Abstract:
Methods, apparatuses, and systems are presented for compensating for independent clocks in relayed modem communications involving receiving data packets from a first network device, forwarded over a packet-based network, at a second network device, the data packets representing data in a first modulated signal from a first modem, the first modulated signal being associated with a first symbol rate, placing data derived from the data packets into a queue in the second network device, generating a second modulated signal at the second network device for transmission to a second modem, the second modulated signal being generated using data retrieved from the queue and being associated with a second symbol rate, obtaining at least one queue size measurement associated with data in the queue, and effectively adjusting the second symbol rate based on at least one queue size measurement associated with data placed in the queue in the second network device.

Description:
BACKGROUND OF THE INVENTION 
     FIG. 1  depicts an illustrative environment in which relayed modem communication is carried out over a packet-based network. As shown in the figure, a modem  102  is connected to network device  104 , which is in turn connected to a packet-based network  106 . Also connected to the packet-based network  106  is another network device  108 , which in turn is connected to another modem  110 . 
   Each of the modems  102  and  110  is capable of generating and receiving modulated signals that convey digital information. A wide range of modulation schemes may be implemented. These include different types of amplitude modulation such as quadrature amplitude modulation (QAM), different types of phase shift keying modulation such as quadrature phase shift keying modulation (QPSK), different types of frequency shift keying modulation (FSK), etc. The appropriate type of modulation scheme or schemes used may depend on the specific needs of a particular implementation. Some examples of data transfer standards that may be followed by modems  102  and  110  may include the V. 26, V.32, V.34, and other standards as recommended by the Comité Consultatif International Téléphonique et Télégraphique (CCITT), or Telecommunication Standardization Sector of the International Telecommunications Union (ITU). Modems  102  and  110  may be capable of either transmitting modulated signals in a bi-directional fashion or a uni-directional fashion. 
   Traditionally, modems  102  and  110  would be capable of transmitting modulated signals to one another over an analog medium. For example, the modulated signals may be transmitted over a co-axial cable, on twisted-pair wires, over the air, and so on. The modulated signals may also occupy different portions of the frequency spectrum. For example, the modulated signals may transmitted at baseband, intermediate frequency (IF), radio frequency (RF), etc. 
   However, an environment such as that shown in  FIG. 1  allows modulated signals generated by modems  102  and  110  to be converted to packetized data and relayed over a packet-based network. The relay of modem communications over packet-based networks provides significant benefits by allowing modem communications to be carried out while taking advantage of additional bandwidth, efficient routing, improved control, and other significant benefits that may be associated with a packet-based network.  FIG. 1  illustrates bi-directional communications between modems  102  and  110 , relayed over packet-based network  106 . 
   Communication of data in one direction, from modem  102  to modem  110 , is described in detail below. However, communication of data in the other direction, from modem  110  to modem  102 , may be carried out in a similar fashion. Referring  FIG. 1 , modem  102  receives digital data from a data source (not shown). The data source may be a computer, a voice-based device such as a telephone, a data connection, etc. Modem  102  generates a modulated signal  112  that represents the data received from the data source. For example, modulated signal  112  may be generated based on the data according to a particular modulation scheme. Modem  102  may also implement additional features such as secure transmission and error correction. For example, modem  102  may apply encryption and error correction coding to the data received from the data source. The data, which may be encrypted and coded with error correction codes, may then be represented by symbols in the relevant modulation scheme. Each symbol may correspond to one bit of data. Alternatively, each symbol may represent multiple bits of data. Just as an example, a QAM modulation scheme may utilize each of the four different types of symbols to represent two bits of data. The possible bit pairs represented by a particular symbol may be (0,0), (0,1), (1,0), and (1,1). The symbols are modulated according to the modulation scheme at a particular symbol rate. Typically, the symbol rate is derived from some sort of clock signal. The clock signal may be based on an external clock source, and internal oscillator, etc. 
