1. WLAN

2. What is ?
A family of wireless LAN (WLAN) specifications developed by a working group at the Institute of Electrical and Electronic Engineers (IEEE)
Versions: a, b, g, e, f, i

3. WLANs – 802.11 Protocol Architecture
Physical Layer (PHY)
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
Normal Data Traffic
(Asynchronous)
Real Time Traffic
MAC

4. IEEE 802.11 - Physical Medium Specification
Three Physical Media:
INFRARED
Narrowband Microwave
Spread Spectrum

5. Infrared
Infrared signals used to transmit data (similar to TV remotes!)
Higher data rates possible (than spread spectrum)
Line of sight point-to-point configuration required (or reflection surface will reflects signals)
Too sensitive to obstacles.
10 m maximum range with no sunlight or heat interfere

6. Narrowband Microwave
Typically used to link two WLANs together (for example, to link WLANs in two buildings)
Microwave dishes required at both ends of link
Unlike spread spectrum which operates in the unlicensed ISM band, narrowband microwave requires FCC licensing
Exclusive license typically effective within a 17.5 mile radius

7. Spread Spectrum
Distributed signals over multiple frequencies (to avoid eavesdropping or jamming)
Frequency Hopping Spread Spectrum (FHSS)
Sender transmits over a seemingly random series of frequencies
Intended receiver aware of sequence of frequencies and hops accordingly
Allows the coexistence of multiple networks in the same area by using different hopping sequences
Direct Sequence Spread Spectrum (DSSS)
DS systems use a carrier that remains fixed to a specific frequency band. The data signal is spread onto a much larger range of frequencies (at a much lower power level) using a specific encoding scheme.

8. Wireless LAN Classification
Infrared (IR) LANs
An individual cell of an IR LAN is limited to a single room, since infrared light does not penetrate opaque walls.
Spread Spectrum LANs
In most cases these LANs operate in the ISM (industrial, scientific, and medical) bands, so no FCC licensing is required for their use in the United States.
Narrowband Microwave LANs
These LANs operate at microwave frequencies but do no use spread spectrum. Some of these products operate at frequencies that require FCC licensing; others use one of the unlicensed ISM bands.

9. IEEE 802.11 Medium Access Control
MAC layer covers three functional areas:
Reliable data delivery
Access control
Security

10. Reliable Data Delivery
Loss of frames due to noise, interference, and propagation effects
Frame exchange protocol
Source station transmits data
Destination responds with acknowledgment (ACK)
If source doesn‘t receive ACK, it retransmits frame

11. Wireless LAN MAC CSMA as Wireless MAC?
Hidden and Exposed Terminal Problems make the use of CSMA an inefficient technique

12. Hidden Terminal Problem
Collision A B C
A talks to B
C senses the channel
C does not hear A’s transmission (out of range)
C talks to B
Signals from A and B collide

13. Exposed Terminal Problem
Not
possible A B C D
B talks to A
C wants to talk to D
C senses channel and finds it to be busy
C stays quiet (when it could have ideally transmitted)

14. Hidden and Exposed Terminal Problems
Hidden Terminal
More collisions
Wastage of resources
Exposed Terminal
Underutilization of channel
Lower effective throughput

15. CSMA - Collision Avoidance
CSMA-CA uses short signaling packets for collision avoidance
RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet
CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive
Signaling packets contain
sender address
receiver address
packet size
Variants of this method can be found in IEEE as DFWMAC (Distributed Foundation Wireless MAC)

16. Hidden Terminal Revisited …
RTS A B
CTS C
CTS
DATA
A sends RTS
B sends CTS
C overheads CTS
C inhibits its own transmitter
A successfully sends DATA to B

17. Hidden Terminal Revisited
How does C know how long to wait before it can attempt a transmission?
A includes length of DATA that it wants to send in the RTS packet
B includes this information in the CTS packet
C, when it overhears the CTS packet, retrieves the length information and uses it to set the inhibition time

18. Exposed Terminal Revisited
RTS B
RTS C D
Tx not
inhibited
CTS
Cannot hear CTS
B sends RTS to A (overheard by C)
A sends CTS to B
C cannot hear A’s CTS
C assumes A is either down or out of range
C does not inhibit its transmissions to D

19. Collisions Still possible – RTS packets can collide!
Binary exponential backoff performed by stations that experience RTS collisions
RTS collisions not as bad as data collisions in CSMA (since RTS packets are typically much smaller than DATA packets)

20. Access Control
To deal with these two problems supports two modes of access control DCF (Distributed Coordination Function) and PCF (Point Coordination Function).
All implementations must support DCF, but PCF is optional.