Abstract:
A linecard ( 175 ) permits an increased rate connection between a subscriber ( 15 ) and a service provider ( 40 ) over the PSTN ( 50 ) includes an analog interface ( 152 ) a digital interface ( 165 ) coupled to the digital backplane ( 170 ) to the service provider&#39;s host server ( 34 ), a conversion circuit ( 258 ) interspersed between the analog interface ( 152 ) and the digital interface ( 165 ), and a linecard microcontroller ( 300 ) configured to request bandwidth on the backplane ( 170 ) A linecard ( 175 ) incorporates a codec ( 250 ) with a code recognition mechanism ( 200 ) to monitor the Pulse Code Modulated (PCM) input from the provider. The code recognition mechanism ( 200 ) provides a way to dynamically allocate and deallocate timeslots on the backplane ( 170 ).

Description:
This is a division of Application Ser. No. 09/103,496, filed Jun. 24, 1998. 

   TECHNICAL FIELD 
   The invention relates generally to data communications and more particularly to high speed data transmissions over the public switched telephone network. 
   BACKGROUND OF THE INVENTION 
   The sudden popularity of the Internet as a communication tool has led to an intense push for higher data transmission rates over the Public Switched Telephone Network (PSTN). As a result, the demand for increased data transmission rates over analog twisted pair wiring is at an all time high. The most recent widespread standard is “56K” analog modem technology developed by U.S. Robotics and Rockwell/Lucent. While these technologies generally have not generated true 56 kbps performance under typical subscriber line conditions, they do provide a boost in performance from the previous standard of bidirectional 33.6 kbps. 
   Theoretically, a connection of 64 kbps should be attainable between the subscriber and the Internet Service Provider (ISP) via a standard Plain Old Telephone Service (POTS) connection. This is because 64 kbps is the rate at which data is transferred from the Central Office (CO) linecard to the ISP or other remote terminal. Several factors prevent this from happening including imperfect line conditions and varying local loop lengths common to POTS analog networks. The primary reason, however, for this less than the theoretical transmission rate is that the PSTN was designed to carry voiceband frequencies in the range of 300 Hz–3.4 kHz. 
   With the advent of digital voice systems, the decision was made to use a “companded” (compressed/expanded) data to reduce the number of bits per digital sample from a nominal 13-bits to 8-bits. Companding schemes use higher resolution at low signal amplitudes and lower resolution at high amplitudes. Companded signals are suitable for voice frequencies but not for analog modems since they limit their apparent bandwidth to a ceiling of 33.6 kbps. In practice, most analog modems are only able to achieve rates of 46–48 kbps downstream due to less-than-perfect analog line conditions. 
   The Analog-to-Digital (“A/D”) portion of the linecard coder/decoder (“codec”) is where the analog signal is converted to its 8-bit companded representation. Hence, the linecard codec acts as a bottleneck in the entire data communications chain. One way of avoiding this bottleneck is by removing the A/D conversion in the downstream direction. This is accomplished by requiring a digital connection between the provider and its CO and increasing the data throughput of the modem signals to capitalize on the extra capacity. This is the basis of 56 k technologies. 
   Moreover, while the use of 56K standards results in downstream data throughput of 56 kbps under ideal local loop conditions, the upstream direction must still contend with an A/D conversion into 8-bit companded data and is still limited to 33.6 kbps. Imperfect conditions in the analog local loop further degradate the signal resulting in less than the 56/33.6 kbps maximums. 
   Additionally, while 56K standards offer improvements over the older V.34+ standard, bandwidth is still needed to keep pace with upcoming technologies such as video conferencing, remote server access, and other high rate transmission protocols. If higher data throughput is to be achieved, the limitations in the CO need to be overcome. Overhauling the PSTN by replacing the 8-bit companded data scheme could solve the problem, but this is not a feasible solution since the cost of such as effort would be enormous. 
   SUMMARY OF THE INVENTION 
   The invention overcomes the limitation in bandwidth of prior communications standards including 56K by offering increased transmission rates using an analog modem communicating over the PSTN. 