   Network device  104  receives modulated signal  112  and demodulates it to produce bits of data. This data may still include the encryption and error correction coding applied by modem  102 . The demodulation process may be implemented by digital signal processing (DSP) hardware  114 . Furthermore, the data may be processed by a secure interface unit  116 , which may apply an additional level of security to the data before network transmission. Different types of data security may be implemented, including known security protocols and variations thereof. For example, Secure Telephone Unit—third generation (STU-III), Future Narrowband Digital Terminal (FNBDT), and Secure Communications Interoperability Protocol (SCIP) are some known protocols that may be used. In some implementations, the data is encrypted according to the relevant security protocol by secure interface unit  116 . In other implementations, the data is encrypted using the relevant security protocol by modem  102 , and secure interface unit  116  merely passes along the encrypted data without modifying it. In yet other implementations, modem  102  applies one type of encryption, and secure interface unit  116  applies an additional level of encryption. In any case, network transport processing  118  converts the data into packets  120  appropriate for forwarding over packet-based network  104 . This may involve dividing the data into portions that are then incorporated into different packets. Each packet may include different parts such as headers, body, error correction codes, etc. The packets may be constructed according to protocols defined at multiple levels of networking technology, as is known in the art. Just as one example, packets  120  may be Internet Protocol (IP) packets. Network device  104  thus transmits packet  120  to network  106 . 
   Packets  120  may traverse network  106  via circuitous routes in accordance with the relevant protocols associated with network  106 . Continuing with the previous example, network  106  may be implemented as an IP network, and packets  120  may be routed utilizing IP address information contained in packets  120 . 
   Network device  108  receives packets  120  and converts the packets back into a format suitable for transmission to modem  110 . Specifically, network device  108  may utilize network transport processing  121  to extract relevant data from packets  120 . The data may be processed by a secure interface unit  122 , which may remove any securing encoding or encryption applied by secure interface  116 . Network device  108  then generates a modulated signal  126  that represents this data. The modulation process may be implemented by DSP hardware  124 . Modulated signal  126  may be generated according to a particular modulation scheme at a particular symbol rate. As in the case of modulation signal  112 , the symbol rate associated with modulated signal  126  may be derived from some sort of clock signal. The clock signal may be based on an external clock source, and internal oscillator, etc. 
   Modem  110  receives modulated signal  126  and performs demodulation to generated demodulated data. The demodulated data may incorporate encryption, error correction coding, etc., implemented by modem  102 . Modem  110  may thus perform the relevant decryption and/or error correction decoding to the demodulated data to generate data bits suitable for transmission to a data destination (not shown). The data destination may be a computer, a voice-based device such as a telephone, a data connection, etc. 
   In the course of the data communication from modem  102  to modem  110  described above, the clock signal used to control the symbol rate of modulated signal  112  at the transmit end may differ from the clock signal used to control the symbol rate of modulated signal  126  at the receive end. This is because the clocks signals at the two ends may be independently generated (e.g., from independent oscillators). Even if the clock signals are adjusted to be nominally the same rate and are only slightly different, the resulting difference between the symbol rates can accumulate over time, which is sometimes referred to as “clock drift.” This eventually leads to data underrun or data backup. 
   For instance, the symbol rate of modulated signal  126  may be faster than the symbol rate of modulated signal  112 , which leads to a data underrun condition. Alternatively, the symbol rate of modulated signal  126  may be slower than the symbol rate of modulated signal  112 , which leads to a data backup condition. The practical effects of such conditions can be quite significant. Just as an example, in an system carrying voice data, a data back up condition may cause noticeable audio delay. 
   Referring still to  FIG. 1 , communication of data in the other direction, from modem  110  to modem  102 , may be carried out in a similar fashion and may suffer from the same problem. That is, modem  110  may send a modulated signal  128  to network device  108 . Network device  108  may forward corresponding packets  130  over packet-based network  106  to network device  104 . Network device  104  may send a modulated signal  132  to modem  102 . Because of different clock signals, the symbol rate of modulated signal  126  may be different from the symbol rate of modulated signal  132 . This again can lead to data underrun or data backup conditions. 
