1. What is ISDN?
　　 ISDN (Integrated Services Digital Network) is an all digital communication line that allows for the transmission of voice, data, video and graphics, at very high speed, over standard communication lines. ISDN provides a single, common interface with which to access digital communications services that are required by varying devices, while remaining transparent to the user. Due to the large amounts of information that ISDN lines can carry, ISDN applications are revolutionizing the way of businesses communication . ISDN is not restricted to public telephone networks alone; it may be transmitted via packet switched networks, telex, CATV networks, etc.

　　ISDN, or Integrated Services Digital Network, is a suite of internationally-adopted standards for end-to-end digital communication over the public telephone network. As this network has been traditionally oriented towards voice services, ISDN is an important step forward in the adaptation of the network to handle the increasing global demand for computer-to-computer data communications. ISDN brings us closer to the goal of a ubiquitous multi-service network, integrating voice, data, video and image services in a digital format over a common, global network.

　　While ISDN differs fundamentally from conventional telephone service (called Plain Old Telephone Service or POTS), ISDN has been designed to allow end-to-end compatibility for voice services. Voice calls can be made to or from an ISDN line from a POTS line anywhere in the world. Equally important is the fact that ISDN service can be carried over the existing telephone network infrastructure. This infrastructure represents a massive global investment over the past century in central office switches, which route calls and handle billing; in transmission systems which carry the large volume of calls to remote destinations, largely over optical fiber cable today; and millions of miles of twisted-pair copper cabling to carry services to our homes and offices.

　　While ISDN standards are still evolving, and indeed work is continuing on even higher data-rate services such as full-motion video, two well-defined ISDN interface standards are in common use today:

　　Basic Rate Interface (BRI) - The term ISDN or ISDN line is often used synonymously with the Basic Rate Interface. The Basic Rate Interface defines a digital communications line consisting of three independent channels: two Bearer (or B) channels, each at 64 Kilobits per second, and one Data (or D) channel at 16 Kilobits per second. For this reason the ISDN Basic Rate Interface is often referred to as 2B+D.

　　The B channels are used for carrying the digital information, whether computer data, digitized voice, or motion video with appropriate equipment (such as the ISDN NuBus boards for the Macintosh) these B channels can be bonded or linked together to provide an aggregate 128 Kilobits per second data channel. The D channel is used to carry signaling and supervisory information to the network, and can also be used to carry packet-mode data over an X.25 network.

　　Each of the two B channels is treated independently by the network, allowing for simultaneous voice and data, or data only, connections to different locations. With specialized hardware and software, multiple B channel connections can be aggregated to achieve file transfer rates of several Megabytes of data per minute or more.

　　Primary Rate Interface (PRI) - The Primary Rate standard is a higher-level network interface defined at the rate of 1.544 Megabits per second (for North America). This particular rate was selected for compatibility with T1 digital lines commonly used today. The Primary Rate is comprised of 23 B channels, each at 64 Kbps, and one 64 Kbps D channel for signaling. These B channels can interconnect with the Basic Rate Interface, or when carrying voice services to any POTS line.

2. Why Digital Communications?

　　There are a number of reasons why it is advantageous to carry information, particularly computer data, in a digital format.

　　For one, digital lines provide a far cleaner, error-free connection that can ensure reliable transmission worldwide. Secondly, digital lines allow equipment that processes data digitally, such as computers or networking routers, to be directly connected, and without the 4 Kilohertz bandwidth limitations imposed by POTS (voice) telephone lines. An ISDN line can carry data at nearly five times the fastest rate achievable using analog modems over POTS lines.

　　Further, while a POTS line can carry only limited signaling information between the network and the end device (telephone or modem, for example), ISDN lines can carry detailed messages back and forth. This information can be used to define multiple incoming callers, to specify the type of incoming data, or to convey useful diagnostic information.

VOIP networks convert the voice information to packet data and transmit it over an efficient, cost effective IP data network. This IP data network can reach all the way to the customer's phone set or just to their traditional phone system. The VOIP network connects to other VOIP networks as well as the traditional Public Switched Telephone Network (PSTN) in order to connect calls to those who may or may not have VOIP service. VOIP networks are connected either through dedicated circuits or through the Internet. Therefore, a VOIP user in India can talk to a VOIP user in Nashville without ever touching the traditional voice network.

Those same users can also talk to anyone by accessing the PSTN. VOIP ensures that connections are made without the user having to decide which network to traverse.