1. Networking Fundamentals

1. OSI and DoD Models

1.1 OSI Models and its function

The 7 layers of OSI model are:

1. The Application Layer: Application layer is responsible for identifying and establishing the availability of intended communication partner and verifying sufficient resources exist for communication. Some of the important application layer protocols are: WWW, SMTP,FTP,etc.

2. The Presentation Layer: This layer is responsible for presenting the data in standard formats. This layer is responsible for data compression, decompression, encryption, and decryption. Some Presentation Layer standards are: JPEG, MPEG, MIDI, PICT, Quick Time, TIFF. The presentation layer of the OSI model handles the conversion of data between a Standards-based or platform independent formats to a format understood by the local application. This allows for data to be transported between devices and still be understood by applications on either end of the communication link.

The presentation layer performs the following functions:

Communication with the application layer above.

Translation of data conforming to cross-platform standards into formats understood by the local machine.

Communication with the session layer below.

Examples of Presentation Layer Functions

Conversion of a Sun .RAS raster graphic to JPG.

Conversion of ASCII to IBM EBCDIC

Conversion of .PICT on a MAC to .jpg

Conversion of .wav to .mp3

3.The Session Layer: Session Layer is responsible for co-ordinating communication between systems/nodes. The following are some of the session layer protocols and interfaces: a) Network File System (NFS),SQL,RPC(Remote Procedure Call), X-Windows, ASP, DNA SCP.

4. The Transport Layer: The Transport Layer is responsible for multiplexing upper-layer applications, session establishment, and tearing-down of virtual circuits. This layer does "flow control" to maintain data integrity. Flow Control prevents the problem of a sending host on one side of the connection overflowing the buffers in the receiving host.

5. The Network Layer: There can be several paths to send a packet from a given source to a destination. The primary responsibility of Network layer is to send packets from the source network to the destination network using a pre-determined methods. Routers work at Network layer.

Functions of Network Layer :