CWNA Guide to Wireless LANs, Second Edition

1
CWNA Guide to Wireless LANs, Second Edition

• Chapter Twelve
• Personal, Metropolitan, and Wide Area Wireless
  Networks

2
WPANs Radio Frequency ID (RFID)
Figure 12-8 RFID tag
3
WPANs Radio Frequency ID (continued)

• Passive RFID tags No power supply
• Can be very small
• Limited amount of information transmitted
• Active RFID tags Must have power source
• Longer ranges/larger memories than passive tags

Table 12-4 RFID tags
4
WPANs IrDA

• Infrared Data Association
• IrDA specifications include standards for
  physical devices and network protocols they use
  to communicate
• Devices communicate using infrared light-emitting
  diodes
• Recessed into device
• Many design considerations affect IrDA performance

5
WPANs IrDA (continued)
Figure 12-9 IrDA diodes in device
6
WPANs IrDA (continued)

• IrDA drawbacks
• Designed to work like standard serial port on a
  personal computer, which is seldom used today
• Cannot send and receive simultaneously
• Strong ambient light can negatively impact
  transmissions
• Angle and distance limitation between
  communicating devices

7
Infrared

• Many wireless devices, such as PDAs, use infrared
  (IR) technology
• Two common uses of infrared wireless technology
  are IrDA and wireless local area networks (WLANs)

8
Communications Models and Standards

• International Organization for Standardization
  (ISO) began work in 1970s to develop
  specifications for communication by
  computer-based networks
• Goal was to create an abstract model of
  networking rather than official physical standard
• Completed in 1983, these conceptual
  specifications are known as Open System
  Interconnect (OSI) model

9
Communications Model OSI

• Breaks complex functions into seven basic layers
• Each layer performs specific function that
  involves different tasks
• See Table 4-1

10
OSI Layers and Functions
11
OSI Model

• Tasks may be performed using hardware and
  software
• Each layer must cooperate with layer immediately
  above and immediately below it
• Data travels down layers from sending device, and
  then up layers to receiving device
• See Figure 4-2

12
OSI Data Flow
13
Communications Standards IEEE 803

• Institute of Electrical and Electronics Engineers
  (IEEE) began Project 802 to create standards that
  would ensure interoperability among data networks
  
• While OSI model is theoretical, Project 802
  created standards for actual practice
• Several standards emerged from Project 802
  including 802.3 (Ethernet) and 802.5 (Token Ring)

14
Project 802

• Project 802 subdivided OSI model Layer 2, Data
  Link, into two sublayers
• Logical Link Control (LLC)
• Media Access Control (MAC)
• For wireless networks, defined by 802.11, IEEE
  also subdivided Physical layer into two parts
• Physical Medium Dependent (PMD)
• Physical Layer Convergence Procedure (PLCD)
• See Figure 4-3

15
OSI Model versus IEEE Project 802
16
PLCP Sublayer

• PMD sublayer
• Includes standards for wireless medium such as IR
  and RF
• Defines how medium transmits and receives data
• PLCD sublayer
• Reformats data received from MAC layer into
  packet or frame that PMD sublayer can transmit,
  as shown in Figure 4-4
• Listens to medium to determine when data can be
  sent

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PLCD Sublayer Reformats MAC Data
18
Summary of PMD and PLCD Sublayers
19
Network Protocol Stacks

• Protocols are rules network must follow for
  communications to pass between devices
• Protocols are also divided into layers, generally
  corresponding to the OSI model
• Variety of network protocol stacks
• Transmission Control Protocol/Internet Protocol
  (TCP/IP)a standard protocol for the Internet
• Internet Packet eXchange/Sequenced Packet
  eXchange (IPX/SPX)an older Novell NetWare
  protocol
• AppleTalkused by Apple Macintosh computers

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Infrared WLANs

• Several different IR WLANs have been developed
  during past 20 years
• Infrared WLANs use part of electromagnetic
  spectrum just below visible light
• IR shares these characteristics
• Operates at high frequencies
• Travels in straight lines
• Does not penetrate physical objects

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Other IR Characteristics

• Has an abundance of available bandwidth that is
  unregulated
• Operates at high data rates
• Is more secure than radio frequency transmissions
• Avoids many kinds of interference that affect RF
  signals
• Components are small and consume little power

