Computer Network

Unit-2 Network Models

AMPICS,

Ganpat University

Design Issues of the Layer

• A number of design issues exist for the layer to layer approach of computer networks. Some of the main design issues are as follows −.
• Reliability
• Scalability
• Addressing
• Flow Contro
• Error Contro
• Resource Allocation
• Statistical Multiplexing
• Routing
• Security

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Reliability

• Network channels and components may be unreliable, resulting in loss of bits while data transfer. So, an important design issue is to make sure that the information transferred is not distorted.

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Scalability

• Networks are continuously evolving. The sizes are continually increasing leading to congestion. Also, when new technologies are applied to the added components, it may lead to incompatibility issues. Hence, the design should be done so that the networks are scalable and can accommodate such additions and alterations.

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Addressing

• At a particular time, innumerable messages are being transferred between large numbers of computers. So, a naming or addressing system should exist so that each layer can identify the sender and receivers of each message.

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Error Control

• Unreliable channels introduce a number of errors in the data streams that are communicated. So, the layers need to agree upon common error detection and error correction methods so as to protect data packets while they are transferred.

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Flow Control

• If the rate at which data is produced by the sender is higher than the rate at which data is received by the receiver, there are chances of overflowing the receiver. So, a proper flow control mechanism needs to be implemented.

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Resource Allocation

• Computer networks provide services in the form of network resources to the end users. The main design issue is to allocate and deallocate resources to processes. The allocation/deallocation should occur so that minimal interference among the hosts occurs and there is optimal usage of the resources.

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Statistical Multiplexing

• It is not feasible to allocate a dedicated path for each message while it is being transferred from the source to the destination. So, the data channel needs to be multiplexed, so as to allocate a fraction of the bandwidth or time to each host.

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Routing

There may be multiple paths from the source to the destination. Routing involves choosing an optimal path among all possible paths, in terms of cost and time. There are several routing algorithms that are used in network systems

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Security

• A major factor of data communication is to defend it against threats like eavesdropping and surreptitious alteration of messages. So, there should be adequate mechanisms to prevent unauthorized access to data through authentication and cryptography.

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Protocol Hierarchies

• protocol, in computer science, a set of rules or procedures for transmitting data between electronic devices, such as computers.
• The three aspects of a protocol are −
• Syntax − It defines the format of data that is to be sent or received.
• Semantics − It defines the meaning of each section of bits that are transferred.
• Timings − It defines the time at which data is transferred as well as the speed at which it is transferred.

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• Most networks are organized as a stack of layers, one on the top of another.
• The number of layers and their names vary from network to network.
• Each layer has a specified function and adheres to specified protocols.
• Thus we obtain a stack of protocols.

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• The above figure represents communication between Device A and Device B. The data stream from one device to the other is not sent directly but has to pass through a number of layers. The layers in the same levels are called peers and have a set of protocols for communication. Between each adjacent layer is an interface that defines the services that are being offered by a lower layer to the next higher layer. The dotted arrows depict virtual communication between peer layers, while the solid arrows represent the physical communications between the adjacent layers.

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• Let us consider a situation where Device A wants to send a message to Device B. Device A passes its information to the highest layer. As soon as a data stream reaches a layer, it performs some specified functions on it and passes it to the layer below. This continues until the data stream reaches the lowest layer. Layer 1 passes a bit stream of 0s and 1s to the physical medium that communicates it to the Layer 1 of the receiving end. Each layer in the receiving end performs certain functions on the data stream adhering to the protocol with its peer and passes it to the layer above. This continues until the information reaches the highest layer. The highest layer then conveys the message to Device B in the same format sent by Device A.

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OSI Model

• OSI stands for Open Systems Interconnection. It has been developed by ISO – ‘International Organization for Standardization‘, in the year 1984.
• It is a 7-layer architecture with each layer having specific functionality to perform.
• All these 7 layers work collaboratively to transmit the data from one person to another across the globe.
• OSI model acts as a reference model and is not implemented on the Internet because of its late invention. The current model being used is the TCP/IP model.

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Layers of OSI Model

• Physical Layer
• Data Link Layer
• Network Layer
• Transport Layer
• Session Layer
• Presentation Layer
• Application Layer

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Layer 1- Physical Layer

• The lowest layer of the OSI reference model is the physical layer.
• It is responsible for the actual physical connection between the devices.
• The physical layer contains information in the form of bits.
•  It is responsible for transmitting individual bits from one node to the next. 

