ECE124 Data Communications - Research Work No. 6 - Public Data Network

ECE 124

Research Work

Public Data Network

Public Data Network (PDN)

• It is a network established and operated by a telecommunications administration, or a recognized private operating agency, for the specific purpose of providing data transmission services for the public.
• It is a packet or circuit-switched service provided by carriers. Examples include X.25, frame relay, SMDS, Switched 56 Kb/s, ISDN and ATM service.
• A communications network provided by a carrier organization that makes its transport available to companies. Customers attach to such networks using a variety of protocols including frame relay, ATM, IP, etc., and pay by the month or on a per-byte basis.

The basic principle behind a PDN is to transport data from a source to a destination through a network of intermediate switching nodes and transmission media. The switching nodes are not concerned with the content of the data, as their purpose is to provide end stations access to transmission and other switching nodes that will transport data from node to node until it reaches its final destination. The switching nodes are interconnected with transmission links (channels). The end-station devices can be personal computers, servers, mainframe computers, or any other piece of computer hardware capable of sending or receiving data. End stations are connected to the network through switching nodes. Data enter the network where they are routed through one or more intermediate switching nodes until reaching their destination.

Some switching nodes connect only to other switching nodes (sometimes called tandem switching nodes or switchers switches), while some switching nodes are connected to end stations as well. Node-to-node communications links generally carry multiplexed data (usually time-division multiplexing). Public data networks are not direct connected; that is, they do not provide direct communication links between every possible pair of nodes.

Public switched data networks combine the concepts of value-added networks (VANs) and packet switching networks.

Value-Added Network

A value-added network “adds value” to the services or facilities provided by a common carrier to provide new types of communication services. Examples of added values are error control, enhanced connection reliability dynamic routing, failure protection, logical multiplexing, and data format conversions. A VAN comprises an organization that leases communications lines from common carriers such as AT&T and MCI and adds new types of communications services to those lines.

Packet Switching Network

Packet switching involves dividing data messages into small bundles of information and transmitting them through communications networks to their intended destinations using computer-controlled switches. Three common switching techniques are used with public data networks: circuit switching, message switching, and packet switching.

Circuit switching is used for making a standard telephone call on the public telephone network. The call is established, information is transferred, and then the call is disconnected.

Message switching is a form of store-and-forward network. Data, including source and destination identification codes, are transmitted into the network and stored in a switch. Each switch within the network has message storage capabilities. The network transfers the data from switch to switch when it is convenient to do so.

With packet switching, data are divided into smaller segments, called packets, prior to transmission through the network. Because a packet can be held in memory at a switch for a short period of time, packet switching is sometimes called a hold-and-forward network.