1. 1 Lecture 30 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 (cont’d).

2. 2 Lecture 30 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. 3 Lecture 30 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. 4 Lecture 30 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. 5 Lecture 30 Major Issues in Data Communications (cont’d)  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.

6. 6 Lecture 30 Major Issues in Data Communications (cont’d)  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. 7 Lecture 30 Networking: Interconnecting Computers  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.  For computers to be useful these days, they must be connected to other computers with which they can interchange information.

8. 8 Lecture 30 Computer Networks: Basics (Cont’d) File Server Client 1 Client N Client 2 Client 3 Client N-1

9. 9 Lecture 30 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.

10. 10 Lecture 30 Network Connections: Definition (Cont’d)  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.

11. 11 Lecture 30 Network Connections: Types Point-to-PointShared Line

12. 12 Lecture 30 Network Topologies l Networks come in many shapes, each with different levels of connectivity.

13. 13 Lecture 30 Internets l An internet is a collection of two or more distinct networks, joined by routers. l The Internet is one example.

14. 14 Lecture 30 Network Protocols l A protocol is a set of rules that governs the communication of information. l Examples: »SMTP: Simple Mail Transfer Protocol »HTTP: HyperText Transfer Protocol »FTP: File Transfer Protocol l 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. 15 Lecture 30 Client/Server Applications l A client/server application divides a task between two computers, client and server. l Examples: HTTP, SMTP, FTP. l 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. 16 Lecture 30 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.

17. 17 Lecture 30 Special Networks: LANs and WANs (Cont’d) Long-distance connection (Wireline or Wireless) LAN 2 LAN 1

18. 18 Lecture 30 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. 19 Lecture 30 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.Internet

20. 20 Lecture 30 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.