1. Wireless Personal Communications Systems – CSE5807
Lecture: 07
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. Locating a WLAN Service Set Identifier (SSID): Beacons:
Unique, case sensitive, alphanumeric value from 2-32 characters long.
Used as a network name.
Sent in beacons, probe requests, probe responses and other types of frames.
Beacons:
To organize and synchronize wireless communications.
From AP to station in infrastructure mode.
From station to station in ad hoc mode.
Provide functions including
Time synchronization
FH or DS parameters
SSID information
Traffic Indication Map
Supported rates

3. Locating a WLAN Scanning: Passive Scanning Active Scanning
Process of listening for beacons on each channel for a specific period of time.
Continuing process even after association.
Active Scanning
Sending of probe request frame by a wireless station when it seeks a network to join.
The probe request frame contains either a particular network’s SSID or broadcast SSID.

4. Authentication
Wireless client’s identity is verified by the network/access point.
Access Point => Accept/Deny
Authentication Methods:
Open System Authentication
Based on SSID only.
Option of using WEP for only encrypting data.
Shared Key Authentication
Use WEP. AP
LAN
Authentication Request Frame
Authentication Response Frame

5. Association
Allowed to pass data through access point => “Associated”.
Authentication => Association
Wireless client can authenticate more than one access point at a time but can associate only one access point. AP
LAN
Association Request Frame
Association Response Frame

6. Roaming AP Wireless client determines based on the signal strength.
Disassociation Frame
Reassociation Frame
Wireless client determines based on the signal strength.
IEEE does not define how should be performed.
But some basic building blocks for this process.
Active and passive scanning, reassociation process.
New IEEE802.11f standard for roaming.
Inter Access Point Protocol (IAPP).

7. Load Balancing A B
Multi-cell structure with co-located access points creating a common coverage area.
Wireless clients automatically associate with the access point that is less loaded and provides the best signal quality.

8. Adaptive Rate Selection (ARS)
11 Mbps
2 Mbps
Speed adjustment with varying distance and interference.
Switched between specified data rates.
Important in planning:
Network throughput
Cell sizes
Power outputs of access points and wireless clients
Security

9. Power Management Continuous Aware Mode: Power Save Polling (PSP):
Uses full power and no sleep mode.
Wireless client determines based on the signal strength.
Power Save Polling (PSP):
Wireless client powers down for a very short amount of time.
In BSS, traffic indication map (TIM) is used to notify buffered traffic.
In ad hoc, “Ad hoc traffic indication messages” are used to notify buffered traffic.

10. IEEE802.11 Physical Layer MAC PLCP MAC Management PHY LLC PMD
Data Link Layer
Physical Layer
MAC
PLCP
MAC Management
PHY
LLC
PMD
Management
Physical Layer Convergence Protocol (PLCP):
Responsible for carrier sensing assessment and forming packets for different physical layers.
Physical Medium Dependent (PMD) protocol:
Defines modulation and coding technique for signaling.
Physical Layer Management:
Decides on channel tuning to different options for each physical layer.

11. IEEE802.11 Standards IEEE802.11 IEEE802.11b IEEE802.11a IEEE802.11g
Frequency
2.4GHz
5GHz
Max. Rate
2Mbps
11Mbps
54Mbps
Modulation
FHSS
DSSS
OFDM

12. IEEE802.11 Physical Layer: FHSS
SYNC (80) SFD (16) PLW (12) PSF (4) CRC (16) Whitened MPDU (<4096 Bytes)
PLCP (always 1Mbps)
1 or 2 Mbps)
Preamble
Header
MPDU: MAC Protocol Data Unit
SYNC: Alternating 0 and 1
SFD: Start of Frame Delimiter – specific pattern of 16 bits ( )
PLW: Packet Length Width
PSF: Packet Signaling Field
CRC: Cyclic Redundancy Check – to protect the PLCP bits

13. IEEE802.11 Physical Layer: FHSS
SYNC (80) SFD (16) PLW (12) PSF (4) CRC (16) Whitened MPDU (<4096 Bytes)
PLCP (always 1Mbps)
1 or 2 Mbps)
Preamble
Header
FHSS PMD hops over 78 channels of 1 MHz in the center of the 2.44 GHz ISM bands.
Modulation: Gaussian Frequency Shift Keying (GFSK)
1Mbps – Two levels of GFSK
2Mbps – Four levels of GFSK
Three patterns of 26 hops => Selection by PHY Management layer.
0, 3, 6, 9,……75
1, 4, 7, 10, …..76
2, 5, 8, 11, …..77
Minimum hop rate 2.5 hops per second.
Maximum transmitted power is 100mW.

