Introduction

• Data communications
• Computer-based communication systems
• Networks (local-area and wide-area)
• Internet

Data Communications

• Data communication systems use communication lines to transfer information between computer systems
• The communication lines can be public (telephone lines, cables, microwave, satellites) or private (network cabling)
• Types of communication systems:
  • centralised data processing
  • distributed data processing
  • micro-to-mainframe links
  • local area networks (LANs)
  • wide area networks (WANs)

Data Transmission

• Internal communication within the computer is digital
• Communication across public lines is (mostly) analog (using modulation of electric signals)
• Types of communication line:
  • wire (telephone cables - "copper")
  • coaxial cables ("coax")
  • fiber optic cables
  • microwave transmissions
  • satellites transmissions
• So communication between computers needs to convert information from digital to analog (and vice versa)
• Basic device to do this is a modem (modulation - demodulation)
• Communications software is also required to:
  • control the modem
  • allow the computer to act as a terminal
  • send and receive files
• 'Speed' at which data is transferred is defined in bits per second (bps) - sometimes called a baud. Most modems allow baud rates of at least 28,800 baud (approx. 3000 characters per second).
• Data communications must use a agreed method so that each part of the system is "talking the same language"; these agreed methods are called protocols.

Computer-based communication systems

• Communication systems provide services to users; these include:
  • electronic mail (e-mail, email)
  • voice mail
  • teleconferencing (voice and/or video)
  • facsimile transmission
  • bulletin boards
  • Internet-specific services (FTP, gopher, WWW)
  • electronic funds transfer (EFT)
  • electronic data interchange (EDI)
  • communication service providers (Viatel, Compuserve, Prodigy)
  • public and commercial information service providers (Medline, HEAPS, Dialog)
• Some services use "general purpose" communication software (telnet and terminal emulation)
• Some services require specialised software (clients)

Networks

A computer network is a means of transmitting data/information between various users, pieces of hardware and software in a computer (or information) system.

• Major advantage of being part of a network: it allows users to share the various components of the system:
  1. DATA
  2. peripherals (printers, modems, scanners, fax…)
• It also allows the managers of the networks to
  1. control access
  2. monitor use and performance
• Networks can be described (classified) in terms of
  • topology: the "organization" of the network
  • scale: the geographical "extent" of the network

Network topology

• A local area network (LAN) is a network of computers (usually micros) connected together, in reasonably close proximity
• LANs facilitate sharing of information and/or hardware
• The network has components and topology

topology

1. How the components of the network are physically linked
2. How the transfer of information is organized

LANs usually have one of the three major topologies:

1. bus

   

2. ring

   

3. star

   

Network Components

• A LAN has three types of components:
  1. Workstations
  2. Servers
  3. Connections
• Workstations are made up of
  • a microcomputer (or terminal)
  • a network interface (usually an interface card) that handles the messages to and from workstation
• Servers
  • computers (either a dedicated micro, a 'real' workstation, or a minicomputer) that provide services to the network:
    1. dividing access to resources
    2. controlling hardware access
    3. providing security
    4. managing simultaneous data access
• Connections
  • the medium through which communication between the components can take place
  • connections run between network interfaces
  • "physical" form can be cabling [twisted pair, coaxial], fibre optic or microwave (and others)

Wide area networks

• A wide area network (WAN) is like a LAN, but covering a larger area; it is also likely to have some important differences:
  • greater mixture of connections
  • greater range of services
  • multiple servers, some dedicated to a specific service
  • greater emphasis on security
  • different protocols
• The most "visible" WAN is the Internet

The Internet

• The Internet is the largest (yet) WAN; it is a global (but not universal!) network
• It has some specific characteristics that set it apart from smaller WANs
  • its own set of protocols (compatible with LAN protocols) called TCP/IP
  • specific services (and clients) have been developed to be used on the Internet, notably FTP, gopher, IRC ('chat') and the World Wide Web (WWW)
  • no definable topology (LANs attached to the Internet retain their own)
  • very large numbers of servers; in principle any workstation can also be a server, and many are - server software is freely available
  • little emphasis (until recently) on security
  • no central "authority" to manage (control) the network