker chip.

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CCK

• Complimentary Code Keying
• 64 eight-bit code words
• The code words have unique mathematical
  properties that allow them to be correctly
  distinguished from one another by a receiver even
  in the presence of substantial noise and
  multi-path interference.
• The number of like pairs of one sequence matches
  that of the other

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CCK
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Applications

• The applications of 802.11b has revolutionized
  internet with private and public wireless
  internet access
• The system provides a (relatively) secure, fast
  and easy system for wireless data transfer

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Links

• IEEE 802.11b
• Simulink 802.11b physical layer

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References

• http//www.pulsewan.com/data101/802_11_b_basics.ht
  m
• http//mathworld.wolfram.com/BarkerCode.html
• http//wi-fiplanet.webopedia.com/TERM/C/CCK.html
• http//www.3dsp.com/ieee80211b.shtml
• http//grouper.ieee.org/groups/802/11/Tutorial
• http//www.arcelect.com/DSSS_FHSS-Spead_spectrum.h
  tm
• http//www.wi-fiplanet.com

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References (cont)

• Stallings, William Wireless Communication
  Networks (2nd Edition) Prentice Hall, 2005
• Tanenbaum, Andrew Computer Networks (4th Edition)
  Prentice Hall, 2004
• IEEE 802.11b 1999
• LINK