Central office line card with code recognition for increasing data rates over PSTN

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'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).

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's modem. The linecard incorporates a digital interface to the digital backplane leading to the service provider'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'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.

References in the figures correspond to like numerals in the detailed description unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first toFIG. 1, therein is illustrated an example communication systems10in which the invention can be practiced according to one embodiment. The communication system10includes at least one subscriber15communicating with a service provider40. In general, an individual subscriber15has the transmission/reception and data processing equipment enabling access to the service provider40.

A first processing system17is operably coupled to a modem21via an interface19. The interface19provides a communications pathway for unmodulated data transfers between the processing system17and the modem21. In other embodiments the modem21is internally fixed inside the processing system17and coupled through a standard interface of the processing system17. As shown, the modem21transmits and receives analog signals20over communications link23to and from the central office25. Typically the communication link23between the modem21and the central office25is twisted pair wiring of the type commonly employed in many Public Switched Telephone Networks (PSTN).

Data from the processing system17is transferred through the interface19to modem21. 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 link23as modulated analog signals20. As used herein, the analog signals20represent analog waveforms transmitted to and received from the central office25. Various modulation methods can be employed including Quadrature Amplitude Modulation (QAM), Trellis Code Encoding (TCE) and Frequency Shift Keying (FSK) among others.

Modulated analog signals20from the modem21are received at the central office25by the Analog to Digital Converter (ADC)27. At this point the signals20are converted to a digital bit stream sequence which is communicated on the PSTN30. Preferably, the sequence is transmitted on a digital link28permitting high rate data transfers between the central office25and the PSTN30.

Next, the digital bit stream sequence is communicated to the host server34maintained by the service provider40over the communications link32coupling the host server34to the PSTN30. Preferably, communications between the host server34and the PSTN30are bidirectional so that digital data31emanating from the host server34is transferred to the PSTN on downstream link36.

The DAC26at the central office25converts the bit stream sequence31from the host server34to analog equivalent signals using the modulation rules applicable to the communication protocol between the modem21and the central office25. Thus, modulated analog signals20representatives of the digital bit stream sequence31from the host server34is transmitted to the modem21where it is received, demodulated and transferred to the processing system17of the subscriber15.

Preferably, the communications system10supports both downstream and upstream communications. The modem21includes an Analog Front End (AFE) which acts as the interface to the central office25through communications link23. Typically, a universal asynchronous receiver transmitter (UART) or other similar data flow control device is employed in the modem21for handling communication between the processing system17and the modem21.

The processing system17contains suitable application programs, storage devices, memory and processing capabilities to operate the modem21and provide other user functions known to those of ordinary skill. The transmit and receive functions of the modem21and central office25can be implemented using known methods and devices. For example, the communications protocols between the modem21and the central office25may 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 modem21and the central office25as well as the data rate between the central office25and the host server34. In particular, the invention is specifically directed at a method and linecard device that increases the bandwidth of the communications link23between the modem21and the central office25and the communications links28,31,32and36between the central office25and the host server34.

Preferably, the modem21is any industry accepted modulation/demodulation device available through common industry channels. In other embodiments, the modem21supports communications protocols not yet defined. For example, the modem21can support both digital and analog connections in some embodiments.

As discussed above, various line conditions on the communications link23often prevent the theoretical full data rate between the modem21and the central office facility25to be achieved. Preferably, the linecard of the present invention is installed at the central office facility25to permit increased bandwidth between the modem21and the central office25. 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 modem21are employed. In other embodiments, the central office25is 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 inFIGS. 2 and 3.

Referring toFIG. 2, an example PSTN50is shown. The PSTN50includes a two wire central office switch60which is communicably accessible by a plurality of subscribers55on the near end of the PSTN60. The near end of the PSTN50is associated with the point of origination for a particular connection. Some of the subscribers55are coupled to the central office switch60via analog local loops57. The analog local loops57are typically twisted pair wiring connection extending from the users hand set or analog modem to the central office switch60. Other end users55may be coupled to a concentrator62which functions as a local hub to the central office switch60within a given geographic area. The concentrator62may then be coupled to the central office switch60using 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 switch60and the far end central office switch75occur over a four wire network consisting of four wire toll switches65and70and bidirectional communications lines68and69. Preferably, the four wire network67is a high speed digital link such as a T1 or E1 connection or other similar communications channel. Thus, the two wire central office switch60is coupled to the four wire toll switch65which forms the front end of a hybrid circuit between the central office switches60and75. Preferably, the four wire toll switch65supports high speed bidirectional communications with the four wire toll switch70which, in turn, is coupled to the two wire central office switch75. At the far end of the PSTN50, other subscriber55are coupled to the far end two wire central office switch75via local two wire analog loops79.

