What is ISDN?
The early telephone network consisted of a pure analog system that connected users by an interconnection of wires, which was inefficient and prone to breakdown. As a result, telecommunications companies began converting to a packet-switched digital switching system for their internal connections rather than using just plain electrical switching. Keep in mind that the external connections from the Central Office (the Telephone Exchange) to the customers' equipment (homes and offices) are still largely analog lines.   The changing needs of the telecommunications customer for bandwidth on demand at a cost effective price spawned the Integrated Services Digital Network ISDN. It is designed for high speed digital switching integrating the telephone network with voice, data image and video on the same line. ISDN is independent of the Internet and is designed for customers who wish high quality, low latency, controlled connections to sites and branch offices.

BRI-ISDN uses the Plain Old Telephone System (POTS) copper line with two pairs of wires. The line is split up by a process called Inverse Multiplexing producing two Bearer channels for all digital signals i.e. voice, data video and image along with one Delta channel for call setup and user data. Each Bearer channel has a throughput of 64kbps combining to give 128kbps whereas the Delta channel has a throughput of 16kbps. The Bearer channels use only one pair of wires so one phone line can be used efficiently.

PRI-ISDN uses specialized ISDN cable and provides twenty-three Bearer channels adding up to maximum throughput of 1.544 mbps and one Delta channel for call setup and user data.

Benefits: ISDN is most effective for Telephony and Video Conferencing.

Cost Effectiveness:
• ISDN provides the opportunity to pay as you use.
• Calling party pays
• It uses the existing phone line

Efficiency:
• ISDN provides higher data speeds hence downloads will be faster
• All types of data can be integrated on the same line
• Simultaneous use of Bearer channels give rise to simultaneous use of voice and data applications via the one line

Reliability:
• Better stability and higher quality
• Connections are virtually error free, no disruptions
• Much better voice telephone service

Availibility:
In Trinidad and Tobago ISDN services are only offered by one company which is TSTT.
TSTT only offers BRI-ISDN via telephone lines with each line providing a maximum throughput of 128kbps. However if a higher connection speed is required more than one telephone line can be integrated - for example: 4 lines will provide 4 x 128kbps totaling 512kbps.

TSTT’s ISDN service is not available throughout Trinidad and Tobago due to its sensitivity and limitation of 18,000 feet from the exchange office. It is available only in the following regions:
• Region 1: Chaguaramas, Port of Spain, Maraval, Diego Martin West, St. James, St. Clair and parts of Woodbrook.
• Region 2: San Juan, St. Augustine, Arima, Mausica, Piarco, Trincity and Sangre Grande.
• Region 3: Siparia, Point Lisas, San Fernando, Galeota, Penal, Point Fortin, Marabella, Rio Claro, Mayaro and Gasparillo.
• Region 4: Scarborough and Mt. Pleasant.
• Region 5: Chaguanas, Couva andClaxton Bay.

Cost:
The cost of installation is $600.00
With a monthly charge of $485.00 per month per Bearer channel
Each line has 2 Bearer channels, hence $990.00 per month per line.

This paper was prepared by Bharat Anand Seetahal of Zandolie Networking
June 2006

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