Introduction to Computer
Networks
Lecture 1
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Textbook
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Topics Covered
• Introduction and
Overview.
• Physical Layer.
• Medium Access Control
(MAC).
• Link Layer.
• Network Layer.
• Routing.
CMPE 150- Introduction to Computer Networks
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Internetworking and IP.
IP Routing and Control.
Transport Layer.
Application Layer.
Putting It All Together!
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Course Outline
• Introduction
– History, basic concepts, terminology.
– More, “not-so-basic” concepts:protocols, layering,, etc.
• Physical layer
– Transmitting data.
• Data link layer
– Reliable transmission.
– Accessing the communication medium
• Medium access control protocols.
• LANs
– Ethernet, token ring, wireless LANs.
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Course Outline (cont’d)
• Network layer
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Types of network services.
Circuit- vs. packet switching.
Virtual circuits and datagrams.
Routing.
Addressing.
Unicast and multicast.
• Internetworking
– IP.
– The Internet.
– IP Routing and Control.
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Course Outline (cont’d)
• Transport layer
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E2E communication..
Types of transport service.
Connectionless versus connection-oriented.
UDP.
TCP.
• Application layer
– DNS, ssh, telnet, ftp, news, e-mail.
– The Web.
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HTTP.
HTML.
Search engines.
Proxy and caches
– Peer-to-peer.
– Security.
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What’s a network?
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What’s a network?
• Merriam-Webster Dictionary:
– “|A fabric or structure of cords or wires that
cross at regular intervals…”
– “A system of computers, terminals and
databases connected by communication lines”
• “A computer network is defined as the
interconnection of 2 or more independent
computers.” [Ramteke,”Networks”, pg. 24].
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Why network?
• Before networks:
– One large computer (mainframe) used for all
processing in businesses, universities, etc.
• Smaller, cheaper computers…
– Personal computers or workstations on
desktops.
– Interconnecting many smaller computers is
advantageous! Why?
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Ubiquitous Computing
• Computers everywhere.
• Also means ubiquitous communication.
– Users connected anywhere/anytime.
– PC (laptop, palmtop) equivalent to cell phone.
• Networking computers together is critical!
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Computer Network
• Provide access to local and remote resources.
• Collection of interconnected end systems:
– Computing devices (mainframes, workstations,
PCs, palm tops)
– Peripherals (printers, scanners, terminals).
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Why network?
• Resource sharing!
– Hardware: printers, disks, terminals, etc.
– Software: text processors, compilers, etc.
– Data.
• Robustness.
– Fault tolerance through redundancy.
• Load balancing.
– Processing and data can be distributed over
the network.
• Location independence.
– Users can access their files, etc. from
anywhere in the network.
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Problems?
• Security!
– It’s much easier to protect centralized
resources than when they are distributed.
– Network itself as the target..
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Some History
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Before the Internet
• Postal network.
– Delivers different types of objects (letters,
packages, etc.) world-wide.
– Relatively high delay but relatively cheap.
– Sender and receiver identified by their postal
address (name, number, street, city, etc.).
• Telephone network.
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Engineered to deliver real-time voice.
Also world-wide.
Low delay but more expensive.
Users identified but telephone number.
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The Telephone Network
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The Telephone Network
• Telephone was patented by G. Bell in 1876.
• For one telephone to be able to talk with
another telephone, a direct connection
between the two telephones was needed.
– Within one year, cities were covered with a
wild jumble of wires!
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The Telephone Network (cont’d)
• In 1878, the Bell Telephone company opened
its first switching office (in New Haven, CT).
• Each user would connect to the local
switching office.
– When a user wanted to make a call, s/he rang
to the office, and would be manually
connected to the other end.
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The Telephone Network (cont’d)
• To allow for long-distance calls, switching
offices (switches) were connected .
• Several connections can go through interswitch trunks simultaneously.
• At some point, there were too many
connections between switching offices!
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The Telephone Network (cont’d)
• Thus, a second-level hierarchy was added.
• The current telephone system has at least
five levels of hierarchy.
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Addressing
• Uniquely identifies users.
• Examples:
– Postal address, telephone number.
• Types of addresses:
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Flat.
Hierarchical.
Are postal addresses flat or hierarchical?
And phone numbers?
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POTS or PSTN
• For over 100 years, the POTS (Plain Old
Telephone System) a.k.a. PSTN (Public
Switched Telephone Network) handles voiceband communications.
• The PSTN is well designed and engineered for
the transmission and switching of voice
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Real-time.
Low latency.
High reliability.
Moderate fidelity.
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Evolution of Communications
Networks
• About 30 years ago, a second communications
network was created with the goal of providing a
better transport mechanism for data.
• In this class, we will study the technology
underpinning data networks.
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Communication Model
Network
Source
CMPE 150- Introduction to Computer Networks
Destination
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Simplified Communication Model
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Components
• End systems (or hosts),
• Routers/switches/bridges, and
• Links (twisted pair, coaxial cable, fiber, radio,
etc.).
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Components (cont’d)
• Source
– generates data to be transmitted
• Transmitter
– Converts data into transmittable signals
• Transmission System
– Carries data
• Receiver
– Converts received signal into data
• Destination
– Takes incoming data
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Simplified Data Communications
Model
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Key Tasks
• Transmission.
• Signal Generation.
• Synchronization.
• Error detection and correction.
• Addressing and routing
• End-to-end Recovery.
• Security.
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Networking
• Point to point communication not usually
practical
– Devices are too far apart.
– Large set of devices would need impractical
number of connections.
• Solution is a communications network.
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Simplified Network Model
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