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The Functions of the Physical Layer

• Bit synchronization: The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at the bit level.
• Bit rate control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.
• Physical topologies: Physical layer specifies how the different, devices/nodes are arranged in a network i.e. bus, star, or mesh topology.
• Transmission mode: Physical layer also defines how the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex and full-duplex.

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Layer 2- Data Link Layer (DLL)

• The data link layer is responsible for the node-to-node delivery of the message.
• The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer.
• When a packet arrives in a network, it is the responsibility of the DLL to transmit it to the Host using its MAC address
• The Data Link Layer is divided into two sublayers:  

Logical Link Control (LLC)

Media Access Control (MAC)

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The Functions of the Data Link Layer

• Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.
• Physical addressing: After creating frames, the Data link layer adds physical addresses (MAC addresses) of the sender and/or receiver in the header of each frame.
• Error control: The data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.
• Flow Control: The data rate must be constant on both sides else the data may get corrupted thus, flow control coordinates the amount of data that can be sent before receiving an acknowledgment.
• Access control: When a single communication channel is shared by multiple devices, the MAC sub-layer of the data link layer helps to determine which device has control over the channel at a given time.

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Layer 3- Network Layer

• The network layer works for the transmission of data from one host to the other located in different networks.
• It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available.
• The sender & receiver’s IP addresses are placed in the header by the network layer. 

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Functions of the Network Layer

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• Routing: The network layer protocols determine which route is suitable from source to destination. This function of the network layer is known as routing.
• Logical Addressing: To identify each device on Internetwork uniquely, the network layer defines an addressing scheme. The sender & receiver’s IP addresses are placed in the header by the network layer. Such an address distinguishes each device uniquely and universally.

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Layer 4- Transport Layer

• The transport layer provides services to the application layer and takes services from the network layer.
• The data in the transport layer is referred to as Segments.
• It is responsible for the End to End Delivery of the complete message.
• The transport layer also provides the acknowledgment of the successful data transmission and re-transmits the data if an error is found.

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Functions of the Transport Layer 

• Segmentation and Reassembly: This layer accepts the message from the (session) layer, and breaks the message into smaller units. Each of the segments produced has a header associated with it. The transport layer at the destination station reassembles the message.
• Service Point Addressing: To deliver the message to the correct process, the transport layer header includes a type of address called service point address or port address. Thus by specifying this address, the transport layer makes sure that the message is delivered to the correct process.

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Layer 5- Session Layer

• This layer is responsible for the establishment of connection, maintenance of sessions, and authentication, and also ensures security.

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Functions of the Session Layer

• Session establishment, maintenance, and termination: The layer allows the two processes to establish, use and terminate a connection.
• Synchronization: This layer allows a process to add checkpoints that are considered synchronization points in the data. These synchronization points help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.
• Dialog Controller: The session layer allows two systems to start communication with each other in half-duplex or full-duplex.

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Layer 6- Presentation Layer

• The presentation layer is also called the Translation layer.
• The data from the application layer is extracted here and manipulated as per the required format to transmit over the network. 

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The Functions of the Presentation Layer are

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• Translation: For example, ASCII to EBCDIC.
• Encryption/ Decryption: Data encryption translates the data into another form or code. The encrypted data is known as the ciphertext and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.
• Compression: Reduces the number of bits that need to be transmitted on the network.

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Layer 7- Application Layer

• At the very top of the OSI Reference Model stack of layers, we find the Application layer which is implemented by the network applications.
• These applications produce the data, which has to be transferred over the network. This layer also serves as a window for the application services to access the network and for displaying the received information to the user. 

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The Functions of the Application Layer are

• Network Virtual Terminal: It allows a user to log on to a remote host.
• FTAM- File transfer access and management : This application allows a user to�access file in a remote host, retrieve files in remote host and manage or�control files from a remote computer.
• Mail Services : Provide email service.
• Directory Services : This application provides distributed database sources�and access for global information about various objects and services.

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Computer Networking Terminology

• Network: A collection of interconnected devices, such as computers, printers, and servers, that can communicate with each other.
• Node: Any device connected to a network, such as a computer, printer, or router.
• Protocol: A set of rules and standards that define how devices on a network communicate with each other.