22
Other IR Characteristics

• IR transmissions can be directed or diffused
• Directed transmission uses a narrow beam and line
  of sight
• Both emitter and detector must be aimed directly
  at one another
• Diffused transmission uses a wide beam and
  reflected light
• Both emitter and detector point at a reflection
  point on the ceiling
• Limited to 4 Mbps with a range of 30 to 50 feet

23
IEEE 802.11 Infrared WLANs

• IEEE 802.11 outlines specifications for infrared
  WLANs
• Uses diffused transmission
• PHY layer both reformats data from PLCP layer and
  transmits light impulses (PMD)

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Diffused Infrared Physical Layer Convergence
Procedure Standards

• Frame size is measured in time slots rather than
  bits

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Parts of the Infrared PLCP Frame

• Synchronization field allows emitter and receiver
  to synchronize
• Start Frame Delimiter defines beginning of frame
  by transmitting 1001
• Data Rate value sets transmission speed

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Parts of the Infrared PLCP Frame

• Direct Current Level Adjustment lets receiving
  device determine signal level

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Parts of the Infrared PLCP Frame

• Length field indicates time to transmit entire
  frame
• Header Error Check has value to determine if
  data was transmitted correctly
• Data field can be from 1 to 20,000 time slots

Although the current IEEE 802.11 standard allows
data transmission rates of 1 or 2 Mbps, the
preamble and header are always transmitted at 1
Mbps to accommodate slower devices
28
Diffused Infrared Physical Medium Dependent
Standards

• PMD layer translates 1s and 0s into light pulses
  for transmission
• A 1 bit has a higher intensity signal than a 0
  bit
• Transmissions at 1 Mbps use a 16-pulse position
  modulation (16-PPM), as shown in Table 4-4
• Transmissions at 2 Mbps use a 4-pulse modulation
  (4-PPM), as shown in Table 4-5

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16-PPM Values
30
4-PPM Values
A time slot is always one nanosecond (ns) or a
billionth of a second, but a 4-PPM transmission
contains four times as much data as a 16-PPM
transmission
31
IrDA

• Infrared Data Association (IrDA) is the most
  common infrared connection today
• It links notebook computers, Personal Digital
  Assistants (PDA) handheld devices, cameras,
  watches, pagers, and kiosks
• IrDA specifications include both physical devices
  and network protocols used for communication

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Overview

• IrDA devices have common characteristics
• Communicate with minimal preconfiguration
• Provide point-to-point data transfer between only
  two devices at a time
• Devices include broad range of computing and
  communicating technology
• Inexpensive technology
• Three versions of IrDA specifications are shown
  in Table 4-6

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Three Versions of IrDA Specifications
34
Multiple Infrared Connections

• Single IrDA link can establish multiple
  simultaneous connections
• Two IrDA devices can simultaneously send and
  receive mail, update calendar and contact
  information, and print documents
• A separate program controls each activity
• IrDA devices use infrared light emitting diodes
  (LEDs) to send and photodiodes to receive signals
• See Figure 4-6

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Infrared LEDs and Photodiodes
36
Diodes in Device
37
Design Factors Improve IrDA Communication

• Transparent window in front of IR module should
  be flat instead of curved
• Window should be violet to minimize loss of
  signal
• Module should be recessed several millimeters
  into device case to minimize interference from
  ambient light

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IrDA Protocol Stack

• IrDA Protocol stack has several layers

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Functions of the Layers of the IrDA Protocol Stack

• IrDA Physical Layer Protocol (IrPHY) controls
  hardware
• IrDA Link Access Protocol (IrLAP) encapsulates
  frames and defines how connections are
  established
• IrDA Link Management Protocol (IrLMP) allows
  devices to detect other devices
• IrDA Transport Protocol (Tiny TP) manages
  channels, corrects errors, divides data into
  packets, and reassembles original data

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IrDA Physical Layer Protocol (IrPHY)

• IrPHY controls hardware
• Function depends on which one of two standard is
  used
• Serial Infrared (Version 1.0)
• Fast Infrared (Version 1.1)

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Serial Infrared (Version 1.0)

• SIR transmitter works like standard serial port
  on a PC
• Figure 4-9 shows block diagram of SIR transmitter
• Uses UART (Universal Asynchronous
  Receiver/Transmitter) chip on PC
• Serial port transmits bits one after another
• Parallel port transmits all eight bits as a byte
• See Figure 4-10

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SIR Transmitter Block Diagram
43
Parallel and Serial Transmission
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Functions of the UART

