IEEE 802.11 Standard

1
IEEE 802.11 Standard

• Why we study this standard
• overall architecture
• physical layer spec.
• direct sequence
• frequency hopping
• MAC layer spec.
• channel access
• mobility support

2
802.11 Architecture
3
802.11 Features

• CSMA/CA based MAC protocol
• support for both time-critical and non-critical
  traffic
• support multiple priority levels
• spread spectrum technology (no licensing)
• power management allows a node to doze off

4
802.11 Protocol Entities

• MAC entity
• basic access mechanism
• fragmentation encryption
• MAC layer management entity
• synchronization
• power management
• roaming
• Physical layer convergence protocol (PLCP)
• PHY-specific, common PHY SAP support
• provides carrier sense
• Physical medium dependent sublayer (PMD)
• modulation coding
• PHY layer management
• channel tuning PHY MIB

MAC Sublayer
MAC layer Management
PLCP sublayer
PHY layer Management
PMD sublayer
5
PHY spec

• Infrared PHY
• diffuse infrared
• 1 and 2Mbps
• Frequency hopping PHY
• Direct Sequence PHY
• CCA how to sense a channel is clear
• energy level is above a threshold
• can detect a signal
• use both

6
Frequency Hopping Spread Spectrum

• Pseudo-random frequency hopping
• spreads the power over a wide spectrum
  -gtspread spectrum
• narrowband interference cannot jam
• developed initially for military
• 2.4Ghz ISM band, 1-2Mbps 2GFSK, 4GFSK hop over
  79 channels

7
Frequency Hopping
8
Direct Sequence Spread Spectrum

• Spreading factor code bits/data bit, 10-100
  commercial (min 10 by FCC).
• Signal bandwidth gt 10data bandwidth
• code sequence synchronization
• correlation between codes -gt interference -
  orthogonal
• 2.4Ghz band, 1,2Mbps DBPSK, DQPSK 11 chip
  barker sequence

9
Direct Sequence Spread Spectrum
10
802.11b Frequency Channels

• In US, the 2.4ISM band is from 2400MHz to
  2483.5MHz
• Divided up to 11 channels from 24122462MHz,
  spaced 5MHz apart
• Each 802.11b channel is 22MHz
• Channel 1 centered at 2412MHz, 24002423MHz
• Channel 2 centered at 2417MHz,
• Channel 6 centered at 2437MHz, 24262448MHz
• Channel 11 centered at 2462MHz, 24512473MHz
• 3 channels (e.g., Channels 1, 6, 11) are safe to
  use simultaneously
• 3MHz of buffer zone between channels

11
Basic MAC Features

• Carrier sense multiple access with collision
  avoidance (CSMA/CA) based
• based on carrier sense function in PHY called
  Clear Channel Assessment (CCA)
• CSMA/CAACK for unicast frames, with MAC level
  recovery
• parameterized use of RTS/CTS to protect against
  hidden nodes
• frame formats to support both infrastructure and
  ad-hoc networks
• supports time-critical traffic in addition to
  traditional data traffic

12
CSMA/CAACK 4-way handshake
13
Frame Format

• MAC headers format differs per type
• control frames RTS, CTS, ACK
• management frames, e.g. beacon, probe/probe
  response, (re)-association request/response,
• data frames

14
802.11 frame priorities
15
CSMA/CAACK explained

• Reduce collision probability where mostly needed
• stations are waiting for medium to become free
• select random backoff after a defer, resolving
  contention to avoid collision
• efficient backoff algorithm stable at high loads
• defer access based on carrier sense
• CCA from PHY and virtual carrier sense state
• direct access when medium is sensed free longer
  than DIFS, otherwise defer and backoff
• receiver of directed frames to return ACK when
  CRC correct

16
4-Way Handshake
17
Time-critical service via PCF
18
PCF Access Procedure

• PC senses the medium at the beginning of each CFP
• PC transmits a beacon containing CF parameter
  set element when idle gt PIFS
• each station presets its NAV to the
  CFPMaxDuration from the CF Parameter Set Element
  in beacons from the PC.
• after a SIFS period, PC sends one of the
  following a data frame, CF-Poll frame,
  DataCF-Poll frame, CF-end frame (when no traffic
  buffered no polls to send at the PC)
• PC maintains a polling list to select stations
  that are eligible to receive CF-Polls during
  contention-free periods.
• A CF-Pollable station always responds to a
  CF-Poll if no data from the station, responds
  with a Null Frame or a CF-ACK (no data) frame
  (when ACK is required)
• piggyback ACK or Poll in the data frame
  whenever possible

19
Further details

• Alternating Contention free and contention
  operations under PCF control
• NAV prevents contention traffic until reset by
  the last PCF transfer -gt variable length
  contention free period per interval
• both PCF and DCF defer to each other causing PCF
  burst start variations
• CF-burst by polling bit in CF-down frame
• immediate response by station on a CF_Poll

20
Synchronization in 802.11

• All stations maintain a local timer
• Timing synchronization function (TSF)
• keeps timers from all stations in synch
• AP controls timing in infrastructure networks
• timing conveyed by periodic beacons
• beacons contain timestamp for the entire BSS
• timestamp from beacons to calibrate local clocks
• not required to hear every beacon to stay in
  synch
• used for power management
• beacons sent at well known intervals
• all station timers in BSS are synchronized

21
Roaming in 802.11
22
Roaming Approach

• Station decides that link to its current AP is
  poor
• station uses scanning function to find another AP
• station sends Reassociation Request to new AP
• if Reassociation Response is successful
• then station has roamed to the new AP
• else station scans for another AP
• if AP accepts Reassociation Request
• AP indicates Reassociation to the Distribution
  System
• Distribution System information is updated
• normally old AP is notified thru distributation
  system

23
Scanning

• Scanning required for many functions
• finding and joining a network
• finding a new AP while roaming
• initializing an ad hoc network
• 802.11 MAC uses a common mechanism
• passive or active scanning
• Passive scanning
• by listening for Beacons
• Action Scanning
• probe response

24
Active scanning
25
Power Management
26
802.11a Standard

• PHY layer
• 12 nonoverlapping channels in 5GHz band
• OFDM
• Offers rate up to 54Mbps
• MAC
• Roughly the same as 802.11b
• 802.11g
• Backward compatible with 802.11b, operating at
  2.4Ghz, fall back to 11Mbps with 802.11b AP
• OFDM based