Business Data Communications,
Fourth Edition
Chapter 4: Communications Equipment
Learning Objectives
• Describe the role of multiplexers in signal
transmission
• Describe the differences among frequency
division multiplexing, time division multiplexing,
statistical time division multiplexing, and
wavelength division multiplexing
• Explain how frequency division multiple access,
time division multiple access, and code division
multiple access are used in wireless
transmission systems
Chapter 4: Communications Equipment
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Learning Objectives
• Describe the differences among hubs, bridges,
switches, routers, front-end processors, and
controllers
• Explain how front-end processors are used in a
data communications network
• Define protocol conversion and why it is needed
in data communications networks
• List the equipment needed to monitor a network
• Define a computer port and explain how data
can be directed to different ports
Chapter 4: Communications Equipment
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Introduction
• Networks are made up of servers, clients
and other equipment
• Physical equipment makes up an interface
between servers, clients and network
• Functions
– Combine data signals
– Direct and monitor network traffic
– Conversion of data to correct format
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Multiplexers
• A device that combines data from several
devices into a single stream
• Multiplexers are used in pairs
• Increases efficiency, reduces cost
• Idle time is reduced
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Chapter 4: Communications Equipment
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Multiplexers
• Used in WANS, LANs, wireless networks
– In WANs, used on point-to-point, multipoint
lines
– Connection multiplexer in LANs
– Multiple radio signals combined on wireless
networks
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Chapter 4: Communications Equipment
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Multiplexers
• Frequency Division Multiplexing (FDM)
– Bandwidth divided into narrow bandwidths
– Example of FDM circuit
– Guardbands used to separate the signals
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Chapter 4: Communications Equipment
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Chapter 4: Communications Equipment
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Multiplexers
• Frequency Division Multiplexing
– Voice-grade circuits
• AT&T separates into groups
– Supergroup – 60 channels
– Mastergroup – 600 channels
– Jumbo group – 3600 channels
– Used in cable television systems
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Chapter 4: Communications Equipment
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Multiplexers
• Time Division Multiplexing (TDM)
– Transmission line divided into time segments
– Guard time separate signals
– Used on Dataphone Digital Service
• Leased digital lines
• Maximum speed of 56 Kbps
– Used on T-1 lines (1.55 Mbps)
– Used on fiber optic networks
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Chapter 4: Communications Equipment
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Multiplexers
• Statistical Time Division Multiplexing
(STDM)
– Time slots allocated dynamically
– Eliminates empty time slots used in TDM
– Provides advanced functions over TDM
• Data compression
• Accumulation and reporting of network statistics
• Some error detection and correction
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Chapter 4: Communications Equipment
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Multiplexers
• Wavelength Division Multiplexing (WDM)
– Used for analog and digital transmission over
fiber optic cables
– Optical equivalent of FDM
– Allows up to 400 Gbps on a single cable
– Problems connecting to copper cables
• Conversion between electrical and optical signals
• Optical amplifiers – amplifies optical signal
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Multiplexers
• Wavelength Division Multiplexing (WDM)
– Future may include fiber optic to private
homes
– Fiber exhaust
• Traffic exceeds network bandwidth capacity
– Dense Wavelength Division Multiplexing
(DWDM)
• May allow up to 50,000 Gbps
• Same as 33 million T-1 lines
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Multiplexers
• Frequency Division Multiple Access
– Multiple access for analog cellular
communication
– One user on a frequency at one time
– Earliest system
• Advanced Mobile Phone Service (AMPS) – US
and Canada
• Total Access Communication System (TACS) –
United Kingdom
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Multiplexers
• Time Division Multiple Access
– Used for today’s cellular systems
– Operates similarly to TDM
– Multiple users on a single frequency
simultaneously (3 at a time)
– Used in conjunction with FDMA
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Multiplexers
• Time Division Multiple Access
– North American Digital Cellular (NADC)
– Global System for Mobile Communications
(GSM)
– Japanese Digital Cellular (JDC)
– Personal Digital Cellular (PDC)
– Personal Communication Service (PCS)
• Assigns phone number to person, not telephone
• Number is encoded in a circuit card
• Uses GSM and CDPD
