Codec Supporting PCM Modem Communications Over a Universal Digital Loop Carrier

1. Field of the Invention

The present invention relates generally to the field of telephony. More specifically, the present invention discloses a codec for supporting PCM modem communications and conventional voice communications over a universal digital loop carrier.

2. Statement of the Problem

Various types of PCM modem communications have gained wide acceptance in recent years in response to the limited data transfer rates that are possible with analog modems. For example, the V.90 protocol adopted by the International Telecommunications Union (ITU) provides a specification for downstream modem data transfer rates up to 56 Kbps over the standard public switched telephone network (PSTN). It should be noted that the terms "public switched telephone network" or "PSTN" are used herein as these terms are commonly used in the United States to refer generally to the digital network between telephone company central offices. V.90, X2, K56flex, and other PCM modem systems overcome the limitations imposed on previous analog modems by exploiting the digital connection that most data service providers and internet service providers (ISPs) use at their end to connect to the PSTN.

The PSTN was designed for voice communications. By artificially limiting the sound spectrum to just those frequencies relevant to human speech understanding, the required bandwidth for each voice channel could be reduced, thereby increasing the number of voice channels that can be handled simultaneously when multiplexing is used. While this works well for voice communications, it imposes limits on data communications. In particular, existing V.34 modems that use conventional phase and amplitude modulation are optimized for the situation where both ends connect by analog lines to the PSTN, and are based on the assumption that both ends of the connection suffer impairment due to quantization noise introduced by analog-to-digital converters (ADCs).

The ADC samples the incoming analog waveform 8,000 times per second and outputs a pulse code modulation (PCM) code for each sample. The sampling system uses 256 discrete 8-bit PCM codes. The PCM codes sent across the PSTN can only approximate the original analog waveform because the sample values must be one of the 256 discrete codes. The amplitude difference between the original waveform and the reconstructed quantized waveform is called quantization noise.

Quantization noise and other impairments limit V.34 communications to about 35 Kbps. However, quantization noise affects only analog-to-digital conversion, not digital-to-analog conversion, which results in the principal advantage of the PCM modem protocols. FIG. 1 provides a diagram of a typical V.90 communications configuration. The high speed data direction for V.90 modems is from the central server modem 11 to the subscriber modem 14 (i.e., left to right in FIG. 1). The server modem 11 could be a gateway to an internet service provider (ISP) as this example shows. If there are no analog-to-digital conversions between the V.90 digital modem 11 and the PSTN 12, and since the V.90 digital modem uses only the 256 discrete PCM codes available on the digital portion of the phone network, then this exact digital information reaches the subscriber's CO line interface 13, and can be correctly decoded by the subscriber's analog modem 14, without information being lost in the conversion process. The encoding process in the server modem 11 uses only the 256 PCM codes (or a subset) used in the digital portion of the phone network. This avoids the quantization noise that would be associated with an analog-to-digital conversion. The PCM codes are converted at the local telephone company's central office (CO) 13 to an analog signal that is sent to the subscriber's analog modem via a copper pair. The subscriber's analog modem 14 reconstructs the PCM codes from the analog signals it receives, decoding what the ISP 11 sent with no resulting loss of information.

Eight bit PCM codes are transmitted by ISP's V.90 digital modem 11 at the rate of the 8000 codes per second, so that the ISP modem's symbol rate is equal the phone network's sample rate. At the subscriber's end, the V.90 analog modem 14 must discriminate among the 256 analog voltages to recover the PCM codes (symbols). If this could be done at a rate of 8000 codes per second, the resulting download speed would be 64 Kbps (8000.times.8 bits per code). However, this is not often possible. Even though the quantization noise problem is removed, other noise sources exist. These arise principally from thermal noise, switching noise, and cross-talk. In addition, network DACs are not linear, but follow a non-linear conversion rule (.mu.-law in North America and A-law elsewhere) with very small step sizes near zero. These problems can make it impractical to use all 256 discrete codes, because the corresponding DAC output voltage levels near zero are too closely spaced to accurately represent data on an analog loop having a degree of noise. Therefore, the V.90 encoder uses various subsets of the 256 codes to eliminate detection errors between DAC output signals most susceptible to noise. Furthermore, the output of the DAC is band limited to about 3500 Hz by a reconstruction filter and connecting circuits that reduce the channel capacity (twice the available bandwidth) to approximately 7000 symbols per second. The limited bandwidth makes the symbols more difficult to distinguish from one another, a problem known as inter-symbol interference, or ISI. Therefore, the V.90 downstream modulator interleaves symbols periodically that contain lower information content, but serve to mitigate ISI.

