Integrated Services Digital Network

Integrated Services Digital Network (ISDN) is a set of communications standards allowing a single wire or to carry voice, digital network services and video. ISDN was intended to replace legacy telephone systems. It has since been superseded by other broadband protocols, such as digital subscriber lines (DSL).

History
ISDN was first published as one of the 1984 "Red Book" recommendations. The 1988 "Blue Book" recommendations added many new features. ISDN uses mostly existing (PSTN) switches and wiring, upgraded so that the basic "call" is a 64 kilobits per second, all-digital end-to-end channel. and modes are also provided in some places.

Deployment
Apple's first ISDN card was introduced in 1991 for Macintosh II models with NuBus slots. The Apple ISDN NB Card contained its own Motorola 68000 processor, 512KB of memory, and cost $1,100. It was compatible with AT&T's 5ESS and 's DMS-100 telephone exchanges. However, it was discontinued in the following year due to low adoption. / later released a Planet ISDN NuBus card, but at a list price of $2.995. In 1996, Farallon introduced its Netopia ISDN Modem as an external device at far lower prices.

ISDN was offered by local telephone companies in Australia, France, Japan and Singapore, with limited availability in areas of the United Kingdom and United States. In March 1994, ISDN was deployed in Germany under a specification called. launched Numeris, which used a non-standard protocol called VN4 (the 4th version), though with high pricing. Belgium launched a V1 experimental system. The European nations worked to unify these standards under.

Specifications
There are different kinds of ISDN connections of varying bandwidth (see DS level): DS0 =    1 channel  PCM at   0.064 Mbps T1 or DS1  =   24 channels PCM at   1.54  Mbps T1C or DS1C =  48 channels PCM at   3.15  Mbps T2 or DS2  =   96 channels PCM at   6.31  Mbps T3 or DS3  =  672 channels PCM at  44.736 Mbps T4 or DS4  = 4032 channels PCM at 274.1   Mbps

Each channel is equivalent to one voice channel, with DS0 as the lowest level of the circuit. However, 64 kbps was never referred to as "T0". T1C, T2 and T4 were less commonly used, except for T2 over microwave links.

A (BRI) is two 64K "bearer" channels and a single "delta" channel ("2B+D"). A (PRI) in North America and Japan consists of 24 channels, usually 23 B + 1 D channel with the same physical interface as T1. Elsewhere, the PRI usually has 30 B + 1 D channel and an interface. A (TA) can be used to connect ISDN channels to existing interfaces such as  and.

Different services may be requested by specifying different values in the "Bearer Capability" field in the call setup message. One ISDN service is "telephony" (i.e. voice), which can be provided using less than the full 64 kbps bandwidth (64 kbps would provide for 8192 eight-bit samples per second) but will require the same special processing or as ordinary PSTN calls. Data calls have a Bearer Capability of "64 kbps unrestricted".