14. IEEE802.11 Physical Layer: DSSS
SYNC (128) SFD (16) Signal (8) Service (8) Length (16) FCS (8) MPDU
PLCP (always 1Mbps)
1 or 2 Mbps)
Preamble
Header
MPDU: MAC Protocol Data Unit
SYNC: Alternating 0 and 1
SFD: Start of Frame Delimiter – specific pattern of 16 bits ( )
Signal: Data rate
Service : Reserved for future use
Length: Length of MPDU in microsecond
FCS: PLCP header coding

15. IEEE802.11 Physical Layer: DSSS
SYNC (128) SFD (16) Signal (8) Service (8) Length (16) FCS (8) MPDU
PLCP (always 1Mbps)
1 or 2 Mbps)
Preamble
Header
Barker code of length 11.
Uses non-overlapping pulses at chip rate of 11Mcps occupying 26 MHz.
Modulation:
1Mbps => DBPSK
2Mbps => DQPSK
ISM band at 2.4 GHz divided into 11 overlapping channels spaced by 5 MHz.
Maximum transmit power is 100mW.

16. IEEE802.11b Physical Layer
Defines a new coding, Complementary Code Keying (CCK) to support data rates of 5.5 Mbps and 11Mbps.
1Mbps => Barker Code and DBPSK
2Mbps => Barker Code and DQPSK
5.5Mbps => CCK and DQPSK
11 Mbps => CCK and DQPSK
Uses the same PLCP as the IEEE DSSS standard.
Interoperates with IEEE networks.

17. IEEE802.11a Physical Layer Based on OFDM scheme.
Operates at 5 GHz UNII bands.
Eight non-overlapping channels of 20 MHz at the two lower bands of the 5 GHz UNII band.
Each channel is divided into 52 subcarreirs, each approximately 300 kHz.
Data is transmitted in parallel on each subcarrier.
Forward Error Correction (FEC) codes are used to correct errors.
Data rates: 6, 9, 12, 18, 24, 36, 48 and 54
Modulation: BPSK, QPSK, 16-QAM and 64-QAM

18. IEEE802.11g Physical Layer Based on OFDM scheme.
Operates at 2.4 GHz ISM bands.
Backward compatibility with IEEE802.11b.
Switch automatically to CCK/Other modulations.
Data rates: 1, 2, 5.5, 6, 9, 11, 12, 18, 22, 24, 33, 36, 48 and 54 Mbps.
Use optional CCK-OFDM.

19. IEEE802.11a and HiperLAN-2 IEEE802.11a HiperLAN-2 Frequency 5 GHz
Max. trans. Rate
54 Mbps
Medium Access
Through sensing
Centralized scheduling
QoS support
PCF
ATM/802.1p/RSVP
Wired backbone
Ethernet
Ethernet/ATM/ UMTS/PPP/IP
Connectivity
Connectionless
Connection-oriented

20. Wireless LAN: Deployment
Requirements:
- Facility (Building plan).
- Applications.
- Users.
- End user devices.
- Battery longevity.
- Coverage areas.
- Security.
Design:
- System Architecture.
- Identifying standards.
- Selecting devices.
Installation and Testing.

21. Wireless LAN: Deployment
RF interference (from other devices).
Interoperability issues (eg.: IEEE a & IEEE 802.b).
Security holes.
Application interfaces/requirements.
Unclear requirements.

22. Wireless LAN: Design Technical Considerations:
Adequate radio coverage throughout the service area.
Adequate capacity to handle traffic load.
Network performance.
Main design steps:
Selection of AP locations.
Assignment of radio frequencies to APs.

23. Wireless LAN: Design Radio propagation is mostly unpredictable.
Design is iterative process.
Steps involved:
Initial selection of AP locations.
Test and redesign.
- Adjusting the AP locations based on signal strength measurements.
Creation of coverage map.
Assignment of frequencies (or channels) to APs.
Signal strength measurements and minimizing co-channel coverage overlap.

24. WLAN Design: Access Points
Based on measurements.
Layout and construction of buildings determine the coverage area of each AP.
Must avoid coverage gaps.
Space APs as far apart as possible to minimize:
- the cost of equipment and installations.
- the co-channel overlap.

25. WLAN Design: Access PointsR D

26. WLAN Design: Access PointsR D

27. WLAN Design: Channel Allocations
Once APs are located and their coverage areas are identified, radio channels are assigned to the APs.
Radio channels are assigned in a way that provides the smallest possible co-channel overlap.
In high-density areas:
Multiple radio channels.
Reducing the coverage areas of each APs.
Coverage-oriented design: In low density areas, minimizing the number of APs.
Capacity-oriented design: In high-density areas, assuring adequate capacity to serve all users.

28. Wireless Metropolitan Area Networks

29. Wireless Metropolitan Area Networks
Defined in IEEE standard.
Use licensed spectrum in 10 GHz – 66 GHz.
Provide public network service to fee-paying customers.
Use point-to-multipoint architecture with stationary rooftop or tower-mounted antennas.
Provide efficient transport of heterogeneous traffic supporting quality of service (QoS).
Are capable of broadband transmissions.

30. IEEE 802.16 Protocol Architecture

31. IEEE 802.16a Support to mesh network topology.
Line of sight is not required.
Also operates at frequencies between 2 and 11 GHz.
Dynamic Frequency Selection (DFS) to avoid interference with WLAN.
Further MAC and QoS support.
Three radio technologies:
Single carrier modulation format.
OFDM
OFDMA
Centralized and distributed MAC mechanism.

32. 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.