AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Overview of IEEE 802.11b Wireless LAN
`S-72.4210 Postgraduate course in Radio Communications
`10.1.2006
`
`Tommi Koivisto
`tommi.koivisto@tkk.fi
`
`Overview of IEEE 802.11b Wireless LAN 1
`
`Bell Northern Research, LLC, Exhibit 2011, Page 1 of 27
`
`
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Overview of IEEE 802.11b Wireless LAN
`S-72.4210 Postgraduate course in Radio Communications
`10.1.2006
`
`Tommi Koivisto
`tommi.koivisto@tkk.fi
`
`Overview of IEEE 802.11b Wireless LAN 1
`
`Bell Northern Research, LLC, Exhibit 2011, Page 1 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Outline
`
`• Introduction
`• Standardization
`• Physical layer (PHY)
`− Direct-sequence spread spectrum
`− Complementary Code Keying
`− Physical layer convergence protocol (PLCP)
`• Medium access control layer (MAC)
`− CSMA/CA
`− MAC frames
`• Conclusion
`• References
`• Homework
`
`Overview of IEEE 802.11b Wireless LAN 2
`
`Bell Northern Research, LLC, Exhibit 2011, Page 2 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Introduction
`• IEEE 802.11b is a wireless LAN standard that defines a physical
`layer and MAC layer for wireless communications within a short
`range (up to 300 meters) and with low power consumption.
`• IEEE 802.11b provides a substitute for wired LAN and also offers
`flexibility in terms of mobility.
`• The 802.11b is an extension for the original 802.11 and provides up
`to 11 Mbps transmission rates over the air interface.
`• Devices have been on the market for several years now. Currently
`the dominating WLAN standard seems to be 802.11g, but most of
`those devices are compatible also with 802.11b.
`• WLAN networks can be either infrastructured networks, when there
`is an access point (AP) that controls access to the (wired) network,
`or ad hoc networks that are composed solely of the stations
`transmitting to each other.
`
`Overview of IEEE 802.11b Wireless LAN 3
`
`Bell Northern Research, LLC, Exhibit 2011, Page 3 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`IEEE 802.11 standards
`• IEEE 802.11: up to 2 Mbps, 2.4 GHz, approved in 1997
`• IEEE 802.11a: up to 54 Mbps, 5 GHz, approved in 1999
`• IEEE 802.11b: up to 11 Mbps, 2.4 GHz, approved in 1999
`• IEEE 802.11g: up to 54 Mbps, 2.4 GHz, approved in 2003
`• IEEE 802.11e: new coordination function for QoS, not yet approved
`• IEEE 802.11f: IAPP, inter-AP protocol, approved in 2003
`• IEEE 802.11h: use of 5 GHz band in Europe, approved in 2003
`• IEEE 802.11i: new encryption standards, approved in 2004
`• IEEE 802.11n: MIMO physical layer, not yet approved
`
`Standards are available at http://standards.ieee.org/getieee802/portfolio.html
`
`Overview of IEEE 802.11b Wireless LAN 4
`
`Bell Northern Research, LLC, Exhibit 2011, Page 4 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`IEEE 802.11 standards
`IEEE 802.11 standards specify MAC and PHY layers. PHY layer is
`further divided into PLCP (physical layer convergence procedure) and
`PMD (physical medium dependent) sublayers.
`
`Application
`
`Presentation
`
`Session
`
`Transport
`
`Network
`
`Data link
`
`Physical
`
`ISO OSI
`layers
`
`Logical link
`control (LLC)
`
`Medium access
`control (MAC)
`
`Physical (PHY)
`
`IEEE 802
`standards
`
`IEEE 802.11(b)
`
`Overview of IEEE 802.11b Wireless LAN 5
`
`Bell Northern Research, LLC, Exhibit 2011, Page 5 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Physical layer
`• IEEE 802.11b WLAN operates at the ISM frequency band which is
`2.4 GHz - 2.4835 GHz in USA and Europe and 2.471-2.497 GHz in
`Japan.
`• The frequency band is divided into 14 partially overlapping
`channels each 22 MHz wide. In Europe, 13 of these are available,
`11 in USA and only one in Japan.
`• All devices within the same BSS (basic service set) use the same
`channel.
