ISDN stands for Integrated Service Digital Network, but it also stands for reliability and constant connectivity. Using your phone line, ISDN transfers data much faster than analogue technology.

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ISDN Overview

Welcome to ISDN (Integrated Service Digital Network) - one of the most extensively used digital communication tools in the world today. ISDN provides digital connectivity; enabling customers to establish digital end-to-end connections through the Public Switched Telephone Network (PSTN).

This digital dial-up service enables users to transmit information such as voice, data, video and images over a telephone line. Communicating with digital, instead of traditional analogue technology, results in higher speed, capacity and productivity, as well as crystal clear voice and sound quality.

What are the features of ISDN?

Telkom offers three ISDN products

ISDN2, ISDN2a and ISDN30

They are classified as:

  • Basic Rate Access (BRA) - ISDN2 & ISDN2a or,
  • Primary Rate Access (PRA) - ISDN30.

Basic Rate Access (BRA) - ISDN2 & ISDN2a

The ISDN BRA service (ISDN2 & ISDN2a) allows different information such as telephone calls, Internet access, video conferencing and faxes to be transmitted with digital technology that is more advanced than traditional analogue lines.

Two B Channels are available and can be used simultaneously and independently, for example; you may use the Internet, receive a phone call, send or receive a fax - any two at the same time. The speed of each B Channel is 64 kbps and may be bonded to provide the speed of 128 kbps.

Supplementary services provide additional functions to your existing ISDN2 BRA service. One such service is Multiple Subscriber Numbers, (MSN), and allows up to 4 additional telephone numbers.

A network terminator (NT) unit will be installed at your premises. Multiple devices may be simultaneously connected to the network terminator, including telephones, personal computers or fax machines

The only difference between ISDN2 and an ISDN2a service is the NT unit. A NT1 is installed for ISDN2 service and consist of two digital ports; therefore only digital equipment may be used (unless a Terminal Adaptor (TA) is purchased to allow connectivity to analogue equipment). For and ISDN2a service, a NT2 is installed and consist of two digital and two analogue ports (digital as well as analogue equipment will function on the ISDN 2a service).

ISDN2/2a as classified under Basic Rate Access

Basic Rate Access enables the user to transmit up to 128 kbps on a standard twisted copper pair. This bandwidth is divided into three logical channels, namely two independent B Channels of 64 kbps each and one signalling D Channel of 16 kbps. BRA can also be referred to as 2B + D. The B Channels are simultaneously available for voice, data, text or image calls, or any combination of these. These B Channel calls are seen as normal, independent telephone calls to separate destinations. The D Channel, on the other hand, will set up and control these B Channel calls. Therefore, a Basic Rate Access will allow up to two terminals to communicate simultaneously, using the two B Channels.

With only a digital line Terminating Card in the exchange and a Network Terminator (NT) on the customer's premises, Basic Rate Access of 128 kbps can be provided over the standard twisted copper pair used for standard telephone connections. This interface (twisted copper pair) is specified by ITU-T and is commonly referred to as the U bus.

On the customer's side of the NT, ITU-T has specified another interface, called the So bus. Customer Premises Equipment (CPE) may be connected to the S bus of an ISDN BRA. CPE can be categorised into two main functional groups namely ISDN S bus compatible CPE and non-compatible CPE. Compatible CPE is equipment which can be connected directly to the S bus, and which interfaces with the ISDN signalling protocol (e.g. ISDN PC-cards, ISDN digital telephone). Non-compatible CPE is equipment that cannot be connected to the S bus and which needs a mediation device to convert its protocol to an ISDN signalling protocol (e.g. V24, X25, analogue telephones and facsimile). This mediation device is called a Terminal Adapter (TA) and is applicable to ISDN2.

Basic Rate Access Network Terminator (NT)

A network terminator may consist of two functional components (NT1 and NT2). The NT1 has two digital ports for digital equipment. Should customers require analogue equipment to function a terminal adapter needs to be purchased.

The NT2a has two analogue and two digital ports, and both analogue and digital equipment can function. The network terminator form part of the ISDN basic rate access, and is supplied by Telkom.

Primary Rate Access (PRA) - ISDN30

The ISDN30 service offers integrated voice, data, video and image transmission, or any combination thereof, with vastly increased quality and data speeds due to digital transmission.

The service provides 30 independent B Channels of 64 kbps and one D Channel of 64 kbps. B Channels are used to carry virtually any kind of digital information. The B Channel calls are seen as normal, independent telephone calls to separate destinations. The D Channel will set up and control these B Channel calls.

Telkom ISDN30 usually terminates on medium to large PBXs with integrated ISDN support, Multiplexers or independent Primary Rate network terminators. An ISDN PBX will then provide Basic Rate Access extensions. When using Telkom ISDN30 for voice applications each Primary link from each PBX can support 30 simultaneous calls and these calls can be to 30 different locations.

By connecting Telkom ISDN30 to an ISDN multiplexer several types of equipment such as PBXs, Routers and Codecs can be connected to the multiplexer. The multiplexers will allow for n x 64 kbps bandwidth-on-demand applications such as video conferencing and LAN interconnection.

ISDN30 as classified under Primary Rate Access

The Primary Rate Access has a bandwidth of up to 2 mbps over balanced symmetrical pairs of copper cable or a coaxial cable, optic fiber cable, digital microwave or radio links (G.703 interface), 120 (impedance) and provides 30 independent logical B Channels of 64 kbps and one D Channel of 64 kbps (30B + D). PRA is used to interconnect PABXs with ISDN capabilities, routers or PRA network terminators with the digital exchange. An ISDN PABX will then either provide BRA or analogue extensions.


What is the price of ISDN and how is it Billed?

When applying for ISDN, you will pay the following type of charges.

Once- off Installation OR conversion charge

Should you have no line, an installation fee will be charged. When you have a normal exchange line, and want to convert to ISDN, a conversion charge is applicable.

Monthly rentals

Separate monthly rental charges apply for Residential Customers, and Business Customers. Monthly rental includes free maintenance.


ISDN usage is billed at the same call rates as for normal analogue lines. ISDN is a dial-up service, and you will be charged for actual time spent on the B Channels.

Installation of temporary ISDN services

You may require your ISDN services to be connected only for a certain time period. In this instance you may view the following tariffs applicable.


Tariff List

For a detailed breakdown of all ISDN tariffs, please click here.

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