   Thus, there is a need to improve modem communications relayed over packet-based networks to compensate for unintended effects relating to the use of independent clocks. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention relates to methods, apparatuses, and systems for compensating for independent clocks in relayed modem communications involving (1) receiving data packets from a first network device, forwarded over a packet-based network, at a second network device, the data packets representing data transmitted in a first modulated signal from a first modem to the first network device, the first modulated signal including symbols associated with a first symbol rate based on a first clock, (2) placing data derived from the data packets into a queue in the second network device, (3) generating a second modulated signal at the second network device for transmission to a second modem, the second modulated signal being generated using data retrieved from the queue, the second modulated signal including symbols associated with a second symbol rate based on a second clock, (4) obtaining at least one queue size measurement associated with data placed in the queue in the second network device, and (5) effectively adjusting the second symbol rate, based on at least one queue size measurement associated with data placed in the queue in the second network device. 
   In one embodiment of the invention, the at least one queue size measurement comprises an average queue size measurement calculated from a first plurality of individual queue size measurements, and wherein the second symbol rate is adjusted based on a comparison of the average queue size measurement against a baseline value. The baseline value may be calculated from an initial plurality of individual queue size measurements. 
   The at least one queue size measurement may further comprise a subsequent average queue size measurement calculated from a second plurality of individual queue size measurements, and the second symbol rate may be effectively adjusted again based on a comparison of the subsequent average queue size measurement against the baseline value. According to one embodiment, the second plurality of individual queue size measurements include no common measurements included in the first plurality of individual queue size measurements. According to another embodiment, the second plurality of individual queue size measurements include some measurements included in the first plurality of individual queue measurement as well as at least one new measurement. 
   Each individual queue size measurement may be taken to coincide with a retrieval of data from the queue. For example, each individual queue size measurement may be taken prior to a retrieval of data from the queue. Alternatively, each individual queue size measurement may be taken after a retrieval of data from the queue. 
   The at least one queue size measurement may corresponds to at least one measured size of a buffer. The second symbol rate may be effectively adjusted by introducing a phase adjustment in the second modulated signal. An adjustment made to the second symbol rate may be restricted to a predetermined maximum limit. Upon occurrence of a known event, the at least one queue size measurement may be cleared and re-obtained to effectively adjust the second symbol rate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts an illustrative environment in which relayed modem communication is carried out over a packet-based network. 
       FIG. 2  is a block diagram of a network device capable of making adjustments to symbol rate for a modulated signal sent to a modem, in order to avoid data underrun and data backup conditions, in accordance with an embodiment of the present invention. 
       FIG. 3  shows an average queue size measurement in accordance with one embodiment of the invention. 
       FIG. 4A  depicts the calculation of average queue size measurements using non-overlapping windows. 
       FIG. 4B  depicts the calculation of average queue size measurements using overlapping windows. 
       FIG. 5  illustrates use of average queue size measurements to adjust symbol rate, in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  is a block diagram of a network device  108  capable of making adjustments to symbol rate for a modulated signal sent to a modem, in order to avoid data underrun and data backup conditions, in accordance with an embodiment of the present invention. Network device  108  may receive packets  120  from packet-based network  106  and generate a modulated signal  126  that is sent to modem  110 . Network device  108  includes network transport processing  121 , security interface unit  122 , and DSP hardware  124 , as discussed previously. While network device  108  is described in detail below, a network device such as network device  104  having similar capabilities may also be employed to receive packets of data in the other direction. Thus, the present embodiment of the invention may be applied to bi-directional communications over a network. 