`• The chip rate over the radio interface is 11 MHz. Supported
`transmission rates in are 1 Mbps, 2 Mbps, 5.5 Mbps and 11 Mbps.
`• 1 Mbps and 2 Mbps rates are obtained using direct-sequence
`spread spectrum (DSSS) as specified in IEEE 802.11.
`• 5.5 Mbps and 11 Mbps rate are obtained using complementary
`code keying (CCK) modulation.
`
`Overview of IEEE 802.11b Wireless LAN 6
`
`Bell Northern Research, LLC, Exhibit 2011, Page 6 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Direct-sequence spread spectrum
`• In DSSS, the transmitted signal is spread in bandwidth using a
`spreading code. Each symbol is multiplied by the spreading code:
`
`Spreading in IEEE 802.11 1 Mbps WLAN using Barker code (length 11)
`
`1
`
`2
`
`3
`
`1/11
`
`2/11
`
`3/11
`
`4/11
`
`5/11
`
`6/11
`
`7/11
`
`8/11
`
`9/11
`
`10/11
`
`1
`
`1
`
`Time (µs)
`
`2
`
`3
`
`Overview of IEEE 802.11b Wireless LAN 7
`
`01
`
`−1
`
`0
`
`01
`
`−1
`
`0
`
`01
`
`−1
`
`0
`
`Orig. symbols
`
`Spreading code
`
`After spreading
`
`Bell Northern Research, LLC, Exhibit 2011, Page 7 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Direct-sequence spread spectrum
`• Since the chip rate after spreading is faster than the symbol rate,
`the bandwidth is increased (by a factor that equals the length of the
`spreading code).
`• At the receiver, the signal is despread by a filter that is matched to
`the spreading code.
`• This reduces interference and introduces processing gain to the
`desired signal.
`• The amount of processing gain is calculated as
`
`G =
`
`Chip rate
`Symbol rate
`
`• Thus, DSSS tolerates interference well which is especially
`important when operating at the ISM band.
`
`Overview of IEEE 802.11b Wireless LAN 8
`
`Bell Northern Research, LLC, Exhibit 2011, Page 8 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`DSSS-based PHY
`• The DSSS-based PHY specified in IEEE 802.11 can be used for
`data rates of 1 Mbps and 2 Mbps.
`• The used spreading code is an 11-chip Barker sequence
`+1,-1,+1,+1,-1,+1,+1,+1,-1,-1,-1.
`• Barker sequence has very good autocorrelation properties and is
`thus ideal for environments with interference.
`• The processing gain for a code of length 11 is 10.4 dB.
`• In 1 Mbps PHY, differential BPSK is used as modulation method
`after spreading.
`• In 2 Mbps PHY, differential QPSK is used (the same code is used in
`both I- and Q-branches).
`• IEEE 802.11b devices support also DSSS-based PHY.
`
`Overview of IEEE 802.11b Wireless LAN 9
`
`Bell Northern Research, LLC, Exhibit 2011, Page 9 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Complementary Code Keying
`• IEEE 802.11b defines PHY layer for higher data rates 5.5 Mbps and
`11 Mbps. This is called HR/DSSS for high rate/DSSS.
`• In HR/DSSS, complementary code keying is used as a modulation
`method.
`• CCK is an M-ary orthogonal keying modulation method where one
`of the M unique (almost orthogonal) signal code words are chosen
`for transmission.
`• The length of a code word is 8 => the duration of one symbol is 8
`complex chips. The chip rate is still 11 MHz, so the radio parts of
`the transmitter stay the same as in 802.11.
`
`Overview of IEEE 802.11b Wireless LAN 10
`
`Bell Northern Research, LLC, Exhibit 2011, Page 10 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Complementary Code Keying
`• The 8-chip code words are defined in both case of 5.5 Mbps and 11
`Mbps as
`
`c = hej(φ1+φ2+φ3+φ4), ej(φ1+φ3+φ4), ej(φ1+φ2+φ4),
`−ej(φ1+φ4), ej(φ1+φ2+φ3), ej(φ1+φ3), −ej(φ1+φ2), ejφ1i
`• The first two bits b0 and b1 encode φ1 based on DQPSK, i.e. the
`phase φ1 is relative to the phase φ1 in the previous symbol.