   In one embodiment, an improved linecard device is disclosed. The linecard permits increased rate communications between a subscriber and a service provider over the PSTN. An analog interface couples the subscriber modem to the PSTN over a twisted pair connection from the subscriber&#39;s modem. The linecard incorporates a digital interface to the digital backplane leading to the service provider&#39;s modem. A converter is interspersed between the analog interface and the digital interface. A linecard microcontroller is configured to request bandwidth on the digital backplane. 
   According to one embodiment, the linecard incorporates a codec with a pattern recognition mechanism that receives code patterns from the service provider modem via the digital backplane. The code patterns from the service provider modem are decoded by the mechanism and if a predetermined code pattern is detected, a strobe signal is transmitted to the linecard microcontroller which interfaces with the digital backplane to request bandwidth. 
   In one embodiment, a code recognition function monitors the Pulse Code Modulated (PCM) input from the provider modem in the downstream direction. A certain amount of intelligence is employed in the code recognition mechanism to handle simple handshaking and act on the PCM codes received. The general instruction architecture places the provider modem in the master position and the codec in the slave position. The code recognition mechanism provides a way to dynamically allocate and deallocate timeslots during data communications based on code patterns received from the provider modem. 
   According to another embodiment, the linecard microcontroller can request more or less timeslots from the network administrator based on the code patterns received from the service provider or the subscriber. During periods of inactivity, the timeslots are deallocated to make room for other connections on the same backplane. 
   According to another embodiment, the codec includes a strobe terminal that permits sending interrupts to the linecard microcontroller for “on-the-fly” timeslot allocation. 
   A technical advantage of the invention is that it provides the subscriber with much more bandwidth than currently available with analog modems while maintaining much of the same equipment and connection methods. 
   Another technical advantage of the invention is that it eliminates the need to employ a direct customer interface with the CO and thus no equipment needs to be installed at the subscriber&#39;s residence. Thus, the invention allows the telecom infrastructure to be built up gradually accommodating other high rate communications protocols to support additional traffic. 
   Yet another technical advantage of the invention is that it permits replacement of the existing linecard in the CO with the linecard of the present invention enabling hardware and software changes at the CO to provide the increased bandwidth. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects of the invention including specific implementations are understood by reference to the following detailed description taken in conjunction with the appended drawings in which: 
       FIG. 1  is a diagram illustrating a communications system wherein the invention can be practiced; 
       FIG. 2  is a block diagram of a typical public switch telephone network; 
       FIG. 3  is a block diagram at a communications system illustrating the arrangement of a central office with respect to an Internet service provider and subscribers; 
       FIG. 4  is a block diagram of a prior art linecard codec; 
       FIG. 5  is a block diagram for linecard according to one embodiment of the invention; and 
       FIG. 6  is a circuit diagram for a linecard codec according to one embodiment. 
   

   References in the figures correspond to like numerals in the detailed description unless otherwise indicated. 
   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring first to  FIG. 1 , therein is illustrated an example communication systems  10  in which the invention can be practiced according to one embodiment. The communication system  10  includes at least one subscriber  15  communicating with a service provider  40 . In general, an individual subscriber  15  has the transmission/reception and data processing equipment enabling access to the service provider  40 . 
   A first processing system  17  is operably coupled to a modem  21  via an interface  19 . The interface  19  provides a communications pathway for unmodulated data transfers between the processing system  17  and the modem  21 . In other embodiments the modem  21  is internally fixed inside the processing system  17  and coupled through a standard interface of the processing system  17 . As shown, the modem  21  transmits and receives analog signals  20  over communications link  23  to and from the central office  25 . Typically the communication link  23  between the modem  21  and the central office  25  is twisted pair wiring of the type commonly employed in many Public Switched Telephone Networks (PSTN). 
   Data from the processing system  17  is transferred through the interface  19  to modem  21 . The data can be temporally stored in a buffer memory or other similar device for further processing. The data is then converted from its digital format to an equivalent analog signal using the appropriate modulation rules that apply to the transmission protocol. A Digital-to-Analog Conversion (DAC) circuit or other similar signal processing device can be used for this purpose. The converted data is transmitted via the communications link  23  as modulated analog signals  20 . As used herein, the analog signals  20  represent analog waveforms transmitted to and received from the central office  25 . Various modulation methods can be employed including Quadrature Amplitude Modulation (QAM), Trellis Code Encoding (TCE) and Frequency Shift Keying (FSK) among others. 
