Wireless Personal Communications Systems – CSE5807

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): • 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 • 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 Request Frame LAN AP Authentication Response Frame 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.

5. Association Request Frame LAN AP Association Response Frame 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.

6. AP AP Reassociation Frame Disassociation Frame Roaming • Wireless client determines based on the signal strength. • IEEE802.11 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. A B B A B A B Load Balancing • 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. A A 11 Mbps 2 Mbps Adaptive Rate Selection (ARS) • 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: • 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. LLC Data Link Layer MAC MAC Management PLCP PHY Physical Layer PMD Management IEEE802.11 Physical Layer • 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

12. 1 or 2 Mbps) PLCP (always 1Mbps) SYNC (80) SFD (16) PLW (12) PSF (4) CRC (16) Whitened MPDU (<4096 Bytes) Preamble Header IEEE802.11 Physical Layer: FHSS MPDU: MAC Protocol Data Unit SYNC: Alternating 0 and 1 SFD: Start of Frame Delimiter – specific pattern of 16 bits (0000110010111101) PLW: Packet Length Width PSF: Packet Signaling Field CRC: Cyclic Redundancy Check – to protect the PLCP bits

13. 1 or 2 Mbps) PLCP (always 1Mbps) SYNC (80) SFD (16) PLW (12) PSF (4) CRC (16) Whitened MPDU (<4096 Bytes) Preamble Header IEEE802.11 Physical Layer: FHSS • 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. 1 or 2 Mbps) PLCP (always 1Mbps) SYNC (128) SFD (16) Signal (8) Service (8) Length (16) FCS (8) MPDU Preamble Header IEEE802.11 Physical Layer: DSSS MPDU: MAC Protocol Data Unit SYNC: Alternating 0 and 1 SFD: Start of Frame Delimiter – specific pattern of 16 bits (1111001110100000) Signal: Data rate Service : Reserved for future use Length: Length of MPDU in microsecond FCS: PLCP header coding

15. 1 or 2 Mbps) PLCP (always 1Mbps) SYNC (128) SFD (16) Signal (8) Service (8) Length (16) FCS (8) MPDU Preamble Header IEEE802.11 Physical Layer: DSSS • 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 IEEE802.11 DSSS standard. • Interoperates with IEEE802.11 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

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 802.11 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. R D WLAN Design: Access Points

26. R D WLAN Design: Access Points

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 IEEE802.16 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 • 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.