   Network device  108  monitors a queue of data that is derived from packets  120  and used to generate modulated signal  126 , in order to adjust the symbol rate associated with modulated signal  126  send to modem  110 , thereby avoiding data underrun and data backup conditions. Here, the queue of data is represented as a buffer  202  implemented in DSP hardware  124  and used to store data prior to modulation by a modulator  204 . According to the present embodiment of the invention, both buffer  202  and modulator  204  are implemented in DSP hardware  124 . In other implementations, functions performed by buffer  202  and modulator  204  may be implemented in software. Buffer  202  serves as a first-in-first-out (FIFO) buffer that receives data originating from packets  120  and outputs data to modulator  204 . Network device  108  monitors the buffer size, i.e., the amount of data in buffer  202 . Based on the buffer size, network device  108  makes appropriate adjustments to the symbol rate at which modulator  204  is operated. 
   While buffer  202  is used in the present embodiment of the invention to represent the queue of data monitored for purposes of making symbol rate adjustments, the queue need not be defined as a particular buffer in other embodiments. In fact, the queue of data monitored may span multiple buffers and may even include data being processed that is not stored in a buffer. Just as an example, the data queue may begin with the data that is extracted from packets  120  by network transport processing  121  and end with data that is about to be forwarded to modulator  204  by network device  108 . In such an example, the queue of data would include data being processed by security interface unit  122 , buffer  202 , and any intermediate storage or processing components. Thus, demarcation of the beginning and end of the data queue that is monitored may vary depending on implementation. 
   As mentioned previously, the reason that modulator  204  may be operating at a symbol rate that does not coincide with the rate at which data is arriving via packets  120  is that modulator  204  may be utilizing a clock signal that is independent from the clock signal used to generate modulated signal  112  on the other side of packet-based network  106 . The slight difference between the independent clock signals may be difficult to eliminate. Thus, according to the present embodiment of the invention, adjustments are made to the symbol rate of modulator  204 , without changing the clock signal on which this symbol rate is based. 
   In one specific embodiment of the invention, the symbol rate is adjusted by controlling modulator  204  to adjust the phase of modulated signal  126 . Changing the phase of modulated signal  126  alters the duration of each affected symbol within modulated signal  126 . This effectively changes the symbol rate associated with modulated signal  126 . 
     FIG. 3  shows an average queue size measurement in accordance with one embodiment of the invention. Here, data is placed into the data queue each time a packet  120  arrives from network  120 . Meanwhile, data is retrieved from the data queue each time a portion of data is sent to modulation unit  204  to generate modulated signal  126 . Thus, the size of the data queue is constantly changing as data is inserted into the queue and retrieved out of the queue. 
   Changes in the queue size associated with inputs into the queue may occur at irregular time intervals. Generally speaking, packets  120  may not arrive at network device  108  from packet-based network  106  on a predictable, regular schedule. Indeed, the arrival of packets  120  can appear to be “jittery” or “asynchronous” because it is often difficult to predict the specific route taken through network  106 , as well as the time of arrival, of each packet. Because packets  120  arrive asynchronously, the increases in queue size associated with packet arrivals also occur in an asynchronous fashion. 
   Given such irregularly timed increases in the size of the queue, a single measurement of the queue size can vary simply depending on when the measurement is taken. A measurement of the queue size taken just before a packet arrives would be different from a measurement taken just after the packet arrives. So, a single measurement may not be a dependable measure of the general status of the queue, e.g., a queue size indicating a underrun condition or a backup condition. 
   As shown in  FIG. 3 , an average queue size is used to “average out” variations in the queue size caused by the asynchronous manner by which packets arrive from the packet-based network. Here, the timing of when each individual measurement of the queue size is made coincides with a retrieval of data from the data queue. In the embodiment illustrated in  FIG. 3 , every time data is retrieved from the data queue for forwarding to modulation unit  204 , just prior to the retrieval, a measurement is taken of the size of the data queue. In an alternative embodiment, every time data is retrieved from the data queue to forward to modulation unit  204 , just after to the retrieval, a measurement is taken of the size of the data queue. 
   In this example, 24 symbols of data is retrieved from the data queue and sent to modulation unit  204  every 10 msec. Prior to each retrieval, an individual measurement of the queue size is taken.  FIG. 3  depicts ten such individual queue size measurements taken at 0 msec, 10 msec, 20 msec, . . . , and so on. These ten individual queue sizes are (in symbols): 80, 56, 80, 56, 80, 104, 80, 56, 80, 56, 80. The first nine measurements are shown as being used to calculate an average queue size measurement of 74.666 symbols:
 
