IEEE 802'11 Physical Layer

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IEEE 802.11 Physical Layer
Project Group WS 03/04 - SS04Mobile Ad-Hoc
Networks Based On Wireless LAN

• Andrew Hines

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Where is the Physical Layer situated?

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What is IEEE 802.11?

• IEEE
• Institute of Electrical and Electronics Engineers
• Develop international standards for 802.x suite
  of networking protocols
• 802.11
• Family of standards set forth by IEEE to define
  specs for WLANs
• Local, high-speed wireless connectivity for
  fixed, portable and moving stations
• Defines
• Medium Access Control (MAC)
• Physical Layer (PHY) Specs

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PHY MAC Layer Transparency
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IEEE 802.11 Protocol Architecture (PHY Layer)
802.11
802.11b
802.11g
802.11a
Propogation Method
Data Rates supported
Propogation Medium
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Overview

• What 802.11 PHY involves
• Types of signal propagation
• Infra-Red vs. Radio Waves
• Focus on RF Transmission
• DSSS
• Channels
• Frequency bands legislation
• Antennas
• Factors at PHY layer affecting WLAN performance

• Focus on Protocol
• IEEE 802.11 System Architecture
• IEEE 802.11 PHY Layer
• Components
• Services
• IEEE 802.11 Protocol Architecture
• 802.11 DSSS
• 802.11b
• 802.11g

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Frequencies used for communications

• IEEE 802.11 allows for propagation of data via
• Radio wave transmission in UHF and SHF bands
• Infrared transmission

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Infrared Transmission

• 850 - 950 nanometer range
• Uses diffuse (reflected) light or directed light
  if LOS exists between sender receiver
• Senders LEDs or laser diodes
• Receivers photodiodes
• Data rates supported 1 and 2 Mbps
• Advantages
• Cheap senders receivers
• Integrated into almost all mobile devices
• PDAs, laptops, mobiles have an infra-red data
  association (IrDA) Interface
• No licences needed
• Electrical devices dont interfere with
  transmission
• Disadvantages
• Usable broadcast range of about 10 m
• Quite easily shielded gt cannot penetrate walls
• For good transmission quality and high data
  rates, LOS typically required
• IR PHY will operate only in indoor environments.

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Radio Wave Transmission

• Mainly use 2.4 - 2.4835 GHz region of the
  unlicensed ISM band
• Max. broadcast power of 1000 mW in USA, 100 mW in
  Europe 10 mW/MHz in Japan
• Transmitted data superimposed (modulated) on a
  carrier wave
• Demodulated and extracted at receiving end

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Radio Wave Transmission

• Methods exist to split available bandwidth into
  separate channels
• Senders Receivers Antennas
• Data Rates depends on IEEE 802.11 spec used
• Range from 1 to 54 Mbps
• Advantages
• Previous experience with Wide Area Networks (e.g.
  microwave links) and mobile cellular links
• Transmission covers larger areas, and can
  penetrate walls (thinner)
• Radio does not need LOS if frequencies not too
  high
• Disadvantages
• Shielding not simple gt radio transmission can
  interfere with other senders or electrical
  devices can destroy data
• Limited ranges of licence-free bands available
  worldwide, not same in all countries

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Antennas

• What they do
• Radiation and reception of electromagnetic waves
• Coupling of wires to space for radio transmission
• Antenna specs
• Radiation pattern measurement of radiation
  around an antenna
• Gain maximum power in direction of the main lobe
  compared to power of an isotropic radiator (with
  same average power)
• The higher the sensitivity (or gain) of an
  antenna (measured in dBi), the lower the signal
  strength needs to be in order to be received
• dBi  (dB isotropic) The gain an antenna has over
  a theoretical isotropic (point source) antenna.
• The higher the power of the antenna, the further
  distance the signal will travel