   Modulated analog signals  20  from the modem  21  are received at the central office  25  by the Analog to Digital Converter (ADC)  27 . At this point the signals  20  are converted to a digital bit stream sequence which is communicated on the PSTN  30 . Preferably, the sequence is transmitted on a digital link  28  permitting high rate data transfers between the central office  25  and the PSTN  30 . 
   Next, the digital bit stream sequence is communicated to the host server  34  maintained by the service provider  40  over the communications link  32  coupling the host server  34  to the PSTN  30 . Preferably, communications between the host server  34  and the PSTN  30  are bidirectional so that digital data  31  emanating from the host server  34  is transferred to the PSTN on downstream link  36 . 
   The DAC  26  at the central office  25  converts the bit stream sequence  31  from the host server  34  to analog equivalent signals using the modulation rules applicable to the communication protocol between the modem  21  and the central office  25 . Thus, modulated analog signals  20  representatives of the digital bit stream sequence  31  from the host server  34  is transmitted to the modem  21  where it is received, demodulated and transferred to the processing system  17  of the subscriber  15 . 
   Preferably, the communications system  10  supports both downstream and upstream communications. The modem  21  includes an Analog Front End (AFE) which acts as the interface to the central office  25  through communications link  23 . Typically, a universal asynchronous receiver transmitter (UART) or other similar data flow control device is employed in the modem  21  for handling communication between the processing system  17  and the modem  21 . 
   The processing system  17  contains suitable application programs, storage devices, memory and processing capabilities to operate the modem  21  and provide other user functions known to those of ordinary skill. The transmit and receive functions of the modem  21  and central office  25  can be implemented using known methods and devices. For example, the communications protocols between the modem  21  and the central office  25  may include those supported and standardized by the International Standard Organization (ISO), the International Telegraph and Telephone Consultative Committee (CCITT) and the Electronics Industry Association (EIA) among others. 
   The invention has particular application with respect to the data rate between the modem  21  and the central office  25  as well as the data rate between the central office  25  and the host server  34 . In particular, the invention is specifically directed at a method and linecard device that increases the bandwidth of the communications link  23  between the modem  21  and the central office  25  and the communications links  28 ,  31 ,  32  and  36  between the central office  25  and the host server  34 . 
   Preferably, the modem  21  is any industry accepted modulation/demodulation device available through common industry channels. In other embodiments, the modem  21  supports communications protocols not yet defined. For example, the modem  21  can support both digital and analog connections in some embodiments. 
   As discussed above, various line conditions on the communications link  23  often prevent the theoretical full data rate between the modem  21  and the central office facility  25  to be achieved. Preferably, the linecard of the present invention is installed at the central office facility  25  to permit increased bandwidth between the modem  21  and the central office  25 . In one embodiment, the invention can be applied in any communications system where an analog carrier between a centralized communications hub and an analog communications device such as modem  21  are employed. In other embodiments, the central office  25  is capable of providing multiple time slots and at least one terminal on the commercial time system supplying data in the form of a digital bit sequence. Examples of such other communications systems are illustrated in  FIGS. 2 and 3 . 
   Referring to  FIG. 2 , an example PSTN  50  is shown. The PSTN  50  includes a two wire central office switch  60  which is communicably accessible by a plurality of subscribers  55  on the near end of the PSTN  60 . The near end of the PSTN  50  is associated with the point of origination for a particular connection. Some of the subscribers  55  are coupled to the central office switch  60  via analog local loops  57 . The analog local loops  57  are typically twisted pair wiring connection extending from the users hand set or analog modem to the central office switch  60 . Other end users  55  may be coupled to a concentrator  62  which functions as a local hub to the central office switch  60  within a given geographic area. The concentrator  62  may then be coupled to the central office switch  60  using a digital linecard, pair/gain link or other similar connection medium according to various known configurations. 
