Introduction to Networking and Data Communication

Network Software

By the end of this lesson, you should be able to: state the network application software state and explain the seven layers of OSI model

Network software


  • network protocols and Network Operating System

Network Operating Systems

Most servers run on Network Operating System. This OS is used to control and serve client computers

Functions of Network Operating Systems

  • securing all network resources
  • sharing network resources

Examples of Network Operating System

  • Novell Netware
  • Windows NT/2000/2003
  • various versions of Unix
  • Linux

Network Protocols

A protocol is a set of rules used that guide communication of networked devices

Due to the fact that computers and various devices on a network may be running on the different operating systems, the ISO came up OSI (Open Systems Interconnection ) model to enable such devices to interoperate.

The OSI model has 7 main layers:

  • Application layer
  • Presentation layer
  • Session layer
  • Transport layer
  • Network layer
  • Datalink layer
  • Physical layer

NB: Each of the above layers uses a package of protocols:

Application Layer

This layer uses protocols such as HTTP, SMTP, FTP, Telnet, etc. It provides services for network applications. This is the layer at which network applications operate e.g  internet browsers, email apps,

HTTP/S - used for web surfing through browsers

SMTP - used for email through email apps

FTP - File transfer

Presentation Layer

This layer receives data from the application layer. It converts this data to machine-readable form. So, this layer does more of data translation. It also compresses the translated data so as to be transmitted faster. This layer encrypts data before transmission. Secure Socket  Layer (SSL) Protocol is for encryption and decryption.

Session Layer

This layer helps in:

  • session management
  • authentication
  • Authorization

This layer uses APIs to help it perform the above functions. NETBIOS is an example of API(Application Programming Interface)

Note: The web browsers perform all functions of the application, presentation, and session layer

Transport Layer

Controls the reliability of communication through:

  • segmentation: data received from the session layer is divided into smaller data units each containing a destination's sequence number and port number. The port number helps to direct each of the segment to the correct application while the sequence number helps to reassemble the segments to form the correct message
  • error control: if some data do not arrive at the destination transport layer uses an automatic repeat request scheme to retransmit the data
  • flow control: controls the amount of data transmitted from the server to the DTE depending  on the DTE's processing power

Protocols at this layer include: TCP and UDP

UDP(User Datagram Protocol)  is faster than TCP

TCP - Deals with a connection-oriented transmission

  • lost data can be retransmitted.

UDP - Deals with a connectionless transmission

  • lost data not retransmitted.

Network Layer

  • Receives data segments from Transport Layer
  • Transmit data segments in form of a packet from one computer in a given network to another computer in a different network
  • This is the layer where routers fall
  • This layer assigns the senders IP and receiver IP to the data segments to form data packets

Functions of the network layer:

  • Routing - the process of moving data packets from source to destination
  • Logical addressing: IPv4 and IPv6
  • path determination: network layer uses Open Shortest path First(OSPF), Border Gateway Protocol(BGP), and Intermediate System to Intermediate System(IS-IS) to determine the shortest optimal path

Data Link Layer

  • receives packets from the network layer
  •   physical addressing is done at the data link layer where MAC addresses of the sender and receiver computers are assigned to data packets to form a frame.
  • the data link layer is embedded in a computer's NIC as a software
  • involved in the transmission of data through local media


  • Accesses the media by use of framing
  • Controls how data is placed and received from media

Physical layer

  • Converts bits to signals and vice versa

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