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Types of Antennas

• Isotropic radiator equal radiation in all
  directions (3D)
• theoretical reference antenna
• Real antennas are dipoles
• shape of antenna proportional to wavelength
• Omnidirectional vs. Directional
• Omnidirectional propagates signal at equal power
  in all directions
• Directional antennas propagate the signal in a
  single direction
• capable of extending the range of signal by
  concentrating the signal power in that direction
• Multi-element antenna arrays
• Grouping of 2 or more antennas
• Antenna Diversity
• receiver chooses antenna with largest output
• Diversity Combining
• combine output power to produce gain

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Antennas

• The Laws of Radio Dynamics
• Higher data rates shorter transmission range
• Higher power output increased range lower
  battery life
• Higher frequency radios higher data rates
  shorter ranges
• Real antennas always have directive effects
  (vertically and/or horizontally)
• Signal strength decreases by the square of a
  given distance
• The signal strength of WLANs is restricted by
  legislation
• PCMCIA Card Antennas
• Omnidirectional (but have directional properties)
• Mostly Half-duplex

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Factors affecting WLAN performance

• Interference
• Cause higher loss-rates
• Acts like noise
• Unprotected from outside signals
• Many sources
• Electrical appliances, analog TV, surveillance
• Home RF, Bluetooth ISM band interference
• Any products transmitting in same frequency
  spectrum

• Distance (range)
• 100-300m open air 20-100m office
• Depends on propagation path and strength of the
  transmitting antenna
• Also, sensitivity of receiving antenna
• Obstacles between the antennas affect the ranges
  and make them shorter
• Data rates signal strength affected
• Propagation
• characteristics dynamic unpredictable
• time-varying and asymmetric
• fading (frequency dependent)
• reflections

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Factors affecting WLAN performance

• Multipath propagation
• Reflections of signals can change signal
  strength, affecting data throughput
• Depend on number of reflective surfaces, distance
  from transmitter to the receiver, product design
  and radio technology
• Can take many different paths between sender and
  receiver due to reflection, scattering,
  diffraction
• Time dispersion signal is dispersed over time
• Phase shifting
• Distortion

• Dynamic Topology
• Well-defined coverage areas do not exist
• Small changes in position or direction may result
  in dramatic differences in signal strength
• Similar effects occur whether a STA is stationary
  or mobile (moving objects may impact station-
  to-station propagation)
• Bandwidth can vary greatly with clustering to and
  removal of nodes from a Service Set
• Unreliable propagation medium
• Lack of full connectivity may produce hidden
  nodes

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Spread Spectrum Technology

• Problem of radio transmission
• Frequency dependent fading can wipe out narrow
  band signals for duration of interference
• Solution spread the narrow band signal into a
  broad band signal using a special code
• Protection against narrow band interference
• Alternatives Direct Sequence, Frequency Hopping

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DSSS (Direct Sequence Spread Spectrum)

• XOR of signal with pseudo-random number (chipping
  sequence)
• Many chips per bit (e.g. 128) result in higher
  bandwidth of signal
• Signal is spread using an 11-bit chipping code
  called Barker Code
• Spreads signal across a larger segment of
  available spectrum
• Introduces processing gain that provides good
  noise and interference rejection
• Continuous transmission allows higher data
  throughput

• Advantages
• reduces frequency selective fading
• Disadvantages
• precise power control necessary

tb bit period
tc chip period
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DSSS (Direct Sequence Spread Spectrum)
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DSSS Channelization

• Single channel occupies 22MHz wide segment of
  spectrum, with side lobes
• Output power at side lobes typically below
  noise threshold
• A channel is referenced by its centre frequency
• Channels are separated by 5 MHz, and as a result
  adjacent channels overlap and interfere with each
  other.
• APs on adjacent channels cannot be used in close
  proximity to one another
• The minimum useable channel separation is 3
  allowing, for example, channels 1, 4, 7, 10 and
  13 to be used in close proximity (channel 13 is
  only available in Europe)
• The recommended channel separation is 5, allowing
  adjacent APs to use channels 1, 6 and 11

