There is an ever-present desire to maximize the speed of communications via networks, and particularly telecommunication networks such as public telephone systems. The Public Switched Telephone Network (PSTN) has been upgraded so that it is now almost completely digital. The only analog portion of the telephone network remaining is the “local loop,” the connection between the central office (CO) and the end-user (e.g., a residential telephone subscriber using a telephone set and/or client modem). The analog local loop is the weak link in an otherwise robust system, and it imposes limitations on the speed with which communications can be transmitted over the PSTN, particularly with respect to data transmissions using modems.
Nonetheless, various ways to take advantage of the fact that a large portion of the telecommunications network is digital have been developed. Examples of such developments can be found in U.S. Pat. No. 5,394,437 to Ayanoglu et al., U.S. Pat. No. 5,406,583 to Dagdeviren, and U.S. Pat. No. 5,528,625 to Ayanoglu et al. (each of which are assigned to AT&T/Lucent and all of which are hereby incorporated by reference herein in their entireties).
The International Telecommunication Union Telecommunications Standardization Sector (ITU-T) adopted the V.90/92 standard for the purpose of standardizing Pulse Code Modulation (PCM) type modems, and this standard is utilized today by virtually all modem manufacturers. The V.90/92 standard assumes that one end of the modem session (the ISP/server side) has a digital connection to the phone network and takes advantage of the high speed digital connection.
The V.90/92 standard requires the provision of a probing signal, also known in the art as digital impairment learning or “DIL”. One purpose of the DIL is to give the receiving (analog) modem the opportunity to measure digital network impairments, which includes the digital attenuation or “PAD loss”, which is the digital loss LD across the digital portion of the telephone network. The measurement of LD made by the receiving modem is used by the receiving modem in formulating an appropriate constellation for the transfer of data. The constellation formulated by the receiving modem is transmitted back to the transmitting modem as a CP/CPt sequence as set forth in section 8.5.2 of the V.90 standard, and this constellation is used to set the downstream data signal power. U.S. Pat. No. 6,178,200 to Okunev et al., incorporated herein fully by reference, provides an overview of constellation design and improvements thereof.
Under the V.90/92 standard, in the downstream direction, the server transmitter transmits 8-bit binary numbers (octets) which correspond to a total of 256 (128+ and 128−) μ-law or a-law levels. Each one of the six phases of the constellation tables consists of a subset of the 256 levels. The calculation of average power of the constellation set is defined in section 8.5.2 of the V.90 standard. For any given modem connection, the noise level is fixed and can be measured during the training period prior to the constellation design. The minimum distance of the constellation levels is required to be larger than the fixed noise level. The rule of thumb is that the higher the average power of the constellation set, the higher the data connection speed.
Up until recently, Federal Communication Commission (FCC) rules constrained the maximum power of the encoded analog signal to −12 dBm in the United States (to prevent interference with other electrical devices). FCC rules imply that the power of a signal from the transmitter end to the receiver end must be within −12 dBm measured at any point of the transmitted path. Recently, the rules were changed so that the maximum constrained power for U.S. systems has been increased to −6 dBm, as it already exists in Europe's A-law digital network.
Significant attention has been paid in the prior art to the downstream transmitter path. Modems currently in service interpret the old rules such that the signal measured at the output of the CODEC=s D/A converter is within −12 dBm. Thus, prior art systems can only partially take advantage of the new −6 dBm maximum constrained power by only compensating for the digital loss over the PAD in the CO (Central Office). For example, if the PAD loss is 6 dBm, the constellation will be designed with the average power not larger than −6 dBm. Thus, the average power of the downstream signal measured at the output of the CODEC=s D/A at the CO is less than −12 dBm. If the PAD loss is 0 dBm, the average constellation power will be less than −12 dBm. Even in countries where the digital network is A-law, which implies 0 dBm PAD loss and −6 dBm constrained maximum, the existing modems still use the constellation having a power not larger than −12 dBm. The likely reason for this is to standardize operations so that μ-law and A-law situations are treated in the same way, thereby allowing the same hardware to be used for both systems.
With the changes in the FCC rules, however, there are two factors that are not currently being considered in the constellation design, but which now can be considered and thus result in better constellation design. First, in prior art constellation design, the maximum average power of an analog signal input to the client modem CODEC ADC (A/C converter) is not considered. Every CODEC ADC has a dynamic range (Cmax, Cmin) for the input signal. The V.90/92 downstream signal has a peak-to-average ratio RP/A. The maximum average signal power of the client modem's CODEC ADC, denoted as PADC, is calculated as Cmax−RP/A. As long as the average power level of an analog signal input to the ADC is less than PADC, the larger the average power of an analog signal input to the ADC is, and the better the S/N (signal-to-noise ratio) of the digital signal output from the ADC. When the average power level of an analog signal input to the ADC equal to PADC, the S/N of the digital signal output from the ADC is maximized. Existing client modems today have a PADC equal to −9 dBm, and some are −6 dBm.
Second, the analog attenuation over the analog line from the CO to the decoder (client modem) are not considered in present existing modems. This analog loss varies from 0 dBm to 30 dBm, depending on the particular local loop being used.
Since the existing art only compensates the digital loss (pad loss) in the downstream constellation design, it fails to take full advantage of the −6 dBm maximum constrained power. For example, assume that the PAD loss is 0 dBm and that the analog loss is −3 dBm while the decoder's maximum average signal power, PADC, is −9 dBm. Existing modems will design a constellation with its power less than −12 dBm at the transmitter end. Once the constellation arrives to the decoder as an analog signal input to the modem's ADC, its average power will be −15 dBm while the decoder's ADC actually can take a −9 dBm signal.
Accordingly, it would be desirable to have a modem data constellation design that includes power compensation for analog loss and utilization of maximum dynamic range of a CODEC, in view of the newly changed FCC rules.