1. Learning Objectives  Tell how IEEE 802.11a networks function, and how they differ from 802.11b networks  List the advantages and disadvantages of an IEEE 802.11g network  Describe the HiperLAN/2 networks  Compare low-speed and high-speed WLANs  Explain basic and enhanced WLAN security features

2. High Speed WLANs  Three standards for high-speed WLANs that transmit at speeds over 15 Mbps  IEEE 802.11a  IEEE 802.11g  HiperLAN/2  All WLANs are concerned with security  How to prevent unauthorized access

3. IEEE 802.11a  Approved in 1999, 802.11a transmits at speeds of 5.5 Mbps and 11 Mbps  Great demand for 802.11a WLANS, also called Wi-Fi5, with maximum speed of 54 Mbps  Devices use gallium arsenide (GaAs) or silicon germanium (SiGe) rather than CMOS semiconductors  Increased speed achieved by higher frequency, more transmission channels, multiplexing techniques, and more efficient error-correction

4. U-NII Frequency Band  802.11b uses unlicensed Industrial, Scientific, and Medical (ISM) band and specifies 14 frequencies  802.11a uses Unlicensed Information Infrastructure (U-NII) band  Table 7-1 compares ISM and U-NII  U-NII is divided into three bands, shown in Table 7-2  U-NII provides more bandwidth, faster transmission, and increased power  Efforts underway to unify 5 GHz bands globally

5. ISM vs. U-NII

6. U-NII Spectrum

7. Channel Allocation  802.11a WLANs have have 11 channels in USA but requires 25 MHz passband  See Figure 7.1  Figure 7-2 shows 8 channels in Low and Medium Bands with 20 MHz channel supporting 52 carrier signals, each 200 KHz wide  Supports eight networks per AP, as shown in Figure 7-3  IEEE 802.11e Task Group is working on standard that supports quality of service (QOS)

8. 802.11b Channels

9. 802.11a Channels

10. Orthogonal Frequency Division Multiplexing  Electromagnetic waves reflect off surfaces and may be delayed in reaching their destination  Figure 7-4 illustrates multipath distortion  Receiving device waits until all reflections are received before it can transmit  Increasing speed of WLAN only causes longer delays waiting for reflections  802.11a uses Orthogonal Frequency Division Multiplexing (OFDM) to solve this problem

11. Orthogonal Frequency Division Multiplexing  Dating to 1960s, OFDM’s primary role is to split high-speed digital signal into several slower signals running in parallel  Sending device breaks transmission into pieces and sends it over channels in parallel  Receiving device combines signals to re-create the transmission  See Figure 7-5

12. Multiple Channels of OFDM

13. OFDM Breaks 802.11B Ceiling Limit  Slowing down transmissions actually delays reflections, increases total throughput, and results in faster WLAN  See Figure 7-6  802.11a specifies eight overlapping channels, each divided into 52 subchannels that are 300 KHz wide  OFDM uses 48 subchannels for data and the remaining four for error correction

14. OFDM vs. Single Channel

15. Modulation Techniques Vary Depending on Speed  6 Mbps—phase shift keying (PSK)  Encodes 125 Kbps of data on each of 48 subchannels, resulting in 6Mbps data rate  See Figure 7-7  12 Mbps—quadrature phase shift keying (QPSK)  Encodes 250Kbps per channel for 12 Mbps data rate  See Figure 7-8

16. PSK

17. QPSK

18. Modulation Techniques Vary Depending on Speed  24 Mbps—16-level quadrature amplitude modulation (16-QAM)  16 different signals can encode 500 Kbps per subchannel  See Figure 7-9  54 Mbps—64-level quadrature amplitude modulation (64-QAM)  Transmits 1,125 Mbps over each of 48 subchannels  See Figure 7-10

19. 16-QAM

20. 64-QAM

21. Higher Speeds  Official top speed of 802.11a is 54 Mbps  Specification allows for higher speeds known as turbo mode or 2X mode  Each vendor can develop 2X mode by combining two frequency channels  Produces 96 subchannels and speeds up to 108 Mbps  Other 2X mode techniques include increasing and reallocating individual carriers and using different coding rate schemes

22. Error Correction  802.11a transmissions significantly reduce errors  Minimizes radio interference from outside sources  801.11a has enhanced error correction  Forward Error Correction (FEC) transmits secondary copy of information that may be used if data is lost  Uses 48 channels for standard transmissions and 4 for FEC transmissions

23. 802.11a Physical Layer  802.11a changed only physical layer  PHY layer is divided into two parts  Physical Medium Dependent (PMD) sublayer defines method for transmitting and receiving data over wireless medium  Physical Layer Convergence Procedure (PLCP) reformats data received from MAC layer into frame that PMD sublayer can transmit

24. PLCP  Based on OFDM, PLCP frame has three parts  Preamble—allows receiving device to prepare for rest of frame  Header—provides information about frame  Data—information to be transmitted  See Figure 7-11

25. 802.11a PLCP Frame

26. Fields in PLCP Frame  Synchronization  Rate  Length  Parity  Tail  Service  Data  Pad

27. 802.11a Rate Field Values

28. Advantages and Disadvantages  Advantages  Good for area that need higher transmission speeds  Disadvantages  Shorter range of coverage  Approximately 225 feet as compared with 375 feet for 802.11b WLAN

