Introduction to Computer Networks

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• Introduction to Computer Networks

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Focus

• Intro to data networks from an engineering
  perspective.
• Broad coverage.
• Network architectures.
• Network protocols,
• Layered design.
• Protocol stack.
• TCP/IP and the Internet

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Topics Covered

• Introduction and Overview.
• Physical Layer.
• Medium Access Control (MAC).
• Link Layer.
• Network Layer.
• Routing.

• Internetworking and IP.
• IP Routing and Control.
• Transport Layer.
• Application Layer.
• Putting It All Together!

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Whats a network?
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Whats 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!
• Its much easier to protect centralized resources
  than when they are distributed.
• Network itself as the target..

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Some History, 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.
• 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

• 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 (contd)

• 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 (contd)

• To allow for long-distance calls, switching
  offices (switches) were connected .
• Several connections can go through inter-switch
  trunks simultaneously.
• At some point, there were too many connections
  between switching offices!

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The Telephone Network (contd)

• 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
• 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 voice-band communications.
• The PSTN is well designed and engineered for the
  transmission and switching of voice
• 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
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 (contd)

• 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