   As shown, communications between the near end central office switch  60  and the far end central office switch  75  occur over a four wire network consisting of four wire toll switches  65  and  70  and bidirectional communications lines  68  and  69 . Preferably, the four wire network  67  is a high speed digital link such as a T1 or E1 connection or other similar communications channel. Thus, the two wire central office switch  60  is coupled to the four wire toll switch  65  which forms the front end of a hybrid circuit between the central office switches  60  and  75 . Preferably, the four wire toll switch  65  supports high speed bidirectional communications with the four wire toll switch  70  which, in turn, is coupled to the two wire central office switch  75 . At the far end of the PSTN  50 , other subscriber  55  are coupled to the far end two wire central office switch  75  via local two wire analog loops  79 . 
   Turning to  FIG. 3 , a communications system supporting both voice and digital data transmissions is shown and denoted generally as  100 . A Plain Old Telephone System (POTS)  110  is coupled to the near end central office  25  via the analog link  111 . Generally, the POTS is a standard telephone set used by subscribers  55  in their home, business or other location. Like, a subscriber can use an analog modem  21  to establish a connection with the near end central office  25  through communications line  23 . The modem  21  can be used to gain access to the World Wide Web (WWW) via Internet Service Provider (ISP)  130 . 
   Typically, a user may employ both his POTS  110  and the modem  21  for both voice band and digital communications on the same line  111 . This configuration is represented by the dashed line in  FIG. 3 . In other configurations, separate communications lines ( 23 ,  111 ) are used between the central office  25  to the POTS  110  and modem  21 , respectively. In any case, the central office  25  is the near end hub for all such communications. 
   In general, the ISP  130  may be accessed by a large number of users on communications system  100 . A dedicated digital connection such as an Integrated Service Data Network (ISDN) T1 or E1 link may be used. Likewise, the communications link  105  between the central office  25  and the ISP 130  is often a high speed digital connection providing bidirectional communications between the central office  102  and the ISP  130 . A plurality of host servers  135 ,  140 ,  145  are coupled to the ISP  130  providing all the information and services available on the WWW. 
   A far end modem  118  is likewise coupled to the far end central office  104  via analog line  119 . With the communications system  100 , modem  21  and modem  118  provide subscriber access to the ISP  130  using known signaling protocols and communication standards. The invention can be used to increase the effective data rate between the modems  21  and ISP  130 . The invention can also be used to increase the data rate between the ISP  130  and the central office  25 . 
   Specifically the invention incorporates several features that enable it to multiply the amount of data that can pass through communications link  105  between the central office  25  and ISP  130 . This is accomplished by replacing the existing codec section of the linecard device in the central offices  25  and  104 , with an advanced codec according to the invention. 
   Referring to  FIG. 4 , a block diagram of a prior art linecard is shown and denoted generally as  150 . The linecard  150  is typically used in central offices  25  and  104 , concentrator  62  or other similar central hub connection. The Tip/Ring (T/R) lines  152  carry analog waveforms from an analog modem into the linecard  150 . The signals enter ring relays  155  which performs the appropriate ring detect, on-hook/off-hook functions of the linecard  150 . 
   The ring relays  152  are coupled to the Single Line Interface Circuit (SLIC)  157  which manages the local loop between an individual subscriber and the central office point of connection. The codec/filter section of the linecard  150   160  provides the coding, decoding and filtering functions on the incoming analog signal. Since most PSTNs carry signals within the voiceband frequency range of 300–3.4 KHz, the codec/filter section  160  provides the appropriate cut-off functions that remove extraneous signal components outside the voiceband. The codec/filter section  160  may be implemented as an analog or digital filter using known designs and techniques. 
   The ring relays  155 , SLIC  157  and codec/filter section  160  are operably controlled by the control logic  162  of the linecard  150 . Codec/filter section  160  interfaces with the digital backplane  170  through local loop interface lines  165  which provide the digital interface of the linecard  150 . In this way analog signals from the subscribers  55  are transmitted on the digital backplane  170 , after conversion, coding and filtering. The service provider  40 , which is often an ISP  130 , is coupled to the digital backbone  170 . 
