1. Wireless Personal Communications Systems – CSE5807
Lecture: 06
Stephen Giles and Satha K. Sathananthan
School of Computer Science and Software Engineering
Monash University
Australia
These slides contain figures from Stallings, and are based on a set developed by Tom Fronckowiak .

2. Wireless LAN Applications
LAN Extension:
Wireless LAN linked into a wired LAN on same premises.
Cross-building interconnect:
Connecting wired or wireless LANs by point-to-point wireless link.
Devices connected are typically bridges or routers.
Nomadic Access:
Wireless link between LAN hub and mobile data terminal equipped with antenna.
Ad hoc networking:
Temporary peer-to-peer network set up to meet immediate need.

3. Wireless LAN AP
WAN
Backbone LAN
Router

4. Wireless LAN Ad hoc networks Infrastructure networks WAN LAN
Access Point
LAN
WAN

5. Wireless LAN Requirements
Throughput
Number of nodes
Connection to backbone LAN
Service area
Battery power consumption
Transmission robustness and security
License-free operation
Handoff/roaming
Dynamic configuration

6. Wireless LAN Categories
Microwave Radio
Infrared (IR)
Directed
Omini-directional
Diffused
Spread spectrum
Narrowband

7. Infrared Data Transmission Techniques
Directed Beam Infrared:
Used to create point-to-point links.
Range depends on emitted power and degree of focusing.
Focused infrared data link can have range of kilometers.
Cross-building interconnect between bridges or routers.
Ominidirectional:
Single base station within line of sight of all other stations on LAN.
Base station broadcasts signal that can be received by infrared transceivers.
Infrared transceivers transmit with directional beam aimed at base station.
Diffused:
All infrared transmitters focused and aimed at a point on diffusely reflecting ceiling
Infrared radiation strikes ceiling and reradiated omnidirectionally.
Picked up by all receivers.

8. Infrared Transmission
Advantages:
Spectrum for infrared virtually unlimited => High data rates.
Infrared spectrum unregulated.
Equipment inexpensive and simple.
Reflected by light-colored objects.
Ceiling reflection for entire room coverage.
Doesn’t penetrate walls.
More easily secured against eavesdropping.
Less interference between different rooms.
Disadvantages:
Indoor environments experience infrared background radiation.
Transmitters of higher power required.
Limited by concerns of eye safety and excessive power consumption
Limits range.

9. Spread Spectrum LAN Configuration
Multiple-cell arrangement.
Within a cell, either peer-to-peer or hub.
Peer-to-peer topology:
No hub
Access controlled with MAC algorithm => CSMA
Appropriate for ad hoc LANs.
Hub topology:
Mounted on the ceiling and connected to backbone.
May control access and act as multiport repeater.
Automatic handoff of mobile stations.
Stations in cell either:
Transmit to / receive from hub only.

10. Narrowband Microwave LAN
Use of a microwave radio frequency band for signal transmission.
Relatively narrow bandwidth.
Licensed:
Licensed within specific geographic areas to avoid potential interference.
Motorola licenses in 18-GHz range
Encrypted transmissions prevent eavesdropping.
Unlicensed:
RadioLAN introduced narrowband wireless LAN in 1995.
Uses unlicensed ISM spectrum => Operates at 10 Mbps in the 5.8-GHz band
Used at low power (0.5 watts or less).
Range = 50 m to 100 m.

11. License-Free Bands No permission required for bandwidth usage.
No licensing cost.
Limit on power of transmission exists.
Potential interference is high.
Industrial Scientific Medical (ISM) bands:
900 MHz ISM Band
902 MHz – 928 MHz => Used in wireless home phones and wireless camera systems.
2.4 GHz ISM Band
2.4 GHz – 2.5 GHz => Used by IEEE802.11, IEEE802.11b and IEEE802.11g devices.
5.8 GHz ISM Band
5.725 GHz– GHz
Unlicensed national Information Infrastructure (UNNI) Bands:
Lower Band
5.15 GHz – 5.25 GHz
Middle Band
5.25 GHz – 5.35 GHz
Upper Band
5.725 GHz – GHz

12. WLAN Organizations
Institute of Electrical and Electronic Engineers (IEEE).
European Telecommunications Standards Institute (ETSI)
The Wi-Fi Alliance
Wireless LAN Association (WLANA)
Infrared Data Association (IrDA)

13. WLAN Standards IEEE802.11 family of standards. HiperLAN HomeRF
IEEE802.11a
IEEE802.11b
IEEE802.11g
HiperLAN
HiperLAN 1
HiperLAN 2
HomeRF

14. Other IEEE802.11 Standards IEEE802.11c IEEE802.11e IEEE802.11f
Define MAC procedure for the bridge operation.
IEEE802.11e
Enhance current MAC to expand support for applications with QoS requirements.
IEEE802.11f
Define procedure for Inter Access Point Protocol (IAPP).
IEEE802.11i
Enhance the MAC to enhance security and authentication mechanisms.

15. IEEE 802 Protocol Layers

16. Protocol Architecture
Functions of physical layer:
Encoding/decoding of signals.
Preamble generation/removal (for synchronization).
Bit transmission/reception.
Includes specification of the transmission medium and topology.

