ISO/OSI Model of Networking

ISO / OSI MODEL! 

The Open Systems Interconnection (OSI) model is a standard set by ISO to cover all aspects of network communications. It was introduced in the late 1970s. An Open System is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture.

The purpose of OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software. The OSI model is a layered framework for the design of network systems that allows communication between all types of computer systems. It consists of seven separate but related layers, each of which defines a part of the process of moving information across the network.

Layered Architecture:

The OSI model is composed of seven ordered layers:

1. Physical Layer (layer 1)
2. Data Link Layer (layer 2)
3. Network Layer (layer 3)
4. Transport Layer (layer 4)
5. Session Layer (layer 5)
6. Presentation Layer (layer 6)
7. Application Layer (layer 7)

All these layers can be thought of as belonging to three sub groups:

1. Lower layer (layers 1, 2 and 3)
2. Middle layer (layer 4)
3. Upper layers (layer 5, 6 and 7)

The Upper layers – Session, Presentation and Application – can also be thought of as the user support layers; they allow interoperatability among unrelated software systems. These layers are almost always implemented in software.

The Lower layers – Physical, Data Link and Network – are also known as network support layers, because they deal with the physical aspects of moving data from one device to another (such as electrical specifications, physical connections, physical addressing, transport timing and reliability). These layers are implemented both in hardware and software, except for the physical layer, which is mostly hardware.

The Middle layer is the Transport layer, which links the two sub layers (upper and lower) and ensures that the data passed by the lower layers are in a form that can be used by the upper layers. It is named after its function of transporting data between lower and upper layers.

Peer – to – Peer Process:

As messages travel from a source (A) to a destination (B), it may pass through many intermediate nodes. These intermediate nodes usually involve only the first three layers (lower layers) of the OSI model; but the source and the destination may involve all three sub layers. Between machines, layer x on one machine communicates with the layer x on another machine. This process of communication between two machines at a given layer is called peer-to-peer process.

The communication between layers of two different machines is governed by an agreed-upon series of rules and conventions called protocols. The following figure shows the layers involved when a message is sent from device A to device B.

Fig.: The interconnection between layers of OSI model

At the physical layer, communication is direct: device A sends a stream of bits to device B. At the higher layers, however, communication must move down through the layers on device A, over to device B, and then back up through the layers.

Each layer in the sending device adds its own information to the message it receives from the layer just above it and passes the whole package to the layer just below it. At layer 1 the entire package is converted to a form that can be transmitted to the receiving device.

At the receiving machine, the message is unwrapped layer by layer, with each process receiving and removing the data meant for it. The following figure shows this process of exchanging the message between two computers using OSI model:

Fig. : An exchange using the OSI model

Interfaces between Layers:

Within a single machine, each layer calls upon the services of the layer just below it. Layer 3, for example, uses the services provided by layer 2 and provides services for layer 4. The passing of the data and network information down through the layers of the sending device and back through the layers of the receiving device is made possible by an interface between each pair of adjacent layers.

Each interface defines the information and services a layer must provide for the layer above it. Well defined interfaces and layer functions provide modularity to a network. As long as a layer provides the expected services to the layer above it, the specific implementation of its functions can be modified or replaced without requiring changes to the surrounding layers.

The process of communication at layer 7 of the sender machine (the application layer), then moves from layer to layer in descending, sequential order. At each layer, a header, or possibly a trailer, can be added to the data unit. Commonly, the trailer is added only at layer 2. When the formatted data unit passes through the physical layer (layer 1), it is changed into an electromagnetic signal and transported along a physical path.

Upon reaching its destination, the signal passes into layer 1 and is transformed back into digital form. The data units then move back up through the OSI layers. As each block of data reaches the next higher layer, the headers and trailers attached to it at the corresponding sending layer are removed, and actions appropriate to that layer are taken. By the time it reaches layer 7, the message is again in a form appropriate to the application and is made available to the recipient.