The V.90 protocol is asymmetrical. As previously discussed, the V.90 protocol is capable of downstream data rates of 56 Kbps because little information is lost in the digital-to-analog conversion. However, upstream communications go through an analog-to-digital conversion in the telephone company central office line interface, which under V.90 does not have the ability to support PCM symbols and thus limits the channel to lower data rates. For this reason, V.90 modems limit upstream communications to V.34 speeds.

Communications from the local telephone central office (CO) to the subscriber's premises have traditionally been carried over a pair of copper wires using analog signals. This is commonly referred to as an "analog loop." In recent years, several types of situations have arisen that require extending digital communications beyond the central office. For example, where a group of subscribers 26-29 are geographically close to one another but relatively remote from the central office, the local exchange carrier may extend a digital carrier or a digital subscriber line (DSL) 25 from the central office to a remote terminal 24 in the subscriber's area, as illustrated in FIG. 2. Communications over the digital facility are handled using any of a number of standard digital protocols (e.g., T1 or T3). Analog loops are then extended from the remote terminal 24 to each of the subscribers 26-29 in the carrier service area. When dealing with a large number of subscribers (e.g., up to 1000 or more), this type of system is commonly known as a digital loop carrier or DLC. Smaller configurations are sometimes referred to as a "mini DLC."

The universal DLC (or UDLC) shown in FIG. 2 works well for voice communications and is satisfactory for data communications at V.34 speeds or lower. But, this type of DLC is incapable of V.90 communications due to the analog or "universal" interface between the central office switch 21 and the central office terminal 22 (COT) at the headend of the digital facility. As previously discussed, this analog interface requires an analog-to-digital conversion at the CO terminal 22 resulting in quantization noise. The analog interface is largely the result of the historical design for CO switches, that were based on directly providing analog loops to individual subscribers 23. Many newer CO switches are capable of providing a digital interface for a DLC, but many existing switches are not, and would require expensive retrofitting or replacement.

Another related situation arises when a subscriber wants to add a second or third telephone line to accommodate a fax or modem. The local exchange carrier has a limited number of wire pairs in each geographic area and installing new wires is often very expensive. To address this problem, a subscriber line multiplexer, universal digital carrier (UDC), or digital added main line, can be used to provide additional lines over a single pair of wires as depicted in FIG. 3. Here again, a digital facility, in this case a DSL 35, is extended from the central office to a remote terminal (RT) 34 typically located on the subscriber's premises. Several analog loops extend from the RT 34 for telephone 36, fax, or modem 37 connections. Communications for each of the analog loops are multiplexed over the DSL 35 using any of a number of standard digital communications protocols (e.g., 2B1Q). One type of subscriber line multiplexer is commercially available from Raychem Corporation under the name MINIPLEX.TM.. Here again, the problem is the analog interface between the CO switch 31 and the central office terminal 32 (COT) at the headend of the DSL 35. This analog interface requires an analog-to-digital conversation at the COT 32, resulting in quantization noise.

Due to their conceptual similarities, a subscriber line multiplexer may be considered to be a type of DLC. Any references throughout the remainder of this application to DLCs should be interpreted as including subscriber line multiplexers. The problem addressed here is often aggravated in the case of a subscriber line multiplexer since these systems are frequently installed to provide a second line for use with a modem.

A subscriber line multiplexer can be connected by an analog interface to a central office switch 31, as illustrated in FIG. 3. Alternatively, a subscriber line multiplexer can be used at the remote end of a DLC to provide an additional telephone line from the DLC remote terminal to a subscriber in the carrier service area. Here again, a subscriber line multiplexer could be connected by an analog interface to the DLC remote terminal. This would result in two analog-to-digital conversions in the downstream path. The first occurs at the CO terminal of the DLC, and the second occurs at the CO terminal of the subscriber line multiplexer.

Other related art in the field includes the following:

Ayanoglu et al., "An Equalizer Design Technique for the PCM Modem: A New Modem for the Digital Public Switched Network," IEEE Transactions on Communications, vol. 46, no. 6, pages 763-774 (June 1998).

U.S. Pat. No. 5,494,437 (Ayanoglu et al.) discloses a high-speed modem synchronized to a remote codec. The modem operates reliably at symbol rates up to twice its bandwidth when it is controlled by the clock of the receiving ADC.

U.S. Pat. No. 5,528,625 (Ayanoglu et al.) discloses a quantization-level-sampling (QLS) modem that includes means for separately equalizing each loop in an end-to-end digital telephone system network connection by employing a plurality of transmitter filters and a plurality of receiver filters in such a way that, in the upstream direction, the voltage samples seen by the codecs are equivalent to the network quantization levels transmitted by the modem, and in the downstream direction, the voltage samples seen by the modem are equivalent to the network quantization levels encoded by the codecs. These patents by Ayanoglu et al. relate to the fundamental principles employed in V.90 and other PCM modems and contribute to the understanding of the issues in the present disclosure.

The article by Ayanoglu et al. in IEEE Transactions on Communications discuss equalization techniques for use with PCM modems.

3. Solution to the Problem

The present invention solves the problem of providing PCM modem communications over a DLC having an analog interface to the central office. In particular, the present invention accomplishes this by employing a codec in the CO terminal that includes at least one adaptive equalizer and synchronizing the codec to the CO clock to minimize resampling error. The CO line interface and codec are typically housed within the same building at the central office and are located a relatively short distance from the CO switch. This results in an environment that is relatively noise-free and unchanging, so that adaptive training is only infrequently required. These features allow the codec to accurately recreate PCM codes at data rates sufficient to support V.90, X2, and K56flex communications, and also to support conventional voice communications and data communications. More generally, the present invention can be used whenever PCM coding is used in communications.

This invention provides a codec capable of supporting voice, conventional modem, and PCM modem communications (e.g., V.90, X2, or K56flex protocols) over a universal digital loop carrier (UDLC) having an analog interface to the central office (CO). In the downstream direction, the codec, which has been synchronized to the CO clock, includes an anti-aliasing filter, an analog-to-digital converter (ADC), and an equalizer that has been trained to compensate at the sampling times for the distortion introduced by the downstream channel. These features allow the codec to output a PCM stream that accurately reproduces the PCM data at the central office, at data rates sufficient to support V.90 communications. These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings.

The present invention can be more readily understood in conjunction with the accompanying drawings, in which:

FIG. 1 is a simplified block diagram of telecommunications from an internet service provider (server) to a subscriber using the V.90 protocol.

FIG. 2 is a simplified block diagram of a digital loop carrier (DLC).

FIG. 3 is a simplified block diagram of a digital added main line, or subscriber line multiplexer.

FIG. 4 is a block diagram of the present codec 42 interfaced to a CO line interface 41.

FIG. 5 is a conceptual graph comparing the useable bandwidth available to a conventional subscriber modem, with the bandwidth available to the present codec at the central office.

FIG. 6 is a block diagram of an array of adaptive equalizers 61-63 that can be used in place of a single equalizer 425 in the present codec.

FIG. 7 is a block diagram of an alternative embodiment of the codec 42 using a four-wire interface to the CO line interface 41.

FIG. 8 is a block diagram of the present codec 42 interfaced to a CO line interface 41 showing where these elements reside in a typical telephone network.

Turning to FIG. 4, a schematic diagram is provided showing the present codec 42 connected to a central office line interface 41. FIG. 8 is a block diagram of the codec 42 interfaced to a CO line interface 41 showing where these elements reside in a typical telephone network. The CO line interface 41 is conventional in design, and is widely used to provide an analog or universal interface between a digital central office switch and an analog loop extending to the subscribers premises. A CO line interface 41 can also be used to interface the CO terminal of a DLC as shown in FIG. 2, or the CO terminal of a subscriber line multiplexer, as previously described with regard to FIG. 3. In either case, the CO line interface 41 receives pulse code modulated (PCM) signals from the CO switch that are converted to analog voltages by a digital-to-analog converter (DAC) 411. The analog signal passes through a reconstruction filter 412. A hybrid 413 is used to both send and receive analog signals over a single conventional pair of wires 43. The received analog signal is band-limited by an anti-aliasing filter 415. The filtered signals are sampled and converted into digital data by an analog-to-digital converter 414 (ADC), and output to the CO switch. It should be noted that the CO line interface 41 samples synchronously with the CO clock. This clock is used, for example, by both the DAC 411 and ADC 414.

