COMMUNICATION NETWORKS
There are five types of communication networks:
1. Private Branch Exchanges (PBXs)
2. Integrated Services Digital Networks (ISDNs)
3. Local Area Networks (LANs)
4. Wide Area Networks (WANs)
5. Value Added Networks (VANs)

Private Branch Exchanges (PBXs)
 PBX (private branch exchange) is a telephone system within an enterprise that switches calls between enterprise users on local lines while allowing all users to share a certain number of external phone lines.
 The main purpose of a PBX is to save the cost of requiring a line for each user to the telephone company’s central office.
 The PBX is owned and operated by the enterprise rather than the telephone company (which may be a supplier or service provider).
 Private branch exchanges used analog technology originally.
 Today, PBXs use digital technology (digital signals are converted to analog for outside calls on the local loop using plain old telephone service (POTS ).
 A PBX includes:
• Telephone trunk (multiple phone) lines that terminate at the PBX
• A computer with memory that manages the switching of the calls within the PBX and in and out of it.
• The network of lines within the PBX.
• A console or switchboard for a human operator (optional)
 Integrated Services Digital Networks (ISDNs):
 The ISDN is a digital communication network that uses the public telephone network and allows users to transmit data, voice, text, and video communication in digital form over telephone lines. So it is called a digital network.
 It eliminates the need for a modem to convert analog signals into digital signals and vice versa.
 It does not require any rewiring. It uses the coaxial or fiber optic cables,
 It promotes uniformity and standardisation through a set of standard interfaces.
 The basic building block of ISDN is a 64 kbps (thousands bits per second) channel, referred to as a B-channel.
 Each B-channel is used to transmit user information.
 Another channel called the D-channel, carries signaling and control information used to initiate, redirect, or terminate cells.
 ISDN works basically on five principles:
 Openness;
 Modularity;
 Communication based intelligence;
 Network management and control;
 Integrated products and services;
 Openness: It means that all ISDN products will be standardized. This brings order and uniformity to the complex task of developing a network.

 Modularity: It enables the organization to upgrade or replace any part in a network without replacing the entire network.
 Communication based intelligence: It provides users with a way to configure their network connections to meet their requirements.
 Network management and control: It is one of the complex and challenging tasks that information system managers face.
 Integrated products and services: It allows the use of wide variety of products and services on the network such as voice networking, data networking, teleconferencing, etc. Recent computerized railway reservation network throughout the country is one of the examples of ISDN.

Local Area Networks (LANS)
 A local area network (LAN) is a group of computers and associated devices that share a common communications line or wireless link.
 It is connected devices share the resources of a single processor or server within a small geographic area (for example, within an office building).
 The server has applications and data storage that are shared in common by multiple computer users.
 A local area network may serve as few as two or three users (for example, in a home network) or as many as thousands of users (for example, in an FDDI network).
 Major local area network technologies are: Ethernet, Token Ring, and FDDI.
 Ethernet is the most commonly used LAN technology.
 A number of corporations use the Token Ring technology.
 FDDI is sometimes used as a backbone LAN interconnecting Ethernet or Token Ring LANs.
 Another LAN technology, ARCNET, once the most commonly installed LAN technology, is still used in the industrial automation industry.
 A suite of application programs can be kept on the LAN server.
 Users who need an application frequently can download it once and then run it from their local hard disk.
 Users can order printing and other services as needed through applications run on the LAN server.
 A user can share files with others at the LAN server; read and write access is maintained by a LAN administrator.
 A LAN server may also be used as a Web server if safeguards are taken to secure internal applications and data from outside access.

Wide Area Networks (WANS)
 It is a collection of computers and network resources connected via a network over a geographic area.
 Wide-Area Networks are commonly connected either through the Internet or special arrangements made with phone companies or other service providers.

 A WAN is different from a MAN because of the distance between each of the networks. In a
WAN, one network may be anywhere from several hundred miles away, to across the globe
in a different country.
 There are different types of set up a WAN are:-direct distance dialing (DDD), Wide area
telephone service lines (WATS), leased lines and satellites.
 Example:-banking networks, airline reservation systems and railway reservation systems.
Value Added Networks (VANS)
 The public data networks that add value to the basic communication services provided by common carriers by offering specialised services are termed as value added networks.
 The specialized services include access to commercial databases and software, correction of transmission errors, providing compatibility between incompatible computers and terminals, Email, and video conferencing.
 A subscriber to a VAN service pays monthly fee depending on its level of use.

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