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
There are Several:
Different for 802.11b and 802.11a
Contain Fields For:
Data rate, length, synchronization, and other things
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)]