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Multiplexers
• Time Division Multiple Access
– Standards
•
•
•
•
TIA/CTIS IS-54 (first standard)
TIA/CTIS IS-54B (digital voice channels added)
TIA/CTIS IS-136A (advanced features)
TIA/CTIS IS-136B (short message service)
– FDMA/TDMA – advantages and limitations
– Enhanced TDMA
• Dynamically allocates time slots to users
• Makes it twice as efficient as TDMA
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Multiplexers
• Code Division Multiple Access (CDMA)
– Each cellular conversation is assigned a code
– Signals are identified by the code
– Uses direct sequence spread spectrum
– Makes higher speed transmission possible
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Multiplexers
• Code Division Multiple Access (CDMA)
– Standards
• TIA/EIA/IS-95 (cellular radio)
• ANSI J-STD-008 (CDMA with PCS)
– Higher capacity than FDMA and TDMA
– CDMA is a controversial technology
– Equipment is very expensive
– Unproven at this time
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Multiplexers
• Configurations
– Multiplexer and demultiplexer used together
– Inverse multiplexer
• Combines high-speed lines between two servers
• AKA bonding
• Common carriers use it for high-speed services
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Chapter 4: Communications Equipment
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Multiplexers
• Configurations
– Connect communications lines to another
multiplexer
– Cascading multiplexers
• Using several levels of multiplexers at several sites
• Reduces costs
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Chapter 4: Communications Equipment
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Interconnecting Devices
• Hubs (concentrator)
– Combines data from several devices on a
single communications line
– Not necessarily used in pairs
– Placed between clients and the server
– Home users may have one to connect
multiple computers to an Internet connection
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Chapter 4: Communications Equipment
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Interconnecting Devices
• Bridges
– Connects two local area networks
– Filters data to determine if it should be passed
to another segment
• Switches
– Adds intelligence to functions of a hub
– Backplane – hardware portion of switch
– Operates much faster than a hub
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Chapter 4: Communications Equipment
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Interconnecting Devices
• Routers
– Connects networks that use different
protocols (rules for communication)
– Does conversion of data between the
networks
– Can act as a firewall
– Functions
• Monitor network traffic
• Collect and report network statistics
• Provide quality of service functions
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Front-End Processor (FEP)
•
•
•
•
•
Another type of communications controller
Located close to the server
Performs network management tasks
FEP Functions (Table 4-1)
Types of Front End Processors
– Nonprogrammable
– Programmable
• Network Control Program (NCP)
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Controllers
• Scaled-down version of an FEP
– Handles up to 64 devices
– Cluster controller
• Specifically used with IBM servers
– Remote controller
• Located a distance from the server
– Local controller
• Located near the server
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Chapter 4: Communications Equipment
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Protocol Converters
• Translates between protocols on different
computers and networks
• Hardware converter
– Special device
• Software converter
– Installed on the server
• Gateway
– Used with LANs
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Chapter 4: Communications Equipment
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Ancillary Equipment
• Diagnostic Equipment
– Line monitors
• Monitors data as it travels over the medium
• Data displayed on a screen
– Network sniffer
• Monitors all network traffic
• Can compare data to historical data for trends
– Breakout box
• Checks voltage levels
• Can isolate a circuit for testing
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Ancillary Equipment
• Port Concentrators and Selectors
– Port concentrator
• Multiple devices connected through a single port
• Reduces the need to add additional server ports
– Port Selector
• Used with dial-up lines to direct data to a port
• Special Purpose Modems
– RF modem
– Fiber-optic modem
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Chapter 4: Communications Equipment
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Summary
• Multiplexers used in pairs
to combine data
• Concentrator also
combines data
• Front end processor is a
special version of a
concentrator
• Controllers combine data
at remote or local sites
• Protocol converters used
to translate between
different devices
Chapter 4: Communications Equipment
• Line monitors and
breakout boxes are
diagnostic equipment
• Port concentrators and
selectors are used to
allow access to computer
ports
• RF modems are used for
wireless connections
while fiber optic modems
are used for fiber optic
lines
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