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• IP Address: A unique numerical identifier assigned to each device on a network, used to identify and communicate with other devices.
• Router: A networking device that connects multiple networks together and forwards data packets between them.
• Switch: A networking device that connects devices on a network and forwards data packets between them.
• Firewall: A security device or software that monitors and controls incoming and outgoing network traffic, based on a set of predefined security rules.

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• DNS (Domain Name System): A system that translates domain names (such as www.example.com) into IP addresses, allowing devices to locate and connect to websites and other network resources.
• LAN (Local Area Network): A network that connects devices within a limited geographical area, such as a home, office, or building.
• WAN (Wide Area Network): A network that connects devices over a large geographical area, such as multiple offices in different cities or countries.

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• DHCP (Dynamic Host Configuration Protocol): A protocol that automatically assigns IP addresses and network configuration settings to devices on a network.
• TCP/IP (Transmission Control Protocol/Internet Protocol): A set of protocols used to communicate over the internet and other networks.
• A host can act as a Client when he is requesting information.
• A host can act as a Server when he provides information.

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• Packet is a container of data where it send over the network is known as “Packet”.
• Bandwidth: It is the measurement of data speed which can be transmitted at a single point of time. we all known this word with internet connection, If we talked about the internet bandwidth that means it describe the speed of the internet connection which can be transmitted into our devices e.g Phones, laptop etc.

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Connection-Oriented and Connectionless Service

• Data communication is a telecommunication network to send and receive data between two or more computers over the same or different network.
• There are two ways to establish a connection before sending data from one device to another, that are Connection-Oriented and Connectionless Service.
• Connection-oriented service involves the creation and termination of the connection for sending the data between two or more devices.
• In contrast, connectionless service does not require establishing any connection and termination process for transferring the data over a network.

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Connection-Oriented Service

• A connection-oriented service is a network service that was designed and developed after the telephone system.
• A connection-oriented service is used to create an end to end connection between the sender and the receiver before transmitting the data over the same or different networks.
• In connection-oriented service, packets are transmitted to the receiver in the same order the sender has sent them.
• It uses a handshake method that creates a connection between the user and sender for transmitting the data over the network. Hence it is also known as a reliable network service.

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Suppose, a sender wants to send data to the receiver. Then, first, the sender sends a request packet to a receiver in the form of an SYN packet. After that, the receiver responds to the sender's request with an (SYN-ACK) signal/packets. That represents the confirmation is received by the receiver to start the communication between the sender and the receiver. Now a sender can send the message or data to the receiver.

What is a TCP?

• TCP (Transmission Control Protocol) is a connection-oriented protocol that allows communication between two or more computer devices by establishing connections in the same or different networks. It is the most important protocol that uses internet protocol to transfer the data from one end to another. Hence, it is sometimes referred to as TCP/IP. It ensures that the connection is established and maintained until the data packet is transferring between the sender and receiver is complete.

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Connectionless Service

• A connection is similar to a postal system, in which each letter takes along different route paths from the source to the destination address.
• Connectionless service is used in the network system to transfer data from one end to another end without creating any connection.
• So it does not require establishing a connection before sending the data from the sender to the receiver.
• It is not a reliable network service because it does not guarantee the transfer of data packets to the receiver, and data packets can be received in any order to the receiver.
• Therefore we can say that the data packet does not follow a defined path. In connectionless service, the transmitted data packet is not received by the receiver due to network congestion, and the data may be lost.

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• For example, a sender can directly send any data to the receiver without establishing any connection because it is a connectionless service. Data sent by the sender will be in the packet or data streams containing the receiver's address. In connectionless service, the data can be travelled and received in any order. However, it does not guarantee to transfer of the packets to the right destination.

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What is UDP?

• The UDP (User Datagram Protocol) is a connectionless protocol that allows communication between two or more devices without establishing any connection. In this protocol, a sender sends the data packets to the receiver that holds the destination address. A UDP does not ensure to deliver the data packets to the correct destination, and it does not generate any acknowledgment about the sender's data. Similarly, it does not acknowledge the receiver about the data. Hence, it is an unreliable protocol.

