OSI background
The “Open System Interconnection (OSI)” was created by the “International Organization for Standardization (ISO)” to represent an international standardization program that facilitates multi-vendor equipment interoperability.
There are four significant architectural entities in an OSI network namely:
1. Hosts – the one that provides data information which will be exchange..
2. Areas – lies within a domain.
– a logical entity, it is formed by a set of contiguous routers and the data links that connect them.
3. A Backbone – areas that are connected together.
4. A Domain – any portion of an OSI network that is under common administrative operation.
7 OSI layers
Layer 1 – Physical
This is the most basic network layer which only provides the means of transmitting raw bits rather than packets over a physical data link connecting network nodes. It provides an electrical, mechanical, and procedural interface to the transmission medium.
Major Functions and Services:
- Bit-by-bit node-to-node delivery
- Providing a standardized interface to physical transmission media, including
- Mechanical specification of electrical connectors and cables (labels)
- Electrical specification of transmission line, signal level and impedance
- Radio interface, including electromagnetic spectrum frequency allocation
- Specifications for IR over optical fiber or a wireless IR communication link
- Modulation
- Line coding
- Bit synchronization in synchronous serial communication
- Start-stop signaling and flow control in asynchronous serial communication
- Circuit mode multiplexing
- Carrier sense and collision detection utilized by some level 2 multiple access protocols
- Equalization filtering, training sequences, pulse shaping and other signal processing of physical signals.
Examples:
1. USB physical layer
2. DSL
3. Bluetooth physical layer
Layer 2 – Data Link
This defines the format of data on the network. It responds to service requests from the network layer and issues service requests to the physical layer. This layer is responsible in transferring data between adjacent network nodes in a wide area network or between nodes on the same local area network segment. It provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the Physical layer. Data link is all about getting information from one place to other places. It gives the convenience of getting information not within the place easily. This is often implemented in software as a “network card driver”. This layer is responsible for node to node (hop to hop) packet delivery.
Examples:
1. Ethernet
2. ATM
Layer 3 – Network
This responds to service requests from the transport layer and issues service requests to the data link layer. This layer is responsible for end to end (source to destination) packet delivery. It provides the functional and procedural means of transferring variable length data sequences from a source to destination via one or more networks while maintaining the quality of service requested by the transport layer.
Network layer performs the following:
- network routing
- flow control
- network segmentation/desegmentation
- error control
This layer deals with transmitting information all the way from its source to its destination. OSI Network layer can be either connection oriented or connectionless.
Examples:
1. Internet Protocol
2. Datagram Delivery Protocol
Layer 4 – Transport
This layer subdivides user-buffer into network buffer sized datagrams and enforces desired transmission control. The transport layer provides transparent of data between hosts. It is responsible for end-to-end connection, error recovery, flow control, and ensuring complete data transfer.
In this layer, there are two transport protocols:
- Transmission Control Protocol (TCP) – establishes connections between two hosts on the network through sockets which are determined by the IP address and port number.
- User Datagram Protocol (UDP) – provides low overhead transmission service, but with less error checking.
Examples:
1. TCP, Transmission Control Protocol
2. UDP, User Datagram Protocol
Layer 5 – Session
This layer defines the format of the data sent over the connections. It responds to service requests from the presentation layer and issues service requests to the transport layer. This layer is typically completely unused, but it does have a few places where it is useful. The idea is to allow information on different streams, perhaps originating from different sources, to be properly combined. It deals with synchronization issues, and ensuring nobody ever sees inconsistent versions of data, and similar things like “web conference”.
Examples:
1. ZIP, Zone Information Protocol
2. NetBIOS, Network Basic Input Output System
Layer 6 – Presentation
This layer provides independence from differences in data representation by translating from application to network format, and vice versa. It transforms data into the form that the application layer can accept giving it easy access to the data. It relieves the application layer of concern regarding syntactical differences in data representation within th eend-user systems. This layer deals with issues like how strings are represented – whether they use the Pascal method or the C/C++ method.
Examples: 1. AFP, AppleShare File Protocol
Layer 7 – Application
This layer supports application and end-user processes. It provides application services for file transfers.
This layer does the following:
1. Communication partners are identified
2. Quality of service is identified
3. User authentication and privacy are considered
4. Any constraints on data syntax are identified
Examples:
1. Mail
2. Telnet
