Practical-1 - ABOUT OSI MODEL AND TCP/IP MODEL AND THEIR DIFFERENCE PDF

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ABOUT OSI MODEL AND TCP/IP MODEL AND THEIR DIFFERENCE...

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PRACTICAL-1 AIM: Study of OSI and TCP/IP Layers with their comparison.

Solution:

➔ There are two important network architectures, the OSI reference model and the TCP/IP reference model. ❖ OSI REFERENCE MODEL : ➔ The OSI model is based on a proposal developed by the International Standards Organization (ISO) as a first step toward international standardization of the protocols used in the various layers.The model is called the ISO OSI (Open Systems Interconnection) Reference Model because it deals with connecting open systems—that is, systems that are open for communication with other systems. OSI MODEL:

➔ The OSI model has seven layers. The principles that were applied to arrive at the seven layers can be briefly summarized as follows: 1. A layer should be created where a different abstraction is needed. 2. Each layer should perform a well-defined function.

3. The function of each layer should be chosen with an eye toward defining internationally standardized protocols. 4. The layer boundaries should be chosen to minimize the information flow across the interfaces. 5. The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy.

FIG : ISO/OSI REFERENCE MODEL ➔ LAYER 1 : The PHYSICAL LAYER deals with transmission of individual bits from one node to another over a physical medium. The main functions of the physical layer are as follows:● It defines the physical characteristics and functions of the physical devices and interfaces so that transmission can occur. It states the number of pins in each of the devices as well as the purpose of each pin. ● It lays out the transmission medium and type of signal for transmitting the bits, i.e., electrical pulses, optical pulses or radio signals.

● It defines the procedure of encoding of the bits, for example, how many volts should represent a 0 bit and 1 bit in case of electrical signals. ● It states the data transmission rate, i.e., number of bits transmitted per second; and the duration of a bit, i.e., how long a bit stays. ● It defines the topology, i.e., physical layout, of the network devices. ● It also states the direction of transmission, i.e., whether the transmission is in simplex mode, half-duplex mode or full-duplex mode. ➔ LAYER 2 : The DATA LINK LAYER converts the raw transmission facility provided by the physical layer to a reliable and error-free link. The main functions of the data link layer are as follows − ● It breaks up the stream of bits into data frames having sizes from a few hundred to a few thousand bytes. ● It ensures distribution of the frames to the different systems. For this, it adds a header to the frame containing the address of the sender and the receiver. ● In case of reliable connection, this layer ensures that the receiver sends an acknowledgement frame. In absence of acknowledgement frames, frame retransmission is done. It also has mechanisms to identify duplicate frames. ● The data link layer ensures flow control of transmission so that a fast sender does not drown a slow receiver. ● In the case of broadcast networks, this layer also controls access to the shared channel.

➔ LAYER 3 : The NETWORK LAYER controls the source to destination delivery of data packets across multiple hops (nodes). It controls the operation of the subnet. The main functions of the network layer are as follows − ● It is responsible for routing packets from the source host to the destination host. The routes can be based upon static tables that are rarely changed, or they can be automatically updated depending upon network conditions. ● The data link layer assigns the physical address locally. When the data packets are routed to remote locations, a logical addressing scheme is required to differentiate between the source system and the destination system. This is provided by the

network layer. ● This layer also provides mechanisms for congestion control. ● The network layer tackles issues like transmission delays, transmission time, avoidance of jitters, etc.

➔ LAYER 4 : The TRANSPORT LAYER is responsible for delivery of an entire message from an application program on the source device to a similar application program on the destination device. The main functions of the transport layer are as follows:● It delivers a message from a specific process of one computer to a specific process in another computer. The transport layer adds a port address to the header of the data packet. ● It divides a message into smaller segments such that each segment contains a sequence number along with the port address. It ensures that the segments arrive correctly at the receiver’s end and then reassembles them. ● It provides an error-free point-to-point channel for both connectionless and connection-oriented services. ● It isolates the upper layers, i.e., the user support layers from any changes in hardware technology in the lower layers, i.e., network support layers. ● It identifies errors like damaged packets, lost packets, and duplication of packets, and provides adequate error-correction techniques.

➔ LAYER 5 : The SESSION LAYER is responsible for establishing, managing, synchronizing and terminating sessions between end-user application processes. The main functions of the session layer are as follows:● It works as a dialog controller. It allows the systems to communicate in either half-duplex or full-duplex mode of communication. ● It is responsible for token management. Through this, it prevents the two users to simultaneously attempt the same critical operation. ● It synchronizes communication. It adds synchronization points or checkpoints in data streams for long communications. This ensures that data streams up to the

checkpoints are successfully received and acknowledged. In case of any failures, only the streams after the checkpoints have to be re-transmitted. ➔ LAYER 6 : The PRESENTATION LAYER ensures that the message is presented to the upper layer in a standardized format. It deals with the syntax and the semantics of the messages. The main functions of the presentation layer are as follows:● It encodes the messages from the user dependent format to the common format and vice versa, for communication among dissimilar systems. ● It is responsible for data encryption and decryption of sensitive data before they are transmitted over common channels. ● It is also responsible for data compression. Data compression is done at the source to reduce the number of bits to be transmitted. It reduces the storage space and increases the file transfer rate. It is particularly useful for transmission of large multimedia files.

➔ LAYER 7 : The APPLICATION LAYER is the topmost layer of the OSI model. It specifies the interfaces and supports services to the end users for network access. The main functions of the application layer are as follows:● It provides a virtual network terminal through which a user can log on to the remote host on the network. ● It provides facilities so that a user can access and retrieve files in a remote computer for local usage, as well as manage and store files on a remote computer from the local computer. ● It provides email services. ● It helps to retrieve global information and services by providing access to distributed database resources.

