Data Packets

To improve the efficiency of transferring information over a shared communication line, messages are divided into fixed-sized, numbered packets

Network devices called routers are used to direct packets between networks.
The data packets is a unit of data made into a single package that travels along a given network path. Data packets are used in Internet Protocol (IP) transmissions for data that navigates the Web, and in other kinds of networks.

Most network packets are split into three parts:

Header - The header contains instructions about the data carried by the packet. These instructions may include:

Payload - Also called the body or data of a packet. This is the actual data that the packet is delivering to the destination. If a packet is fixed-length, then the payload may be padded with blank information to make it the right size.

Trailer - The trailer, sometimes called the footer, typically contains a couple of bits that tell the receiving device that it has reached the end of the packet. It may also have some type of error checking. The most common error checking used in packets is Cyclic Redundancy Check (CRC).

It is important to understand that each packet carries the information that will help it get to its destination.

Network interface card (NIC)

Network interface card (NIC)-

Provides physical interface between computer and network medium
It Performs two crucial tasks

      -Establishes and manages computer’s network connection

     -Translates data into signals for outgoing messages and translates signals for incoming messages




Most computers use parallel data lines, called bus, to send data between CPU and adapter cards
Most networking media transmits data in single line, called serial transmission
NIC translates parallel into serial for outgoing messages and serial into parallel for incoming messages

Network Addresses

Hostname: A unique identification that specifies a particular computer on the Internet
For example
Fong.Sierr.Mora.edu
Ortichirino.develocorp.com


Network software translates a hostname into its corresponding IP address

For example
205.39.145.18



An IP address can be split into:

Network address, which specifies a specific network
host number, which specifies a particular machine in that network

A hostname consists of the computer name followed by the domain name
Fong.Sierr.Mora.edu  is the domain name
-A domain name is separated into two or more sections that specify the organization, and possibly a subset of an organization, of which the computer is a part.
-Two organizations can have a computer named the same thing because the domain name makes it clear which one is being referred to.

The very last section of the domain is called its top-level domain (TLD) name:

Organizations based in countries other than the United States use a top-level domain that corresponds to their two-letter country codes

The domain name system (DNS) is chiefly used to translate hostnames into numeric IP addresses.

Network OS

Network Operating Systems not only allow communication across a network, they also allow a network administrator to organize resources, control access, and ensure that the network is operating efficiently. it includes special functions for connecting computers and devices into a local-area network (LAN).

Sharing of network resources can be peer-to-peer or client/server. Which one is the best is dependent on the end goal of the network. The term network operating system is generally reserved for software that enhances a basic operating system by adding networking features.

Novell Netware, Artisoft's LANtastic, MicrosoftWindows Server, and Windows NT are examples of a NOS.

Transmission media/medium

The transmission medium is the physical path between transmitter and receiver.

Transmission and reception of data is performed in four steps:
   1-the data is coded as binary numbers at the sender end;
   2-a carrier signal is modulated as specified by the binary representation of the data
   3-at the receiving end, the incoming signal is demodulated into the respective

binary numbers; and 

   4- decoding of the binary numbers is performed.

A physical medium in data communications is the transmission path over which a signal propagates. Many transmission media are used as communications channels.

Connectivity device

Connectivity devices are those devices used to make physical network connections. Connectivity devices operate at the physical layer of the Open Systems Interconnection Reference Model (OSI) model. The OSI model describes how computer services and procedures are standardized. This standardization allows computers to share information and enables the interconnection of various networking connectivity devices regardless of vendor. 

The seven layers of the OSI model are:



7————–Application

6————–Presentation

5————–Session

4————–Transport

3————–Network

2————–Data Link

1————–Physical



The functions of the seven layers of the OSI model are:


7-The Application Layer of the OSI model is responsible for providing end-user services, such as file transfers, electronic messaging, e-mail, virtual terminal access, and network management. This is the layer with which the user interacts.


6-The Presentation Layer of the OSI model is responsible for defining the syntax which two network hosts use to communicate. Encryption and compression should be Presentation Layer functions.


5-The Session Layer of the OSI model is responsible for establishing process-to-process commnunications between networked hosts.


4-The Transport Layer of the OSI model is responsible for delivering messages between networked hosts. The Transport Layer should be responsible for fragmentation and reassembly.


3-The Network Layer of the OSI model is responsible for establishing paths for data transfer through the network. Routers operate at the Network Layer.


2-The Data Link Layer of the OSI model is responsible for communications between adjacent network nodes. Hubs and switches operate at the Data Link Layer.


1-The Physical Layer of the OSI model is responsible for bit-level transmission between network nodes. The Physical Layer defines items such as: connector types, cable types, voltages, and pin-outs

Inside of a Network

Now that we have talked about COMPUTER NETWORK, it is important to know what is :


Host: A computer that mediates multiple access to databases mounted on it or provides other services to a computer network.


Server: Computer program that manages access to a centralised resource or service in a network. there are:
                   File server A computer that stores and manages files for multiple users on a network
                   Web server A computer dedicated to responding to requests (from the browser client) for web pages

Node: A piece of equipment, such as a PC or peripheral attached to a network.


User: A person who uses or operates something.

Client: A dedktop computer or workstation that is capable of obtaining information and applications from a server.


Computer networks have opened up an entire frontier in the world of computing called 

the client/server model