Blog Post

Layer Architecture:

The Communication Architecture is a strategy for connecting host computers and other communicating equipment. It defines necessary elements for data communication between devices. The 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. A complex problem is divided into a number of pieces of manageable and comprehensible size. It provides structured modular approach. Each module can be developed and tested independently. It allows easy enhancement and implementation of the functions of a particular layer without affecting other layers. 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.

   #  Use optimum number of layers.
   
   #  Put similar functions in the same layer.
   
   #  Create a layer where there is need for different levels of 
   abstraction.
   
   #  Allow changes of functions to be made with in a layer without 
   affecting others.
   
   #  Create layer boundaries for each layer with its upper and lower 
   layers.
   
   #  Choose layer boundaries to minimize information flow across the 
   boundaries.

International standard organization (ISO) established a committee in 1977 to dev elop the architecture for computer communication. Open Systems Interconnection (OSI) reference model is the result of this effort. In 1984, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture. The term “open” denotes the ability to connect any two systems which conform to the reference model and associated standards. The OSI model is now considered the primary Architectural model for inter-computer communications. The OSI model describes how information or data makes its way from application programs (such as spreadsheets) through a network medium (such as wire) to another application program located on another network. The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems. This separation into smaller more manageable functions is known as layering.

OSI Reference Model: A 7 Layered Network Model:

The process of breaking up the functions or tasks of networking into layers reduces complexity. Each layer provides a service to the layer above it in the protocol specification. Each layer communicates with the same layer's software or hardware on other computers. The lower 4 layers (transport, network, data link and physical-Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network. The upper four layers of the OSI model (application, presentation and session-Layers 7, 6 and 5) are orientated more toward services to the applications. Data is encapsulated with the necessary protocol information as it moves down the layers before network transit.

Physical layer provides physical interface for transmission of information. It defines rules by which bits are passed from one system to another on a physical communication medium. This layer covers all mechanical, electrical, functional and procedural aspects for physical communication. 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 spec ifications. Devices working on at physical layer are hubs, repeaters, cables, modems etc.

Data link layer attempts to provide reliable communication over the physical layer interface. It breaks the outgoing data into frames and reassembles the received frames. Data link layer create and detect frame boundaries. It handles errors by implementing an acknowledgement and retransmission scheme. It implements flow control, supports points-to-point as well as broadcast communication. This layer also supports simplex, half-duplex or full-duplex communication.

Network Layer implements routing of frames (packets) through the network. This layer defines the most optimum path the packet should take from the source to the destination. It also defines logical addressing so that any endpoint can be identified. This layer handles congestion (crowding or jamming) in the network, facilitates interconnection between various networks (Internetworking). The network layer also defines how to fragment a packet into smaller packets to accommodate different media. Device working on Network layer is router.

Purpose of this layer is to provide a reliable mechanism for the exchange of data between two processes in different computers. This layer ensures that the data units are delivered error free, data units are delivered in sequence and there is no loss or duplication of data units. It provides connectionless or connection oriented service. This layer also provides the connection management and multiplex multiple connection over a single channel. It is also called as the heart of the OSI layers.

Session layer provides mechanism for controlling the dialogue between the two end systems. It defines how to start, control and end conversations (called sessions) between applications. This layer requests for a logical connection to be established on an end-user’s request. Any necessary log-on or password validation is also handled by this layer. Session layer is also responsible for terminating the connection. This layer provides services like dialogue discipline which can be full duplex or half duplex. Session layer can also provide check-pointing mechanism such that if a failure of some sort occurs between checkpoints, all data can be retransmitted from the last checkpoint.

Presentation layer defines the format in which the data is to be exchanged between the two communicating entities. Presentation layer is responsible for converting data into standard format. Examples: ASCII, JPEG, MPEG, BMP, WAV, MP3 etc.

It also handles data encoding - data decoding, data compression data - decompression and data encryption - data decryption (cryptography).

Application layer interacts with application programs and is the highest level of OSI model. Application layer contains management functions to support distributed applications. Examples of application layer are applications such as file transfer, electronic mail, remote login etc. Application layer is responsible for providing network services to user. It is also known as desktop layer. Here identification of services is done using port numbers. Ports are nothing but sockets i.e. entry and exit point to the layer.

A message begins at the top application layer and moves down the OSI layers to the bottom physical layer. As the message descends, each successive OSI model layer adds a header to it. A header is layerspecific information that basically explains what functions the layer carried out. Conversely, at the receiving end, headers are striped from the message as it travels up the corresponding layers.

OSI & TCP/IP Models:

TCP/IP Model: