Networks

Unit I

Network architecture – layers – Physical links – Channel access on links – Hybrid multiple access techniques - Issues in the data link layer - Framing – Error correction and detection – Link-level Flow Control.

Network Architecture

Network architecture that guides the design and implementation of networks. Two of the most widely referenced architectures—the OSI architecture and the Internet architecture.

Layering

In OSI Architecture, there are 7 layers which can be combined into basic four layers as shown in the below figure.

Example of a layered network system

Layering Characteristics

 Each layer relies on services from layer below and exports services to layer above
 Hides implementation - layers can change without disturbing other layers

Abstraction

Abstraction hides the complexity of layering.
The idea of an abstraction is to define a unifying model that can capture some important aspect of the system.
Abstractions naturally lead to layering, especially in network systems.
The following figure depicts the layered system with alternative abstraction available at a give layer.

Layered system with alternative abstractions available at a given layer

In the above figure, two layers of abstraction sandwiched between the application program and the underlying hardware, as illustrated in above figure. The services provided at the high layers are implemented in terms of the services provided by the low layers.

The layer immediately above the hardware in this case might provide host-to-host connectivity and the next layer up on the available host-to-host communication is Process to Process channel that provides support for application to application communication services. From the above fissure, the process to process channel is a combination of two abstract channels; one is Request/Reply channel for sending or receiving the request or reply message. The Message stream abstract channel is used for sending or...