1. Wireless Local Area Networks (LANs)

2. Outline Introduction to wireless LANs Wireless LAN physical layer
Wireless LAN medium access control (MAC)
Distributed coordination function (DCF)
Point coordination function (PCF)

3. Review of Related Lectures
Local area networks (LANs)
Share resources and information
Low-cost, high speed, and error-free communications
Ethernet; token ring networks
Transmission medium

4. Review of Related Lectures (Cont’d)
Random access protocols
ALOHA: “just do it”
A station transmits whenever it has data to transmit
Throughput: 18%
Slotted ALOHA
Time is slotted
Only transmit at the beginning of a time slot
Throughput: 36%
Carrier sensing multiple access with collision detection (CSMA-CD)
Ethernet
Sense before transmission; if channel busy, wait
Continue to sense during transmission
If collision  abort

5. Introduction to Wireless LANs
What is wireless LAN?
An extension of the wired LAN
Compatible
Coverage: ~ 100 feet
Merits
Convenience
Fast installation
User mobility
Challenges
Smaller bandwidth
Interference/noise  not reliable
Broadcast medium  intercepted by snoopers

6. Wireless LAN Standards
HiperLAN - European Telecommunications Standards Institute 5 GHz unlicensed frequency band
IEEE IEEE Worldwide Standard 2.4 GHz or 5 GHz unlicensed frequency band
* IEEE: Institute of Electrical and Electronics Engineers

7. IEEE 802.11 Family for Wireless LANs
Specify air interface between access points (APs) and stations, or between two stations
Difference: radio frequency band, transmission speed, modulation scheme
(see slide #34 for more details)
original wireless LAN standard
1 - 2 Mbps
802.11a
Orthogonal frequency division multiplexing (OFDM)
5 GHz radio frequency
High speed: up to 54 Mbps
802.11b
DS-SS at 2.4 GHz
Up to 11 Mbps
802.11e
Support quality-of-service
802.11g
OFDM
High speed standard at 2.4 GHz
Up to 54 Mbps

8. WLAN Architecture Two modes: Ad hoc networks & Infrastructure networks
Basic service set (BSS)
a group of stations that can communicate with each other
Ad hoc network
No infrastructure; temporary
Peer-to-peer
Conference meetings,
distributed computer games
Ad hoc network

9. Infrastructure Network
Server
An AP in each BSS
Distribution system: interconnect BSSs to form an extended service set (ESS)
Portal: bridge to other networks
Gateway to
the Internet
Portal
Portal
Distribution system
ESS
AP1
AP2 B2 A1 B1 A2
BSS A
BSS B

10. Road Map Introduction to wireless LANs Wireless LAN physical layer
Wireless LAN medium access control (MAC)
Distributed coordination function (DCF)
Point coordination function (PCF)

11. Wireless LAN Physical Layers
Physical layer: transfer of bits over a communication channel
IEEE wireless LAN physical layer (We discuss)
Infrared
Spread spectrum (SS) at 2.4 GHz
Application
Presentation
Session
Transport
Network
Data link
Physical
OSI model

12. Infrared Physical Layer
Coverage: 10 – 20 m
Pulse-position modulation (PPM)
Each transmitted symbol has 16 time slots, one contains a pulse
Four bits  integer in [1, 16] (‘0000’1, ‘1111’16)
The integer determines which slot is used for the pulse
An example
‘0000’  1
Slot 1
Slot 16
‘1111’  16
Slot 1
Slot 16
Symbol

13. Infrared Physical Layer (Cont’d)
Advantages
Simple & inexpensive
Constrained by walls  Secured against eavesdropping, low interference
Disadvantages
Interference (sunlight, indoor lighting)
Limited range
Not popular

14. Spread Spectrum Physical Layer
Spread spectrum: spread the signal energy over a wide frequency band (recall: CDMA)
Frequency hopping (FP) & direct sequence (DS)

