Chapter 1

Introduction to Data Communications


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Summary

This chapter is the first in the book, it covers the OSI model and defines the different types of LAN's.  The OSI model is very important in networking and should be understood.  


Elements of Communication

When communicating their are four essential requirements.  A sender, receiver, message, and medium.   While these are required, their are a few things that are helpful, error control and a common language.  When talking in a room the sender would be the person talking, the receiver would be the person listening, the message would be what is said, and the medium would be the air that the sound travels on.  In this example error control would be if the receiver asked the sender to repeat the message.  In order for the receiver to understand the sender a common language is necessary.  In the computer world the same rules apply.  The computers are the senders and receivers, and the message is sent thru the cable, or air (in wireless networks).


Network Definition

This course is primarily about Local Area Networks.  Their are many types of networks so before we talk about why and how people use them will define two basic types of networks.

  • Terminal Network - This type of network consists of a single host computer with attached terminals.  The host computer does all or most of the processing.  The terminal may be a standard PC with emulation software.

  • Network of Computers - Two or more nodes connected by a data communications medium.  In this course whenever a network is mentioned we are referring to a network of computers


OSI Model

In 1983, the International Standards Organization (ISO) developed a model that would allow the sending and receiving of data between two computers. It works on a layer approach, where each layer is responsible for performing certain functions.

In the Open Systems Interconnect model there are SEVEN distinct layers.  Below is a picture of the OSI model.

The Seven Layer OSI ModelThe seven layer OSI model

Listed below is the function of each layer.

  • Application Layer
    Provides Applications with access to network services.

  • Presentation Layer
    Determines the format used to exchange data among networked computers. i.e. changing the date format from mm/dd/yyyy to dd/mm/yyyy

  • Session Layer
    Allows two applications to establish, use and disconnect a connection between them called a session. Provides for name recognition and additional functions like security which are needed to allow applications to communicate over the network.

  • Transport Layer
    Ensures that data is delivered error free, in sequence and with no loss, duplications or corruption. This layer also repackages data by assembling long messages into lots of smaller messages for sending, and repackaging the smaller messages into the original larger message at the receiving end.

  • Network Layer
    This is responsible for addressing messages and data so they are sent to the correct destination, and for translating logical addresses and names into physical addresses. This layer is also responsible for finding a path through the network to the destination computer.

  • Data-Link Layer
    This layer takes the data frames or messages from the Network Layer and provides for their actual transmission. At the receiving computer, this layer receives the incoming data and sends it to the network layer for handling.  The Data-Link Layer also provides error-free delivery of data between the two computers by using the physical layer. It does this by packaging the data from the Network Layer into a frame that includes error detection information. At the receiving computer, the Data-Link Layer reads the incoming frame, and generates its own error detection information based on the received frame data. After receiving all of the frame, it then compares its error detection value with that of the incoming frames, and if they match, the frame has been received correctly.

  • Physical Layer
    Controls the transmission of the actual data onto the network cable. It defines the electrical signals, line states and encoding of the data and the connector types used. An example is 10BaseT. Repeaters are an example of devices that work at the Physical Layer.

As the data goes thru the stack information is added at each level.  This process is known as enveloping or encapsulating.  Click on the picture to see an example.

Encapsulation Encapsulation at work


General Network Implementations

Below is a list of the different network implementations.  Under each is a list of the characteristics that define the different types.  The title of the book used for this class is Local Area Networks.  The majority of the class will revolved around the technologies used in a local area network.  We will discuss some basics about the others.

  • LAN - Local Area Networks

    • Limited geographic area

    • High speeds (10Mbps or higher)

    • Locally owned media

    • Generally made up of Workstations, and Servers

      • Workstations - A LAN user's computer, a.k.a. Client or Host

      • Servers - A network node that is dedicated to providing services to client nodes. I.e. Network printing, application sharing, and database sharing.  We will discuss more about server hardware in Chapter 5

  • WAN - Wide Area Network

    • Wide geographical area, such as a state, country or world. Note: A WAN is not limited to a wide geographical area, it can be in a local environment such as a site.  More on this in Chapter 13.

    • Lower speeds (Usually 1.5Mbps or slower)

    • Media is not usually locally owned

  • MAN - Metropolitan Area Network

    • Larger area then a LAN (approximately 100 miles)

    • Hi speeds (100Mbps or higher)

    • FDDI media (Fiber Distributed Data Interface)

  • VAN - Value Added Network

    • This is a network owned by a communications utility that sells it's service of the network to other companies.   In this type of network the subscriber would pay a fee depending on the amount of data that is sent.  In situations were a lot of data is transmitted it might be cheaper to lease a line and create a WAN.

  • Enterprise Network

    • An Originations complete network.  This would include interconnected WAN's and LAN's.  I.e. If you have an organization with two sites and both contain a LAN and are connected to form a WAN the whole network is the Enterprise Network.

  • internet

    • Note the lower case i.  This is the interconnection of two network.  An Enterprise Network is an internet.

  • The Internet

    • This is a specific instance of an internet that is global.  The Internet uses many standards to allow global communication.  More on the Internet in Chapter 15

  • Intranet

    • An organizations internal Web.  A web is a way of sharing information using Hypertext.  This page that you are viewing now is written in HTML (HyperText Markup Language).  A link that brings you to another location is considered hypertext.  With an Intranet the information that is shared is not meant for the outside word, it is meant to be internal.  An example might be a car parts store web based parts database.


More Information

 

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