29. IEEE 802.11g  In 2001, IEEE proposed 802.11g draft standard to combine stability of 802.11b with faster data transfer rates of 802.11a  Operates in 2.4 GHz ISM frequency  Has two mandatory modes: Complementary Code Keying (CCK) mode and Orthogonal Frequency Division Multiplexing (OFDM)  Offers two optional modes: Packet Binary Convolutional Coding (PBCC-22) and CCK-ODFM  802.11g products not expected until 2003

30. HiperLAN/2  Similar to 802.11a, HiperLAN/2 was standardized by European Telecommunications Standards Institute  Figure 7-12 shows protocol stack for HiperLAN/2  Has three basic layers: Physical, Data Link, and Convergence  Products based on HiperLAN/2 may appear in 2003

31. HiperLAN/2 Protocol Stack

32. Physical Layer  PHY layers of IEEE 802.11a and HiperLAN/2 are almost identical  Operate in 5 GHz band  Use OFDM  Transmit up to 54 Mbps  Connect seamlessly to wired Ethernet networks

33. Data Link Layer  HiperLAN/2 centralizes control of RF medium to access point (AP)  AP informs clients, known as mobile terminals (MTs), when they may send data  Channel allocation is based on dynamic time-division multiple access (TDMA) that divides bandwidth into several time slots  Quality of Service (QOS) refers to dynamically allocated time slots based on needs of MT and condition of network

34. Radio Link Control (RLC) Sublayer  Three primary functions of RLC sublayer  Connection setup procedure and connection monitoring—authentication and encryption  Radio resource handling, channel monitoring, and channel selection—automatic transmission frequency allocation (known as Dynamic Frequency Selection (DFS)  Association procedure and reassociation procedure—standardized handoff to nearest AP by roaming MTs  Logical Link Control (LLC) sublayer, also part of Data Link Layer, performs error checking

35. Convergence Layer  HiperLAN/2 offers seamless high-speed wireless connectivity up to 54 Mbps  Can connect to cellular telephone systems  Can connect to Asynchronous Transfer Mode (ATMs) systems using fiber-optic media and transmitting at 622 Mbps  Can connect to IEEE 1394 (also known as FireWire) high speed external serial bus transmitting at 400 Mbps

36. Summary: High- and Low-Speed WLANs  May compare different types of WLANs  Do not consider them as competing technologies  Rather, they are complementary technologies, each with its strengths and weaknesses and market niche  HomeRF—combines wireless data, cordless telephony, and streaming media for home networks  Supports QoS and transmits from 1/6 Mbps to 10 Mbps

37. WLAN Summary  IEEE 802.11—provides cable-free access for mobile or fixed location at rate of 1 or 2 Mbps  802.11b (Wi-Fi)—popular choice for business wireless networks  Transmits at 11 Mbps on three simultaneous channels but offers no QoS and uses crowded ISM band

38. WLAN Summary  802.11a—current leader in business WLANs  Uses U-NII frequency, allows 8 simultaneous channels, and transmits at 54 Mbps standard, can be increased to 108 Mbps  802.11g—offers faster data rates while remaining compatible with 802.11b networks  Uses only three channels and crowded ISM frequency

39. WLAN Summary  HiperLAN/2—uses dynamically allocated time slots and dynamic frequency selection for high-speed communications  Popular in Europe  Table 7-4 compares WLANs

40. WLAN Comparison

41. 802.11 Security  Greatest strength of WLANs is ability to roam freely  Greatest weakness is risk of unauthorized user receiving RF signals  Some flawed IEEE WLAN security provisions  Basic Security involves two areas:  Authenticating users  Keeping transmissions private

42. Authentication  Verifies user has permission to access network  Each WLAN client can be given Service Set Identifier (SSID) of network  Only clients that know SSID may connect  SSID may be entered manually into wireless device, but anyone with device has access to network  Access points (APs) may freely advertise SSID to any mobile device within range

43. Privacy  IEEE standard provides optional Wired Equivalent Privacy (WEP) specification for data encryption  Two types of keys used for encryption  Public key cryptography uses matched public and private keys  IEEE uses shared key cryptography with same key used for encryption and decryption  The longer the key, the more secure it is  See Figure 7-13

44. WEP

45. WEP Privacy Concerns  In late 2000, researchers revealed “initialization vector” used to encrypt transmissions with WEP were reused about once every five hours  Makes it easy for anyone to collect data to break WEP encryption  Researches recovered 128-bit WEP key in less than 2 hours  Many think IEEE WLANs should be treated as insecure

46. Enhanced Security  Administrators must use enhanced security measures to prevent WLAN attacks  Four kinds of WLAN attacks  Hardware theft  Access point impersonation  Passive monitoring  Denial of service

47. Additional Security Procedures  IEEE task group working on draft known as IEEE 802.1x to allow centralized authentication of wireless clients  Uses Extensible Authentication Protocol (EAP)—client negotiates authentication protocols with separate authentication server  Uses Remote Authentication Dial-In User Service (RADIUS)—server on wired network sends security keys to wireless client  See Figure 7-14

48. 802.1x Security

49. Other Security Steps  Use an access control list with MAC addresses of approved clients, as seen in Figure 7-15  Use digital certificates issued by trusted third party for secure, encrypted online communication  Use digital wrapper or gatekeeper that secures data by wrapping around another program or file  Use a Virtual Private Network (VPN), a secure, encrypted connection between two points

50. Access Control List

51. Higher Levels of Security  Reduce transmission power used in WLANs  Decreases distance radio waves travel, thus limiting range where hackers can pick up signals  Change default WLAN security settings  Keep WLAN traffic separate from that of wired network  Use 128-bit WEP keys rather than default 40-bit keys