Turning toFIG. 3, a communications system supporting both voice and digital data transmissions is shown and denoted generally as100. A Plain Old Telephone System (POTS)110is coupled to the near end central office25via the analog link111. Generally, the POTS is a standard telephone set used by subscribers55in their home, business or other location. Like, a subscriber can use an analog modem21to establish a connection with the near end central office25through communications line23. The modem21can be used to gain access to the World Wide Web (WWW) via Internet Service Provider (ISP)130.

Typically, a user may employ both his POTS110and the modem21for both voice band and digital communications on the same line111. This configuration is represented by the dashed line inFIG. 3. In other configurations, separate communications lines (23,111) are used between the central office25to the POTS110and modem21, respectively. In any case, the central office25is the near end hub for all such communications.

In general, the ISP130may be accessed by a large number of users on communications system100. A dedicated digital connection such as an Integrated Service Data Network (ISDN) T1 or E1 link may be used. Likewise, the communications link105between the central office25and the ISP130is often a high speed digital connection providing bidirectional communications between the central office102and the ISP130. A plurality of host servers135,140,145are coupled to the ISP130providing all the information and services available on the WWW.

A far end modem118is likewise coupled to the far end central office104via analog line119. With the communications system100, modem21and modem118provide subscriber access to the ISP130using known signaling protocols and communication standards. The invention can be used to increase the effective data rate between the modems21and ISP130. The invention can also be used to increase the data rate between the ISP130and the central office25.

Specifically the invention incorporates several features that enable it to multiply the amount of data that can pass through communications link105between the central office25and ISP130. This is accomplished by replacing the existing codec section of the linecard device in the central offices25and104, with an advanced codec according to the invention.

Referring toFIG. 4, a block diagram of a prior art linecard is shown and denoted generally as150. The linecard150is typically used in central offices25and104, concentrator62or other similar central hub connection. The Tip/Ring (T/R) lines152carry analog waveforms from an analog modem into the linecard150. The signals enter ring relays155which performs the appropriate ring detect, on-hook/off-hook functions of the linecard150.

The ring relays152are coupled to the Single Line Interface Circuit (SLIC)157which manages the local loop between an individual subscriber and the central office point of connection. The codec/filter section of the linecard150160provides 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 section160provides the appropriate cut-off functions that remove extraneous signal components outside the voiceband. The codec/filter section160may be implemented as an analog or digital filter using known designs and techniques.

The ring relays155, SLIC157and codec/filter section160are operably controlled by the control logic162of the linecard150. Codec/filter section160interfaces with the digital backplane170through local loop interface lines165which provide the digital interface of the linecard150. In this way analog signals from the subscribers55are transmitted on the digital backplane170, after conversion, coding and filtering. The service provider40, which is often an ISP130, is coupled to the digital backbone170.

The codec/filter section160of the linecard150provides the coding and decoding functions that translate analog signals received over the T/R lines152to digital bit stream sequences transmitted on the backplane170to the service provider40. According to the invention, the codec/filter section160is 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 backplane170to be achieved between the central office25and the subscriber modem21.

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 linecard150is shown inFIG. 5with the code recognition function200of the present invention.FIG. 5is a detailed block diagram of a linecard device175according to the invention is shown coupled to a modem21through T/R lines152. The T/R lines152feed into a line over voltage protection device180which performs the voltage suppression and line protection functions of the linecard175. The ring and test relays182and supervision circuit186are operably coupled through the relay drivers184permitting an interface between the two wire portion of the linecard175and the four wire connection165to the digital backbone170. A battery feed187maintains the central office supply levels across the linecard175. A 2-wire to 4-wire hybrid circuit185provides the exchange mechanism between the 2-wire subscriber side and the 4-wire network side. Together, the supervision circuit186, hybrid circuit185and battery feed187provide the SLIC157functions.

On the transmit side of the linecard175, the codec/filter section160is now coupled to a code recognition mechanism200. As before, the codec/filter section160is configured to massage the outgoing Pulse Code Modulation (PCM) signals which are transmitted on the backplane170to other entities on the network on the digital interface. Likewise, PCM signals from the digital backbone170are received by the linecard175, decoded, filtered and reconstructed using appropriate signal communications protocols.