`• In case of 11 Mbps, bits b2-b7 encode φ2,φ3 and φ4 in the same way
`based on DQPSK.
`• In case of 5.5 Mbps, only 4 bits are transmitted during one code
`period. The codeword set is a subset of the codewords used in 11
`Mbps mode, so same hardware can be used for generating them.
`
`Overview of IEEE 802.11b Wireless LAN 11
`
`Bell Northern Research, LLC, Exhibit 2011, Page 11 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Complementary Code Keying
`• Only six bits need to be fed to the code generation block, since the
`first two bits (φ1) affect all chips:
`
`Q
`
`I
`
`DQPSK
`
`Pick one of
`64 complex
`codes
`
`1 1
`
`6
`
`Data input
`
`MUX
`1:8
`
`1.375 MHz
`
`11 MHz
`
`• At the receiver, the transmitted bits are detected by finding the
`correct codeword using a bank of 64 correlators. Also, phase
`detection for the code that gave the largest correlator output is
`needed.
`
`Overview of IEEE 802.11b Wireless LAN 12
`
`Bell Northern Research, LLC, Exhibit 2011, Page 12 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Physical layer convergence protocol
`• Physical layer convergence protocol defines a method of mapping
`the MAC layer data units into frames suitable for transmitting and
`receiving across the air interface.
`• IEEE 802.11b specifies two different PLCP preambles and
`headers: the long PLCP preamble and header that are mandatory
`and an optional short preamble and header that can be used in
`order to get maximum data throughput.
`• The preamble and header are always carried using 1 Mbps DBSPK
`mode. The header determines the transmission rate for the service
`data units (the actual data).
`
`Overview of IEEE 802.11b Wireless LAN 13
`
`Bell Northern Research, LLC, Exhibit 2011, Page 13 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Long PLCP frames
`
`128 bits
`
`SYNC
`
`16 bits
`
`SFD
`
`8 bits
`
`8 bits
`
`16 bits
`
`SIGNAL
`
`SERVICE
`
`LENGTH
`
`16 bits
`
`CRC
`
`1 Mbps DBPSK
`
`144 bits
`
`48 bits
`
`PLCP preamble
`
`PLCP header
`
`PSDU
`
`1 Mbps DBPSK
`2 Mbps DQPSK
`5.5 Mbps, 11 Mbps CCK
`
`• PLCP frames contain a preamble, a header and the carried data
`(PSDU).
`• The preamble consists of
`− A 128-bit SYNC field that consists of scrambled "1" bits and is
`used for synchronization.
`− A 16-bit SFD field that only indicates the start of PHY-dependent
`parameters.
`
`Overview of IEEE 802.11b Wireless LAN 14
`
`Bell Northern Research, LLC, Exhibit 2011, Page 14 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Long PLCP frames
`• The header consists of
`− An 8-bit SIGNAL field that indicates the transmission rate
`(modulation) that is used for carrying the data units.
`− An 8-bit SERVICE field that is mostly reserved for future use.
`− A 16-bit LENGTH field that indicates the length of the PSDU.
`− A 16-bit CRC check that is calculated for the SIGNAL,SERVICE
`and LENGTH fields.
`• The PSDU field carries the MAC frames.
`
`Overview of IEEE 802.11b Wireless LAN 15
`
`Bell Northern Research, LLC, Exhibit 2011, Page 15 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Medium access control layer
`• Medium access control (MAC) layer controls the access of the
`stations to the medium (radio interface).
`• In IEEE 802.11(b), the access to the medium is controlled through
`coordination functions:
`− Distributed coordination function (DCF): All stations participate in
`the medium access control using CSMA/CA access scheme.
`− Point coordination function (PCF): An access point controls the
`medium access by polling the stations periodically. This is an
`optional feature that is not very widely implemented.
`• DCF provides contention-based access whereas PCF can be used
`to provide contention-free services.
`• MAC layer also handles ARQ, addressing and authentication,
`among others.
`
`Overview of IEEE 802.11b Wireless LAN 16
`
`Bell Northern Research, LLC, Exhibit 2011, Page 16 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`CSMA/CA
`• The basic multiple access scheme used in IEEE 802.11(b) is a
`DCF called carrier sense multiple access with collision avoidance
`(CSMA/CA).
