Data communications
Think! Think!
Telecommunications
Networks
Lecture
12
Lecture
??
12
What makes it possible
to communicate from
point A to point B??
Telecommunications
Long-Distance
Transmission Media
Modems: to analog and back
Telephone lines use
analog transmission
sender
If you place a call outside the local transport area, an
interchange carrier (IXC)—a long-distance telephone
company—comes into play
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Lecture
Transmission Media
copper wire T1, 1.54 Mbps
fiber-optics T3, 43 Mbps
microwaves, 30 miles
satellites
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12
Lecture
12
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• modulation
• demodulation
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Asynchronous Communication
• asynchronous: used by modems
• Synchronous: faster but less resilient
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internal modem
external modem
A modem transforms the
computer's digital signals
into analog tones that can
be conveyed through the
telephone system.
The last mile is the hardest for
digital information
Synchronicity and Protocols
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How modems work
Telecommunications
Telecommunications
Lecture
Modem 2
Modem 1
Fiber optic cables consist of
thin strands of glass that
transmit data by means of
pulses of light.
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receiver
Modulation Protocols
• data transfer rate measured in bps (bits per
second)
• V.90 maximum rate of 56 Kbps
• V.34 maximum rate of 28800 bps (obsolete)
• modems automatically fall back to the
highest speed that both modems can handle
Lecture
12
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Bandwidth (amount of information
that can be on the line)
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Last mile technologies
• measured in Kbps, Mbps, or Gbps
• Integrated Services Digital Network
(ISDN)
• Digital Subscriber Line (DSL)
• These need an adapter (“modem”)
BellSouth is one of the first to offer ADSL service. Follow
the links to FastAccess to learn more.
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Digital Subscriber Line
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Lecture
12
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Internet and network at speeds up to 50
times faster than a 28.8kbps modem on a
standard phone line.
384Kbps - 1.5Mbps downstream and
128Kbps upstream
1.5Mbps - 6Mbps down/384Kbps up
DSL vs.
vs. Modem
Lecture
12
Telecommunications
Direct Broadcast Satellite
Alternatives to phone system
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Direct Broadcast Satellite (DBS)
• needs phone line to upload - 500 Kbps
down
Lecture
12
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Lecture
Cable Modems
• coaxial cable (TV cable) - 500 Kbps
12
down
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Electrical Power Systems
• not very compatible with the US grid
Networks
Networks
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Lecture
12
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Introducing Computer Networks
Network Fundamentals
Local Area Networks (LANs)
Wide Area Networks (WANs)
Introducing Computer Networks
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What is a computer network?
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Types of computer networks: LANs & WANs
• 2 or more computers connected
• local area network
• wide area network
Lecture
12
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Networking advantages
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reduction of hardware costs
enabling shared applications
building massive information resources
connecting people
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Networks
Understanding the Works out
of Networks
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Network Physical Media
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Packet vs. Circuit switching
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• packets: multiple routes
• circuit: a single route
Lecture
Networks
Lecture
Physical media: T1, T3, wireless
Technologies and Topologies
Protocols
Routing
Domain Name Servers
12
Networks
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Twisted pair
Coaxial cable
Fiber-optic cable
Infrared
Radio, microwave
Networks
LAN Topologies
LAN Technologies
Lecture
Lecture
12
12
By far the most popular
LAN standard is
Ethernet
Like to know the latest news about LANs? Check out LAN
Times, the premiere LAN trade journal
Like to learn more about the Ethernet? Check out Charles
Spurgeon’s Ethernet Web Site which covers all the Ethernet
technologies used today and includes a practical guide for do-ityourselfers.
Most networks use a bus, star, or ring topology.
Networks
LAN Protocols
Lecture
12
Router
page
one
page
one
Client
page
one
TCP/IP
send
me
TCP/IP
Client
Protocol stacks
“package wrapping and opening”
AppleTalk
IPX/SPX
NetBEUI
TCP/IP
send
me
Router
stack
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Networks
send
me
Router
Like AppleTalk, TCP/IP can be used with a variety of
lower-level protocols, such as Ethernet. A LAN that uses
TCP/IP is called an intranet, a term that suggests that it’s an
Internet designed for internal use within an organization.
Lecture
protocol
layers
12
A message starts at the "top" of a stack of layers and moves
down through the various layers (protocol stack) until it reaches
the "bottom" or physical media.
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Network Hardware and
Software
Networks
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Networking software
• peer-to-peer networks
• client/server networks
How WANs work
• Point of Presence (POP)
• backbones
Networking hardware:
Network Interface Cards (NICs)
• workstation
• node
Lecture
Wide Area Networks
Networks
Lecture
12
WANs are like
long-distance
telephone
systems
How do homes and businesses establish a presence
on the internet?
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