OSI Reference Model Prepared by Siddique Ibrahim S

OSI Reference Model Prepared by Siddique Ibrahim S P AP/CSE Kumaraguru college of Technology

Objectives Data communication among heterogeneous systems – difficulties and solutions n The need for layered architecture n Design issues for the layers n The OSI model n 2

Network complexities Different types of hardware and software n Different operating systems n Different types of data to be transferred – text, images, music, video, etc n Data must be transferred without errors n Many different paths may have to be taken n 3

Network complexities n Data formats and data exchange conventions vary between manufacturers ¨ E. g. , ASCII, UNICODE, EBCDIC(Extended Binary Coded Decimal Interchange Code), etc. 4

Solution: n This can be resolved only if computers follow certain common set of rules or protocols 5

What is a protocol? n It is a formal description of message formats and the rules that two computers must follow in order to exchange messages. n This set of rules describes how data is transmitted over a network. 6

Why are protocols needed? n Protocols are needed for communication between any two devices. ¨ In what format will the messages be transmitted? ¨ At what speed should messages be transmitted? ¨ What to do if errors take place? ¨ What to do if parts of a message are lost (or) Delayed? 7

Protocols in daily life n How does conversation take place between human beings ¨ “Hello” ¨ “Goodbye” ¨ Handshake ¨ Good n Bye Letters ¨ “Dear Sir” ¨ “Yours faithfully” ¨ No splleing mistakes ! 8

Network Model n Network model - A method of describing and analyzing data communications networks by breaking the entire set of communications process into a number of layers. n Each layer has a specific function 9

Open Systems Interconnect (OSI) Model n Who made: ¨ International Standards Organization (ISO) A Model of How Protocols and Networking Components Could be Made n “Open” means the concepts are nonproprietary; can be used by anyone. n OSI is not a protocol. It is a model for understanding and designing a network architecture that is flexible and robust. n E. g: POC n 10

n n n OSI (Open Systems Interconnection) is reference model for how applications can communicate over a network. A reference model is a conceptual framework for understanding relationships. The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools. Most vendors involved in telecommunications make an attempt to describe their products and services in relation to the OSI model. And although useful for guiding discussion and evaluation, OSI is rarely actually implemented, as few network products or standard tools keep all related functions together in well-defined layers as related to the model. The TCP/IP protocols, which define the Internet, do not map cleanly to the OSI model. 11

Open Systems Interconnect (OSI) Model n n The OSI model describes how data flows from one computer, through a network to another computer The OSI model divides the tasks involved with moving information between networked computers into 7 smaller, more manageable sub-task. A task is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. 12

7 -layer OSI model n Why so many layers? ¨ To reduce complexity, networks are organized as a stack of layers, one below the other. ¨ Each layer performs a specific task. It provides services to an adjacent layer ¨ This is similar to the concept of a function in programming languages – function does a specific task 13

Layered Approach Peers 14

Design Issues for the Layers n n n n Addressing Error control Order of messages must be preserved Flow control – fast sender and slow receiver ! Disassembling, transmitting, and reassembling large messages Multiplexing / de-multiplexing Routing 15

OSI model did not fully implemented. n TCP/IP was used and tested extensively in the internet. n 16

The internet is based on TCP/IP which is a "simplified" implementation of theoretical OSI model. n it is not used at all. OSI is just a theoretical model, it has never been implemented 1: 1 in a real world scenario. TCP/IP is an "implementation" of the OSI model. n 17

n The OSI model was developed by researchers. It only exists on paper. Afterwards TCP/IP was created by using the principles that were described in the OSI model. TCP/IP in it's various versions is what the internet is based on today 18

The Layers of the OSI Model Application Presentation Session Transport Network Data Link Physical 19

The Layers of the OSI Model Some Mnemonics All People Seem To Need Data Processing Application Presentation Session Transport Network Data Link Physical Please Do Not Tell Secret Passwords Anytime 20

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Physical layer Coordinates the functions required to carry a bit stream over physical medium. n It deals with a electrical and mechanical(medium types, thickness) specifications of the interface and transmission medium(type of medium). n It responsible for movements of individual bits from one hop(node) to the next. n 24

Physical layer • • Specifications for the physical components of the network. Functions of Physical Layer: • • Bit representation – encode bits(0’s and 1’s) into electrical or optical signals Transmission rate – The number of bits sent each second Physical characteristics of transmission media Synchronizing the sender and receiver clocks(both are not have same bit rate). Line Configuration The physical layer is concerned with the connection of devices to the media. (point-to-point (or) multipoint) Transmission mode – simplex, half-duplex, full duplex Physical Topology – how devices are connected – ring, star, mesh, bus topology Application Presentation Session Transport Network Data Link Physical 25

Physical Layer 26

Data Link Layer Responsible for delivery of data between two systems on the same network(hop to hop delivery) Main functions of this layer are: • Framing – divides the stream of bits received from network layer into manageable data units called frames. • Physical Addressing – Add a header to the frame to define the physical address of the source and the destination machines(MAC). • Flow control – If the receiver is less than rate of at which data are produced in the sender. [The flow control mechanism to avoid overwhelming the receiver] Application Presentation Session Transport Network Data Link Physical 27

