Hello World! An Invitation to Computer Science

Lecture Slides

Computer Networks
Monday April 28, 2008

Key ideas from last time

• operating system as resource manager
• virtual memory
• concurrency and deadlock

What is a network?

• entities connected by transmission media
• abstractly: nodes and links between them
• Examples:
    cities & interstate highways
    airports & air-traffic routes
    electricity grid
    social networks

Why networks?

• sharing scarce resources
    printers, secondary storage, or other peripherals
• early computer networks: dumb terminals share processor
• "sneakernet"
• goals: save money, save time, or both

Why computer networks?

• sharing information
• client-server architecture
• remote collaboration
• save time, reduce redundancy

Communication networks

• voice and sign
• telegraph: action at a distance
• radio (and eventually television)
• telephone
• cable
• wireless
• streamline business cycle: UPS example
• videoconferencing: communication/transportation

Where does the Internet fit in all this?

Networking for QoL

• rise of personal computer
• 1980s: word-processing and games
• 2000s: e-mail, Web, games,...
• instant messaging
• goal: do everything from home

Ubiquity of networks

• everything "on demand"
• peer-to-peer
• mobile computing
• wireless everywhere
• continuing miniaturization

Very brief history of Internet

• still evolving: "Internet" today not same as 15, 30 years ago
    is it tangible? who owns it?
    software vs. hardware
    no single inventor
  
• cold-war incentive: survivable communication systems
• economic incentive: allow disparate networks to communicate
• distributed network: decentralized with redundancy
    (see Baran's papers at RAND)
• message-switching over circuit-switching
• store and forward: works well for off-line communication
    interactivity requires: smaller messages - packets

Packet-switched networks

• packets: source, destination, size, number, checksum, data
• long messages broken into packets, each can take different route
• no dedicated link necessary
• time-sharing systems led to community atmosphere on networks
• protocols
• distributed, democratic development of software
• early applications were asymmetric (client-server):
    telnet and ftp
• Network Control Program (NCP) - led to TCP/IP
    symmetric: machines on network have equal status
• RFC mentality led to open source

Network protocols

Maybe the finest application of abstraction yet.

1. physical layer
2. data-link layer (Ethernet)
3. network layer (IP)
4. transport layer (TCP)
5. application layer (ftp, email, Web, etc.)