ISDN Defined for the Telemessaging Industry

By T. Clint Hurd

Over the past several years much has been said about ISDN (Integrated Services Digital Network) – the good, the bad and the uninformed. The intent of this article is to focus on ISDN for telemessaging applications.

Simplistically, ISDN is a set of standards which provides a common architecture for the development and deployment of digitally integrated communication services by one of several media – copper pair, fiber optic cable, or microwave.

The key to ISDN is out-of-band signaling which permits the users’ equipment and the network to exchange control and signaling information over a separate channel from the channel carrying user data information or voice.

ISDN BRI (Basic Rate Interface): There are 3 channels, referred to as 2B+D. The 2 “B” channels (bearer channels) may be utilized for voice and the “D” channel carries data (data channel for out-of-band signaling) that monitors the voice channels. In the past, both “B” channels were used collectively for data transmission to connect PCs to the Internet through an Internet Service Provider at 128 kbps. This application is losing popularity with the advent of DSL and cable services. These channels may also be used for video; however, the resolution is marginal.

ISDN PRI (Primary Rate Interface): PRI provides 24 channels, 23 of which may be used for voice and 1 for data (23B+D). As with BRI, all channels may be used collectively for data or video transmission at a rate of 1.544 Mbps. Practically speaking, an ISDN PRI is a T-1 circuit transporting the ISDN protocol by a chosen medium — copper pair, fiber optic cable.

Local Switch Telemessaging Systems: Most telemessaging systems available today utilize their own local switch. Until recently, the majority of these systems functioned with an analog switch, but now a digital switching platform is common. Answer, hold, disconnect, and patching are all performed in the TAS equipment’s local switch.

Currently, printed circuit boards with associated software can be installed in PCs that interface directly with the telephone company’s digital network, providing a digital to digital connection. Using these component circuit boards, T-1, ISDN PRI and ISDN BRI may be interfaced with the local switch of telemessaging systems to provide:

  • A method of transporting DID truck numbers over a digital circuit
  • Elimination of multiple DID trunk cable pairs
  • The speed of digital telephony
  • Caller I.D. over the “D” channel (only with ISDN)
  • Remote (Central Office) Switch Telemessaging Systems

Also on the market are several telemessaging systems that utilize ISDN BRI to perform call handling by utilizing the telephone company’s switch. Answer, hold, disconnect, call transfer, and conference are remotely performed in the central office switch by the telemessaging equipment utilizing the out-of-band “D” channel signaling. All ISDN lines are “two way” (incoming and outgoing calls). Calls can be put on hold and the same line is used for other call handling. Patching, or call transfer, is accomplished in the CO switch; therefore, no TAS lines are required for the connection. Instead of using DID numbers for account indexing, the customer’s telephone number references their account. The telephone company sends the customer’s forwarded telephone number and the caller’s phone number whenever a call is re-directed to the TAS. For this reason, all customers are able to call forward to the same number. Systems having these capabilities generally fall into two categories.

  1. Non-Integrated System: With this equipment, commercially available ISDN telephones are used. An incoming, forwarded call initiates the display of the forwarded telephone number on the set’s LCD screen. Using a separate computer system, digits from the displayed number (customer’s number) are entered to activate the display of customer’s account information. Messages are entered and dispatched.
  2. Integrated System: This system functions somewhat like the Non-Integrated, except that incoming call information (forwarding phone number), customer account data, and message dispatch functions are integrated into one system. Specially designed hardware (with embedded software) and workstation terminals function as versatile ISDN telephones. These “phones” are networked to form a system.

Summary: The great majority of telemessaging systems can support ISDN, usually PRI. The application, using a local switch, provides a transport medium for DID numbers and caller I.D.; however many flexible ISDN features are not utilized. Systems using remote (CO) switching take advantage of many available ISDN-BRI features with lower recurring telephone company charges.

Myths about ISDN: “ISDN is old technology and is being replaced by DSL and cable” DSL and cable are replacing ISDN BRI as a data link for PCs to the Internet. This is not the case when the application involves voice capability.

“Fiber optics are far superior to ISDN” Fiber optic cable is a transport medium for digital transmissions not a protocol for digital data management.

“ISDN – ‘It Still Does Nothing'” This is true, only if no effort is made to investigate the great potential of ISDN digital telephony.

For more information on ISDN call Morgan Comtec, Inc. at 800-239-3949.

[From Connection Magazine – September 2001]

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