MAC(Medium Access control) 28

MAC(Medium Access control) 29

MAC(Medium Access control) 30

MAC(Medium Access control) 31

Data Link Layer Error Control – Adds relaibility mechanisms to detect and retransmit damaged or lost frames. This is achieved by adding a trailer to the end of a frame Also, recognize duplicate frames. • • Access Control –When two (or) more devices connected to the same link, then this layer protocol determine which devices has control over the link at any given time. Application Presentation Session Transport Network Data Link Physical 32

Data Link Layer 33

Data Link Layer 34

Network Layer Main functions of this layer are: Responsible for delivery of packets across multiple(sou to des delivery) networks[Note: if two systems are connected to the same link there is no need for a n/w layer] • Logical Addressing: The physical addressing implemented by the data link layer handles the addressing problem locally(MAC) • • When it passes the boundary the n/w layer adds a header to the packet(IP) coming from the upper layer(include logical address of sender and receiver) Application Presentation Session Transport Network Data Link Physical 35

Network Layer Main functions of this layer are: Application Presentation Routing – Independent networks (or) large n/w, the connecting device(Router or switch) • Router the packets to the final destrination. • Network layer is responsible only for delivery of individual packets and it does not recognize any relationship between those packets. • Session Transport Network Data Link Physical 36

Network Layer 37

Network Layer 38

Transport Layer Responsible for source-to-destination delivery of the entire message. Application • This layer manages packetization of data, then the delivery of the packets, including checking for errors in the data Presentation once it arrives. Main functions of this layer are: Session • Segmentation and reassembly – divide message into smaller Transport segments, add sequence number Network them and transmit. Reassemble these messages at the receiving end. Data Link • Error control – make sure that the entire message arrives without errors Physical – else retransmit. 39 •

Transport Layer Service point addressing: Compuer often run several programs at a same time. Sou-to-des not only from one computer to next. The transport layer header must therefore include a address called service point address[port address] Connection control: Connectionless- Each segment as a individual & delivery to the destination. Connection oriented- Fist establish a connection with the transport layer at the destination before the packets send. After all data packets delivered, then the connection is terminated. Application Presentation Session Transport Network Data Link Physical 40

Transport Layer A port is identified for each address and protocol by a 16 -bit number, commonly known as the port number. A port number is a 16 -bit unsigned integer, thus ranging from 1 to 65535 (port number 0 is reserved and can't be used). Transport Layer protocols, such as the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP), specify a source and destination port number in their packet headers. 41

Transport Layer 42

Transport Layer 43

Session Layer Application First three layers not sufficient for some Presentation processes. session layer establishes, coordinate maintains and terminate Session conversations, exchanges and synchronizes Transport the interaction among communicating systems. Network Main functions of this layer are: Data Link • Dialog control – allows two systems to enter into a dialog, keep a track of whose turn it is Physical to transmit • Synchronization – adds check points (synchronization points) into stream of data. 44

Session Layer From Presentation Layer To Presentation Layer Session Layer H 5 syn syn To Transport Layer syn syn From Transport Layer 45

Presentation Layer n This layer is usually part of an operating system (OS) and converts incoming and outgoing data from one presentation format to another (for example, from clear text to encrypted text at one end and back to clear text at the other). Application Presentation Session Transport Network Data Link Physical 46

Presentation Layer Responsibilities of this layer are: • Translation • • Different computers use different encoding systems (bit order translation) Convert data into a common format before transmitting. Syntax represents info such as character codes - how many bits to represent data – 8 or 7 bits Compression – reduce number of bits to be transmitted Application Presentation Session Transport Network Data Link Physical 47

Presentation Layer Application Encryption – transform data into an unintelligible format at the sending Presentation end for data security • Decryption – at the receiving end Session • n Layer 6 Presentation examples include encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MIDI. Transport Network Data Link Physical 48

Application Layer Contains protocols that allow the users to access the network (FTP, HTTP, Application SMTP, etc) • Does not include application programs Presentation such as email, browsers, word processing applications, etc. Session • it is the set of services (DNS, FTP, Transport HTTP)an application should be able to make use of directly, although some Network application functions. Data Link • Protocols contain utilities and networkbased services that support email via Physical SMTP, Internet access via HTTP, file 49 •

n n Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is applicationspecific. This layer provides application services for file transfers, email, and other network software services. Telnet and FTP are applications that exist entirely in the application level. 50

Application Layer To Presentation Layer From Presentation Layer 51

Summary of Functions of Layers Application To translate, encrypt and compress data To allow access to network resources Presentation Session To provide reliable end-to-end message delivery Transport To organise bits into frames Data Link Network Physical To establish, manage & terminate sessions To move packets from source to destination To transmit bits over a medium & provide electrical specs. 52

Lyer 1 -3 -are the network support layer. n (they deal with the physical aspects of moving data from one device to another) n Layer 4 - links the two subgroups n Layer 6 -7 -can be thought of as the user support layers(Allow interoperability among unrelated software system) n 53

n n n n When troubleshooting a network problem we often go up the OSI mode. Physical - Is the network cable plugged in? Data Link - Do you have a link light? Network - Are you getting an IP? Transport - Can you ping (or more accurately as Save. The. Rbtz pointed out telnet to) your default gateway? Session - Do you have DNS server information? Can you ping 4. 2. 2. 2 but not google. com? & 7. Presentation & Application - Can you browse to a site? 54

References n “Computer Networks”, ¨ Tanenbaum A (PHI) n “Data Communications and Networking”, ¨ Forouzan B (TMH) n “Local Area Networks”, ¨ Keiser (TMH) 55

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