15. 11-chip Barker sequence Symbols Transmitted chips Received chips+1
+1 +1
11-chip Barker sequence -1 -1
+1 (for bit ‘1’)
Symbols
-1 (for bit ‘0’)
Transmitted
chips +1
+1 +1 +1 +1
Modulation +1 -1
+1 +1
-1 -1
Received chips
Barker sequence
at the receiver
Products
+11
-11
Demodulation
Receiver -1 -1 -1
-1 -1
Sender

16. Spread Spectrum Physical Layer (Cont’d)
Code-division multiple access (CDMA) channelization is also based on spread spectrum
What’s the difference of spread spectrum (CDMA vs. wireless LANs)?

17. Spread Spectrum in CDMA
Each station has a unique sequence
Stations’ transmissions can be distinguished by their sequences
Support multiple transmissions
Symbols
Station 1
Sequence
Station 2
Station 3

18. Spread Spectrum in Wireless LANs
All the stations use the same Barker sequence
Multiple transmissions  collision
Spread spectrum: overcome interference from other networks
Unlicensed frequency band share by Bluetooth, cordless phones, …

19. Road Map Introduction to wireless LANs Wireless LAN physical layer
Wireless LAN medium access control (MAC)
Distributed coordination function (DCF)
Point coordination function (PCF)

20. Where is MAC in OSI Model?
IEEE 802 wired/wireless LAN
OSI
Network layer
Network layer
Logical Link Control
Data link
layer
LLC
802.3
Ethernet
802.5
Token ring
802.11
Wireless
LAN
Other
LANs
MAC
Various physical layer
(Infrared, spread spectrum, cable)
Physical
layer
Physical
layer
Data link layer: logic link control (LLC) + MAC
MAC: coordinating the access to the shared medium.
LLC: operate over all MAC standards (802.3, 802.5, and ), and offer the network layer a standard set of services

21. A Similar Scenario The instructor is holding an office hour…
Who asks the first questions? who next?
Listen and wait  contention based
Polling based on an order  contention free
Coordination functions: determine when to transmit/receive
Distributed coordination function (DCF): “listen and wait”  contention service
Point coordination function (PCF): “polling”  contention-free service

22. Distributed Coordination Function (DCF)
Mandatory in IEEE family
Distributed manner
Asynchronous data transfer & best effort
All stations contend
Recall: Ethernet has carrier sensing multiple access with collision detection (CSMA-CD)
Why not use CSMA-CD in wireless LANs?
Sense the channel before transmission
Channel busy  wait for some time, then try again
During transmission, continue to sense (detect collision)
Collision detected  abort
transmit and sense at the same time

23. Distributed Coordination Function (DCF) (Cont’d)
Drawbacks of CSMA-CD over wireless LANs
“Collision detection” problem: half-duplex  unable to transmit and sense simultaneously
“Hidden-station” problem (also called “hidden-terminal” problem)
A transmits data frame
C senses medium; hears nothing
C transmits data frame
C collides with A at B
A new MAC: CSMA with collision avoidance (CSMA-CA)
Data frame
Data frame A C B

24. Solution to “Collision Detection” Problem
Acknowledgement (ACK)
No ACK  collision
Information exchange handshake: Data + ACK
New problem: ACK collision
Data A C B A C B
ACK
Data

25. Solution to “Collision Detection” (Cont’d)
Two kinds of carrier sensing
Physical carrier sensing
Virtual carrier sensing: tell others how long I need
Sender: set duration field in the MAC header of the transmitted frames
Indicate the amount of time needed to complete the Data-ACK handshake
Other stations wait until the completion of the exchange, and the waiting time is called network allocation vector (NAV).
When the NAV value is set, a station will not attempt to initiate any transmission for that interval, and if any station is running a back-off counter (see slide #28), the counter will be frozen for that interval.
We need 500 us to complete
Data frame
Ok, I can wait 500 us until your ACK
Duration
=500
ACK
MAC header A C B
Receiver
Sender