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DSSS Channelization

• FCC (US), IC (Canada), and ETSI (Europe) specify
  operation from 2.42.4835 GHz
• For Japan, operation is specified as 2.4712.497
  GHz
• France allows operation from 2.44652.4835 GHz
• Spain allows operation from 2.4452.475 GHz

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IEEE 802.11 System Architecture

• First some definitions .........
• Station (STA)
• IEEE 802.11 compliant device containing PHY
  interface to wireless medium
• The nodes of the wireless network
• Access Point (AP)
• Provide stations with a connection point to other
  wireless/wired networks
• Basic Service Set (BSS)
• Set of stations and access points within the same
  radio coverage
• Disribution System (DS)
• Connects several BSSs via access points, forming
  a single network
• Extended Service Set (ESS)
• The extended wireless coverage area made possible
  via a DS
• Independent Basic Service Set (IBSS)
• A self contained BSS with no access to a DS (PG
  MANET!)
• Portal
• Logical point at which frames from a non-IEEE
  802.11 network enter the DS of an ESS, and vice
  versa

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IEEE 802.11 System Architecture
Infrastructure network

• Ad Hoc network

PG MANET
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IEEE 802.11 Physical Layer

• Physical Medium Dependant (PMD) sublayer
• Modulation encoding/decoding of the
  transmission frame
• Interfaces directly to wireless transmission
  medium
• Each PMD sublayer may require the definition of a
  unique PLCP
• Physical Layer Convergence Protocol (PLCP)
  sublayer
• Provides carrier sense signal called Clear
  Channel Assessment (CCA)
• Common PHY Service Access Point (SAP) for
  interfacing MAC sublayer
• Map MPDU into a framing format suitable for
  sending/receiving user data and management data
  between STAs using associated PMD system.

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Service Access Points

• Across which defined primitives are exchanged
• Double arrows show interactions that are not
  defined explicitly within IEEE 802.11 standard
• Specific manner in which Management Entities
  integrated into layers also not specified

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Clear Channel Assessment (CCA)

• Algorithm used to determine if channel is clear
• Measure RF energy at antenna
• determine if strength of received signal below
  specified threshold
• or different carrier type than 802.11
  transmitters
• Channel then declared clear MAC layer can be
  given clear channel status for data transmission
• CCA Modes
• CCA Mode 1 Energy above threshold
• CCA reports a busy medium upon detection of any
  energy above the ED threshold.
• CCA Mode 2 Carrier sense only
• CCA reports a busy medium only upon detection of
  a DSSS signal (above or below the ED threshold)
• CCA Mode 3 Carrier sense with energy above
  threshold
• CCA reports a busy medium upon detection of a
  DSSS signal with energy above the ED threshold

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Clear Channel Assessment (CCA)

• Energy detection status given by PMD primitive,
  PMD_ED
• Carrier sense status given by PMD_CS
• The status of PMD_ED and PMD_CS is used to
  indicate activity to the MAC through the PHY
  interface primitive PHY-CCA.indicate
• A busy channel is indicated by PHY-CCA.indicate
  of class BUSY
• A clear channel is indicated by PHY-CCA.indicate
  of class IDLE
• Should a loss of carrier sense occur in middle of
  reception, the CCA indicates a busy medium for
  the intended duration of the transmitted frame

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PLCP State Machines

• 3 State machines
• Carrier Senses determine the state of the medium
• Transmit send the data frame
• Receive receive the data frame

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Carrier Sense Function
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Transmit Function
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Receive Function
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IEEE 802.11 Physical Layer

• PHY Layer Management Entity (PLME)
• Channel choice/tuning
• Admin of PHY MIB
• Management Information Base (MIB)
• Hierarchical data structure
• describes all "objects" that device can report
  status of set value of
• Managed objects represent resources of a system
• may be monitored and modified by a (remote)
  manager
• MIB contains name, object identifier, data type
  and whether objects values can be read from
  and/or written to
• Managed objects of a system usually defined in
  multiple MIB definitions

