Learning Objectives

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, OFDMs 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 Mbpsphase shift keying (PSK)
• Encodes 125 Kbps of data on each of 48
  subchannels, resulting in 6Mbps data rate
• See Figure 7-7
• 12 Mbpsquadrature phase shift keying (QPSK)
• Encodes 250Kbps per channel for 12 Mbps data rate
• See Figure 7-8

16
PSK
17
QPSK
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Modulation Techniques Vary Depending on Speed

• 24 Mbps16-level quadrature amplitude modulation
  (16-QAM)
• 16 different signals can encode 500 Kbps per
  subchannel
• See Figure 7-9
• 54 Mbps64-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
• Preambleallows receiving device to prepare for
  rest of frame
• Headerprovides information about frame
• Datainformation to be transmitted
• See Figure 7-11

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802.11a PLCP Frame
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Fields in PLCP Frame

• Synchronization
• Rate
• Length
• Parity

• Tail
• Service
• Data
• Pad

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802.11a Rate Field Values
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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
  monitoringauthentication and encryption
• Radio resource handling, channel monitoring, and
  channel selectionautomatic transmission
  frequency allocation (known as Dynamic Frequency
  Selection (DFS)
• Association procedure and reassociation
  procedurestandardized 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
• HomeRFcombines 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 802.11provides 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

• 802.11acurrent leader in business WLANs
• Uses U-NII frequency, allows 8 simultaneous
  channels, and transmits at 54 Mbps standard, can
  be increased to 108 Mbps
• 802.11goffers faster data rates while remaining
  compatible with 802.11b networks
• Uses only three channels and crowded ISM frequency

39
WLAN Summary

• HiperLAN/2uses 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

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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

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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