   The codec/filter section  160  of the linecard  150  provides the coding and decoding functions that translate analog signals received over the T/R lines  152  to digital bit stream sequences transmitted on the backplane  170  to the service provider  40 . According to the invention, the codec/filter section  160  is replaced with an enhanced codec supporting code recognition and multiple dynamic timeslot allocation functions. The fact that the enhanced codec can support code recognition and multiple timeslot allocation allows an increased bandwidth on the backplane  170  to be achieved between the central office  25  and the subscriber modem  21 . 
   In practice, multiple linecard configurations may be employed at a central office facility as is known to those of ordinary skill. A variation of the linecard  150  is shown in  FIG. 5  with the code recognition function  200  of the present invention.  FIG. 5  is a detailed block diagram of a linecard device  175  according to the invention is shown coupled to a modem  21  through T/R lines  152 . The T/R lines  152  feed into a line over voltage protection device  180  which performs the voltage suppression and line protection functions of the linecard  175 . The ring and test relays  182  and supervision circuit  186  are operably coupled through the relay drivers  184  permitting an interface between the two wire portion of the linecard  175  and the four wire connection  165  to the digital backbone  170 . A battery feed  187  maintains the central office supply levels across the linecard  175 . A 2-wire to 4-wire hybrid circuit  185  provides the exchange mechanism between the 2-wire subscriber side and the 4-wire network side. Together, the supervision circuit  186 , hybrid circuit  185  and battery feed  187  provide the SLIC  157  functions. 
   On the transmit side of the linecard  175 , the codec/filter section  160  is now coupled to a code recognition mechanism  200 . As before, the codec/filter section  160  is configured to massage the outgoing Pulse Code Modulation (PCM) signals which are transmitted on the backplane  170  to other entities on the network on the digital interface. Likewise, PCM signals from the digital backbone  170  are received by the linecard  175 , decoded, filtered and reconstructed using appropriate signal communications protocols. 
   Typically, the modem  21  forms one end of a single connection loop with the linecard  175 . The SLIC  157  provides the supervision  186  and hybrid  185  functions of the interface card  175  which are dedicated to the modem  121  during a single connection to the central office facility  25 . 
   Turning to  FIG. 6 , a circuit diagram for a linecard codec  250  according to one embodiment is shown. The codec  250  is of the sigma-delta variety although a successive approximation codec may also be used. As shown, analog waveforms enter the codec  250  through input terminal  252  and reach the gain block  254  where they are amplified to compensate for any line losses. After appropriate amplification at the gain block  254 , the analog waveforms are passed through filter  256  which prevents aliasing of the converter  258 . In one embodiment, the filter  256  is a low pass filter with a cut-off frequency in the high KHz range. The converter  258  typically operates at a sampling rate of at least a few MHZ. 
   The smoothed and filtered analog signal  257  is passed through converter  258  which implements a well known analog to digital conversion function on the signal  257  using a sampling rate at least twice the modulation frequency of the analog signal  257 . Preferably the converter  258  is over sampled. The output of the converter  258  is a digital bit stream sequence  259  which is passed to the digital filter  260  for further digital signal processing. 
   The digital filter  260  performs a voiceband shaping function and sigma-delta decimation on the incoming sequence  259 . The digital filter  260  may also be used to compensate for any loss data bits in the digital signal  259  from the convertor  258 . Other processing functions may be performed by the digital filter  260  as are known to those of ordinary skill. 
   Next, the digital filtered signal  261  is passed to an output register  264 . The output from the output register  264  is a PCM output signal  266  which is transferred on the digital backplane  170  through interface  165 . The transmission protocol and methods used to relay the PCM output  266  on the digital backbone  170  are well known. 
   As shown, PCM signals  210  from the digital backplane  170  enter a register  270  having a code recognition mechanism  272 . The fact that register  270  has a code recognition mechanism  272  enables the increased bandwidth advantages of the invention. According to one embodiment, predetermined code patterns are transmitted by the service provider  40  on the digital backplane  170  and arrive at the PCM interface  165 . The patterns enter the register  270  which performs a decode function on the patterns. 
   Next, the code recognition mechanism  272  decodes the patterns to determine if a predetermined sequence is contained in the pattern. If a predetermined sequence is detected, an interrupt signal is transmitted on strobe terminal  274  to the code register  310  of the linecard microcontroller  300 . As shown, the strobe terminal  274  extends from the code recognition mechanism  272  to the code register  310  and the interface  312 . 
