Abstract:
A method and system for decreasing the probability of blocked access to a local exchange carrier&#39;s telecommunications network comprises a multi-mode line card for determining whether a particular call is a voice or data call. If a call is determined to require data transmission, the line card assigns time slots for interconnection of the line card to a serving digital switch. If no meaningful data is received, no time slots are assigned. In this manner, time slots interconnecting the serving digital switch to a line card are established only if data or voice is being transmitted from a subscriber line.

Description:
TECHNICAL FIELD 
     This invention relates to telecommunications systems, and more particularly, to providing subscriber line cards deployed in such telecommunications systems. 
     BACKGROUND OF THE INVENTION 
     Telecommunications has evolved from transmitting only voice to a multimedia vehicle for communication. Modern users of telecommunications services are supported by subscriber lines which not only transmit voice but also send data transmissions generated by facsimiles and Internet access. A common implementation of a modem subscriber line interconnects a twisted cable pair from a subscriber&#39;s customer premises equipment to a subscriber line card associated with a digital switching system. The line card provides functions of battery, overvoltage protection, ringing, supervision, hybrid and testing (commonly referred to as the BORSHT functions) and converts analog voice band frequencies into a stream of pulse code modulation octets carried at 64 kilobits per second across the digital switching system and the rest of the public-switched telecommunications network (PSTN). 
     Current line card implementations operate in a mode for optimizing voice transmission. The transmission of data signals requires use of modulator/demodulator devices (modems) for transforming digital data signals into analog signals which are carried within the standard voice band frequency limitations of the telecommunications network. Current voice band frequency limitations of the PSTN hold the per subscriber line modem data transmission rate to approximately 56 kilobits per second. Unfortunately, the 56 kilobits per second transmission rate is unacceptably slow for many telecommunications service subscribers who routinely send data via the PSTN. To obtain higher transmission rates, it is common for these subscribers to bind two or more analog subscriber lines together through special modems working in the role of inverse multiplexers. Special modems, or any increase in the number of lines used per subscriber, are problematic because of the increase in average holding time of all telephone calls served by the switch. Further, increased subscriber line usage invalidates current standards for concentration of subscriber lines per switch and the provisioning of intraswitch time slot facilities. 
     The real-life impact of an increase in concentration of subscriber lines per switch is the more frequent denial of dial tone or the receipt of busy signals by subscribers who are accustomed to consistent and reliable access to the PSTN. The current state of subscriber line concentration is especially troubling since switch resources are unnecessarily occupied. This is because a significant amount of data transmissions between a subscriber line and the PSTN carry idle data (that is, non-useful packets of information) which is a well known and common phenomenon of data connections. Therefore there is a need in the art for decreasing the occurrence of blocked access to the telecommunications network while efficiently accommodating prolonged data transmissions. 
     SUMMARY OF THE INVENTION 
     This need is addressed and a technological advance is achieved in the telecommunications art by a multi-mode line card for determining and establishing a mode of operation consistent with subscriber needs. More particularly, a multi-mode subscriber line card operates in a packet transmission mode (hereinafter referred to as “mode  2 ”) for allowing a faster bit rate transmission from a line card to a digital switching system but can be automatically switched to a voice only transmission mode (hereinafter referred to as “mode  1 ”). Other modes of operation may accommodate new voice companding techniques or packet transmission of compressed voice signals. 
     In the preferred embodiment, the multi-mode line card serves a plurality of subscribers in a local telecommunications system. When off-hook status is detected by a subscriber line interface circuit, a determination is made as to whether a distinct packet mode initiation signal is received within a predetermined time period. If no such signal is received, the subscriber line operates in mode  1  in which well-established filtering, sampling and companding standards are used for voice transmissions. 
     If a packet mode initiation signal is received, the multi-mode line card establishes filters, analog-to-digital (A/D) sampling and line coder/decoders for receiving and decoding packet transmissions in mode  2 . The parameters of the filters, A/D sampling and coding are established in an exchange with customer premises equipment initiating the packet call. A packet protocol exchange, which occurs between the line card and the customer equipment for the duration of the packet mode state, allows the removal and insertion of packets to and from the subscriber line via a packet queue. The packet queue is shared across multiple subscriber line interface circuits and reduces the number of circuit resources which must be held active during packet mode calls. A shared transmission capacity (hereinafter referred to as a “packet link”) is used for reading and writing packets of data into and out subscriber line associated queues. Advantageously, prolonged periods of non-useful data transmission, such as when a user is simply viewing information received on a personal computer but not actively sending data, are not transmitted to the digital switch. The transmission of only meaningful data decreases the inefficient use of intraswitch time slot facilities and minimizes the prevention of other users from transmitting useful data In this manner, subscriber line congestion is reduced by effectively allocating switch resources. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a simplified block diagram of a subscriber line card and associated systems in which the present invention may be practiced; and 
     FIG. 2 is a more detailed block diagram of a digital signal processor shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows subscriber line card  100  including per line circuitry  102 - 1 , mode  2  digital signal processor (DSP)  140 , mode  1  DSP  150 , controller  160 , packet queue  170  and concentrator mechanism  190 . In the embodiment shown, per line circuitry  102 - 1  serves subscriber line  101  and is interconnected to a PSTN-connected digital switching system (not shown) via concentrator mechanism  190  and intraswitch time slot facility  191 . In an operational embodiment, there are a plurality of subscriber lines each with its own circuitry served by mode  2  DSP  140 , mode  1  DSP  150 , controller  160 , packet queue  170  and concentrator mechanism  190 . For illustrative purposes, however, circuitry associated with a single subscriber line (that is, per line circuitry  102 - 1 ) is described below but existence of up to “n” subscriber lines and associated circuitry is represented by per line circuitry  102 -n. 