`• Before a station starts to transmit, it senses the medium to
`determine if another station is transmitting:
`− If the medium is idle for a duration >= distributed interframe
`spacing (DIFS), the station starts transmitting.
`− If the medium is busy, the station shall do the following:
`1. Wait until the end of current transmission.
`2. After the medium has been idle for a duration of DIFS, the
`station selects a random backoff interval counter and starts
`decrementing it while the medium is idle.
`3. After the counter reaches zero, the station starts transmitting.
`4. If the medium becomes busy while decrementing the counter,
`the counter is stopped until the medium becomes idle again.
`
`Overview of IEEE 802.11b Wireless LAN 17
`
`Bell Northern Research, LLC, Exhibit 2011, Page 17 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`STA 1
`
`STA 2
`
`STA 3
`
`CSMA/CA
`
`STA 1 transmitting,
`others waiting.
`
`
`
`
`
`
`
`
`
`
`
`Transmitting
`
`Backoff counter
`
`
`
`
`
`
`
`
`
`
`
`
`
`Backoff counter of STA 3
`reaches zero and it starts
`to transmit. Others stop
`decrementing their
`counters
`
`STA 3 generates
`a new backoff
`interval randomly.
`
`STA 2 generates
`a new backoff
`interval randomly.
`
`DIFS
`
`DIFS
`
`DIFS
`
`transmitting stations increases,
`Obviously, as the number of
`throughput of a single station decreases rapidly.
`
`the
`
`Overview of IEEE 802.11b Wireless LAN 18
`
`Bell Northern Research, LLC, Exhibit 2011, Page 18 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Hidden node problem
`• The hidden node problem occurs when there are two stations A
`and B that can not hear each other both trying to send to the same
`access point AP (or any other station).
`• Both A and B sense that the medium is idle and start transmitting.
`They can not hear each other, but AP hears both of them, so
`collision will occur at AP.
`
`STA A
`
`AP
`
`STA B
`
`Station A is hidden from station B
`
`Overview of IEEE 802.11b Wireless LAN 19
`
`Bell Northern Research, LLC, Exhibit 2011, Page 19 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`RTS/CTS
`• To get around the hidden node problem, a refinement to the
`distributed coordination function has been specified.
`• The problem is solved using a RTS (request to send) / CTS (clear
`to send) protocol prior to packet transmission.
`• The station A that wants to transmit first sends an RTS packet to
`the receiving station AP. The receiving station AP then responds
`with a CTS packet if the medium is idle.
`• Other station B that can not hear the RTS packet, can hear the CTS
`packet coming from AP and will thus defer the transmission.
`• RTS and CTS packets are very short, so it is less probable that
`they will collide with RTS packets of other stations.
`
`Overview of IEEE 802.11b Wireless LAN 20
`
`Bell Northern Research, LLC, Exhibit 2011, Page 20 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Point coordination function
`• Point coordination is an optional feature that offers contention-free
`service (i.e. no collisions).
`• Point coordination is used only in an infrastructured network
`topology. The point coordinator is then the access point.
`• The AP uses a round-robin policy to poll each station for data to be
`transmitted.
`• If PCF is implemented, it co-exists with DCF so that PCF and DCF
`alternate, thus creating a contention-free period followed by
`contention period.
`• It is optional for stations to respond to the polls. Those stations that
`do respond are called CF-pollable.
`
`Overview of IEEE 802.11b Wireless LAN 21
`
`Bell Northern Research, LLC, Exhibit 2011, Page 21 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`MAC frame format
`
`Octets:
`
`2
`
`2
`
`6
`
`6
`
`6
`
`2
`
`6
`
`0−2312
`
`4
`
`Frame
`control
`
`Duration/
`ID
`
`Address
`1
`
`Address
`2
`
`Address
`3
`
`Sequence
`Control
`
`Address
`4
`
`Frame
`Body
`
`FCS
`
`MAC header
`
`The MAC frame consists of:
`• MAC header, which comprises frame control, duration, address and
`sequence control information.
`• Frame body, which can be 0-2312 octets long.
