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What is Protocol

There are rules governing how data is transferred overnetworks,

how

they

are

compressed,

how

they

are

presented on the screen and so on. These set of rules arecalled protocols. There are many protocols, each onegoverning the way a certain technology works.

Elements of Protocol

Syntax

:- relates to structure or format of data, the order in

which they are presented. Semantics

:- relates to meaning of each section of bits

Timing

:- When data is to be sent

How fast is to be sent

Types of Protocols

Direct and Indirect ProtocolMonolithic and Structured ProtocolSymmetric and AsymmetricStandard and Non standard

Types of Protocols(Cont.)

Symmetric and Asymmetric

:- If the communication

takes between two peer entities, then protocol is called symmetric protocol

and if the environment like client server

model, then protocol is called

asymmetric protocol.

Standard and Non-Standard

:- If the protocol is

designed that it can be used for any computer in any situationthen protocol is called

standard protocol

. But if it is designed

in such a manner that it can be used only in a specificcommunication situation or for a particular model of computerthen it is called

non standard protocol

.

Functions of a Protocol^ 

Connection Control 

Ordered Delivery 

Flow Control 

Error Control 

Addressing 

Multiplexing 

Transmission services

Functions of a Protocol(Cont.)

Ordered Delivery



PDUs may travel different routes, and may arrive out of orderwith respect to the transmitting order. 

A protocol must be able to reorder the PDUs in the correctorder.Flow Control



A receiver may not be able to able to process the PDUs as fastas the transmitter can send them. 

A receiver require some ways of limiting the rate oftransmitter. 

Flow control function ensures that data sent does notoverwhelm the receiver.

Functions of a Protocol(Cont.)

Error Control



PDU can be lost or damaged. 

Methods for detecting and correcting error is required. 

Retransmission upon failure of acknowledgement ofreceipt is a common method for handling lost PDUs. 

Cyclic redundancy checks(CRC) are often used to detectdamaged PDUs.

Functions of a Protocol(Cont.)

Multiplexing



Multiplexing is used to improve the efficiency and usageof the transmission medium. 

Functions exist to support frequency or time divisionmultiplexing as well as multiplexing the connection.

Functions of a Protocol(Cont.)

Transmission Services



Other type of services to the upper layer exist 

Three common services are : Priority, grade of service,and security

OSI MODEL(Cont.)^ 

Application layer

:- the application layer provides a

means for the uses to access information on the networkthrough an application. Some example of application layerincludes

Telnet

, applications which use

File Transfer

Protocol(FTP)

, applications which use

Simple Mail

Transfer Protocol(SMTP)

and applications which use

Hypertext Transfer Protocol(HTTP)

OSI MODEL(Cont.)

Presentation layer

:- the presentation layer

transforms data to provide a standard interface for theapplication layer.

Coding and DecodingData compressiondata encryption

Session layer

:- the session layer controls the

dialogues/connection (sessions) between computer. Itestablishes, manages and terminates the connectionbetween the local and remote applications.

OSI MODEL(Cont.)

Data Link Layer

:- The data link layer provides the

functional and procedural means to transfer data betweennetwork entities and to detect and possibly correct errors thatmay occur in the physical layer. Best known example of thislayer is

Ethernet

and

HDLC

for

ADCCP

for point to point or

packet- switched networks and

Aloha

for local area networks.

Physical layer :-

The physical layer defines all the

electrical and physical specifications for devices. It includespins, voltages, and cable specifications. Hubs, Repeaters,Network adapters are physical layer devices.

Merits and Demerits of OSI M

odel

Merits

1. It is a general model which supports both connection

oriented and connection less services.

2. It clearly distinguishes between protocols, services and

interfaces.

3. It uses hidden protocols that can easily be replaced by

new protocols as the technology changes. Demerits

1. The session layer and presentation layer are not required.2) Model was invented before invention of the protocols.