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SYSTEM DESCRIPTION

The ISDN subscriber loop represents the conversion of the

``last mile'' of telephone cable from analog signaling to digital

signaling. Today's homes and offices typically receive

their phone service from a Central Office (CO) Switch. This

telephone network provides voice services, but cannot provide

similar data services without the use of high speed modems.

ISDN digitizes all of the information passed on the

subscriber loop, enabling new services and features, such

as Automatic Number Identification, Automatic Call Distribution,

and Computer Screen Sharing and File Transfer to

take place simultaneously with voice transmission. As ISDN

is an international standard, users will have access to the

same features and capabilities worldwide.

In order to digitize the subscriber loop new equipment must

be installed. The analog CO linecards are replaced, and a

new Network Termination, called an NT-1, is installed at the

customer premises. These two pieces of equipment enable

full duplex 160 Kbps transmission over the existing twisted

pair copper lines.

Until recently the availability of advanced silicon solutions

and the establishment of tariffs has impeded the implementation

of ISDN. Advances in mixed mode analog/digital circuitry

has enabled semiconductor manufacturers to introduce

single chip U Interface solutions. The U Interface

transceiver is placed on either end of the subscriber loop,

one in the linecard and one in the NT-1. Multiple U Interface

transceivers may be used in repeaters or regenerators between

the CO and the NT-1. Sophisticated echo cancelling

techniques are used to achieve the high transmission and

low error rates required by the ISDN specification.