Introduction to Data Communications

1
Introduction to Data Communications

• Overview
• Lecture Objectives.
• Data Communications Basics.
• Major Issues in Data Communications.
• Networking Interconnecting Computers.
• Modes of Data Communication Between Computers.
• Preview Data Communications (contd).

2
Lecture Objectives

• To have a breadth coverage of data communications
  technologies.
• To know the basic properties of data
  communications systems.
• To understand the underlying standards allowing
  different data communications systems to interact
  with each other.

3
Introduction to Data Communications

• Data Communication is the transmission of data
  from one agent (can be human or machine) or
  device to another agent (also machine or human).
  The agents could be a humans or machines.
• Basic components in a data communications system
  A data communication system consists of the
  following four main components
• The message to transmit
• The sender of the message
• The medium of transmission
• The receiver of the message

4
Major Issues in Data Communications

• When we think of how to send data from one
  computer to another, there are many different
  things involved. Here are some of the major
  issues
• Computer characteristics
• character set (EBCDIC, ASCII, Unicode etc)
• word size (16-bit, 32-bit, 64-bit etc)
• operating system (Windows, Unix, Linux etc)
• internal representation of data
  (1s-,2s-complement etc for integers alone)
• Transmission media what are the physical media
  on which the transmitted
• message should be carried?
• Transmission mode bit-serial? word-parallel?
• Error control What happens when something goes
  wrong during transmission?
• Flow control What happens when sender/receiver
  operate at different rates?
• Some agreed rules of operation (protocols) and
  standards
• Networking How are the communicating computers
  to be connected?

5
Major Issues in Data Communications (contd)

• From the issued discussed in the preceding
  slide, it is clear that for the different variety
  of computers to communicate successfully, they
  must contain a communication subsystem which can
  resolve those major issues.
• In the mid 1970s, the International Standards
  Organization (ISO) introduced a standard
  architecture for such a communication subsystem.
• This architecture is known as the ISO Reference
  Model for Open Systems Interconnection.
• The ISO Reference Model is concerned with the
  overall structure of the complete communication
  subsystem within each computer.
• The aim of the Reference Model is to allow an
  application program (e.g., an e-mail system) in
  any computer that supports a particular set of
  standards to communicate freely with an
  application program in any other computer that
  supports the same standards, regardless of their
  manufacturers.

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Major Issues in Data Communications (contd)

• The ISO adopted a layered approach to designing
  the Reference Model to make it structured and
  therefore easier to implement and modify.
• The OSI model is a networking architecture broken
  down into seven layers. These layers are designed
  to make the complex networking tasks into a
  series of logically and ordered subtasks. This
  layering provides ease of design and
  implementation.
• The OSI model consists of the following seven
  layers (we explain them later)
• 1. Physical Layer
• 2. Data-Link Layer
• 3. Network Layer
• 4. Transport Layer
• 5. Session Layer
• 6. Presentation Layer
• 7. Application Layer

7
Networking Interconnecting Computers

• For computers to be useful these days, they must
  be connected to other computers with which they
  can interchange information.

• A computer network is two or more computers
  connected together so that information and
  resources can be shared.
• Most computers are connected to some kind of
  network.
• Each computer has its own network address, which
  uniquely identifies it among the others.
• A file server is a network computer dedicated to
  storing programs and data that are shared among
  network users.
• A file server often has a large amount of
  secondary memory.

8
Computer Networks Basics (Contd)
9
Network Connections Definition

• There are many techniques for connecting
  computers into networks
• Point-to-point connections Each computer is
  directly connected to each other. This technique
  is not feasible for more than a few close
  machines.
• Adding a new computer requires a new
  communication line for each computer already in
  the network.

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Network Connections Definition (Contd)

• Most modern networks share a single communication
  line.
• Adding a new computer to the network is
  relatively easy.
• The shared communication line must be managed
  carefully.
• Network users must take turns using the line,
  which introduces delays.
• Often information is broken down into parts, sent
  to the receiving machine, and reassembled.

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Network Connections Types
Point-to-Point
Shared Line
12
Network Topologies

• Networks come in many shapes, each with different
  levels of connectivity.

13
Internets

• An internet is a collection of two or more
  distinct networks, joined by routers.
• The Internet is one example.

14
Network Protocols

• A protocol is a set of rules that governs the
  communication of information.
• Examples
• SMTP Simple Mail Transfer Protocol
• HTTP HyperText Transfer Protocol
• FTP File Transfer Protocol
• HTTP uses a Uniform Resource Locator (URL) to
  specify an address on the Internet

METHOD//HOST/PATH HTTP//www.prenhall.com/morelli
/index.html
15
Client/Server Applications

• A client/server application divides a task
  between two computers, client and server.
• Examples HTTP, SMTP, FTP .

• Client/Server Protocol

Server Set up a service on a particular host
computer. Client Contact the server and request
the service. Server Accept a request from a
client and provide the service.
16
Special Networks LANs and WANs

• A local-area network (LAN) is designed to cover
  small distances and a small number of computers.
• A LAN often connects the machines in a single
  room or building.
• A wide-are network (WAN) connects two or more
  LANs, often over long distances.
• Individual LANs are usually owned by a single
  organization, but WANs often connect LANs from
  many different groups in many different countries.

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Special Networks LANs and WANs (Contd)
18
Communication Modes
There are three common transmission modes for
data interchange between computers namely,
simplex, half-duplex and full-duplex

• Simplex Data in a simplex channel is always one
  way. Simplex channels are not often used because
  it is not possible to send back error or control
  signals to the transmit end.
• Simplex transmission is like a one way street.
  An example of simplex is Television, or Radio.  

19
Communication Modes Half-Duplex

• A half-duplex channel can send and receive, but
  not at the same time. It's like a one-lane bridge
  where two way traffic must give way in order to
  cross.
• Only one end transmits at a time, the other end
  receives. In addition, it is possible to perform
  error detection and request the sender to
  retransmit information that arrived corrupted.
• In some aspects, you can think of Internet
  surfing as being half-duplex, as a user issues a
  request for a web document, then that document is
  downloaded and displayed before the user issues
  another request.

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Communication Modes Full-Duplex

• AIn full-duplex data can travel in both
  directions simultaneously. There is no need to
  switch from transmit to receive mode like in half
  duplex.
• Its like a two lane bridge on a two-lane highway.
  
• An example can be a consumer that uses a cable
  connection to not only receive TV channels, but
  also uses the same cable to support their phone
  and Internet surfing. All these activities can
  occur simultaneously.