Training of level learning modems

DETAILED DESCRIPTION FIG.
1 shows, at the left, an analog modem 101 connected to the public switched digital network (PSTN)150 by a local loop 110.
The current PSTN in the United States is largely a digital network in which transmission from one central office to another is digital.
Internet service providers and others may connect to the PSTN through digital links such as the well-known T-1 system which directly accommodates digital modems.
To transmit data over the local loop, the client modem 101 must include a digital to analog (DAC) converter 102 to send data towards ("upstream") the PSTN and an analog to digital (ADC) converter 104 to receive data ("downstream") from the PSTN.
For simplicity, the digital to analog functions are shown as being performed by a separate DAC 102 and the analog-to-digital functions are shown as being performed by a separate ADC 104.
However, it is well-known that such functions, as well as equalization to minimize intersymbol interference ("ISI") may be incorporated in a digital signal processor DSP 100 within the client modem.
A hybrid 103 is provided in modem 101 to accommodate two-way transmission over the conductors of the local loop.
At the network interface 120, a similar hybrid 121 is provided.
Network interface 120 includes an ADC 122 and DAC 123 which convert between the analog signals used on the local loop and either the mu-law or A-law signal compression used in the public switched digital network.
The mu-law algorithm includes 255 discrete signal conversion values which are more closely spaced together for small analog signal values and spread further apart for large signal values, while A-law uses 256 values.
Each quantization interval is defined by lower and upper analog voltage thresholds, the upper threshold for an interval being the lower threshold for the next highest quantization interval.
When an analog signal transmitted by the client modem over the local loop is received by the ADC 122 in network interface 120, the signal is converted to a unique symbol at a quantization level corresponding to the analog signal value