John's Corner

ISDN Information

Integrated Services Digital Network or ISDN is a digital service that supports both voice and data communication for use by phone, fax and PC.

ISDN operates much like a standard telephone line, but has far greater speed and capacity for transmitting voice, data, video and image at speeds up to four times faster than most analogue modems.

ISDN allows telephones, faxes and PCs to be linked to the network with a common connection, so all communications are made on the same service.

Example ISDN Connection Diagram

One Basic Rate ISDN service on a standard telephone cable carries two 64Kbps ISDN user (B) channels plus a 16Kbps control (D) channel (often called a 2B+D service). This means that up to 128 Kbps can be sent and received at one time. Faster speeds can be achieved if more ISDN lines are combined.

In Australia, Telstra operates ISDN services known as OnRamp. These services are based on globally accepted European ETSI standards and are are compatible with a wide range of hardware.

OnRamp services are offered in a number of channel combinations. The basic service is OnRamp2 which allows 2x64Kbps user channels. Higher capacity Primary Rate services known as OnRamp10, OnRamp20 and OnRamp30 offer 10, 20 and 30 user channels respectively.

OnRamp services are priced in a variety of ways designed to suit different business requirements. As ISDN is fundamentally a dial up service, most of the pricing schemes relate to how often a customer expects to make a call, and how long they expect to remain connected.

In order to cater for customers that require a "permanent" connection, OnRamp Xpress is offered which fixes the annual call charges. A customer dials up and remains connected until something abnormal like a power fail or network dropout occurs.

S/T Interface Connections

The S/T interface uses an RJ-45 connector. The maximum length of an S/T bus is 1000 metres. The RJ-45 pin connector assignments for the S/T interface are shown in Table 1.

Table 1. RJ-45 Pin Connector Assignments

U Interface Connections

The U interface also uses an RJ-45 connector. The connector pin assignments for the U interface are shown in Table 2.

Table 2: U-Interface Pin Assignments