Wi-Fi (IEEE 802.11)

Wi-Fi (IEEE 802.11)
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Background

The Standards:
- IEEE 802.11 (a.k.a. ISO/IEC 8802-11)
The Certification:
- Wireless Fidelity (Wi-Fi)
- Given by the Wi-Fi Alliance [formerly Wireless Ethernet Compatibility Alliance (WECA)]

The Physical Layer

Purpose:
- Defines the radio wave modulation and signalling characteristics for data transmission
Alternative Modes for 802.11b/g:
- Direct Sequence Spread Spectrum (DSSS) - encodes each bit into a bit pattern (called a chipping code or Barker sequence; spreads the energy in a signal over a wider frequency range using 11 channels)
- Frequency Hopping Spread Spectrum (FHSS) - jumps between one of 79 channels (using a fixed hop pattern)
- Infrared
Mode for 802.11a:
- Orthogonal Frequency-Division Multiplexing (OFDM) - multiple narrowband data streams at different frequencies

The Physical Layer (cont.)

Frequency Bands:
- 802.11b and 802.11g DSSS uses 11 channels in the 2.4GHz band, each of which is 22MHz wide
- 802.11a uses 9 channels in the 5.8GHz range
Distance Limitations:
- In theory, 802.11b has a range of 100 meters indoors and 200 meters outdoors
- In practice, many things can interfere with the signal, including microwave ovens that operate at 2.45GHz (recall that this part of the spectrum is unlicensed)

The Physical Layer (cont.)

DSSS Channels in 802.11b/g:
- 1: 2.401-2.423GHz
- 2: 2.405-2.428GHz
- 3: 2.411-2.433GHz
- 4: 2.416-2.438GHz
- 5: 2.421-2.443GHz
- 6: 2.426-2.448GHz
- 7: 2.431-2.453GHz
- 8: 2.436-2.458GHz
- 9: 2.441-2.463GHz
- 10: 2.446-2.468GHz
- 11: 2.451-2.473GHz
An Important Observation:
- Channels 1, 6 and 11 do not overlap so can be used at the same time

Topologies

Completely Connected:
- Ad hoc Wi-Fi networks communicate peer-to-peer
Star:
- Infastructure networks use a Wireless Access Point (AP/WAP) to transmit between devices
- [A basic service set (BSS) consists of one or more stations and an access point.]
- [Multiple APs can be connected to form a distribution system (DS).]

Media Access Control (MAC) Layer

Recall:
- Ethernet uses Carrier Sense Multiple Access/Collision Detection (CSMA/CD)
A Difficulty:
- This is impractical for wireless when an adapter can't transmit and receive at the same time (because you can't listen to determine if a collision has occurred while transmitting)

MAC Layer (cont.)

Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA):
- Listen [for an amount of time called the distributed inter frame space (DIFS)] to see if idle
- If idle, transmit
- After transmitting, wait for an acknowledgment (ACK)
- The recipient transmits an ACK [after waiting for an amount of time called the short inter frame space (SIFS)]
- If no ACK is received, backoff (as in Ethernet) and repeat
A Potential Problem:
- Carrier sense does not work when two devices are too far from each other

MAC Layer (cont.)

Fixing this Problem:
- Replace the "direct carrier sensing" process with a "virtual carrier sensing" process
Virtual Carrier Sensing:
- A device sends a Request To Send (RTS) to the wireless access point
- If appropriate, the WAP sends a Clear To Send (CTS) that identifies the device that can send and the amount of time its been allocated
- All devices receive the CTS and, hence, know whether they can send (i.e., the requesting device) or not (i.e., all other devices)

MAC Layer (cont.)

Terminology:
- CSMA/CA is referred to as the Distributed Coordination Function (DCF)
- Virtual carrier sensing is referred to as the Point Coordination Function (PCF)
Of Interest:
- DCF is required
- PCF is optional

MAC Layer Frames in IEEE 802.11

Frame Control (FC):
- Two octets that indicate whether it is a control, management or data frame
Duration/Connection ID (D/I):
- Two octets indicating the number of milliseconds allocated for transmission
Address 1:
- Six octets that depend on frame type but often containing the MAC address of the transmitter
Address 2:
- Six octets that depend on frame type but often contain the MAC address of the receiver
Address 3:
- Six octets that depend on frame type but often contain the MAC address of the destination
Sequence Control (SC):
- Two octets; a 4-bit fragment number and a 12-bit sequence number
Address 4:
- Six octets that depend on frame type but often contain the MAC address of the source
Data:
- 0-2312 octets
Frame Check Sequence (FCS):
- Four octets used for a cyclic redundancy check (CRC)

MAC Layer Frames in IEEE 802.11 (cont.)

Control Frames:
- RTS, CTS, ACK, Contention-free end (CF-End), CF-ACK, a few others
Data Frames:
- Data, Data+CF-ACK, a few others
Management Frames:
- Used to manage associations between stations and APs

Physical Layer Frames in IEEE 802.11

Security

The Issue:
- Since Wi-Fi uses an unguided medium, it its very easy for "outsiders" to receive the signal
Services:
- Authentication - Used by stations to establish their identity (several schemes are supported)
- Confidentiality/Privacy [e.g., wired equivalent privacy (WEP) which is fairly weak; Wi-Fi protected access (WPA)]
- (Courtesy of Hackles)
- (Courtesy of SomethingOfThatIlk.com)

Some Other 802.11 Standards of Interest