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CWNA Guide to Wireless
LANs, Second Edition
Chapter Twelve
Personal, Metropolitan, and Wide Area
Wireless Networks
WPANs: Radio Frequency ID (RFID)
Figure 12-8: RFID tag
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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
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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
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WPANs: IrDA (continued)
Figure 12-9: IrDA diodes in device
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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
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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)
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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
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Communications Model: OSI
• Breaks complex
functions into
seven basic layers
• Each layer
performs
specific function
that involves
different tasks
• See Table 4-1
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OSI Layers and Functions
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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
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OSI Data Flow
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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)
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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
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OSI Model versus IEEE Project 802
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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
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Summary of PMD and PLCD
Sublayers
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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
– AppleTalk—used 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
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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
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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
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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
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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
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to Wireless LANs, Second Edition
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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
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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
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Diodes in Device
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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
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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
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UART Frame
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NRZ with Same Bit Transmitted
• Standard
RS-232 serial
ports can use
NRZ (non-returnto-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
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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
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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
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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
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Wireless Metropolitan Area Networks
• Cover an area of up to 50 kilometers (31 miles)
• Used for two primary reasons:
– Alternative to an organization’s 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
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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
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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 earth’s 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-ondemand, 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
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