The OSI Model

The Open System Interconnection OSI is a set of rules for developer to create and implements applications that run on network. As a matter of fact, it is not a physical layer.

Similarly, OSI model gives framework to developers for creating and implementing devices, internetworking schemes and networking standards.

The communication functions of a telecommunication or computing system is characterized and standardized by OSI model without underlying internal structure and technology.

OSI Model Layer

There are 7 OSI model Layer.

7. Application Layer

6. Presentation Layer

5. Session Layer

4. Transport Layer

3. Network Layer

2. Datalink Layer

1. Physical Layer

 

Application Layer

Application layer provides end user interface which directly interacts with software application and that implement a communicating component. Additionally, in this layer, quality of services is identified, user authentication, communication couples are identified and constrains data are identified. In fact, the protocol data unit (PDU) of this layer is Data. The protocols used in this layer are (HTTP, DHCP, FTP, SMTP, SNMP,SMPP,Telnet).

Presentation Layer

This layer helps to free from data representation by interpreting between network formats and application. Application accepts the data into different coding formats that computer understand. Eg. ASCII from Presentation Layer. The protocol data unit (PDU) of this layer is data. Protocols are (TLS, MIME, SSL, XDR).

Session Layer

This layer helps to keep data separate of different applications. In fact, it controls and terminates the connection between the local and remote application and control the connections between computers. Moreover, it establishes check pointing, adjournment, termination and restart procedures by providing for full-duplex, half-duplex, or simplex operation. The protocol data unit (PDU) is called Data. Protocols are(PPTP, SAP, L2TP and NerBIOS).

Transport Layer

Transport Layer plays vital role in OSI model. Most importantly, it finds out the solution of error before retransmit and perform reliable and unreliable delivery. In fact, in this layer some of the protocols are connection less while some are connection oriented. The protocols data unit(PDU)  of this layer is segment. Protocols are (ICP, UDP, SPX, DCCP and SCTP).

Network Layer

Router uses logical addressing for path determination which is provided by Network layer.

• Indeed, the functional and procedural of transferring variable length data sequences from one point to another point. In addition, message delivery do not need to be reliable at network layer. In this layer, the protocols data unit(PDU) is Packet. Protocols are (AppleTalk, ICMP, Internet Protocol(IPv4), Internet Protocol(IPv6), IPX, IPsec and IGMP).

Data Link Layer

• Data link layer  provides error detection but not correction. Firstly, it helps the protocols to form a connection between two physically connected devices. In fact, it also provide node-to-node data transfer. Moreover, it also control the flow between protocols. The protocols data unit(PDU) is Frame. Protocols are(ARP, CSLIP, HDLC,PUP,ATM,SLIP)

Physical Layer

In this layer, bits deals with raw data which moves between devices. Moreover, it refers network topology as bus, mesh or ring being some of the common and electrical and physical specification of data connection. In fact, the PDU of this layer is Bit. Protocols are(Bluetooth, PON, OTN, DSL, IEEE.802.11, IEEE.802.3, and TIA 449)

 

Some of the advantages of OSI model are:-

• it breaks complex problem in to smaller pieces.
• Each layer provide the services to above layer.
• Decoupling changes.

Some of the disadvantages of OSI model are:-

• complex to follow
• OSI provide data integrity  which don’t accept by many applications.

Protocol Data Unit(PDU)

It defines the various types of data which are transferred from each layer over a network.

Transmission Modes in Computer Network.

• Simplex Mode :-In simple model the communication is single directional.  From the two devices only one can transmit and other can only receive over a network. Eg. Keyboard.
• Half-Duplex Mode:- In Half-Duplex Mode, not at a same time but each station can both transmit and receive. Eg. Walkie-talkie.

•   Full-Duplex Mode :- In full-duplex mode both stations can transmit and receive at a same time over a  network. In fact, the signals of both directions share the capacity of link. Eg Telephone Network.

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