ISDN

ISDN, or integrated Service 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 services (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 and ISDN line from a POTS line anywhere in the world. Equally important is the fact that ISDN service can be carried over 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 system, 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.

ISDN standards are still evolving and 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) is the term ISDN is often used synonymously with. The BRI 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 MAC) 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 are treated independently by the network, allowing for simultaneous voice and data or data only, connections to different locations. With specialize 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 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 commonaly used today. The primary rate is comprised of 23 B channels, each at 64Kbps, and one 64kbps D channel for signaling. These B channels can interconnect with the basic rate interface, or when carrying voice services to any POTS line.

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

For on, digital lines provide a far clear, 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 of modem, for example), ISDN lines can carry detailed messages back and fort. This information can be used to define multiple incoming callers, to specify the type of incoming data, or to convey useful diagnostic information.

With digital communications it is finally possible to carry multiple service types simultaneous on the same network. ISDN offers the means to realize universal in-box integrating voice, voice mail, email,fax, and video images from a single application.

ISDN equipment varies for different applications from ISDN telephone sets to videoconferencing units to routers. Synapse deals primarly with ISDN basic rate terminal equipment for establishing wide area network connections to personal computers, workstations and Local area networks. This includes internal ISDN boards for the Macintosh and PC machines, stand-alone ISDN terminal adapters (TA) to connect to non-ISDN devices (such as analog telephones or a computer serial port), and ISDN-capable routers.

An NT1 or Network Termination-1, is required to connect ISDN equipment to an ISDN line. NT1 connects the two-wire line that your telephone company has assigned for your ISDN service. This two-wire interface is referred to as the U interface or U reference point, and is accessible via a modular RJ-11 or RJ-45 jack.

The connection point between the NT1 and terminal equipment is the S/T interface, which defines a four wire line with a separate transmit and receive pair (and additional pairs for powering when required). The S/T interface is accessible through RJ-45 jacks on the NT1 and terminal equipment. Most US-made equipment, such as Ascend and Farallon routers, the 3Com ISDN modems, the Motorola BitSURFR Pro of the Adtran ISU Express, incorporate the NT1 directly, in which case the S/T interface is not accessible. While this approach does have advantages in cost and ease of installation, a separate NT1 permits the connection of more than one piece of equipment to the ISDN line, (such as an ISDN telephone).

Analog traffic, such as voice, Group 3 fax, and modem data must be converted to a digital format before it can be carried over an ISDN line. An adapter known as an ISDN terminal adapter or TA does this conversion. By using a TA, you can connect your present analog telephone, fax machine or modem to an ISDN line.

When using ISDN equipment, which interfaces directly to your computer, you cannot connect end to end with an analog modem or fax machine without special software or firmware that emulates the analog modulated waveforms of modems and fax machines. This emulation capability is available with equipment by several companies. This type of Add-on capability would allow an ISDN TA to connect to another ISDN TA or to a standard V.32/V.42 bis analog modem.

ISDN is a faster more efficient way to carry Voice, fax, computer data, images, and video conferencing over the same network. This suite of standards makes use of the existing global network of twisted pairs and fiber optic cables.