17. Protocol Architecture
Functions of medium access control (MAC) layer:
On transmission, assemble data into a frame with address and error detection fields.
On reception, disassemble frame and perform address recognition and error detection.
Govern access to the LAN transmission medium.
Functions of logical link control (LLC) Layer:
Provide an interface to higher layers and perform flow and error control.

18. Separation of LLC and MAC
The logic required to manage access to a shared-access medium not found in traditional layer 2 data link control.
For the same LLC, several MAC options may be provided.

19. IEEE Services

20. MAC Frame Format MAC control Destination MAC address
Contains MAC protocol information.
Destination MAC address
Source MAC address
Data
Cyclic Redundancy Check (CRC)

21. Logical Link Control Properties not shared by other control protocols:
Must support multi-access, shared-medium nature of the link.
Relieved of some details of link access by MAC layer.
LLC Services:
Unacknowledged connectionless service
No flow- and error-control mechanisms
Data delivery not guaranteed
Connection-mode service
Logical connection set up between two users
Flow- and error-control provided
Acknowledged connectionless service
Cross between previous two
Datagrams acknowledged
No prior logical setup

22. IEEE 802.11 Architecture Access point (AP): Basic service set (BSS) :
MAC protocol by a central coordination function.
Basic service set (BSS) :
Stations competing for access to shared wireless medium.
Isolated or connected to backbone distribution system (DS) through AP.
Distribution system (DS):
Can be a switch, a wired network or a wireless network.
Extended service set (ESS):
Two or more basic service sets interconnected by DS.

23. IEEE Architecture AP
WAN
Backbone LAN
Router
BSS
BSS
BSS
ESS

24. IEEE 802.11 Services Service Provider Used to support Association
Distribution System
MSDU delivery
Authentication
Station/AP
LAN access and security
Deauthentication
Disassociation
Distribution
Integration
Privacy
Reassociation
MSDU – MAC Service Data Unit

25. Distribution of Messages Within a DS
Distribution service:
Used to exchange MAC frames from station in one BSS to station in another BSS.
Integration service:
Transfer of data between station on IEEE LAN and station on integrated IEEE 802.x LAN.

26. Association-Related Services
Before DS can deliver data to or accept data from a station, that station must be associated.
Three transition types:
No transition
Stationary or moves only within BSS.
BSS transition
Station moving from one BSS to another BSS in same ESS.
ESS transition
Station moving from BSS in one ESS to BSS within another ESS.

27. Association-Related Services
Establishes initial association between station and AP.
Reassociation
Enables transfer of association from one AP to another, allowing station to move from one BSS to another.
Disassociation
Association termination notice from station or AP.

28. Access and Privacy Services
Authentication
Establishes identity of stations to each other.
Deauthentication
Invoked when existing authentication is terminated.
Privacy
Prevents message contents from being read by unintended recipient.

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

30. Reliable Data Delivery
More efficient to deal with errors at the MAC level than higher layer (such as TCP).
Frame exchange protocol:
Source station transmits data.
Destination responds with acknowledgment (ACK).
If source doesn’t receive ACK, it retransmits frame.
Four frame exchange:
Source issues request to send (RTS).
Destination responds with clear to send (CTS).
Source transmits data.
Destination responds with ACK.

31. Access Control Distributed Foundation Wireless MAC (DFWMAC).
Distributed Access:
Attractive for ad hoc network and bursty traffic.
Handled by lower sublayer of the MAC layer, Distributed Coordination Function (DCF).
Use CSMA.
Ordinary asynchronous traffic directly uses DCF.
DCF includes a set of delays to provide priority scheme.
Optional Centralized Access:
Point Coordination Function (PCF) provides contention free service.
PCF is built on top of DCF.

32. Access Control

33. Medium Access Control Logic

34. Interframe Space (IFS) Values
Short IFS (SIFS):
Shortest IFS
Used for immediate response actions.
Acknowledgment (ACK)
Clear to send (CTS)
Poll response
Point coordination function IFS (PIFS):
Midlength IFS.
Used by centralized controller in PCF scheme when using polls.
Takes precedence over normal contention traffic.
Distributed coordination function IFS (DIFS):
Longest IFS
Used as minimum delay of asynchronous frames contending for access
Used for all ordinary asynchronous traffic.

35. Communication Process
Beacon
Contention-Free Period
(PCF Mode)
Contention Period
(DCF Mode)
Superframe
PIFS
DIFS
Contention Period
time
Stations in DCF mode contend for access here.
AP seizes control of medium here.

36. MAC Frame Format

37. Control Frames Data Frames
Power save – poll (PS-Poll)
Request to send (RTS)
Clear to send (CTS)
Acknowledgment
Contention-free (CF)-end
CF-end + CF-ack
Data Frames
Data-carrying frames
Data
Data + CF-Ack
Data + CF-Poll
Data + CF-Ack + CF-Poll

38. Management Frame Beacon Probe request Probe response Authentication
Deauthentication
Association request
Association response
Reassociation request
Reassociation response
Dissociation
Announcement traffic indication message

39. Required Reading Reference
W. Stallings, “Wireless Communications and Networks” Prentice-Hall, 2000.
>> Chapter 13 & 14
Reference
K. Pahlavan and K. Krishnamurthy “Principles of Wireless Networks”, Prentice-Hall, 2002.