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OSI MODEL Meenakshi Sood ECE NIT

Networking : 

Netprog: OSI Reference Model Networking

Network : 

Netprog: OSI Reference Model Network “ ... communication system for connecting end-systems” End-systems a.k.a. “hosts” PCs, workstations dedicated computers network components

Multiaccess vs. Point-to-point : 

Netprog: OSI Reference Model Multiaccess vs. Point-to-point Multiaccess means shared medium. many end-systems share the same physical communication resources (wire, frequency, ...) There must be some arbitration mechanism. Point-to-point only 2 systems involved no doubt about where data came from !

Slide 5: 

Netprog: OSI Reference Model Multiaccess Point-to-point

LAN - Local Area Network : 

Netprog: OSI Reference Model LAN - Local Area Network connects computers that are physically close together ( < 1 mile). high speed multi-access Technologies: Ethernet 10 Mbps, 100Mbps Token Ring 16 Mbps FDDI 100 Mbps

WAN - Wide Area Network : 

Netprog: OSI Reference Model WAN - Wide Area Network connects computers that are physically far apart. “long-haul network”. typically slower than a LAN. typically less reliable than a LAN. point-to-point Technologies: telephone lines Satellite communications

MAN - Metropolitan Area Network : 

Netprog: OSI Reference Model MAN - Metropolitan Area Network Larger than a LAN and smaller than a WAN - example: campus-wide network - multi-access network Technologies: coaxial cable microwave

Internetwork : 

Netprog: OSI Reference Model Internetwork Connection of 2 or more distinct (possibly dissimilar) networks. Requires some kind of network device to facilitate the connection. Net A Net B


THE NEED FOR STANDARDS Over the past couple of decades many of the networks that were built used different hardware and software implementations, as a result they were incompatible and it became difficult for networks using different specifications to communicate with each other. To address the problem of networks being incompatible and unable to communicate with each other, the International Organisation for Standardisation (ISO) researched various network schemes. The ISO recognised there was a need to create a NETWORK MODEL that would help vendors create interoperable network implementations.

Slide 11: 

Open Systems Interconnection (OSI) is a set of internationally recognized, non-proprietary standards for networking and for operating system involved in networking functions. An open system is a set of protocols that allow two different systems to communicate regardless of their underlying architecture. Open Systems Interconnection model is fundamental to all communications between network devices It is now the theoretical model for how communication takes place between network devices.

Slide 12: 

The OSI reference model is an arbitrary hierarchical stratification (layering) of computer networking functions. The stratification consists of seven layers. There are various ways of implementing the protocols at any given layer. OSI is a CONCEPTUAL model. OSI Model allows complete interoperability between incompatible systems. The communication is governed by an agreed upon series of rules and conventions called PROTOCOLS.

Slide 13: 

The OSI Reference Model The OSI reference model consists of seven layers, not including layer 8, the end user’s application, and layer 0 the physical transmission media. 7. Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer 8. O/S or User Application 0. Physical Transmission Media

Design Issues for the Layers : 

Design Issues for the Layers Addressing Error Control Flow Control Multiplexing Routing




LAYER 1: PHYSICAL The physical layer deals with the physical characteristics of the transmission medium. It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications. Examples :- EIA/TIA-232, RJ45, NRZ.

Slide 17: 

Converts bits into electronic signals for outgoing messages Converts electronic signals into bits for incoming messages This layer manages the interface between the the computer and the network medium (coax, twisted pair, etc.) This layer tells the driver software for the MAU (media attachment unit, ex. network interface cards (NICs, modems, etc.)) what needs to be sent across the medium The bottom layer of the OSI model

Physical Layer cont… : 

Physical Layer cont… Responsibility: transmission of raw bits over a communication channel. Issues: mechanical and electrical interfaces time per bit ,Distances Transmission mode topology

Slide 19: 

Handles special data frames (packets) between the Network layer and the Physical layer At the receiving end, this layer packages raw data from the physical layer into data frames for delivery to the Network layer At the sending end this layer handles conversion of data into raw formats that can be handled by the Physical Layer Layer 2 : DATA LAYER

Slide 20: 

The data link layer provides access to the networking media and it enables the data to locate its intended destination on a network. It provides reliable transit of data across a physical link by using the Media Access Control (MAC) addresses. This layer uses the MAC address to define a hardware or data link address in order for multiple stations to share the same medium and still uniquely identify each other. Concerned with network topology, network access, error notification, ordered delivery of frames, and flow control. Examples :- Ethernet, Frame Relay, FDDI.