The codec 42 connects to the CO line interface 41 over the wire pair 43 by means of its hybrid 44. Downstream analog signals pass through an anti-aliasing filter 422 and are converted into digital signals by an analog-to-digital converter (ADC) 423. This ADC 423 is preferentially controlled by the same CO clock used by the CO line interface 41 to ensure that it has precisely the same sampling rate as the DAC 411 in the CO line interface 41. This is usually possible if the codec 42 is located adjacent to the CO line interface 41 by means of a common timing signal. In this case, the length of wire pair 43 connecting the CO line interface 41 and the codec 42 is typically only a few hundred feet, at most. This creates a predictable environment with low noise that is capable of supporting communications with a high signal-to-noise ratio. Alternatively, the CO clock rate can be inferred by detecting periodic amplitude patterns in the analog signals received from the CO line interface 41. Methods for estimating the clock from the incoming analog signal in a multilevel PAM system are well understood in the art. Several methods may be employed, some of which are described by Proakis, Digital Communications, pages 358-365 (3.sup.rd edition, McGraw Hill, Inc., 1995), which is hereby incorporated by reference.

Conventional analog telephone communications systems assume a band from about 300 to 3500 Hz depending on local conditions, as shown conceptually represented in FIG. 5. However, the close proximity and controlled environment between the CO line interface 41 and the codec 42 in the present invention increases the useable bandwidth available for communications to about 4 KHz. When viewed in terms of either the Shannon capacity formula or the Nyquist limit, the analog connection between the CO line interface 41 and the codec 42 is therefore capable of supporting data rates sufficient for V.90 communications, and beyond.

The output signal from the ADC 423 is then processed by at least one adaptive equalizer 425. In the simplest embodiment, the equalizer 425 has previously been trained to provide a transfer function that substantially compensates for distortions introduced by the reconstruction filter 412 and the signal path between the codec 42 and the reconstruction filter 412. The output of the adaptive equalizer 425 is sampled by a slicer 426 and converted into digital data. The slicer 426 is also synchronized to the CO clock to minimize resampling error. The digital data output by the slicer 426 undergoes conversion by a linear to .mu.-law converter 424 (.mu.-law companding in North America and Japan, A-law companding elsewhere) to generate PCM codes for output over the DLC. This configuration allows the codec 42 to output a PCM stream that accurately reproduces the PCM codes entering the DAC 411 of the CO line interface 41.

The equalizer must be initially trained prior to entering operational mode. This involves having a known training sequence sent through the channel and the equalizer 425, and comparing the equalizer output to the expected sequence. The equalizer filter taps are then adjusted based on an adaptive process such as the least-mean squares (LMS) algorithm to minimize error. This algorithm is widely used in the telecommunications industry and is discussed by Widrow et al., Adaptive Signal Processing, Prentice Hall, Englewood Cliffs, N.J. (1985). After the training period, the equalizer 425 taps are frozen for normal communications. Equalizer training can be repeated, as necessary, to compensate for changes in line conditions and the CO line interface 41.

FIG. 6 illustrates an alternative embodiment in which a plurality of adaptive equalizers 61, 62, . . . 63 are arranged in parallel in place of the single equalizer 425 shown in FIG. 4. Each of the equalizers in the array 61-63 is trained to process one symbol in each frame of N symbols from the ADC 423. This approach is discussed in detail by Ayanoglu et al. in IEEE Transactions on Communications, supra, which is hereby incorporated by reference.

Returning to FIG. 4, upstream digital signals received by the codec 42 from the DLC pass through a .mu.-law to linear converter 421, which linearizes the code space of the companded PCM received from the DLC. The linearized digital output from the .mu.-law to linear converter 421 is converted into analog voltages by a digital-to-analog converter (DAC) 427. The resulting analog signal passes through a reconstruction filter 428 and is transmitted upstream to the CO line interface 41 over the wire pair 43 by the codec's hybrid 44. The upstream digital signal is also used by an echo canceller 429 to generate an echo estimate signal that is subtracted from the output of the ADC 423 to mitigate echo leaking through the hybrid 44.

FIG. 7 is a schematic block diagram of an alternative embodiment of the present invention in which the two-wire interface 43 between CO line interface 41 and codec 42 has been replaced with an analog four-wire interface 431 and 432. One pair of wires 431 is used for downstream communications and the second pair 432 is used for upstream communications. This embodiment allows both of the hybrids 413 and 44 and the echo canceller 429 shown in FIG. 4 to be eliminated.

The preceding discussion has focused on use of the present invention in facilitating PCM modem communications. It should be noted that the present codec can also be employed to improve the performance of voice and earlier modems (V.34 and before) by more accurately reproducing the PCM stream at the COT of the DLC.

The present codec could also be used in the CO line interface to facilitate the accurate reproduction of PCM codes in the upstream direction. For example, this implementation could be used to enable PCM modem data transport in the upstream direction.

The above disclosure sets forth a number of embodiments of the present invention. Other arrangements or embodiments, not precisely set forth, could be practiced under the teachings of the present invention and as set forth in the following claims.