What do you mean by Network architecture? Expalin about layering and protocols.

A computer network must provide general, cost effective, fair and robust among a large number of computers. It must evolve to accommodate changes in both the underlying technologies. To help to deal this network designers have developed general blueprints called network architecture that guide the design and implementation of networks.
To reduce the complexity of getting all the functions maintained by one a new technique called layering technology was introduced. In this, the architecture contains several layers and each layer is responsible for certain functions. The general idea is that the services offered by underlying hardware, and then add a sequence of layers, each providing a higher level of service. The services provided at the higher layers are implemented in terms of the services provided by the lower layers. A simple network has two layers of abstraction sandwiched between the application program and the underlying hardware. 

The layer immediately above the hardware in this case might provide host to host connectivity, and the layer above it builds on the available host to host communication service and provides support for process to process channels.
            Features of layering are: 1. It decomposes the problem of building a network into more manageable components. 2. It provides a more modular design. Addition of new services and modifications are easy to implement.
            In process to process channels, they have two types of channels. One for request\reply service and the other for message stream service.
            A protocol provides a communication service that higher level objects use to exchange message. Each protocol defines two different interfaces. First it defines a service interface to other objects on the same system that want to use its communication services. This interface defines the operations that local objects can perform on the protocol. Second a protocol defines a peer interface to its counterpart on another machine. It defines the form and meaning of message exchanged between protocol peers to implement the communication service.
            There are potentially multiple protocols at any given level, each providing a different communication service. It is known as protocol graph that make up a system.


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