   The linecard microcontroller  300  performs the linecard control functions of the codec  250  that enable an increased throughput connection to be achieved when requested by the service provider  40 . Thus, the strobe terminal  274  has the ability of interrupting the linecard microcontroller  300  based on the code patterns received from the digital backplane  170 . 
   As is known to those of ordinary skill in the art, various means of implementing the code recognition mechanism  272  within a linecard codec  250  can be used. Accordingly, the invention encompasses a codec  250  having a pattern recognition mechanism that monitors the PCM input  165  with a certain amount of intelligence used to handle the handshaking and act on the code patterns as they are received. The provider modem is placed in the position of the transfer device and the codec  250  assumes the slave position. Preferably, the provider modem sends the code patterns and the codec  250  acts upon them. 
   Once the code register  310  receives the strobe signal from the code recognition mechanism  272  it passes the code pattern to the microcontroller interface  312 . Preferably the microcontroller interface  312  is configured to request bandwidth on the digital backplane  170  from the network administrator. Thus, more or less bandwidth may be allocated for a particular connection through the interface  312 . 
   In practice, the linecard obtains the number of timeslots allowed to the requesting channel through the codec  250  using the microcontroller interface  312 . When the linecard microcontroller  300  has determined which timeslots in the backplane  170  have been allocated to that connection, it programs the codec  250  to transmit and receive data on all assigned timeslots. In one embodiment, the codec  250  sends signals to the provider modem indicating that it has been configured for an increased throughput connection along with how many timeslots have been allocated. The provider modem connects to the corresponding timeslots on its line and begins sending a mapping table to the codec  250  which can be stored in the codec  250 . 
   The provider modem can transmit a signal to the subscriber modem  21  indicating that an increased data throughput connection has been achieved and that the connection will progress using the increased connection rate. Likewise, the subscriber modem can be configured to tell the ISP modem that it wants to establish an increased throughput connection. The subscriber modem waits while the provider modem sends a predefined code pattern to the central office codec  250  that interfaces with the subscriber modem  21 . The codec  250  is able to recognize the code patterns and upon receiving then sends a strobe to the linecard microcontroller  300  on strobe terminal  274  indicating that an increased throughput connection has been requested. Through the interface  312 , the microcontroller  300  petitions for additional timeslots from the network. The network obtains the number of timeslots allowed for that subscriber line and allocates them to the requesting channel. 
   In one embodiment, the codec  250  is in “voice” mode by default, operating in the same manner as any standard POTS  110 . This means that the filters  256  pass 300–3400 Hz and the converter  258  samples accordingly using a single timeslot in the backplane  170 . When a subscriber initiates a modem connection, the modem creates a low-rate connection with the modem at the service provider that requires only one timeslot (such as V.34+). 
   During much of a typical modem connection, no activity is taking place. For this reason, the present invention also encompasses a method to deallocate extra timeslots during periods of inactivity. When the provider modem has determined that the line  165  has been idle for a predetermined amount of time, it sends a predetermined code to the subscriber modem initiating a throttle-down to a low-rate protocol that requires only one timeslot on the network. The provider modem then sends a predefined bit sequence to the codec  250 . The codec  250  recognizes this bit sequence and sends a strobe to the microcontroller  300 , giving the microcontroller  300  permission to switch the codec  250  to “single-timeslot” data mode. In this mode, only one timeslot is used. To deallocate the secondary timesiots, the microcontroller  300  interfaces with the network through the interface  312 . Once the timeslots have been deallocated, the linecard microcontroller  300  re-programs the codec  250  for its primary timeslot only. The codec  250  signals sends to the provider modem and immediately begins transceiving on the primary timeslot. 
   A return to “multiple-timeslot” data mode is accomplished in the similar manner as it was during the original establishment of the increased throughput connection. Within a single connection, the protocol may switch between single-timeslot data mode and multiple-timeslot data mode an indefinite number of times. In this way, maximum efficiency over the communication system is achieved. 
   While the invention has been described in conjunction with preferred embodiments, it should be understood that modifications will become apparent to those of ordinary skill in the art and that such modifications are intended to be included within the scope of the invention and the following claims.