`• A frame check sequence that contains a 32-bit cyclic redundancy
`code (CRC).
`
`Overview of IEEE 802.11b Wireless LAN 22
`
`Bell Northern Research, LLC, Exhibit 2011, Page 22 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`MAC frame format
`• In the MAC header, the frame control field controls e.g. frame type,
`fragmentation, power management and WEP (wired equivalent
`privacy).
`• There are four address fields, not all of which are necessarily
`present. They are used for transmitter address, receiver address,
`source address and destination address.
`• The sequence control field is used for frame and fragment
`numbering.
`
`Overview of IEEE 802.11b Wireless LAN 23
`
`Bell Northern Research, LLC, Exhibit 2011, Page 23 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Conclusion
`• IEEE 802.11b extends the IEEE 802.11 WLAN standard by
`providing higher data rates of 5.5 Mbps and 11 Mbps.
`• The key technology enabling this is CCK modulation, in which the
`data bits determine a code word that is transmitted over the air
`interface. At the receiver, the received codeword is compared to
`possible codewords by calculating correlation between them.
`• At the PLCP layer, the most visible change to 802.11 is the addition
`of an optional short preamble that enables maximum data
`throughput.
`• MAC layer has not been changed since IEEE 802.11.
`• IEEE 802.11b devices are fully compatible with IEEE 802.11 and
`thus support also 1 Mbps and 2 Mbps rates.
`
`Overview of IEEE 802.11b Wireless LAN 24
`
`Bell Northern Research, LLC, Exhibit 2011, Page 24 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`References
`[1] IEEE Std 802.11, "Information Technology - Telecommunications
`and Information Exchange between Systems - Local and
`Metropolitan Area Networks - Specific Requirements Part 11:
`Wireless LAN Medium Access Control (MAC) and Physical Layer
`(PHY) Specifications", 2003, 528 pages, available at
`http://standards.ieee.org/getieee802/portfolio.html
`[2] IEEE Std 802.11b, "Supplement to Information Technology -
`Telecommunications and Information Exchange between Systems -
`Local and Metropolitan Area Networks - Specific Requirements
`Part 11: Wireless LAN Medium Access Control (MAC) and Physical
`Layer (PHY) Specifications: Higher-Speed Physical Layer
`Extension in the 2.4 GHz Band", 2003, 96 pages, available at
`http://standards.ieee.org/getieee802/portfolio.html
`
`Overview of IEEE 802.11b Wireless LAN 25
`
`Bell Northern Research, LLC, Exhibit 2011, Page 25 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`References
`[3] E. Ferro and F. Potortì, "Bluetooth and Wi-Fi Wireless Protocols: A
`Survey and a Comparison", IEEE Wireless Communications, vol.
`12, no. 1, Feb. 2005, pp. 12–26
`[4] R. Jordan and C. T. Abdallah, "Wireless Communications and
`Networking: An Overview", IEEE Antennas and Propagation
`Magazine, vol. 44, no. 1, Feb. 2002, pp. 185–193
`
`Overview of IEEE 802.11b Wireless LAN 26
`
`Bell Northern Research, LLC, Exhibit 2011, Page 26 of 27
`
`
`
`AB HELSINKI UNIVERSITY OF TECHNOLOGY
`
`S-72.4210 Postgraduate course in Radio Communications
`
`Homework
`1) Explain the basic principles of CCK modulation and demodulation in
`11 Mbps mode.
`2) 8 bits are transmitted using 11 Mbps mode. Find the I- and
`Q-branch signals at the output of the modulator when the
`transmitted bits are 10 11 01 01. You can use the following table to
`map the bits to DQPSK symbols:
`
`Bits didi+1 Phase Bits didi+1 Phase
`π/2
`00
`0
`01
`3π/2
`π
`
`10
`
`11
`
`3) Direct-sequence spread spectrum techniques are used for signal
`transmission and usually DSSS techniques enable CDMA. Give
`your thoughts on why CDMA is not in the case of IEEE 802.11/11b
`signals well suitable to be used as a multiple access technique.
`
`Overview of IEEE 802.11b Wireless LAN 27
`
`Bell Northern Research, LLC, Exhibit 2011, Page 27 of 27
`
`
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