Data Link Control : 

Data Link Control Responsibility: provide an error-free communication link Hop-to-hop delivery Issues: framing (dividing data into chunks) header & trailer bits Addressing error notification, ordered delivery of frames, and flow control 10110110101 01100010011 10110000001


LAYER 3: NETWORK Defines end-to-end delivery of packets. Defines logical addressing so that any endpoint can be identified. Defines how routing works and how routes are learned so that the packets can be delivered. The network layer also defines how to fragment a packet into smaller packets to accommodate different media. Routers , IP address

Slide 23: 

Handles addressing messages for delivery, Responsible for deciding how to route transmissions between computers This layer also handles the decisions needed to get data from one point to the next point along a network path i.e source to destination delivery. This layer also handles packet switching and network congestion control

The Network Layer : 

The Network Layer Responsibilities: path selection between end-systems (routing). subnet flow control. fragmentation & reassembly translation between different network types. Issues: packet headers virtual circuits


LAYER 4: TRANSPORT The transport layer regulates information flow to ensure process – to- process delivery between host applications reliably and accurately. The transport layer segments data from the sending host's system and reassembles the data into a data stream on the receiving host's system. Layer 4 protocols include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

Slide 26: 

Manages the flow of data between parties by segmenting long data streams into smaller data chunks (based on allowed “packet” size for a given transmission medium) Reassembles chunks into their original sequence at the receiving end Provides acknowledgements of successful transmissions and requests resends for packets which arrive with errors

The Transport Layer : 

The Transport Layer Responsibilities: provides virtual end-to-end links between peer processes. end-to-end flow control Issues: Segmentation and reassembly error detection reliable communication


LAYER 5: SESSION LAYER The session layer defines how to start, control and end conversations (called sessions) between applications. This includes the control and management of multiple bi-directional messages using dialogue control. It also synchronizes dialogue between two hosts' presentation layers and manages their data exchange. Examples :- SQL, ASP(AppleTalk Session Protocol).

Slide 29: 

Enables two networked resources to hold ongoing communications (called to exchange data for the duration of the session responsible for initiating, maintaining and terminating sessions a session) across a network Responsible for security and access control to session information (via session participant identification) Responsible for synchronization services (checkpoints)

The Session Layer : 

The Session Layer Responsibilities: establishes, manages, and terminates sessions between applications. service location lookup


LAYER 6: PRESENTATION The presentation layer ensures that the information that the application layer of one system sends out is readable by the application layer of another system. Concerned with Semantics and syntax of the information Provides encryption and compression of data. Examples :- JPEG, MPEG, ASCII, EBCDIC, HTML.

Slide 32: 

Manages data-format information for networked communications (the network’s translator) For outgoing messages, it converts data into a generic format for network transmission; for incoming messages, it converts data from the generic network format to a format that the receiving application can understand. If necessary, the presentation layer translates between multiple data formats by using a common format. This layer is also responsible for certain protocol conversions, data encryption/decryption, or data compression/decompression.

The Presentation Layer : 

The Presentation Layer Responsibilities: data encryption data compression data conversion

Slide 34: 

The top layer of the OSI model Provides a set of interfaces for sending and receiving applications to gain access to and use network services, such as: networked file transfer, message handling and database query processing LAYER 7: APPLICATION

Slide 35: 

The application layer is the OSI layer that is closest to the user. It provides network access to the user. It differs from the other layers in that it does not provide services to any other OSI layer, but rather, only to applications outside the OSI model. Examples of such applications are spreadsheet programs, word processing programs, and bank terminal programs.

Slide 36: 

7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical Transmission Media User Application User Application Sending Computer Receiving Computer

Layering & Headers : 

Layering & Headers Each layer needs to add some control information to the data in order to do it’s job. This information is typically appended to the data before being given to the lower layer. Once the lower layers deliver the the data and control information - the peer layer uses the control information.

Headers : 

Headers Process Transport Network Data Link Process Transport Network Data Link DATA DATA DATA DATA H H H H H H

What are the headers? : 

What are the headers? Physical: no header - just a bunch of bits. Data Link: address of the receiving endpoints address of the sending endpoint length of the data checksum.

Network layer header - examples : 

Network layer header - examples type of service length of the data packet identifier fragment number time to live protocol header checksum source network address destination network address

Network SoftwareProtocol Hierarchies : 

Network SoftwareProtocol Hierarchies Layers, protocols, and interfaces.

Summary : 

Summary Data-Link: communication between machines on the same network. Network: communication between machines on possibly different networks. Transport: communication between processes (running on machines on possibly different networks).

Protocol Hierarchies : 

Protocol Hierarchies

Services to Protocols Relationship : 

Services to Protocols Relationship The relationship between a service and a protocol.

Reference Models : 

Reference Models The OSI reference model.

Reference Models : 

Reference Models

Hybrid Model : 

Hybrid Model The hybrid reference

A Critique of the OSI Model and Protocols : 

A Critique of the OSI Model and Protocols Why OSI did not take over the world Bad timing Bad technology Bad implementations Bad politics

Bad Timing : 

Bad Timing The apocalypse of the two elephants.

Slide 50: 


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