Abstract: A system and method are provided for real time subscriber billing in a standard network routing path. Account information is stored for at least one subscriber. A determination is made, based on the account information, whether the subscriber has a sufficient balance for a desired service. Service is authorized or denied to the subscriber based on the determination.

Abstract: A data modem system incorporates, digital filters in transmitter and receiver units having intermediate frequency stages which operate at a frequency which is an integral multiple of the modulation symbol rate. This enables filtering, modulation and demodulation to be carried out in a relatively simple manner using digital hardware, with conversion between different forms of modulation such as QPSK, BPSK and offset BPSK being possible without substantial hardware modifications. In one embodiment of the invention, the receiver employs a phase estimating circuit to compensate for phase and frequency errors arising when a free-running local oscillator is used as an alternative to one controlled via a phase-locked loop.

Abstract: A data stream is impressed on a carrier by forming from the data stream a succession of binary symbol words which control the modulation level of the carrier during respective modulation time intervals. To generate the symbol words from the data stream, the latter is first divided into a base bit stream (14) and one or more second bit streams (15). These second streams (15) are then subjected to error control coding before being used to define the least significant bits of the symbol words; the most significant bits are provided by the base bit stream. As a result, although the least significant bits of the transmitted symbol words (IV,III,II,I) are more prone to noise corruption than the most significant bits this tendency is compensated for by the error coding employed. The overall effect is to minimize power requirements while retaining good bandwidth efficiency.