Physical Layer (Layer 1)

The Physical Layer of the OSI model deals with the specifications that are used to interface the network with the devices and that include the combination of pins in connector, specified cables and voltages. After deal with them specifications its job is to transmits the bits to the medium, here we consider some of the Physical layer service as follow

   ·        Conversion of digital data into signals at sender side

   ·        Conversion of signals in to digital data at reviver side

Some of the protocol used at the physical layer

   ·        CSMA/CD (Carrier Sense Multiple Access/Collision Detection)

   ·        DSL (Digital Subscriber Line)

   ·        RS-232

   ·        Infrared Data Association

   ·        Bluetooth

Network Layer (Layer 3)

The Network layer of the OSI Model’s responsibility is to provide node to node communication over the network. The node to node communication happens with the help of two functions

·        Logical Addressing

·        Routing

The logical address is actually the address of the device. The network layer adds two address with the data, one is the sender logical address and second is the reviver logical address. With the help of the logical addresses the next function is happen.

Routing concept is to route the data towards the correct destination.

Some of the network layer protocols are as follow

   ·        IP

   ·        IPX

Transport Layer (Layer 4)

The Transport Layer of the OSI Model’s responsibility is to provide end-to-end communication. There are following functions of Transport Layer

·        Addressing (port number)

·        Transport (Connection-Less and Connection-Oriented)

·        Error Correction

Addressing refer on Layer 4 as port number, suppose when you are using an internet and you are require to do two jobs. One is browsing using HTTP and two is download a file using FTP. Here the transport layer gives a port number to both HTTP and FTP (80 and 20-21) and the result is that the HTTP port 80’s data is not going to mix with the FTP 20-21’s data. Their Sessions at Layer 5 are going to handle separate and as well as at Layer 6 handle the port 80 and 20-21 differently according to the requirements and at Layer 7 port 80’s data compare up with the HTTP on the browser and port 20-21’s data compare up with the FTP.

Transport refer on Layer 4 as the connection-less and connection-oriented. The two terms are use here. One is connection-less which means when one device is sending a data to another, the sending device doesn't going to established a connection with the reviver (no handshaking), it simple send the data to the reviver and does not care weather it is going to reach at the destination or not. UDP, ICMP, DNS, T FTP and IP are some of the protocols of connection-less communication. The other term connection-oriented is basically the opposite of the connection-less, which means its is going to establish a connection before sending a data to the reviver (handshaking). When the data are send, it is going to be acknowledge. By that mean connection-oriented provide a reliable and guaranteed connection between the devices. TCP, Frame Relay, ATM and SCTP are some of the protocols of connection-oriented.

Error Correction is also in the function of the Layer 4. The connection-oriented protocols are mostly used this function, because it provide the data grantee. When the reviver revived the data the following methods it uses for the error detection parity bit, check sum and CRC. When errors are detected then its  going to acknowledge the sender to resend the data.

Session Layer (Layer 5)

At Session Layer of the OSI Model the user data is not being sent, it is just in the stream of bits.  Now these things happen at the session layer are to established, manage (synchronizing etc) and terminate the communication between the two devices. It is actually the introduction between the two devices and in this introduction the two devices that are involve in the communication are going to finds out, do they really can communicate with each other or not?

But remember it is just a preparation that how the devices are going to established, manage and terminate the connections between them. Suppose they can communicate about their speed means at what speed they can easily accept the data and they can communicate about length of the data and also when to cancel the communication. 

 

There are some protocols used in session layer that established, manage and terminate the connection between the two devices are given below

   ·        Remote Procedure Call (RPC)

   ·        Zone Information Protocol (ZIP)

   ·        Session Control Protocol (SCP)

   ·        Apple Talk Session Protocol (ASP)

   ·        Network File Services (NFS)

Presentation Layer (Layer 6)

The presentation layer of the OSI model sometime called conversion layer because when one computer sends data form one computer to another, the data is actually send in the form of stream of bits, they are 1’s and 0’s (on and off of the electrical signal ). When the receiving side receives that stream of bits it actually doesn’t know what kind of data it is (text, video, image, sound) until the data reach at the presentation layer.

At the sending side typically these function happens

   ·        Compression

   ·        Encryption

   ·        Data Formatting

At receiving side thing are opposite

   ·        Decompression

   ·        D-encryption

   ·        Data Formatting

These types of files that we need to do the above mention functions

   ·        ASCII (Alphabetic Text)

   ·        EBCIDC (Text Numbers)

   ·        JPEG, GIF, TIFF (Pictures)

   ·        MPEG (Video)