The IEEE 802.11 SpecificationThe IEEE 802.11 Specification

By Matthew RustonBy Matthew RustonNovember 25, 2007November 25, 2007

Overview Introduction to 802.11 Wireless Networking Specification History High-level Architecture Physical Architecture Data Link Architecture

802.11 Wireless Networking Revolutionized wireless communication Allows compliant devices to communicate using

radio transmissions are specific frequencies. Found in:

AutomobilesDigital CamerasPrintersCell Phones

Specification History First IEEE 802.11 standard released in 1997

Maximum data rate: 2Mbps2.4Ghz bandIndoor Range: 20 metersOutdoor Range: 100 meters

Specification History IEEE 802.11a released in 1999

Maximum data rate: 54Mbps5.1 - 5.8Ghz bandIndoor range: 35 metersOutdoor range: 120 metersLimited indoor effectiveness due to high frequency

use

Specification History IEEE 802.11b also published in 1999

Maximum data rate: 11Mbps2.4Ghz bandIndoor Range: 38 metersOutdoor Range: 140 meters

Specification History IEEE 802.11g released in 2003

The current industry adopted specificationMaximum data rate: 54Mbps2.4Ghz band (backwards compatible with 802.11b)Indoor range: 38 metersOutdoor range: 140 meters

802.11 Architecture A 802.11 compliant network design can be

partitioned into a set of various objects.

802.11 Architecture The 802.11 specification fits perfectly within the

OSI networking model. Thus common place protocols such as TCP or UDP are not effected by the use of a 802.11 network.

Application

Presentation

Session

Transport

Network

Data Link802.3 Ethernet 802.11 Wireless

Physical

802.11 Physical Layer Architecture

Relies on encoding radio waves with binary information to send datagrams.

Multiple methods implemented by the various 802.11 specifications.

802.11 Physical Architecture Uses the Frequency Hopping Spread-Spectrum

(FHSS) specification to transmit encoded information.A device will ‘hop’ between a predetermined and

configured set of frequency channels during the transmission.

If one of the channels has interference or noise this communication protocol will ensure that the majority of the information can still transmit successfully.

802.11 Physical Architecture To encode information into the transmission a

system called Gaussian Frequency Shift Keying (GFSK) is used.Best explained if illustrated…

802.11b Physical Architecture Uses a method called Differential Phase Shift

Keying (DPSK) to encode information into it’s 2.4Ghz band.Uses phase shifts of the carrier wave to encode a

binary message. Design again, best illustrated…

Symbol Phase Shift

00 0

01 Π/2 radians

11 Π radians

10 3 Π/2 or – Π/2 radians

802.11a/g Physical Architecture Although they are not backwards compatible

they use Orthogonal Frequency Division Multiplexing for a transmission protocol.Has been omitted due to complexity.

802.11 Data Link Architecture Much like 802.2, the Ethernet specification,

802.11 relies on Frames to encompass a single datagram.

Ethernet Frame:Preamble Dest.

AddressSource Address

Type Data CRC

8 bytes 6 bytes 6 bytes 2 bytes

46-1500 bytes

4 bytes

802.11 Data Link Architecture 802.11 Frame

802.11 Frame Control Breakdown

Frame Control

Duration Address 1

Address 2

Address 3

Seq. Control

Address 4

Payload CRC

2 bytes 2 bytes 6 bytes 6 bytes 6 bytes 2 bytes 6 byes 0-2312 bytes

2 bytes

Protocol Version

Type Subtype

To DS From DS

More Frag.

Retry Power Mgt.

More Data

WEP Order

2 bits 2 bits 4 bits 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit

802.11 Data Link Architecture To help illustrate the data link architecture,

Wireshark was used to capture and analyze information from a 802.11 datagram.

802.11 Data Link Transport 802.11 uses CSMA/CA for datagram transmission

management.Carrier Sense Multiple Access With Collision

AvoidanceEach wireless device with ‘sense’ the radio channel

before transmission. If it appears busy it will wait a short random amount of time before retesting the channel.

802.11 Data Link Transport ‘Collision Avoidance’ is implemented by using

two special frames between a wireless device and its associated access point (AP).Device wishing to transmit will send a Request To

Send frame to the AP.If the AP deems the channel available it will respond

with a Clear To Send frame to the device.

[1] IEEE Computer Society, IEEE-SA Standards Board, “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”. 1999, Reaffirmed June 2003. IEEE-SA Standards Board. [http://standards.ieee.org/getieee802/download/802.11-1999.pdf]. Accessed Nov. 18, 2007.

[2] IEEE Computer Society, IEEE-SA Standards Board, “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—Amendment 1: High-speed Physical Layer in the 5 GHz band”. 1999, Reaffirmed June 2003. [http://standards.ieee.org/getieee802/download/802.11a-1999.pdf]. Accessed Nov. 18, 2007.

[3] IEEE Computer Society, IEEE-SA Standards Board, “Supplement to IEEE Standard for 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”. 1999, Reaffirmed June 2003. [http://standards.ieee.org/getieee802/download/802.11b-1999.pdf]. Accessed Nov. 18, 2007.

[4] IEEE Computer Society, IEEE-SA Standards Board, “IEEE Standard for 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—Amendment 4: Further Higher-Speed Physical Layer Extension in the 2.4 GHz Band”. 2003. [http://standards.ieee.org/getieee802/download/802.11g-2003.pdf]. Accessed Nov. 18, 2007.