ISDN: Integrated Services Digital Network

By Rose Anne Hurd

Integrated Services Digital Network are some of those “buzz” words used in out industry for years. An unsuccessful search for ISDN information as it relates to the telemessaging industry has prompted this interpretation of the telephone company’s jargon into a layman’s primer.

ISDN Defined: Southern Bell defines ISDN as “a revolutionary communications architecture that integrates voice, data and video transmission into a single access line. It enables a single telephone line to carry voice, data and video information simultaneously.”

Those of us at the elementary level of digital communications may relate to Gary Kessler’s explanation from his book, ISDN, Second Edition: “ISDN is merely an enhancement to the telephone local loop that will allow both voice and data to be carried over the same twisted pair. It is a fully digital network, where all devices and applications present themselves in a digital form. That is, information from the telephone, personal computer, stereo, television, PBX, main frame and ISDN coffee pot are all seen as bit streams by the network switch. “1

Analog Defined: It might be helpful to review analog (sometimes referred to as broadband) voice transmission and digital transmission (also called base band.) An excellent description of analog is found in an article, “The ABC’s of ISDN” written by Lea Meadows, a staff manager for Bell South Telecommunications: “In the beginning, there was analog. Alexander Graham Bell created a means for transmitting voice signals between separate locations. The three parts of transmitting this voice signal were the transmitter, which converts sound waves to a continuous electrical signal: a transmission path, consisting of a pair of wires (or cable pair) to carry the signal to its destination; and a receiver that turned the electrical signal back into sound. A pair of wires for every telephone and a telephone for every pair of wires.”2

Digital Transmission: To carry human voice in a digital form, the analog wave is transmitted into an equivalent digital mode. PCM measures the amplitude of each voice sample as an 8 bit-byte. If each 8 bit-byte requires 8000 samples per second, then it takes 64,000 bits per second to digitize the human voice. When the digital code reaches its destination, a reverse process produces analog voice sound.

Your PC talks in “bits” and to communicate with another PC, the use of a modem is required for the conversion to an analog signal. ISDN will bring the digital signal to your (digital) equipment including your PC, eliminating the need for a modem.

Telephone central offices have utilized digital switching (both voice and signaling information is transmitted in digital form) between central offices for 25 years and yet the signal between the customer and the C/O has remained analog – until recently. The major motivating factor for the telephone company’s accelerated use of digital transmission is economy but equally important is customer’s demand for higher data a transmission speeds. With ISDN, data is transmitted at 64 kilobits per second, while today’s modems transmit data at much slower speed.

ISDN Deployment: End users might wonder why it has taken so long for this digital technology to be available. Divestiture of the “Bells” and the lack of national and international standards are the major reasons. The Consultative Committee of the International Telephone & Telegraph (CCITT) was developed in 1984 to formulate the standards for ISDN for all telephone providers. Then CPE (Customer Premise Equipment) vendors wanted the telcos to commit to wider deployment and the telcos wanted to see more ISDN products before commitment. TRIP 92 (Transcontinental ISDN Project) launched national ISDN and brought about commitments for ISDN products from big name computer companies. The wide spread usage of PC’s has been a major catalyst for the RBOCs current “push” for ISDN implementation. Tariffs have been filed with the Public Utilities Commission to offer “ISDN anywhere”. This would be achieved in a metro area used FX (foreign exchange) from an ISDN-capable C/O switch to the C/O without ISDN capabilities.

ISDN’S Potential: Mr. Ed Klingman, president of ISDN* tek, was quoted in a “Technology Update” story in Communications Week May 16, 1994: “The public policy push for development of the ‘Information Superhighway’ includes ISDN as an-and in many cases as the – access mechanism. Groups as diverse as AT&T, the Electronic Frontier Foundation and the Consumers Federation of America have all endorsed ISDN as the appropriate technology to provide end-user connectivity to the Superhighway.” 3

In a recent article in America’s Network, Dianne Hammer sited Microsoft and IBM as major factors in the re-newed push for digital services. Microsoft will use ISDN lines for 2000 employees to access their corporate network from remote work-at-home offices. IBM has developed a digital modem with 64 kbps speed.

ISDN was utilized for telecommuting after a major earth quake in California. The RBOC was permitted by the PUC, to install “free” ISDN and to lend terminal adapters and phones to people in the area. Employees who were unable to reach work by the normal highway were able to work via a superhighway.

ISDN Interfaces: There are two ISDN access interfaces to the network: Basic and primary rate. Basic Rate ISDN (BRI), uses a single telephone line, comprises of 2 B-channels with transmission speeds of 64 kbps and D-channel at 16 kbps. Primary rate ISDN (PRI) is generally 23B-channels and a single D-channel with all 24 channels operating at 64 kbps.

The “B” (bearer) channels are used to transmit voice or data and the “D” (delta) channel sends signaling information to control the B channels.

Each ISDN BRI line required a Network Termination Type 1 (NT-1),” ISDN device responsible for the termination of the ISDN transmission facility at the customer’s premise.” 4 NT-1’s can be purchased from vendors or your telephone company for $300-$400.

ISDN vs. DID’s: ISDN can replace DID trunks and numbers! A limited number of hardwire accounts can also be supported using a directory number for dedicated assignment of the customer’s line. In average-sized bureaus, each work station would require one B-channel (primary purpose is for voice and data transmission) with the D-channel (the network and user’s equipment exchange signaling messages necessary to request services on the B-channel) for typical messaging tasks. The second B-channel can be added when needed.

ISDN BRI replaces DID trunks, numbers and in some cases, business or patching lines. ISDN eliminates DID and Hardwire hardware: digital switches, DID trunks and business line cards. Unlike DID incoming-only trunks, ISDN lines are used for incoming calls and out dialing, making ISDN more cost effective than DID services. Conferencing or call transfer (patching) is connected at the central office.

In an ISDN telemessaging system, clients would all forward their line to a main directory number (DN) of the telemessaging service bureau. Then when the client’s phone number (DN) was dialed, the call would forward to the Service’s main DN. Additional information delivered with the call, would be cross-referenced to that client’s account file. This client’s DN identification is called the “redirecting number”. Other call management capabilities, currently available only in the central office, will be brought under the user’s control. The reason for call redirection is a feature of interest to messaging services. At present, there are five possible reasons that can be provided for handling the call: call forward busy, no reply, Data Terminal Equipment (DTE) out of order, call forward unconditional and call forward by the called DTE.

In the Southeast, a comparison of the installation charges for ISDN and DID surprisingly confirms that ISDN costs less to install than DID trunks and numbers! Is this our telephone company’s strategy for steering users away from old technology to the new? Monthly costs are competitive (old technology versus new services and features are not easily compared) and the cost of NT-1’s is expected to decline with the upsurge of new ISDN products.

Future of ISDN: Although the US viewed ISDN with skepticism initially, it has taken a leading role in developing standards, applications, products and services. Many in the telecommunications industry predict that more applications and services will be developed for the use of the ISDN. Will our industry play a role in these predictions? I think so!

Rose Anne Hurd and husband, Clint have been co-owners of a telemessaging service and RCC paging company for 13 years. The Hurd’s are involved with Morgan Comtec in the manufacturing of ISDN telemessaging equipment.

[From Connection Magazine, July 1994]

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