2.2 802.11
Wireless LAN 802.11 is designed for use in a limited geographical area (office, campus, etc)
Physical Properties
802.11 was designed to run over three different physical media namely FHSS, DSSS and
infra red. The data rate for spread spectrum currently is 11 Mbps.
IEEE 802.11 FHSS
It uses the frequency-hopping spread spectrum method and operates in 2.4GHz ISM
band The band is divided into 79 sub-bands of 1 MHz (and some guard bands)
In FHSS, signal is transmitted over a random sequence of frequencies
A pseudorandom number generator selects the hopping sequence
The modulation technique used is frequency shift keying (FSK)
The receiver uses the same algorithm and seed to hop frequencies and to remain
synchronized with the transmitter in order to correctly receive the frame.
IEEE 802.11 DSSS
It uses the direct sequence spread spectrum method and operates in 2.4GHz ISM band
For each data bit the sender transmits the XOR of that bit and n random bits known as nbit chipping code (11-bit chipping sequence is widely used)
The sequence of random bits is generated by a pseudorandom number
generator The modulation technique used is called phase shift keying (PSK)
IEEE 802.11 Infrared
It uses infrared light in the range of 800 to 950 nm
The modulation technique is called pulse position modulation
(PPM) The sender and receiver do not need a clear line of sight
It has a range of up to about 10 m and is limited to the inside of buildings only
Hidden and Exposed Station
Collision detection is not feasible, since all nodes are not within the reach of each
other. The two major problems are hidden and exposed terminals.
In figure, each node is able to send and receive signals from the nodes to its immediate
left and right.
Hidden Station
B and C are hidden to each other
Exposed Station
C is exposed to transmission from A to B
Hidden Station
o Suppose that station B is sending data to A. At the same time, station C also has
data to send to station A.
o
Since B is not within the range of C, it thinks the medium is free and sends its
data to A. Frames from B and C collide at A.
o
Stations B and C are hidden from each other.
Exposed Station
o
Suppose station A is transmitting to station B and station C has some data to send
to station D, which can be sent without interfering the transmission from A to B.
o
Station C is exposed to transmission from A and it hears what A is sending and
thus refrains from sending, even if the channel is available
Multiple Access with Collision Avoidance (MACA)
The idea is for the sender and receiver to exchange short control frames with each other,
so that stations nearby can avoid transmitting for the duration of the data frame.
Collision avoidance is done through the use of control frames Request to Send (RTS) and
Clear to Send (CTS).
Any station hearing RTS is close to sender and remain silent long enough for the CTS to
be transmitted back.
Any station hearing the CTS must remain silent during the upcoming data
transmission. Handshake for hidden station:
o B sends an RTS containing name of sender, receiver & duration of
transmission. o It reaches A, but not C.
o
The receiver A acknowledges with a CTS message back to the sender B
echoing the duration of transmission and other information.
o The CTS from A is received by both B and C. B starts to transmit data to A.
o
C knows that some hidden station is using the channel and refrains from
transmitting.
The handshaking messages RTS and CTS does not help in exposed stations.
Handshake for hidden node
Handshake for exposed node
The receiver sends an ACK frame to the sender after successfully receiving a
frame In case of collision of RTS frames from two or more stations:
o
Nodes do not receive CTS. Each node waits for a random amount of time (known
as exponential back-off strategy) and then tries to send RTS again
Distribution System
Mobile nodes are connected to a wired network infrastructure called access points (AP)
Access points are connected to each other by a distribution system (DS) such as ethernet,
token ring, etc
Two nodes communicate directly with each other if they are reachable (for eg, A and C)
Communication between two stations in different APs occurs via the two APs (for
example, A and E)
Active Scanning
The technique for selecting an AP is called active scanning as follows:
o The node sends a Probe frame.
o All APs within reach reply with a Probe Response frame.
o The node selects one of the access points and sends that AP an Association
Request frame.
o The AP replies with an Association Response frame
A node uses this protocol whenever it joins the network or when it wants to switch over
to another AP
Passive Scanning
APs also periodically send a Beacon frame that advertises its features such as
transmission rate. This is known as passive scanning
Frame Format
Frame Control It defines the type of frame and control information. Some sub fields are
Type (management, control or data), subtype (RTS, CTS or ACK), ToDS and FromDS
Duration It either defines duration of the transmission.
Addresses There are four address fields, each 6 bytes long. The meaning of each address
field depends on the value of the To DS and From DS subfields. For example,
o When one node is sending directly to another, both the DS bits are 0, Addr1
identifies the target node, and Addr2 identifies the source node
o When both DS bits are set to 1, the message went from a node onto the
distribution system, and then from the distribution system to another node. Addr1
identifies the ultimate destination, Addr2 identifies the immediate sender, Addr3
identifies the intermediate destination and Addr4 identifies the original source.
Sequence Control It defines sequence number of the frame to be used in flow control.
Frame body This field can be between 0 and 2312 bytes and contains information based
on the type and the subtype defined in the FC field.
FCS The FCS field contains a CRC-32 error detection sequence.