Typically, the modem21forms one end of a single connection loop with the linecard175. The SLIC157provides the supervision186and hybrid185functions of the interface card175which are dedicated to the modem121during a single connection to the central office facility25.

Turning toFIG. 6, a circuit diagram for a linecard codec250according to one embodiment is shown. The codec250is of the sigma-delta variety although a successive approximation codec may also be used. As shown, analog waveforms enter the codec250through input terminal252and reach the gain block254where they are amplified to compensate for any line losses. After appropriate amplification at the gain block254, the analog waveforms are passed through filter256which prevents aliasing of the converter258. In one embodiment, the filter256is a low pass filter with a cut-off frequency in the high KHz range. The converter258typically operates at a sampling rate of at least a few MHZ.

The smoothed and filtered analog signal257is passed through converter258which implements a well known analog to digital conversion function on the signal257using a sampling rate at least twice the modulation frequency of the analog signal257. Preferably the converter258is over sampled. The output of the converter258is a digital bit stream sequence259which is passed to the digital filter260for further digital signal processing.

The digital filter260performs a voiceband shaping function and sigma-delta decimation on the incoming sequence259. The digital filter260may also be used to compensate for any loss data bits in the digital signal259from the convertor258. Other processing functions may be performed by the digital filter260as are known to those of ordinary skill.

Next, the digital filtered signal261is passed to an output register264. The output from the output register264is a PCM output signal266which is transferred on the digital backplane170through interface165. The transmission protocol and methods used to relay the PCM output266on the digital backbone170are well known.

As shown, PCM signals210from the digital backplane170enter a register270having a code recognition mechanism272. The fact that register270has a code recognition mechanism272enables the increased bandwidth advantages of the invention. According to one embodiment, predetermined code patterns are transmitted by the service provider40on the digital backplane170and arrive at the PCM interface165. The patterns enter the register270which performs a decode function on the patterns.

Next, the code recognition mechanism272decodes 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 terminal274to the code register310of the linecard microcontroller300. As shown, the strobe terminal274extends from the code recognition mechanism272to the code register310and the interface312.

The linecard microcontroller300performs the linecard control functions of the codec250that enable an increased throughput connection to be achieved when requested by the service provider40. Thus, the strobe terminal274has the ability of interrupting the linecard microcontroller300based on the code patterns received from the digital backplane170.

As is known to those of ordinary skill in the art, various means of implementing the code recognition mechanism272within a linecard codec250can be used. Accordingly, the invention encompasses a codec250having a pattern recognition mechanism that monitors the PCM input165with 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 codec250assumes the slave position. Preferably, the provider modem sends the code patterns and the codec250acts upon them.

Once the code register310receives the strobe signal from the code recognition mechanism272it passes the code pattern to the microcontroller interface312. Preferably the microcontroller interface312is configured to request bandwidth on the digital backplane170from the network administrator. Thus, more or less bandwidth may be allocated for a particular connection through the interface312.

In practice, the linecard obtains the number of timeslots allowed to the requesting channel through the codec250using the microcontroller interface312. When the linecard microcontroller300has determined which timeslots in the backplane170have been allocated to that connection, it programs the codec250to transmit and receive data on all assigned timeslots. In one embodiment, the codec250sends 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 codec250which can be stored in the codec250.

The provider modem can transmit a signal to the subscriber modem21indicating 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 codec250that interfaces with the subscriber modem21. The codec250is able to recognize the code patterns and upon receiving then sends a strobe to the linecard microcontroller300on strobe terminal274indicating that an increased throughput connection has been requested. Through the interface312, the microcontroller300petitions 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 codec250is in “voice” mode by default, operating in the same manner as any standard POTS110. This means that the filters256pass 300–3400 Hz and the converter258samples accordingly using a single timeslot in the backplane170. 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 line165has 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 codec250. The codec250recognizes this bit sequence and sends a strobe to the microcontroller300, giving the microcontroller300permission to switch the codec250to “single-timeslot” data mode. In this mode, only one timeslot is used. To deallocate the secondary timesiots, the microcontroller300interfaces with the network through the interface312. Once the timeslots have been deallocated, the linecard microcontroller300re-programs the codec250for its primary timeslot only. The codec250signals 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.