26. Solution to “Hidden-Station” Problem
Request-to-send (RTS)/clear-to-send (CTS) handshake
Four-way handshake: RTS-CTS-Data-ACK
Data
Data A C B
RTS
Data
CTS
ACK A C B

27. Basic CSMA-CA Operation
Interframe space (IFS): “idle gap” between two frame transmissions
Short IFS (SIFS): High-priority frames (such as CTS, ACK)
DCF IFS (DIFS) : for distributed coordination function (DCF) to initiate a contention period - RTS
PCF IFS (PIFS): for point coordination function (PCF) to initiate a contention-free period
Frame
Time
IFS
DIFS
PIFS
SIFS
Busy medium
RTS
Time
Sender
DIFS
RTS
Data
SIFS
SIFS
SIFS
Receiver
CTS
ACK

28. Backoff Procedure If channel busy
Schedule a random backoff time (an integer) for retry
After DIFS channel idle, count down the backoff time by one when the channel continues to be idle for one more time slot
Transmit when the backoff time reaches 0
Access time: after DIFS + random backoff time
Can collisions be eliminated completely?
If two stations have the same backoff time?
Backoff time (waiting
time for retry), e.g., 7
Time slot
Backoff
time =7
time =0
Time slot
DIFS
SIFS
SIFS
SIFS
Busy medium
RTS
CTS
Data
ACK
Time
Count down
backoff time

29. Collisions and Retransmissions
Each sender
Interprets non-arrival of ACK as collision
Schedule a new backoff time in a double range,
e.g., [0, 7]  [0, 15]
The backoff time is a random number of slot times within this interval
Retransmit when the backoff time counts down to 0
If collided again, double again
Until ACK or frame dropping at the sender
binary exponential backoff

30. Summary of CSMA-CA Mechanisms
Objective
ACK
“Collision detection” problem
RTS/CTS
“Hidden station” problem
Binary exponential backoff
Collision avoidance and resolution

31. Road Map Introduction to wireless LANs Wireless LAN physical layer
Wireless LAN medium access control (MAC)
Distributed coordination function (DCF)
Point coordination function (PCF)

32. Point Coordination Function (PCF):
Optional
Connection-oriented, contention-free services through polling
Time bounded transfer (e.g., voice over wireless LANs)
Central controller: point coordinator at AP
During the contention free periods the AP polls stations with delay sensitive traffic. The portion of time allocated to the contention free period is variable, and the assignment is made by the AP based on the number of stations requesting contention free service, their transmission requirements and data rates. The AP broadcasts a control message after a PIFS interval causing all stations to reset their NAV to initiate contention free period. As in the RTS/CTS operation, that NAV setting will inhibit stations from sending for the specific amount of time.
Polling table
If polled, transmit without contention
At the end of the contention free period, the network automatically returns to the contention mode.

33. PCF Procedure Mandatory DCF + optional PCF
Contention-free period (CFP): by PCF; contention period (CP): by DCF
CFP and CP alternate
CFP starts with a beacon
SIFS < PIFS < DIFS
Contention-free period (CFP)
Contention period (CP)
SIFS
SIFS
SIFS
SIFS
SIFS AP B D1
+Poll D2
+Poll CF
End B
PIFS
Station 1&2 U1
+ACK U2
+ACK
Reset NAV
Other stations
Network Allocation Vector (NAV)
CF_Max_duration
B: beacon D: downlink frame U: uplink frame

34. Summary Point coordination function Logic link control (LLC) sublayer
Contention-free service
Contention service
Point coordination function
MAC
sublayer
CSMA-CA-based distribution coordination function
1, 2
Mbps
6-54
Mbps
5.5, 11
Mbps
6-54
Mbps
Physical
layer
Infrared
2.4 GHz
DSSS
2.4 GHz
FHSS
5 GHz
OFDM
2.4 GHz
DSSS
2.4 GHz
OFDM
IEEE
802.11a
802.11b
802.11g