     Per line circuitry  102 - 1  comprises subscriber line interface circuit  104  interconnected to receive mode switch  106  and transmit mode switch  108  via low voltage interfaces  105  and  107 , respectively. Subscriber line interface circuit  104  serves as a conduit between subscriber line  101  and the digital switching system. More particularly, subscriber line interface circuit  104  detects on/off hook status of customer premises equipment associated with subscriber line  101  and provides sufficient bandwidth capability so that the subscriber line is operable in either mode  1  or mode  2 . In this regard, links  109  and  113  are designated packet mode data links while links  111  and  115  are voice transmission mode links. All voice transmissions or packet transmissions emanating from subscriber line  101  are directed to receive mode switch  106  and all transmissions directed to subscriber line  101  are handled by transmit mode switch  108 . 
     Mode  1  operation requires receive mode switch  106  to complete a connection between analog-to-digital (A/D) converter  114  and data link  105 . In transmit mode, switch  108  completes a connection to digital-to-analog (D/A) converter  116  via data link  107 . Mode  2  filter and A/D converter  110  is interconnected to receive mode switch  106  via packet mode link  109  while mode  2  filter and D/A converter  112  is interconnected to the switch  108  via packet transmission link  113 . Mode  1  filter and A/D converter  114  is interconnected to receive mode switch  106  via voice transmission link  111  and mode  1  filter and D/A converter  116  is interconnected to transmit mode switch  108  via voice transmission link  115 . 
     Mode  2  is the default mode of operation for receive mode switch  106  and transmit mode switch  108 . In the preferred embodiment, mode  2  operation is commenced when a signal is received from the customer premises equipment associated with subscriber line  101 . Such a packet mode signal is transmitted via filter/converter  110  and mode  2  DSP  140  to controller  160 . If packet mode signal is not received within a predefined time period, controller  160  initiates mode  1  operation by extending a control signal to switch control  119  via control link  117 . Upon receipt of the control signal, switch control  119  issues an alter switch signal to receive mode switch  106  and transmit mode switch  108  such that these switches assume a mode  1  position (that is, a position which completes an interconnection between low voltage interfaces  105 ,  107  and filter/converters  114  and  116 , respectively). 
     Mode  2  DSP  140  is interconnected to filter/converter  110  and filter/converter  112  via links  141  and  143 , respectively. Similarly, mode  1  DSP  150  is interconnected to filter/converters  114  and  116  via data links  151  and  153 , respectively. In the preferred embodiment, mode  2  DSP  140  is interconnected to controller  160  and packet queue  170  via packet link  145 . Controller  160  and packet queue  170  are interconnected via link  165 . In an operational embodiment, mode  2  DSP  140  and mode  1  DSP  150  serve a plurality of subscriber lines. Accordingly, packet link  145  is a conduit between packet queue  170  and a plurality of subscriber lines. Packet queue functionality is described in U.S. Pat. No. 4,577,314 assigned to AT&amp;T Bell Laboratories and incorporated by reference herein. For illustrative purposes, the interaction of a single subscriber line (that is, subscriber line  101 ) and packet queue  170  is described. 
     Packet queue  170  includes a plurality of pairs of subscriber line buffers  172 - 1  and  174 - 1  through  172 -n and  174 -n. Indeed, each subscriber line associated with subscriber line card  100  has its own designated pair of subscriber line buffers. Network buffers  176  and  178  store data received from or going to the serving digital switch via concentrator mechanism  190 . In the embodiment shown, there are two network buffer queues per “n” subscriber lines. Alternative embodiments may include more than two network buffers for the “n” subscriber lines associated with subscriber line card  100 . 