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Connection-Oriented vs Connectionless Service

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TCP/IP Reference model

• The TCP/IP model was developed prior to the OSI model.
• The TCP/IP model is not exactly similar to the OSI model.
• The TCP/IP model consists of five layers: the application layer, transport layer, network layer, data link layer and physical layer.
• The first four layers provide physical standards, network interface, internetworking, and transport functions that correspond to the first four layers of the OSI model and these four layers are represented in TCP/IP model by a single layer called the application layer.
• TCP/IP is a hierarchical protocol made up of interactive modules, and each of them provides specific functionality.

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Each upper-layer protocol is supported by two or more lower-level protocols.

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Physical Network Access 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.

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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.
• Following are the protocols used in this layer.

IP Protocol

ARP Protocol

ICMP Protocol

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IP Protocol: IP protocol is used in this layer, and it is the most significant part of the entire TCP/IP suite.

Following are the responsibilities of this protocol:

• IP Addressing: 
• Host-to-host communication: 
• Data Encapsulation and Formatting: 
• Fragmentation and Reassembly: 
• Routing: 

ARP Protocol: ARP stands for Address Resolution Protocol. ARP is a network layer protocol which is used to find the physical address from the IP address.

• ARP request:  
• ARP reply: 

ICMP Protocol: ICMP stands for Internet Control Message Protocol.

• ICMP Test:   
• ICMP Reply: 

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Transport Layer

• The transport layer is responsible for the reliability, flow control, and correction of data which is being sent over the network.
• The two protocols used in the transport layer are User Datagram protocol and (UDP) Transmission control protocol(TCP).

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• User Datagram Protocol (UDP)It provides connectionless service and end-to-end delivery of transmission.

UDP consists of the following fields:� Source port address: � Destination port address: � Total length: � Checksum: 

• Transmission Control Protocol (TCP)It provides a full transport layer services to applications.
• It creates a virtual circuit between the sender and receiver, and it is active for the duration of the transmission.

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Application Layer

• An application layer is the topmost layer in the TCP/IP model.
• It is responsible for handling high-level protocols, issues of representation.
• This layer allows the user to interact with the application.
• When one application layer protocol wants to communicate with another application layer, it forwards its data to the transport layer.
• There is an ambiguity occurs in the application layer. Every application cannot be placed inside the application layer except those who interact with the communication system. For example: text editor cannot be considered in application layer while web browser using HTTP protocol to interact with the network where HTTP protocol is an application layer protocol.

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Following are the main protocols used in the application layer:

• HTTP: HTTP stands for Hypertext transfer protocol. This protocol allows us to access the data over the world wide web. It transfers the data in the form of plain text, audio, video. It is known as a Hypertext transfer protocol as it has the efficiency to use in a hypertext environment where there are rapid jumps from one document to another.
• SNMP: SNMP stands for Simple Network Management Protocol. It is a framework used for managing the devices on the internet by using the TCP/IP protocol suite.
• SMTP: SMTP stands for Simple mail transfer protocol. The TCP/IP protocol that supports the e-mail is known as a Simple mail transfer protocol. This protocol is used to send the data to another e-mail address.
• DNS: DNS stands for Domain Name System. An IP address is used to identify the connection of a host to the internet uniquely. But, people prefer to use the names instead of addresses. Therefore, the system that maps the name to the address is known as Domain Name System.
• TELNET: It is an abbreviation for Terminal Network. It establishes the connection between the local computer and remote computer in such a way that the local terminal appears to be a terminal at the remote system.
• FTP: FTP stands for File Transfer Protocol. FTP is a standard internet protocol used for transmitting the files from one computer to another computer.

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Comparison of ISO-OSI and TCP/IP Model

Similarities between the OSI and TCP/IP model

• Share common architecture
• Both the models are the logical models and having similar architectures as both the models are constructed with the layers.
• Define standards
• Both the layers have defined standards, and they also provide the framework used for implementing the standards and devices.
• Simplified troubleshooting process
• Both models have simplified the troubleshooting process by breaking the complex function into simpler components.

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• Pre-defined standards
• The standards and protocols which are already pre-defined; these models do not redefine them; they just reference or use them. For example, the Ethernet standards were already defined by the IEEE before the development of these models; instead of recreating them, models have used these pre-defined standards.
• Both have similar functionality of 'transport' and 'network' layers
• The function which is performed between the 'presentation' and the 'network' layer is similar to the function performed at the transport layer.

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Differences between the OSI and TCP/IP model

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