(80+56+80+56+80+104+80+56+80+56)/9=74.666
 
   As  FIG. 3  illustrates, even though packets  1  through  4  arrive at irregular time intervals to introduce jumps in the individual queue size measurements, such variations are “averaged out” by using the average data queue size measurement of 74.666 symbols, instead of any particular one of the individual data queue size measurements. 
   Here, the average measurement is shown as being calculated over a period of 90 msec. However, the average measurement may be calculated over a longer period of time, such as 500 msec or greater. The specific period of time over which an average is calculated can differ, depending on the implementation. Similarly, other parameters may be varied depending on the implementation. For example, the interval at which data is retrieved from the data queue and forwarded to modulation unit  204  is shown here as 10 msec. However, a different interval may be used depending on the specific implementation. 
   The above describes the calculation of a single average queue size measurement. Multiple ones of such average queue size measurements may be calculated over time and used in accordance with embodiments of the present invention. Different manners of obtaining multiple average queue size measurement are discussed in further detail below, including use of non-overlapping windows and overlapping windows. 
     FIG. 4A  depicts the calculation of average queue size measurements using non-overlapping windows. Individual queue size measurements are represented as “x” marks. Here, the first nine individual queue size measurements are considered to be in a first window  410 . These nine individual queue size measurements are used to calculate a first average queue size measurement. The next nine individual queue size measurements are considered a second window  412 . Those nine individual queue size measurements are used to calculate a second average queue size measurement. Windows  410  and  412  are non-overlapping in the sense that they do not contain any common queue size measurements. The process is repeated to calculate additional average queue size measurements. Here, if the individual queue size measurements are taken at 10 msec intervals, a new average queue size measurement can be calculated every 90 msec. In this manner, multiple average queue size measurements may be obtained using non-overlapping windows. 
     FIG. 4B  depicts the calculation of average queue size measurements using overlapping windows. Again, individual queue size measurements are represented as “x” marks. Here, the first nine individual queue size measurements are considered to be in a first window  420 . These nine individual queue size measurements are used to calculate a first average queue size measurement. The last eight individual queues size measurements in window  420 , plus a new individual queue size measurement, are together considered a second window  422 . The nine individual queue size measurements in the second window  422  are used to calculate a second average queue size measurement. Windows  420  and  422  are overlapping in the sense that they share common queue size measurements. The process is repeated to calculate additional average queue size measurements. Here, if the individual queue size measurements are taken at 10 msec intervals, a new average queue size measurement can be calculated every 10 msec. In this manner, multiple average queue size measurements may be obtained using overlapping windows. 
   The use of overlapping windows as shown in  FIG. 4B  may require more computational resources than the use of non-overlapping windows in  FIG. 4A . For one thing, the use of overlapping windows as shown in  FIG. 4B  involves calculating a new average queue size measurement every time a new individual measurement is obtained, e.g., every 10 msec. One advantage to the overlapping windows approach is that average queue size measurements are obtained more frequently, so adjustments to the symbol rate in response to changes in the size of the queue may also be made more frequently. This likely leads to smoother symbol rate adjustments, with individual adjustment having relatively smaller magnitudes. In some implementations, non-overlapping windows may be adopted in view of limited computational resources. In other implementations, overlapping windows may be adopted for better symbol rate adjustment performance. Thus, the approach chosen can depend on the specific needs of a particular implementation. Furthermore, variations on such approaches may also be adopted. 
     FIG. 5  illustrates use of average queue size measurements to adjust symbol rate, in accordance with an embodiment of the present invention. In the example depicted here, it is assumed that a new average queue size measurement is obtained every 500 msec. According to the present embodiment of the invention, the first average queue size measurement is stored as a baseline value against which subsequent average queue size measurements may be compared. In other embodiments, a baseline value may simply be fixed at a predefined value. 
   In any case, once the baseline value is established, subsequent average queue size measurements are subtracted against the baseline value to generate an “absolute delta” value. In this example, the first average measurement, used as the baseline value, is 79.98 symbols. The second average queue size measurement is 80.50 symbols. So, the absolute delta value associated with the second average queue size measurement is +0.52 symbols. The third average queue size measurement is 80.00 symbols. So, the absolute delta value associated with the third average queue size measurement is +0.02 symbols. 
   According to the present embodiment of the invention, each absolute delta value calculated in this manner is used to determine a corresponding adjustment to the symbol rate of the relevant modulated signal. Here, the absolute delta value is multiplied by a scalar X to generate the proper adjustment to the symbol rate. The scalar X can be empirically calculated or determined by trial and error. For example, if a particular absolute delta value is 10 symbols, the scalar value is determined to be 0.061, then the calculation for the adjustment may be: 
   