     During mode  1  operation, analog voice signals are received in subscriber line interface circuit  104  from subscriber line  101  and processed by A/D converter  114  before delivery to mode  1  DSP  150  via data link  151 . Mode  1  DSP  150  is interconnected to data link  197  and concentrator mechanism  190  via connector link  157 . As known in the art, mode  1  DSP  150  processes voice signals and extends these signals to concentrator mechanism  190  via data link  157  so that an appropriate time slot can be assigned for delivery of the signals to the serving digital switch via intraswitch time slot facility  191 . Since voice signal processing is well known, no further discussion of mode  1  operation will be undertaken herein. During mode  2  operation, controller  160  coordinates transmission of data packets between subscriber line  101  and the serving digital switching system via mode  2  DSP  140 . Packet queue  170 , including buffers  172 - 1 ,  174 - 1 ,  176  and  178 , are accessed by mode  2  DSP  140  as directed by control information received from controller  160  via data links  165  and  145 . 
     FIG. 2 shows a more detailed view of mode  2  DSP  140  and its operation in accordance with the preferred embodiment of the present invention. Mode  2  DSP  140  includes processor  142  interconnected to protocol interpreter  144  via data link  147 . Mode  2  data packet transmission operation is commenced soon after a packet mode signal is received from the customer premises equipment associated with subscriber line  101  in subscriber line interface circuit  104 . More particularly, subscriber line interface circuitry  104  transmits the packet mode signal to receive mode switch  106  and filter/converter  110  via interface  105  and data link  109 , respectively. The packet mode signal is extended to mode  2  DSP  140  which passes the signal to controller  160  via packet links  145  and  165 . Upon receipt of the packet mode signal, controller  160  identifies a subscriber line buffer, such as buffer  172 - 1  as a holding queue for receiving data packets transmitted from subscriber line  101  during packet mode transmission. 
     Data received from subscriber line  101  via receive mode switch  106  and filter/converter  110  is processed by digital signal processor  142  using information stored in protocol interpreter  144 . More particularly, protocol interpreter  144  is initialized with information for determining whether packet information received from the subscriber line contains meaningful data (that is, data which should be extended to the serving digital switch via concentrator mechanism  190 ) or whether the packet information received is non-useful data such as idle data packets which do not contain information the subscriber needs. For example, non-useful information may simply be an identification or “hand shake” signal. Protocol interpreter  144  ensures that all meaningful data is extended from digital signal processor  140  to a subscriber line buffer over packet link  145  and data link  165 . At predetermined time intervals, controller  160  accesses subscriber line buffer, such as buffer  172 - 1  and moves data stored therein into a network buffer, such as network buffer  176 . Simultaneously, controller  160  directs mode  2  DSP  140  to remove information stored in network buffer  176  and forward it to concentrator mechanism  190  via packet link  145  and data link  197 . 
     Packet transmissions may also be received from the PSTN in mode  2  operation. These data transmissions are received in concentrator mechanism  190  via intraswitch time slot facility  191  and forwarded to mode  2  DSP  140  over data link  197 . Upon receipt of data transmissions from concentrator mechanism  190 , mode  2  DSP  140  accesses controller  160  for instructions which identify the network buffer in which the received data transmission should be stored. In this example, controller  160  directs mode  2  DSP to store received data transmissions in network buffer  178 . The received data includes address information identifying to which subscriber line the data is directed. Controller  160  reads this address information contained in the received data transmission and transports the received data to an appropriate subscriber line buffer. In this example, controller  160  transfers data currently stored in network buffer  178  to a subscriber line buffer such as buffer  174 - 1 . Each subscriber line has at least two buffers so that one buffer may be designated as an outgoing data queue (e.g.  172 - 1 ) and another designated an incoming data queue (e.g.  174 - 1 ). Simultaneously, controller  160  extends a message to mode  2  DSP  140  via packet link  145  indicating that data directed to subscriber line  101  has been received and is currently stored in subscriber line buffer  174 - 1 . Accordingly, mode  2  DSP  140  retrieves the information stored in subscriber line buffer  174 - 1  and transmits it to subscriber line  101  via filter/converter  112  and transmit mode switch  108 . Significantly, mode  2  DSP  140  uses protocol interpreter  144  to analyze all incoming data transmissions received from concentrator  190  to determine whether the data transmission contains meaningful data or non-useful, idle data. Only meaningful data is transmitted to subscriber line  101 . Non-useful data is discarded (that is, it is saved in an erasable memory). 
     Advantageously, the present invention eliminates the transmission of non-useful data traffic between subscriber lines and a serving digital switching system. Further, a single subscriber line is effectively used to originate voice transmissions or exchange a high speed data transmissions. Indeed, the use of packet queue  170  analysis performed by protocol interpreter  144  to determine the content of data transmission enables intraswitch time slot facilities and resources associated with subscriber lines to be used efficiently. This results in the minimization of blocked calls and subscriber line congestion. 
     Although the invention has been illustrated with respect to a preferred embodiment, those skilled in the art will recognize that numerous other arrangements may be devised without departing from the scope of the invention.