TCP/IP REFERENCE MODEL : ➔ TCP/IP means Transmission Control Protocol and Internet Protocol. It is the network model used in the current Internet architecture as well. Protocols are a set of rules which

govern every possible communication over a network. These protocols describe the movement of data between the source and destination or the internet. They also offer simple naming and addressing schemes.

FIG : TCP/IP REFERENCE MODEL

FIG : PROTOCOLS AND NETWORKS IN THE TCP/IP MODEL INITIALLY Network Access Layer/Host -to -Network Layer : ➔ A network layer is the lowest layer of the TCP/IP model. ➔ A network layer is the combination of the Physical layer and Data Link layer defined in the OSI reference model. ➔ It defines how the data should be sent physically through the network. ➔ This layer is mainly responsible for the transmission of the data between two devices on the same network. ➔ The functions carried out by this layer are encapsulating the IP datagram into frames transmitted by the network and mapping of IP addresses into physical addresses. ➔ The protocols used by this layer are ethernet, token ring, FDDI, X.25, frame relay. Internet Layer : ➔ An internet layer is the second layer of the TCP/IP model. ➔ An internet layer is also known as the network layer. ➔ The main responsibility of the internet layer is to send the packets from any network,

and they arrive at the destination irrespective of the route they take. Transport Layer : ➔ The transport layer is responsible for error-free, end-to-end delivery of data from the source host to the destination host. It corresponds to the transport layer of the OSI model. ➔ The functions of the transport layer are : ● It facilitates the communicating hosts to carry on a conversation. ● It provides an interface for the users to the underlying network. ● It can provide for a reliable connection. It can also carry out error checking, flow control, and verification. ➔ The protocols used in this layer are :● Transmission Control Protocol, TCP − It is a reliable connection-oriented protocol that transmits data from the source to the destination machine without any error. A connection is established between the peer entities prior to transmission. At the sending host, TCP divides an incoming byte stream into segments and assigns a separate sequence number to each segment. At the receiving host, TCP reorders the segments and sends an acknowledgment to the sender for correct receipt of segments. TCP also manages flow control so that a fast sender does not overwhelm a slow receiver. ● User Datagram Protocol, UDP −  It is a message-oriented protocol that provides a simple unreliable, connectionless, unacknowledged service. It is suitable for applications that do not require TCP’s sequencing, error control or flow control. It is used for transmitting a small amount of data where the speed of delivery is more important than the accuracy of delivery. ● Stream Control Transmission Protocol, SCTP −  It combines the features of both TCP and UDP. It is message oriented like the UDP, which provides a reliable, connection-oriented service like TCP. It is used for telephony over the Internet. Application Layer : 1. This is the top most layer of TCP/IP model. 2. It is used for interaction between user and application. 3. There are several protocols used by the application layer for user interaction such as: HTTP, SNMP, SMTP, DNS, TELNET, FTP etc. ➔ HTTP: HTTP stands for Hypertext transfer protocol, it allows the user to interact with world wide web through the browser application. HTTP can be used to transfer various types of data such as plain text, audio, video etc. ➔ FTP: FTP stands for File Transfer Protocol. This protocol is used for transmitting files from one system to another system.

➔ TELNET: TELNET is a client-server protocol. It is a reliable connection oriented protocol. This protocol is used on the internet or on LAN to provide a bidirectional (both ways) text based communication through a virtual terminal connection. ➔ DNS: DNS stands for Domain Name System. Each computer on a network has a different IP address, a computer is known by its IP address. DNS provides a mapping to a name to the IP Address so a name can be used to identify a system on network rather than IP address itself. ➔ SNMP: SNMP stands for Simple Network Management Protocol. It managers the devices connected to the internet using TCP/IP protocol. ➔ SMTP: SMTP stands for Simple mail transfer protocol. It is used for email services, using this protocol an email containing data can be sent to another email address. COMPARISON BETWEEN OSI AND TCP/IP MODEL :

Sr.No

BASIS OF COMPARISON

OSI MODEL

TCP/IP MODEL

1

Expand to

OSI refers to Open System Interconnection

TCP/IP refers to Transmission Control Protocol/Internet Protocol

2

Meaning

It is a theoretical model which is used for computing systems.

It is a client server model used for transmission of data over the internet.

3

Number of layers

The OSI model is made up of seven layers.

TCP/IP model is made up of four/five layers.

4

Developed by

It is developed by ISO It is developed by (International ARPANET(Advanced Standard organisation ). Research Project AgencyNetwork).

5

Approach

OSI follows Vertical approach

TCP/IP follows a horizontal approach

6

Services

In the OSI model the transport layer is only connection oriented

A layer of the TCP IP model is both connection oriented and connectionless

7

Size of header

The minimum size of the OSI header is 5 bytes.

The minimum size of the TCP/IP header is 20 bytes.

8

Distinguishing point

The OSI model provides a clear distinction between interfaces services and protocols.

TC/IP does not have any clear distinguishing points between services interfaces and protocols.

9

Independent/Dependent Model

OSI is a Protocol Independent Model.

TCP/IP is a Protocol Dependent Model.

10

Basis of Internet

The Internet is not based on OSI.

Internet is based onTCP/IP

11.

Delivery of Packets

Transport Layer guarantees delivery of packets.

Transport Layer does not guarantee delivery of packets.

12.

Use of Model

OSI truly is a general model.

TCP/IP cannot be used for any other application.

13.

Replacing the Protocol

The protocols are better hidden and can be easily replaced as the technology changes.

It is not easy to replace the protocols.

14.

Layers. Application

Application

Presentation

Transport

Session

Internet

Transport

Data Link

Network

Physical

Data Link Physical...