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IEEE 802.11 Physical Layer

• Station Management Entity (SME)
• Interacts with both MLME and PLME (layer
  independent)
• coordination of all management functions
• gathering of layer-dependent status from layer
  management entities
• setting the value of layer-specific parameters
• Exact functions not specified in IEEE 802.11
  standard
• Used mainly for STA association with AP
• Perform _at_ AP
• establish AP/STA mapping and enable STA
  invocation of distribution system services (DSSs)
• Otherwise transmission not allowed

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IEEE 802.11 DSSS Frame Format

• PPDU PLCP protocol data unit
• During transmission, MPDU prepended with a PLCP
  Preamble and Header to create the PPDU
• At the receiver, PLCP Preamble and Header are
  processed to aid in demodulation and delivery of
  MPDU
• Entire PLCP Preamble and Header transmitted using
  1 Mbit/s DBPSK modulation
• All transmitted bits are scrambled using
  feedthrough scrambler

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IEEE 802.11 DSSS Frame Format

• Synchronization (SYNC) field
• Consists of 128 bits of scrambled 1s
• Field allows receiver to perform necessary SYNC
  operations
• Also provides for
• gain setting
• energy detection
• antenna selection
• frequency offset compensation
• Start Frame Delimiter (SFD) field
• 1111001110100000
• Indicates start of PHY-dependent parameters
  within PLCP Preamble

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IEEE 802.11 DSSS Frame Format

• SIGNAL field
• Indicates to PHY the modulation used for
  transmission (and reception) of the MPDU
• 0A for 1 Mbit/s DBPSK
• 14 for 2 Mbit/s DQPSK
• Data rate signal field value x 100 kbit/s
• SERVICE field
• Reserved for future use
• Value of 00 signifies IEEE 802.11 device
  compliance

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IEEE 802.11 DSSS Frame Format

• LENGTH field
• Unsigned 16-bit integer
• Indicates time (in µS) required to transmit the
  MPDU
• Header Error Check (HEC) field
• SIGNAL, SERVICE, and LENGTH fields protected with
  a CCITT CRC-16 Frame Check Sequence (FCS)
• All FCS calculations made prior to data scrambling

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IEEE 802.11b High Rate DSSS Frame Format

• Extension of IEEE 802.11 DSSS format
• PLCP service data units (PSDU)
• Each MPDU corresponds to a PSDU carried in a PPDU
• Additionally provides 5.5 Mbit/s and 11 Mbit/s
  data rates
• Chipping rate is 11 MHz
• same as DSSS system
• provides the same occupied channel bandwidth
• Uses same PLCP preamble and header as DSSS
• both PHYs can co-exist in the same BSS
• can use the rate switching mechanism as provided.

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DSSS High Rate DSSS Convergence

• For the 2, 5.5, and 11 Mbit/s specs
• PSDUs converted to and from PPDUs
• During transmission, PSDU appended to PLCP
  preamble and header to create PPDU
• At receiver, PLCP preamble and header processed
  to aid in demodulation and delivery of PSDU
• Two different preambles and headers are defined
• Long Preamble Header
• mandatory supported
• interoperates with the current 1 Mbit/s and 2
  Mbit/s DSSS spec
• Short Preamble Header
• optional
• where maximum throughput is desired
• expected to be used only in networks of like
  equipment

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High Rate DSSS Optional Services

• Mode replacing CCK modulation with packet binary
  convolutional coding (PBCC)
• HR/DSSS/short mode
• Allows data throughput at higher rates (2, 5.5,
  11 Mbit/s) to be significantly increased by using
  a shorter PLCP preamble
• Can coexist with DSSS, HR/DSSS, or HR/DSSS/PBCC
  under limited circumstances
• different channels
• with appropriate CCA
• Channel Agility
• Overcomes inherent difficulty with static channel
  assignments (a tone jammer)
• Can also be used to implement IEEE
  802.11-compliant systems interoperable with both
  FH and DS modulations

