OSI Model

Topics What is the OSI Model? What is a Protocol? Why 7 Layers? The 7 Layers – Application – Presentation – Session – Transport – Network – Data Link – Physical How do the Layers Communicate?

What is the OSI Model? Open Systems Interconnection International Organization for Standardization (ISO) standard for worldwide communications that defines a networking framework for implementing protocols in seven layers Designed to describe layers in a network operating system 7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical

What is the OSI Model? (Continued) Each layer can communicate with the layer directly above it and directly below it The work on OSI was initiated in the late 1970s, and came to a level of maturity in the late 1980s and early 1990s Each layer has a standard defined input and a standard defined output 7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical

What is a Protocol? An agreed-upon format for transmitting data between two devices. The protocol determines the following: – The type of error checking to be used – Data compression method, if any – How the sending device will indicate that it has finished sending a message – How the receiving device will indicate that it has received a message

Why 7 layers? One layer system: – Monolithic specifications or “Link” protocols – Single copper wire or radio link – Application specific – Application aware signals – Inflexible i.e. N times M problem for standards

Why 7 layers? (continued) One to Two layers: – Need to separate Application specific from signaling and routing – Becomes m + n + 1 – One end-to-end Network Service Definition

Why 7 layers? (continued) Two to Four layers defining the Network Service: – Specifying media and signaling – Specifying the operation of a single-link protocol – Specifying the operation of nodes (end and intermediate systems) – Network service splits to 3 layers Physical Layer, concerned with media and signalling Data Link Layer, concerned with the operation of a single link Network Layer concerned with the behaviour of nodes to provide the overall Network Service

Why 7 layers? (continued) Transport layer – Transmission of 1s and 0s carries a chance of corruption – QoS (Quality of Service) Session layer – Dialogue control and Dialogue separation – Check point synchronization Presentation layer – Really the “encoding layer” – Abstract syntax – Transfer syntax

The 7 layers - #7 Application

The 7 layers - #7 Application (Continued)

The Application layer represents the level at which applications access network services. This layer represents the services that directly support applications such as software for file transfers, database access, and electronic mail Uses Terminate and Stay Resident (TSR) programs known as “Network Redirectors” Passes “Operating System File Envelope to Presentation layer Provides protocols for end-user applications Provides standardized services to applications

The 7 layers - #6 Presentation

The 7 layers - #6 Presentation (Continued) Negotiates between abstract syntax and transfer syntax – Transfers data format of sender to data format of receiver Controls security at the file level Data is contiguous and complete Provides code conversion, data compression and encryption services

The 7 layers - #5 Session

The 7 layers - #5 Session (Continued) Establishes and terminates communications sessions between host processes Provides synchronization and translation between name and address databases This layer establishes dialog control between the two computers in a session, regulating which side transmits, plus when and how long it transmits

The 7 layers - #4 Transport

The 7 layers - #4 Transport (Continued) Breaks the file into segments for transport over the network Handles error recognition and recovery Rebuilds segments on receiver Guarantees error free host-to-host transfers Not concerned with routing Works in units of Messages

The 7 layers - #3 Network

The 7 layers - #3 Network (Continued) Works in units of packets Addresses, switches and routes packets between hosts Fragments and reassembles packets Translates logical addresses and names into physical addresses

The 7 layers - #2 Data Link

The 7 layers - #2 Data Link (Continued) Works in units of frames Provides error free node-to-node connections and flow control Provides mapping between network-layer addresses and Data Link layer addresses Waits for acknowledgement from receiver

The 7 layers - #1 Physical

The 7 layers - #1 Physical (Continued) Works in units of bits Regulates the transmission of bits over a physical medium Defines transmission technique used to send data over the wire Handles physical, electrical and procedural specifications Defines connectors, pin outs and electrical voltages and currents

How do the Layers communicate?

What did I just see again? What is the OSI Model? What is a Protocol? Why 7 Layers? The 7 Layers – Application – Presentation – Session – Transport – Network – Data Link – Physical How do the Layers Communicate?

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