• Converts bytes into a single serial bit stream
  for outbound transmission
• Converts serial bit stream into parallel bytes
  for incoming transmission
• Can add an optional parity bit for error checking
• Adds and removes optional start and stop
  delineators called start and stop bits

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Functions of the UART

• Provides some buffering of data to keep computer
  and the serial device coordinated
• May handle other interrupt and device management
  to coordinate speed of computer and device

46
UART Frame
47
NRZ with Same Bit Transmitted

• Standard RS-232 serial ports can use NRZ
  (non-return-to-zero) techniques that keep output
  level the same for the entire bit period

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Return-to-Zero, Inverted (RZI)

• IrDA devices cannot use NRZ technology
• They use RZI that uses the inverse of RZ
• RZI increases voltage for a 0 bit and no voltage
  for a 1 bit
• UARTS have a 16x clock cycle, as shown in Figure
  4-13

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IrDA SIR Transmission
50
Fast Infrared (Version 1.1)

• Specifies data transfer at 4 Mbps
• Retains backward compatibility with SIR devices
• Figure 4-14 shows block diagram of FIR
  transmission

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FIR Transmitter Block Diagram
52
IrDA FIR Transmission

• FIR uses 4 PPM
• Only two bits are transmitted
• Receiving device determines transmitted bit by
  locating pulse within time slot

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Other Considerations

• Several other factors influence infrared
  transmission, including
• Latency
• Ambient light
• Deflection angle

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Half-Duplex and Latency

• IrDA devices cannot send and receive at same time
• Their communication mode is half-duplex
• A time delay is required for device to stop
  transmitting and get ready to receive
• This delay is called latency or receiver set-up
  time
• IrDA specifications allow 10 ms latency

55
Ambient Light

• IrDA specifies test methods for measuring data
  integrity of an IrDA connection under
  electromagnetic fields, sunlight, incandescent
  light, and fluorescent light
• Lux is a photometric measurement of light
  intensity
• If lux values exceed standard, devices may still
  communicate, but they must be placed closer to
  each other

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Deflection Angle

• How sending and receiving IrDA devices align is
  important
• Devices with a deflection angle up to 15 degrees
  can be 3 feet apart, as shown in Figure 4-16
• With deflection angle between 15 and 30 degrees,
  devices must be closer together
• With a deflection angle over 30 degrees, infrared
  transmission will be impossible

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Deflection Angle
58
Wireless Metropolitan Area Networks

• Cover an area of up to 50 kilometers (31 miles)
• Used for two primary reasons
• Alternative to an organizations wired backhaul
  connection
• i.e., T1, T3, T4 lines
• Fiber Optics
• Very expensive to install backhaul connections
• Often less expensive to use a WMAN to link remote
  sites

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Wireless Metropolitan Area Networks (continued)

• Used for two primary reasons (continued)
• Overcome last mile connection
• Connection that begins at a fast Internet service
  provider, goes through local neighborhood, and
  ends at the home or office
• Slower-speed connection
• Bottleneck

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Wireless Metropolitan Area Networks Free Space
Optics

• Optical, wireless, point-to-point, line-of-sight
  wireless technology
• Able to transmit at speed comparable to Fiber
  Optics
• Transmissions sent by low-powered IR beams
• Advantages compared to fiber optic and RF
• Lower installation costs
• Faster installation
• Scaling transmission speed
• Good security
• Atmospheric conditions can affect transmission

61
Wireless Metropolitan Area Networks Local
Multipoint Distribution Service (LMDS)

• LMDS provides wide variety of wireless services
• High-frequency, low-powered RF waves have limited
  range
• Point-to-multipoint signal transmission
• Signals transmitted back are point-to-point
• Voice, data, Internet, and video traffic
• Local carrier determines services offered
• LMDS network is composed of cells
• Cell size affected by line of site, antenna
  height, overlapping cells, and rainfall

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Wireless Metropolitan Area Networks LMDS
(continued)
Figure 12-11 LMDS cell
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Wireless Metropolitan Area Networks Multichannel
Multipoint Distribution Service (MMDS)

• Many similarities to LMDS
• Differs in area of transmission
• Higher downstream transmission, lower upstream
  transmission, greater range
• In homes, alternative to cable modems and DSL
  service
• For businesses, alternative to T1 or fiber optic
  connections
• MMDS hub typically located at a very high point
• On top of building, towers, mountains