     
       
         
           
             
               
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   The adjusted symbol rate that results is the previous symbol rate plus the adjustment. Thus, if the previous symbol rate is 2400 symbols per second, the adjusted symbol is: 
   
     
       
         
           
             
               
                 
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   The adjustment is applied to the relevant modulated signal. For example, in  FIG. 2 , the symbol rate associated with modulated signal  126  may thus be repeated adjusted based on average queue size measurements corresponding to buffer  202 . This process of calculating an adjustment based on a new average queue size measurement may be repeated each time a new average queue size measurement is obtained. Thus, the symbol rate is continually adjusted to control the average queue size, and data underrun and data backup conditions may be avoided. 
   According to one embodiment of the invention, a predetermined maximum limit is be applied to the symbol rate adjustment. An abrupt and sufficiently large change in the symbol rate of a modulated signal can sometimes cause the modem receiving the modulated signal to lose symbol or clock alignment as the modem demodulates the modulated signal. The lost of symbol or clock alignment may lead to bit errors, which can degrade the performance of the data transmission. A maximum limit on each symbol rate adjustment may help to prevent negative impacts such as loss of symbol or clock alignment on the demodulation system. The maximum limit may be determined as a general default value. Alternatively, the maximum limit may be determined based on the specific capabilities of the demodulation system. 
   According to one embodiment of the invention, certain known events can affect the size of the data queue being monitored, without necessarily indicating that an adjustment to the symbol rate of a modulated signal is required to prevent an imminent data underrun or data backup condition. For example, at the beginning of a transmission when the data queue is just beginning to be filled, or at the end of a transmission when the data queue is being emptied, the size of the queue is expected to be lower than any established baseline value. In such a scenario, network device  108  should not respond by making adjustments to decrease the symbol rate of modulated signal  126 . Also, if at some point an interruption occurs in the arrival of packets  120 , the queue size may drop below the baseline value. Again, network device  108  should not necessarily respond by decreasing the symbol rate. Instead, network device  108  may respond to such known events by resetting the symbol rate adjustment process and clearing the individual and/or average queue size measurements. After the reset, acquisition of the individual and/or average queue size measurements and adjustments to the symbol rate may be resumed. This allows fresh data untainted by the known event to be used, to make proper symbol rate adjustments. 
   The various calculations and operations performed with regard to obtaining individual and/or average queue size measurements, appropriate symbol rate adjustments, etc. may be performed using a resource such as DSP  124 . Other hardware, software, or combination of hardware or software may be utilized in addition to or instead of DSP  124  may also be used as is known in the art. 
   While the present invention has been described in terms of specific embodiments, it should be apparent to those skilled in the art that the scope of the present invention is not limited to the described specific embodiments. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that additions, subtractions, substitutions, and other modifications may be made without departing from the broader spirit and scope of the invention as set forth in the claims.