Functions Responsibilities of Layers 1 Physical Layer To

Functions (Responsibilities) of Layers 1. Physical Layer • To carry a bit stream over a physical medium. • It deals with the mechanical and electrical specifications of the interface and transmission medium. Bit Steam

Functions of physical layer • Physical characteristics of interfaces and medium: characteristics of Interface between devices and types of transmission media. • Representation of bits: It encodes the bit stream into electrical or optical signal. • Data rate: Transmission rate, number of bits sent each second. • Synchronization of bits: Sender and the receiver clocks must be synchronized. • Line configuration: Connection of devices to the media. Point-to-point or multipoint connection. • Physical topology: How devices are connected to make a network? • Transmission mode: It defines direction of the transmission between two devices. Ex. Simplex , half-duplex or full duplex mode).

2. Data Link Layer • The data link layer takes a datagram and encapsulated it in a packet called frame. Functions of Data Link Layer • Framing: Data received from network layer the manageable unit is called frame. • Physical addressing : Adds header to the frame defines sender and receiver. • Flow control: Difference between data transmission. • Error control: Data link layer adding mechanism to detect and retransmit damaged or lost frames. • Access control: Multiple links are connected to same link.

3. Network Layer • Responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links). Functions of Network Layer • Logical addressing: The network layer adds a header to the packet coming from the upper layer, includes the logical address of the sender and receiver. • Routing: Network layer route or switch the packets to its final destination in an internetwork.

4. Transport Layer • Transport layer is responsible for process-to-process delivery of entire message. Functions of Transport Layer • Service-point addressing (Port addressing): Computer performs several operations simultaneously. • Segmentation and reassembly: Message is divided into segments. • Each segment contains a sequence number which enables transport layer to reassemble at destination. • Connection control: Transport layer performs connectionless and connection oriented services with the destination machine. • Flow control: Transport layer performed end to end flow control while data link layer performs it across the link. • Error control: Transport layer ensures error free transmission. (error means damage, loss or duplication)

5. Session Layer • The session layer is the network dialog controller. • It establishes, maintains, and synchronizes the interaction among communicating systems. Functions of Session layer • Dialog Control: The session layer is network dialog controller, ie, it establish and synchronizes the interaction between Communication system. • Synchronization: Session layer adds synchronization points into stream of data.

6. Presentation layer • The presentation layer deals with syntax and semantic of the information being exchanged between two systems. Functions of Presentation layer • Translation: Two systems are usually exchanging information in the form of character string, number and so on. • Different computer uses different encoding systems. • The presentation layer maintains interoperability between the two encoding systems.

• Encryption: To carry sensitive information, a system must be able to ensure privacy. • Encryption means to transforms the original information to another form. • Decryption is a reverse process. • Compression: Compression is a technique of reducing number of bits required to represent the data. • Data compression is important in transmission of multimedia such as text, audio and video. 7. Application Layer • It provides user interface and other supporting services such as e-mail, remote file access, file transfer and sharing databases.

Responsibility of Application Layer • Network virtual terminal: It is a software version of physical terminal that allows a user to log onto a remote host. • File Transfer, Access and Management (FTAM): FTAM allows user to access files in remote hosts, to retrieve files and to manage files in remote computer. • Mail Services: E-mail forwarding, storage are the services under this category. • Remote log-in: A user can log into a remote computer and access the resources of that computer. • Directory services: This services include access for distributed database and global information.

Services • The Hypertext Transfer Protocol (HTTP) is a vehicle for accessing the World Wide Web (WWW). • The Simple Mail Transfer Protocol (SMTP) is the main protocol used in electronic mail (email) service. • The File Transfer Protocol (FTP) is used for transferring files from one host to another. • The Terminal Network (TELNET) and the Simple Network Management Protocol (SNMP) is used by an administrator to manage the Internet at global and local levels.

TCP/IP PROTOCOL SUITE • The TCP/IP protocol suite is made of Five layers: Physical, Data link, Network, Transport, and Application. .

TCP/IP PROTOCOL SUITE • The TCP/IP reference model is a set of protocols that allow communication across multiple networks. • It is a hierarchical protocol made up of interactive modules. • The term hierarchical means that each upper level protocol is supported by the services provided by one or more lower level protocols. • The original TCP/IP protocol suite was defined as four software layers built upon the hardware. • Today, however, TCP/IP is thought of as a five-layer model.

Layers in the TCP/IP Protocol Suite • • To better understand the duties of each layer, we need to think about the logical connections between layers. Figure 2. 6 shows logical connections in our simple internet.

