Integrated services refers to ISDN's ability to deliver at minimum two simultaneous connections, in any combination of data, voice, video, and fax, over a single line. Multiple devices can be attached to the line, and used as needed. That means an ISDN line can take care of most people's complete communications needs (apart from broadband Internet access and entertainment television) at a much higher transmission rate, without forcing the purchase of multiple analog phone lines. It also refers to integrated switching and transmission in that telephone switching and carrier wave transmission are integrated rather than separate as in earlier technology.
In Telco's - ISDN line is two channel digital line with ISDN protocol build in, which can be used to access Internet at upto 128Kbps. With proper equipment, It can also be split into two phone lines to make or receive phone calls.
The ISDN was first defined in 1988. Prior to ISDN, the telephone system was viewed as a way to transport voice, with some special services available for data. The key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. The ISDN standards define several kinds of access interfaces, such as Basic Rate Interface (BRI), Primary Rate Interface (PRI), Narrowband ISDN (N-ISDN), and Broadband ISDN (B-ISDN).
ISDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kilobit/s. In some countries, ISDN found major market application for Internet access, in which ISDN typically provides a maximum of 128 kbit/s bandwidth in both upstream and downstream directions. Channel bonding can achieve a greater data rate; typically the ISDN B-channels of three or four BRIs (six to eight 64 kbit/s channels) are bonded.
ISDN is employed as the network, data-link and physical layers in the context of the OSI model, or could be considered a suite of digital services existing on layers 1, 2, and 3 of the OSI model. In common use, ISDN is often limited to usage to Q.931 and related protocols, which are a set of signaling protocols establishing and breaking circuit-switched connections, and for advanced calling features for the user. They were introduced in 1986.
In a video-conference, ISDN provides simultaneous voice, video, and text transmission between individual desktop videoconferencing systems and group (room) videoconferencing systems.
ISDN is used heavily by the broadcast industry as a reliable way of
switching low-latency, high-quality, long-distance audio circuits. In
conjunction with an appropriate codec using MPEG or various
manufacturers' proprietary algorithms, an ISDN BRI can be used to send
stereo bi-directional audio coded at 128 kbit/s with 20 Hz – 20 kHz
audio bandwidth, although commonly the G.722 algorithm is used with a
single 64 kbit/s B channel to send much lower latency mono audio at the
expense of audio quality. Where very high quality audio is required
multiple ISDN BRIs can be used in parallel to provide a higher bandwidth
circuit switched connection. BBC Radio 3 commonly makes use of three
ISDN BRIs to carry 320 kbit/s audio stream for live outside broadcasts.
ISDN BRI services are used to link remote studios, sports grounds and
outside broadcasts into the main broadcast studio. ISDN via satellite is
used by field reporters around the world. It is also common to use ISDN
for the return audio links to remote satellite broadcast vehicles.
In many countries, such as the UK and Australia, ISDN has displaced the older technology of equalized analogue landlines, with these circuits being phased out by telecommunications providers. Use of IP-based streaming codecs such as Comrex ACCESS and ipDTL is becoming more widespread in the broadcast sector, using broadband internet to connect remote studios.
There is a second viewpoint: that of the telephone industry, where ISDN is a core technology. A telephone network can be thought of as a collection of wires strung between switching systems. The common electrical specification for the signals on these wires is T1 or E1. Between telephone company switches, the signaling is performed via SS7. Normally, a PBX is connected via a T1 with robbed bit signaling to indicate on-hook or off-hook conditions and MF and DTMF tones to encode the destination number. ISDN is much better because messages can be sent much more quickly than by trying to encode numbers as long (100 ms per digit) tone sequences. This results in faster call setup times. Also, a greater number of features are available and fraud is reduced.
ISDN is also used as a smart-network technology intended to add new services to the public switched telephone network (PSTN) by giving users direct access to end-to-end circuit-switched digital services and as a backup or fail-safe circuit solution for critical use data circuits.
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