Computer Networks

1
Chapter 12

• Computer Networks

2
Chapter Outline

• Introduction
• Basic concepts in computer networking
• Communication links
• Local area network (LAN)
• Wide area network (WAN)
• Network services
• The Internet
• The World Wide Web

3
Basic Concepts

• A computer network is a set of independent
  computer systems connected by telecommunication
  links that together are called interconnection
  network.
• Individual computers in the network are referred
  to as nodes or hosts.

4
Communication Links

• Vary widely in the physical characteristics,
  error rate, and transmission speed.
• Switched dial-up telephone lines.
• The term switched means that when we dial a
  phone number, a circuit (path) is temporarily
  established between called and the callee. The
  circuit lasts for the duration of the call.

5
Analog vs. Digital

• Voice-oriented dial-up telephone network is (in
  part) an analog medium.
• However, a computer produces digital information.
  
• Use modem (modulator/demodulator) to convert
  binary information into analog and vice versa.
• Telephone line has a rather low bandwidth.
  Physical limits on transmission speed over the
  telephone line are being reached. (56000 bits per
  second)
• Asymmetric Digital Subscriber Line (ADSL)

6
Amplitude Modulation
7
Dedicated Communication Links

• Twisted-pair copper wire
• Coaxial cable
• ISDN 128 Kbps
• T1 1.544Mbps
• T3 44.736 Mbps
• Fiber-optic cable
• OC-3 155.5 Mbps
• OC-12 622 Mbps
• OC-48 2.488 Gbps (gigabit networking)

8
Transmission Time

• 16mb color image

Line type speed time
Dial-up phone line 33.6 Kbps 7.9 min
Dedicated ISDN 128 Kbps 2.1 min
T1 1.544 Mbps 10.4 s
T3 44.737 Mbps 0.36 s
Fiber-optic OC-3 155.5 Mbps 0.1 s
Fiber-optic OC-12 622 Mbps 0.026 s
Fiber-optic OC-48 2.488 Gbps 0.006 s
9
Wireless Data Communication

• Use radio, microwave, infrared signals instead of
  wires/cables.
• Mobile computing
• In the future,
• Fiber-optics to support high-speed error-free
  wired connections
• Radio and microwave-based wireless links to for
  places that fiber does not or cannot reach.

10
Local Area Networks

• A local area network connects hardware devices
  such as computer, printer, and mass storage
  devices that are all in close physical proximity.
• Owner of the computers is also owners of the
  means of communications.
• A private network.
• Most widely used LAN Ethernet.

11
Ethernet

• Developed in the mid-1970s by computer scientists
  at Xerox PARC.
• Originally designed to operate at 10 Mbps using
  coaxial cables.
• Fast Ethernet 100 Mbps.
• Two ways to construct an Ethernet LAN
• Cable, single wire transceiver or multiple
  cable bridge
• Hub

12
Constructing Ethernet

13
Communication Protocol

• The rules describing how messages are transmitted
  between network nodes are called communication
  protocols.
• Protocols are algorithm of communication. They
  describe the step-by-step procedures required to
  guarantee an orderly exchange of information
  across a network.

14
Ethernet Protocol

• Use contention-based transmission technique.
• Users compete for access to a single shared
  communication line.
• When a node wants to send a message, it first
  listens to the line to see if it is currently in
  use.
• Broadcast message if line is not busy.
• Address field attached to the front of the
  message that identifies the destination.
• Every node reads the address filed, if not its
  own, discards remainder of the message. If it is,
  accepts and store it.
• What to do in a collision?
• Control of Ethernet network is distributed. More
  robust.

15
Wide Area Network

• A wide area network (WAN) connects devices that
  may be across town, across the country, or across
  the ocean.
• Cross public property, must purchase services
  from external providers.
• Point-to-point links.

16
WAN Protocol

• Most WANs use a store-and-forward, packet
  switching communication protocol.
• Unit of transmission in WAN is a packet an
  information block with fixed maximum size.
• A message is chopped into N separate packets
  and sent independently through the network.
• At the destination node, the packets are
  reassembled to restore the original message.

17
Packet Switching
18
Routing

• Selecting the specific path to use for relaying
  data.

19
Routing Algorithm

• Shortest path problem O(N2)
• Highly complex because of
• Massive amount of data
• Topological changes routing table must be
  re-computed often.
• Network failures

20
Router

• A router transmits message between two distinct
  networks that uses different protocols.
• LAN/WAN connection.

21
Network Services

• Resource sharing
• Physical resources printer or disks. ? printer
  server
• Logical resources data file or software. ? file
  server
• Client-server computing
• Information sharing electronic data interchange
• Information utility

22
Network Services (contd)

• Groupware support group efforts in producing
  shared documents.
• E-mail
• Bulletin board
• News groups
• Electronic commerce

23
Network Security

• Authentication problem
• Encryption/decryption problem
• Fault-tolerance problem