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Open Systems Interconnection (OSI) Each layer support the layers above it and offers services to the layers below Each layer performs unique and specific task A layer only has knowledge of its neighbour layers only A layer service is independent of the implementation
OSI Model
An attempt for a framework for developing networking technologies OSI became a tool for explaining the Networking in general Before OSI was created people created their Software/Hardware as they wanted it to be. There was not any compatibility. Now OSI is used as a rule set for all vendors to create their Software/Hardware by using the standards.
OSI Model
Strategy for connecting host computers and other communicating equipment. Defines necessary elements communication between devices.
Introduction to the OSI layer How OSI was created and why Comparison with TCP/IP Layers : Application layer Presentation layer Session layer Transport layer Network layer Data link layer Physical layer Conclusion Resources
for
data
A communication architecture, therefore, defines a standard for the communicating hosts. A programmer formats data in a manner defined by the communication architecture and passes it on to the communication software. Separating communication functions adds flexibility, for example, we do not need to modify the entire host software to include more communication devices.
Layer architecture simplifies the network design. It is easy to debug network applications in a layered architecture network. The network management is easier due to the layered architecture. Network layers follow a set of rules, called protocol. The protocol defines the format of the data being exchanged, and the control and timing for the handshake between layers.
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OSI Model
OSI Model
The OSI model is now considered the primary Architectural model for inter-computer communications.
International standard organization (ISO) established a committee in 1977 to develop an architecture for computer communication. Open Systems Interconnection model is the result of this effort.
(OSI)
The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network.
reference
In 1984, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture.
The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems .
Term “open” denotes the ability to connect any two systems which conform to the reference model and associated standards.
This separation into smaller more manageable functions is known as layering.
II OSI Overview 1. OSI - layered framework for the design of network systems that allows communication across all types of computer systems.
Each layer contains a logical grouping of functions Each function receive an input(one or more) and produces an output
2. The OSI 7 Layers. ( Brief functional overview. ) 3. Vertical and horizontal communication between the layers using interfaces. (defines what information and services should the layer provide to the layer above it. )
I History 1. The need for standardization - many vendors, no interoperability - no common framework
Layer abstraction and the path of the message
2. ISO and CCITT came up with OSI (Open System Intercommunication) in 1984. 3. OSI Protocol Suite – unaccepted by vendors and users. (TCP won) 4. OSI – a standard, which allows communication between different systems without requiring changes to the logic of the underlying hardware and software.
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OSI Overview 4. Data Encapsulation a) PDU conception – each protocol on the diff. layer has its own format.
Pretty similar to OSI TCP/IP has less layers(four) Main difference in layers is after layer 4
b) Headers are added while a packet is going down the stack at each layer. c) Trailers are usually added on the second layer.
Applications and Services run on it Enables human network to interface the underlying data network Applications on that layer (E-mail clients, web browsers, Chats, etc.) – top-stack applications (As people are on the top of the stack) Applications provide people with a way to create message Application layer services establish an interface to the network Protocols provide the rules and formats that govern how data is treated Protocols on the destination and the host must match
III The OSI Layers 7. Application Layer ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦
Coding and conversion of Application layer data to ensure that data from the source device can be interpreted by the appropriate application on the destination device. Compression of the data in a manner that can be decompressed by the destination device. Encryption of the data for transmission and the decryption of data upon receipt by the destination. This is the layer at which application programmers consider data structure and presentation Examples: GIF, JPEG, TIFF, etc. Sometimes n distinction is made between the presentation and application layers. For example http/https. HTTP is generally
Provides user interfaces and support for services Resource sharing and device redirection Remote file access Remote printer access Inter-process communication Network management Directory services Electronic messaging (such as mail) Network virtual terminals
regarded as an application layer protocol although it has Presentation layer aspects such as the ability t identify character encoding for roper conversion
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Functions at this layer create and maintain dialogs between source and destination applications Authentication Permissions Session Restoration (Checkpoint or recovery)
Tracking the individual communication between applications on the source and destination hosts Segmenting data and managing each piece Reassembling the segments into streams of application data Identifying the different applications Conversation Multiplexing Segments
Connection-oriented conversations Reliable delivery Ordered data reconstruction Flow control
TCP – Web Browser UDP – Video Streaming Applications
Addressing (IPV4) Encapsulation (Inserts a header with source and destination IPs) Routing (Move a packet over the Internet) Decapsulation (Open the packet and check the destination host) IP is connectionless
The OSI Layers 3. Network Layer ◦ ◦ ◦ ◦ ◦
End to end packet delivery
Logical Addressing Routing (where the packet is destinated to) Datagram Encapsulation Fragmentation and Reassembly (handling too big packets ) Error Handling and Diagnostics ( using status messages for example )
It is the role of the OSI Data Link layer to prepare Network layer packets for transmission and to control access to the physical media. Allows the upper layers to access the media using techniques such as framing Controls how data is placed onto the media and is received from the media using techniques such as media access control and error detection Frame - The Data Link layer PDU Node - The Layer 2 notation for network devices connected to a common medium Media/medium - The physical means for the transfer of information between two nodes Network - Two or more nodes connected to a common medium The Data Link layer is responsible for the exchange of frames between nodes over the media of a physical network.
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The OSI Layers
2. Data Link Layer 2.1. Logical Link Control (LLC ) ◦
Establishment and control of logical links between local devices on a network.
2.2. Media Access Control (MAC) ◦
◦ • • • • • •
The procedures used by devices to control access to the network medium.
The role of the Physical layer is to encode the binary digits that represent Data Link layer frames into signals and to transmit and receive these signals across the physical media that connect network devices. Copper cable Fiber Wireless
Encoding and Signaling (bit representation) Data Transmission and Reception (half duplex, full duplex ) Topology and Physical Network Design (mesh, ring, bus) PDU: bit