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High Rate DSSS Long Frame Format

• Same format as IEEE 802.11 DSSS Frame
• The only exceptions are
• Encoding of the rate in the SIGNAL field
• Use of a bit in SERVICE field to resolve an
  ambiguity in PSDU length in octets, when length
  expressed in µS
• Use of a bit in SERVICE field to indicate if
  optional PBCC mode is being used
• Use of a bit in the SERVICE field to indicate
  that transit frequency and bit clocks are locked

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High Rate DSSS Long Frame Format

• SIGNAL field
• Supports four mandatory rates
• Represent rate in units of 100 kbit/s
• 0A for 1 Mbit/s
• 14 for 2 Mbit/s
• 37 for 5.5 Mbit/s
• 6E for 11 Mbit/s

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High Rate DSSS Long Frame Format

• SERVICE field
• 3 bits defined to support HR extension
• Rightmost bit (bit 7) used to supplement LENGTH
  field
• Bit 3 used to indicate whether modulation method
  is CCK lt0gt or PBCC lt1gt
• Bit 2 used to indicate that transmit frequency
  and symbol clocks are derived from same
  oscillator
• Locked clocks bit set by the PHY layer
• An IEEE 802.11-compliant device shall set values
  of bits b0, b1, b4, b5, and b6 to 0

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High Rate DSSS Short Frame Format

• Short Synch field
• Consists of 56 scrambled 0 bits.
• So receiver can perform necessary SYNC
• Start Frame Delimiter (SFD) field
• Time reverse of long PLCP SFD field
• 0000 0101 1100 1111
• Not detected by non-compliant receivers
• SIGNAL field
• Only 1 Mbit/s removed
• Remaining fields same as HR DSSS Long Frame
  format

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WiFi Standard

• The standard for wireless fidelity (Wi-Fi)
• Given to products which have passed a rigorous
  testing program for 802.11 compatibility
• Most commonly associated with .11b standard
• Mission
• To certify interoperability of Wi-Fi (IEEE
  802.11) products and to promote Wi-Fi as the
  global wireless LAN standard across all market
  segments.
• 83 companies (March 20, 2001)
• 79 certified products (January 29, 2001)
• http//www.wi-fi.org/

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IEEE 802.11g standard

• Ratified in June, 2003
• New supported transmission rates
• 6, 9, 12, 18, 24, 36, 48 54 Mbps
• Backwards compatible with IEEE 802.11b
• Can co-exist in same WLAN network
• Uses OFDM modulation and PBCC encoding
• Additional Benefits
• Lower bandwidth than spread spectrum
• High spectral efficiency
• Resiliency to RF interference
• Lower multi-path distortion

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Factors affecting Scalability

• National international regulations
• Limits on transmit power
• Low power mobile devices running on batteries
• Licence-free operation equipment must operate in
  licence-free bands
• Omni antennas limited in sensitivity
• Interoperability of new legacy systems
• Must use WiFi standard
• Open air medium is significantly less secure than
  wired systems
• Higher delay lower bandwidth
• Interference
• Environmental factors affecting signal
  propagation
• Mobility reduces network stability
• Must be limits on how much bandwidth shared in
  certain area
• Distance between nodes affects signal strength
• Repeaters, fixed nodes with powerful antennas
• Unpredictable propagation medium methods

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Summary

• What 802.11 PHY involves
• Types of signal propagation
• Infra-Red vs. Radio Waves
• Focus on RF Transmission
• DSSS
• Channels
• Frequency bands legislation
• Antennas
• Factors at PHY layer affecting WLAN performance

• Focus on Protocol
• IEEE 802.11 System Architecture
• IEEE 802.11 PHY Layer
• Components
• Services
• IEEE 802.11 Protocol Architecture
• 802.11 DSSS
• 802.11b
• 802.11g

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Any Questions?