Pulse or digital communications – Synchronizers
Reexamination Certificate
1998-05-23
2001-07-24
Chin, Stephen (Department: 2634)
Pulse or digital communications
Synchronizers
C370S523000, C375S233000
Reexamination Certificate
active
06266382
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to communications systems and methods, and more particularly to a communications system and method in which robbed bit signaling is detected and treated in data communications.
BACKGROUND OF THE INVENTION
It is well known that a public switched telephone network (PSTN) comprising T1 facilities can form a basis for a virtual digital network providing 64 kb/s channels. For example, by synchronizing a pulse code modulation (PCM) modem to an 8 kHz sampling rate provided in a central office and using 8-bit PCM words for data transmission, the modem can theoretically achieve a data rate up to 64 kb/s.
However, in practice, due to power constraints and such channel impairments as echo and intersymbol interference, the highest data rate achievable by the PCM modem is about 56 kb/s. This rate may be further reduced as the central office periodically “robs” the least significant bit (LSB) of the PCM words and substitutes it with a signaling bit. As is well known, the robbed bit signaling is necessary for indicating call statuses to effect call administration in the PSTN. While the robbed bit substitution does not cause significant distortion in voice communications, it causes significant degradation in the data communications because of the loss of transmitted bits occasioned thereby. In addition, the robbed bit substitution undermines proper training of an equalizer in the PCM modem, thereby adversely affecting channel equalization for the data communications.
The above problem caused by robbed bit signaling can be fully overcome using the technique disclosed in the co-pending, commonly assigned United States patent application of Y. Lai entitled “Technique for Effectively Treating Robbed Bit Signaling in Data Communications,” Ser. No. 08/962,516, filed Oct. 31, 1997. Specifically, the disclosed technique can be used to detect two types of robbed bit signaling, namely, type A and type B robbed bit signaling, affecting data communications received by a PCM modem. The type(s) of the robbed bit signaling is identified during training of an equalizer in the modem, and the robbed bit signaling of the identified type is taken into account to properly train the equalizer. The robbed bit signaling type identification involves transmission of a four-level training signal through the PSTN to the modem. The signal thus received by the modem is processed by training circuitry therein which includes a level adapter. Based on the received signal, the level adapter utilizes a first mechanism to determine presence of any type A robbed bit signaling, followed by a second, different mechanism to determine presence of any type B robbed bit signaling.
SUMMARY OF THE INVENTION
In accordance with the invention, the usefulness of the above-described technique for detecting and treating the robbed bit signaling is enhanced by implementing in the level adaptor only one mechanism, as opposed to two different mechanisms as used in that technique, to determine the presence of any type A and/or type B robbed bit signaling, thereby improving the efficiency of the level adaptor. Specifically, during the training of the equalizer in a PCM modem, in accordance with the invention, first and second training signals are transmitted to the PCM modem in first and second predetermined intervals, respectively. Such first and second training signals are subject to corruption by the type A and/or type B robbed bit signaling, and each are, for example, a two-level signal. However, the training signals differ from each other in their actual signal levels. Using the same mechanism in the level adapter, the type A robbed bit signaling is identified based on a first effect thereof on the first training signal while the type B robbed bit signaling is identified based on a second effect thereof on the second training signal.
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Agere Systems Guardian Corp.
Chin Stephen
Ghayour Mohammad
LandOfFree
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