1.Overview

A communication network, in its simplest form, is a set of equipment and facilities that provides a service: the transfer of information between users located at various geographical points.

Networks include telephone networks:

• telephone networks
• computer networks
• television broadcast networks
• cellular telephone networks

• Internet

• The ability of communication networks to transfer communication at extremely high speeds allow users to gather information in large volumes nearly instantaneously.
• With the aid of computers, to almost immediately exercise action at a distance.

these two unique capabilities form the basis for many existing services and an unlimited number of future network-based services.

Several services that are examined from the point of view of user requirements: quality of service,features,capabilities.

• Radio and television broadcasting: the most common communication services.

2. Approaches to network design

Different user applications impose different requirements on services provided by the network in terms of:

• transfer delay
• reliability of service
• accuracy of transmission
• volume of information that can be transferred
• cost and convenience

In point of view of network designer, the task of the designer is to develop an overall network design that meets the requirements of the users in a cost-effective manner.

2.1 Network Functions and Network Topology

The essential function of a network is to transfer information between a source and a destination.

• the source and destination typically comprise terminal equipment that attaches to the network.
• This process may involve the transfer of a single block of information or the transfer of a stream of information.

The network must be able to provide connectivity in the sense of providing a means for information to flow among users.

• this basic capability is provided by transmission systems that transmit information by using various media sucha as wires, cables, radio, and optical fiber.
• Networks are typically designed to carry specific types of information representation.

Some analogy:

• roads and high-ways: transmission lines.
• A specific segment of road corresponds to a point-to-point transmission line.
• The intersections: switches.
• routing: decide which exit corresponding to the destination.
• forwarding: move through the exit.

Basic components

We refer to the access transmission lines and the first switch as an access network. These access networks concentrate the information flows prior to entering a backbone network.

Multiplexers are used to concentrate the traffic between the communities into the trunks that connect the switches.

The associated demultiplexer and witch then direct each information flow to the appropriate destination.

Topology

hierarchical network topology: consider of geography, cost considerations, communities of interests among users.

Multi-plexers and trunks are used to interconnect a number of access networks to form a metropolitan network.

• A metropolitan network might correspond to a transportation system associated with a county or large city.
• metropolitan subnetworks are interconnected into a regional network(province).
• national network can be viewed as a network of regional subnetworks hat interconnect metropolitan networks.

community of interest: a set of users who have a need to communicate with each other.

switches: are placed to interconnect a cluster of users where it makes economic sense.

Traffic to more distant users is aggregated into multi-plexers that connect to more distant switches through a backbone network.

These two trends: distance-independent communities of interest and lower communications costs, will in time lead to very different network topologies.

Addressing

Addressing is required to identify which network input is to be connected to which network output.

The use of hierarchical addresses facilitates the task of routing.

In networks, there two types of addressing:

• hierarchical addressing in wide area networks.
• flat addressing in local area networks.

Traffic control

Traffic controls are necessary to ensure the smooth flow of information through the network.

When congestion occurs inside the network as a result of a surge in traffic or a fault in equipment, the network should react by applying congestion or overload control mechanisms to ensure a degree of continued operation in the network.

• Information may be rerouted or prevented from entering the network.

Switching

Network Management

these functions fall under the category of network management:

• monitoring the performance of the network.
• detecting and recovering from faults
• configuring the network resources
• maintaining accounting information for cost and bill puirposes
• providing security by controlling access to the information flows in the network.

Summary

• Basic user service: the primary service or services that the network provides to its user.
• Swicthing approaches: the means of transferring information flows between communication lines.
• Terminal: the end system that connects to the network.
• Information representation: the formate of the information handled by the network.
• Transmission System: the means for transmitting information across a physical medium.
• Addressing: the means for identifying points of connection to the network.
• Routing: the means for determining the path across the network.
• Multiplexing: the means for connecting multiple information flows into shared connection lines.