N ETWORKING Standards and Protocols

S TANDARDS AND P ROTOCOLS The OSI Model

W HAT IS THE OSI M ODEL ?

OSI Stands for Open Systems Interconnection. The OSI model is a layered, abstract description for communications and computer network protocol design. The Open Systems Interconnection model is a set of standard specifications that allows various computer platforms to communicate with each other openly. It is concerned with the interconnection between systems – the way the systems exchange information – and not with the internal functions of the particular system

W HAT IS THE OSI M ODEL ? It divides the network architecture into seven layers: 1. Physical 2. Data Link 3. Network 4. Transport 5. Session 6. Presentation 7. Application

W HAT IS THE OSI M ODEL ? Each of these different layers has its own set of functions and only communicates with the layers directly above and below and with its opposite layer on other computers.

W HY A L AYERED M ODEL ?

Change: When changes are made to one layer, the impact on the other layers is minimized. A layered model defines each layer separately. The layered approach reduces a very complex set of topics, activities, and actions into several smaller, interrelated groupings. This makes learning and understanding the actions of each layer and the model generally much easier. Troubleshooting efforts to be pinpointed on the layer that is the suspected cause of the problem.

W HY A L AYERED M ODEL ? Standards: Probably the most important reason for using a layered model is that it establishes a prescribed guideline for interoperability between the various vendors developing products that perform different data communications tasks. Remember, though, that layered models, including the OSI model, provide only a guideline and framework, not a rigid standard that manufacturers can use when creating their products.

T HE S EVEN L AYERS OF THE OSI M ODEL

P HYSICAL L AYER

The Physical layer is the lowest or first layer of the OSI Model. This layer contains the physical networking medium, such as cabling, connectors, and repeaters. The Physical Layer defines: Physical network structures Mechanical and electrical specifications for using the transmission medium Bit transmission encoding and timing rules

P HYSICAL L AYER The following network connectivity hardware are normally associated with the OSI physical layer: Network interface boards (NIC, adaptors, and so on) Hubs, and repeaters that regenerate electrical signals Transmission media connectors (cables, BNC connectors, etc) Modems and codec's, which perform digital analogue conversions.

D ATA L INK L AYER

The second layer of the OSI Model, the Data Link Layer, controls communications between the Network layer and the Physical layer. Its primary function is to divide data it receives from the Network layer into distinct frames that can be transmitted by the Physical layer.

D ATA L INK L AYER The basic purposes of the data link layer protocol implementations are: Organise the physical layer’s bits into logical groups of information called frames Detect and correct errors Control data flow Identify computers on the network

N ETWORK L AYER

The primary function of the Network Layer, the third layer in the OSI Model has the main objective of moving data to specific network locations.

N ETWORK L AYER Decides on the best route for the data to take from sender to receiver. Similar to what the data link layer accomplishes, however, data link layer addressing only operates on a single network

N ETWORK L AYER The network layer describes methods for moving information between multiple independent networks, called internetworks.

T RANSPORT L AYER

The transport layer provides enhancements to the services of the network layer. Its main tasks is to ensure that data sent form one computer arrives reliably, in the correct sequence and without errors at the receiving computer.

T RANSPORT L AYER This layer is the last chance for error recovery. The transport layer is also responsible for flow control. It’s here that there rate of transmission is determined, based on how fast the receiving computer can accept the data packets being sent to it. Data on the sending computer is broken down into packets that are the maximum size that the type of network can handle.

S ESSION L AYER

The Session Layer is responsible for establishing and maintaining communication between two nodes on the network.

S ESSION L AYER The term session refers to a connection for data exchange between two nodes. Often, this layer also helps the upper layers identify and connect to the services that are available on the network. If a communication session is broken, is the session layer that determines where to restart the transmission once the session has been reconnected

S ESSION L AYER This layer is also responsible for determining the terms of the communication session – it will determine which computer or node can communicate first and for how long It is sometimes known as the ‘traffic cop’ of the network

P RESENTATION L AYER

The Presentation Layer serves as a translator between the application and the network. At the Presentation layer, data become formatted in a schema that the network can understand; this format varies with the type of network used. The Presentation Layer manages data encryption and decryption, such as the scrambling of system passwords.

A PPLICATION L AYER

The top or seventh layer of the OSI Model is the Application layer. The Application provides interfaces to the software that enable programs to use network services.

A PPLICATION L AYER The term “Application Layer” does not refer to a particular software application, such as Microsoft Word, running on the network. Instead, some of the services provided by the Application layer include file transfer, file management, and message handling for electronic mail.

A PPLICATION L AYER Examples of common functions include: Protocols for providing remote file services, such as open, close, read, write, and shared access to files File transfer services and remote database access Message handling services for electronic mail applications Locate resources on a network A uniform way of handling a variety of system devices