TCP/IP Protocol Suite

• The three top most layers in the OSI model, however, are represented in TCP/IP by a single layer called the Application layer (Session, Presentation and Application layer). • At the Transport layer, TCP/IP defines three protocols: Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Stream Control Transmission Protocol (SCTP). • At the Network layer, the main protocol defined by TCP/IP is the Internetworking Protocol (IP). • Internet layer also called Network layer. • Host to network layer is also called Physical and Data link layer.

Encapsulation and De-capsulation • One of the important concepts in protocol layering in the Internet is encapsulation / decapsulation.

Encapsulation at the Source Host At the source, we have only encapsulation. • • • 1. At the application layer, the data to be exchanged is referred to as a message. The message is passed to the transport layer. 2. The result is the transport-layer packet, which is called the segment (in TCP) and the user datagram (in UDP). • The transport layer then passes the packet to the network layer. • 3. The network layer takes the transport-layer packet as data and adds its own header contains the addresses of the source and destination hosts. • The packets are called a datagram passes the packet to the data-link layer. • 4. The data-link layer takes the network-layer packet as data and adds its own header, which contains the link-layer addresses of the host or the next hop (the router). • The result is the link-layer packet, which is called a frame passed to the physical layer for transmission.

De-capsulation and Encapsulation at the Router At the router, both de-capsulation and encapsulation because the router is connected to two or more links. • 1. After the set of bits are delivered to the data-link layer, this layer de-capsulates the datagram from the frame and passes it to the network layer. • 2. The network layer only inspects the source and destination addresses in the datagram header and to find the next hop to which the datagram is to be delivered. • 3. The data-link layer of the next link encapsulates the datagram in a frame and passes it to the physical layer for transmission.

Addressing • The figure shows, there is a relationship between the layer, the address used in that layer, and the packet name at that layer.

Addresses in the TCP/IP protocol suite

• Four levels of addresses are used in an internet employing the TCP/IP protocols: Physical (link) addresses, Logical (IP) addresses, Port addresses, and Specific addresses.

URL: Here FOUR parts http: //172. 16. 17. 20: 8080/Result/IMCA http – Protocol 172. 16. 17. 20 – IP Address 8080 – Service (http) /Result/IMCA – Path and Filename

Addressing • Each address is related to a specific layer in the TCP/IP architecture. • At the transport layer, addresses are called Port numbers, and these define the application-layer programs at the source and destination. a) Port numbers are local addresses that distinguish between several programs running at the same time. • At the network-layer, IP address the addresses are global, with the whole Internet as the scope. • A network-layer address uniquely defines the connection of a device to the Internet. • The link-layer addresses, sometimes called MAC addresses, are locally defined addresses, each of which defines a specific host or router in a network (LAN or WAN).

Multiplexing (Many to One) and Demultiplexing (One to Many) • The TCP/IP protocol suite uses several protocols at some layers, we have multiplexing at the source and demultiplexing at the destination. • Multiplexing in this case means that a protocol at a layer can encapsulate a packet from several next-higher layer protocols (one at a time); • Demultiplexing means that a protocol can decapsulate and deliver a packet to several nexthigher layer protocols (one at a time).

Multiplexing (Many to One) and Demultiplexing (One to Many)

THE OSI MODEL • Almost three-fourths of the countries in the world are represented in the ISO. • An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. • It was first introduced in the late 1970 s. ISO is the organization; OSI is the model. • An open system is a set of protocols that allows any two different systems to communicate. • The purpose of the OSI model is to show to facilitate communication between different systems.

Summary of OSI Layers

OSI versus TCP/IP • When we compare the two models, we find that two layers, session and presentation, are missing from the TCP/IP protocol suite. • The application layer in the suite is usually considered to be the combination of three layers in the OSI model. TCPI/P and OSI model

Services • 21: File Transfer Protocol (FTP) • 22: Secure Shell (SSH) • 23: Telnet remote login service • 25: Simple Mail Transfer Protocol (SMTP) • 53: Domain Name System (DNS) service • 80: Hypertext Transfer Protocol (HTTP) used in the World Wide Web • 110: Post Office Protocol (POP 3) • 119: Network News Transfer Protocol (NNTP) • 123: Network Time Protocol (NTP) • 143: Internet Message Access Protocol (IMAP) • 161: Simple Network Management Protocol (SNMP) • 194: Internet Relay Chat (IRC) • 443: HTTP Secure (HTTPS)

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