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Wireless Metropolitan Area Networks MMDS
(continued)

• Hub uses point-to-multipoint architecture
• Multiplexes communications to multiple users
• Tower has backhaul connection
• MMDS uses cells
• Single MMDS cell as large as 100 LDMS cells
• Receiving end uses pizza box antenna
• Advantages
• Transmission range, cell size, low vulnerability
  to poor weather conditions
• Still requires line-of-site, not encrypted

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Wireless Metropolitan Area Networks IEEE 802.16
(WiMAX)

• High potential
• Can connect IEEE 802.11 hotspots to Internet
• Can provide alternative to cable and DSL for last
  mile connection
• Up to 50 kilometers of linear service area range
• Does not require direct line of sight
• Provides shared data rates up to 70 Mbps
• Uses scheduling system
• Device competes once for initial network entry

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Wireless Metropolitan Area Networks IEEE 802.16
(continued)

• Currently addresses only devices in fixed
  positions
• 802.16e will add mobile devices to the standard
• IEEE 802.20 standard Sets standards for mobility
  over large areas
• Will permit users to roam at high speeds
• WiMAX base stations installed by a wireless
  Internet service provider (wireless ISP) can send
  high-speed Internet connections to homes and
  businesses in a radius of up to 50 km (31 miles)

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Wireless Wide Area Networks (WWANS)

• Wireless networks extending beyond 50 kilometers
  (31 miles)
• Two primary technologies
• Digital cellular telephony
• Satellites

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Digital Cellular Telephony

• Two keys to cellular telephone networks
• Coverage area divided into cells
• Cell transmitter at center
• Mobile devices communicate with cell center via
  RF
• Transmitters connected to base station,
• Each base station connected to a mobile
  telecommunications switching office (MTSO)
• Link between cellular and wired telephone network
• All transmitters and cell phones operate at low
  power
• Enables frequency reuse

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Digital Cellular Telephony (continued)
Figure 12-13 Frequency reuse
70
Satellites

• Satellite use falls into three broad categories
• Acquire scientific data, perform research
• Examine Earth
• Military and weather satellites
• Reflectors
• Relay signals
• Communications, navigation, broadcast

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Satellites (continued)

• Satellite systems classified by type of orbit
• Low earth orbiting (LEO) Small area of earth
  coverage
• Over 225 satellites needed for total coverage of
  earth
• Must travel very fast
• Medium earth orbiting (MEO) Larger area of
  coverage than LEO
• Do not need to travel as fast
• Geosynchronous earth orbiting (GEO) orbit
  matches earths rotation
• Fixed position
• Very large coverage area

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Satellites (continued)
Figure 12-14 LEO coverage area
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The Future of Wireless Networks

• IEEE 802.11 subcommittees currently at work
• 802.11d Supplementary to 802.11 MAC layer
• Promote worldwide use of 802.11 WLANs
• 802.11f Inter-Access Point Protocol (IAPP)
• Will assist with faster handoff from one AP to
  another
• 802.11h Supplement to MAC layer to comply with
  European regulations for 5 GHz WLANs
• 802.11j Incorporates Japanese regulatory
  extensions to 802.11a standard
• 802.11s Defines a mesh wireless network
• Devices configure themselves and are intelligent

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Summary

• WPANs encompass technology that is designed for
  portable devices, typically PDAs, cell phones,
  and tablet or laptop computers at transmission
  speeds lower than the other types of networks
• The IEEE 802.15 standards address wireless
  personal area networks
• RFID is not a standard but is a technology that
  uses RF tags to transmit information
• IrDA technology uses infrared transmissions to
  transmit data at speeds from 9,600 bps to 16 Mbps

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Summary (continued)

• FSO is an optical, wireless, point-to-point
  wireless metropolitan area network technology
• LMDS can provide a wide variety of wireless
  services, including high-speed Internet access,
  real-time multimedia file transfer, remote access
  to local area networks, interactive video,
  video-on-demand, video conferencing, and
  telephone
• MMDS has many of similarities to LMDS, yet has a
  longer distance range

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Summary (continued)

• The IEEE 802.16 (WiMAX) standard holds great
  promise for providing higher throughput rates for
  fixed location and mobile users
• Wireless wide area network (WWAN) technology
  encompasses digital cellular telephony and
  satellite
• The future of wireless networks is hard to
  predict, but most experts agree that wireless
  networks will be faster, more global, and easier
  to use in the years ahead