A Review On Public Switch Data Networks Information Technology Essay

A public data network 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.

A public switched data network (PSDN) is a publicly-available packet-switched network, distinct from the PSTN.

A general wide area network used to provide circuit switched data services for commercial users

A publicly-available network supporting packet-switched data

It is an all digital communications line that allows for the transmission of voice, data, video and graphics, at very high speeds, over standard communication lines

It is a computer network that uses a public telecommunication infrastructure such as the Internet to provide remote offices or individual users with secure access to their organization’s network. It aims to avoid an expensive system of owned or leased lines that can be used by only one organization.

A leased line is a service contract between a provider and a customer, whereby the provider agrees to deliver a symmetric telecommunications line connecting two or more locations in exchange for a monthly rent (hence the term lease). It is sometimes known as a ‘Private Circuit’ or ‘Data Line’ in the UK.

Dial-up Internet access is a form of Internet access that uses the facilities of the public switched telephone network (PSTN) to establish a dialed connection to an Internet service provider (ISP) via telephone lines. The user’s computer or router uses an attached modem to encode and decode Internet Protocol packets and control information into and from analogue audio frequency signals, respectively.


Caller ID, Call Waiting, Voice Mail, and Call Forwarding

Need a connection establishment between end


Connection is maintained until one of end

nodes terminates.

Connection is dedicated to the communication

between two nodes.

Reduces the number and the total length of

the links

Data are transmitted in short messages called packets.

A connection between the two end-nodes is not


A node-to-node link can be dynamically shared by many

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Example : Public Data Network (PDN) likes X.25, Frame


There are two types of PDN: CSPDN and PSPDN.

Mostly used to provide leased-line connections between local area networks and the Internet

Circuit switch network work by allowing multiple sessions to run between end-user by simply switchingto the most direct or available path

Because a circuit switch dedicates bandwidth to each session, it is less effective from packet-switch.

The advantage of circuit switch is transparent, and the path is always open, thus the delay through SCPDN is constant, which is ideal for real time application such as video.

One of the disadvantage of CSPDN is they are very wasteful of bandwidth, because if the device is not transmitting data, the bandwidth will always be availalbe.

Packet switch provide bandwidth on-demand.

Advantage of PSPDN is it saved bandwidth, because they only use the bandwidth when there is data to transmit.

All data in Packet switching is transmitted in frame/packets over a trunk, where each packet or frame can belong to a different session.

All of the data being sent to the network is marked with a sequence number.

This mean PSPDN can correct any data corrupt or loss within a network.

Disadvantage of PSPDN is buffering.

It builds on groups of standard transmission channels. Bearer channels (or B channels) transmit user information at relatively high speeds, while separate Data channels (or D channels) carry call set-up, signaling and other information.

It handles all types of information. Unlike some other digital communications technologies, ISDN handles all types of information – voice, data, studio-quality sound, still and moving images. They are all digitized, and transmitted at high speeds in the same flow of data. It handles many devices and many telephone numbers, on the same line.

It encapsulates data transfers between two or more networked devices which are not on the same private network so as to keep the transferred data private from other devices on one or more intervening local or wide area networks. There are many different classifications, implementations, and uses for VPNs.

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Typically, leased lines are used by businesses to connect geographically distant offices. Unlike dial-up connections, a leased line is always active. The fee for the connection is a fixed monthly rate. The primary factors affecting the monthly fee are distance between end points and the speed of the circuit. Because the connection doesn’t carry anybody else’s communications, the carrier can assure a given level of quality.

Dial-up connections to the Internet require no infrastructure other than the telephone network. Dial-up is often the only choice available for rural or remote areas where broadband installations are not prevalent due to low population and demand. Dial-up access may also be an alternative for users on limited budgets as it is offered free by some ISPs, though broadband is increasingly available at lower prices in many countries due to market competition.

Network Interface


Packet Switching

Packet Switching


Packet Switching

Device-to-device interfaces are called reference points

Interface R Located between Non-ISDN Terminal Equipment (TE2) and a Terminal Adapter (TA)

Interface S/T Located between ISDN user equipment and Network Termination (NT1) equipment

Interface U Located between NT1 equipment and an ISDN central office




Components/ hardware configuration

Consists of telephone lines, fiberoptic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables all inter-connected by switching centers which allows any telephone in the world to communicate with any other.

In a LAN packet-switched environment, such as with an Ethernet network, the transmission of the data packets relies on packet switches, routers, and LAN cables

using permanent virtual circuits (PVCs).

Uses TDM (Time Division Multiplexing), SDH (Synchronous Digital Hierarchy) and PDH (Pleisochoronous Digital Hierarchy)

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Uses Frame Relay and STDM (Statistical Time Division Multiplexing)

TE1- Any ISDN-ready device that connects directly to ISDN or connects to ISDN via an NT1. For example: a digital telephone, a GT 70 Series router.

TE2- Any non-ISDN device that needs to be used with a Terminal Adapter (TA) to work with ISDN. For example: an analog FAX, PC, or analog telephone.

TA-A device that converts non-ISDN communication to ISDN to allow non-ISDN devices to work with ISDN.

NT1- A device located between the S/T and U reference points that allows communication between functional devices.

Secure VPNs use cryptographic tunneling protocols to provide confidentiality by blocking intercepts and packet sniffing, allowing sender authentication to block identity spoofing, and provide message integrity by preventing message alteration.

Leased lines are made up of the following components:

– a router, usually managed by the service provider, is installed into a customer’s comms room. The circuit is presented with an RJ45 connector as standard.

– local loop circuit, , links the router to the service provider’s local point of presence (POP). Network termination equipment (NTE) is attached to the wall in a comms room and is connected to either a fibre optic or copper local loop circuit.

– depending upon location, a back haul circuit may be used to link a customer to their service provider’s point of presence and then onto the internet gateway. This will take place behind the scenes and may run over a third party’s national network.







The LAPD (Link Access Protocol – Channel D) is a layer 2 protocol

Ipsec (Internet Protocol Security)

SSL (Transport Layer Security)

DTLS ( Datagram Transport Layer Security)

SSH (Secure Shell VPN)

Point-to-Point Protocol (PPP)

Multi-Link Point-to-Point Protocol (ML-PPP)

NetWare connect

Point-to-Point Protocol (PPP)

Multi-Link Point-to-Point Protocol (ML-PPP)

NetWare connect


ITU-T, E.1263,






CCITT Q.920/921


Wide Area Network Standard

Wide Area Network Standard

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(275 words)