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130 Chapter 6: Using ISDN and DDR Technologies
Foundation Topics
ISDN refers to a set of digital services that has been available to end users for a number of years.
It involves the digitizing of the telephone network so that carriers can provide end users with
multiple services from a single end-user interface over existing telephone wiring.
ISDN is an effort to standardize subscriber services, user/network interfaces, and network and
internetwork capabilities. The goal of standardizing subscriber services is to give some level of
international compatibility.
Compatibility between International Carrier networks has long been at the forefront of more
than a few heated debates in the global standards committees. Their pain, to a degree, has been
good for the technology. This standardization, as it has evolved, has made reality of the myth
of multivendor interoperability. By no means is it implied that multivendor interoperability is
perfect. It is nowhere near perfect and will never be as long as there are global politics in the
technology fields.
The ISDN community would like to ensure that ISDN networks communicate easily with one
another. ISDN was developed with the idea that it would be used to transport voice calls, data
traffic, and video traffic. The evolution of ISDN as a viable technology moves forward with the
needs of those very different traffic types in mind. ISDN applications include high-speed image
applications, additional telephone lines in homes to serve the telecommuting industry, high-
speed file transfer, and video conferencing. ISDN is also becoming very common in home-
based and small offices as many corporations extend their offices into the residential arena.
POTS Versus ISDN
ISDN is the replacement of traditional analog plain old telephone service (POTS) equipment
and wiring schemes with higher-speed digital equipment. The transition from POTS to ISDN
changes the way connections at the local loop area are processed.
With POTS, a caller would have to dial up the operator and request a call setup. To accomplish
this, the calling party telephone was picked up (that is, went off hook) and a crank was turned
to generate current on the line that would light up an LED on the operator console. The operator
would answer the setup request and begin setting up the call, making a manual connection
between the caller and the called party. The manual connection completed the analog local loop
(that is, the connection between the telco switch and customer devices).
From the local loop, the call typically went through the central office (CO). Once digital
technologies were born and implemented, the operator was replaced with digital facilities,
leaving only the local loop as analog. The transition to ISDN completes the digital link by
replacing the local loop with digital equipment.