Abstract:
An inter-network line level adjustment system includes a switch (SW 1 ) and a processing unit ( 24 ) for identifying calls between a remote telephone network and a local telephone network. Echo cancellers ( 20 ) and/or speech coders ( 22 ) collect line level data associated with the identified calls. The processing unit ( 24 ) determines statistical properties of the collected line level data. Finally, the processing unit ( 24 ) instructs the echo cancellers ( 20 ) and/or speech coders ( 22 ) to adjust, based on the determined statistical properties, line levels of future calls between the remote telephone network and the local telephone network to a desired line level.

Description:
TECHNICAL FIELD 
     The present invention relates to an inter-network line level adjustment method and system that compensate for different line levels in different telephone networks. 
     BACKGROUND 
     The telecommunication market gets more and more deregulated. This means that there usually are several active operators having their own telephone networks in a given geographical region. Sometimes a subscriber belonging to one operator wishes to call a subscriber belonging to another operator. In such a case different networks have to be interconnected. However, often networks belonging to different operators have different line (signal) levels. This results in a difference in perceived speech level for inter-network calls as compared to intranetwork calls. Furthermore, this difference in line level also gives performance problems in level sensitive equipment, such as echo cancellers and speech coders. 
     Reference [1] describes systems for manual line level measurement in a telephone network. 
     Reference [2] describes an automatic level control system based on line level measurements in a telephone network. This system dynamically adjusts the line level to a desired value. A drawback of such a system is that the line level has to be measured before an adjustment is possible. However, such a measurement will take some time. This implies that during the first few seconds of a call the line level may be either too high or too low before a measurement and a consequent adjustment have been made. This is especially annoying if a call from a rather quiet call is followed by a loud one, or vice versa. In this case the completely inappropriate line level of the previous call will be used as starting point for the new call. A similar problem occurs if a loud talker hands over the phone to a more quiet talker (or vice versa) during a call. 
     Furthermore, none of these systems distinguishes between intranetwork and inter-network calls. Therefore they are not capable of individually adjusting line levels for inter-network calls. 
     SUMMARY 
     An object of the present invention is an inter-network line level adjustment method and system that provide individual line level adjustment for inter-network calls. 
     This object is achieved in accordance with the attached claims. 
     Briefly, the present invention collects statistical data of line levels associated with calls between a remote and a local network. Based on these data the system adjusts the line level of future calls between the remote and the local network. This procedure has several advantages. For example, it is automatic, adjusts levels immediately at the start of a call, and may be implemented in already existing equipment, such as echo cancellers or speech coders. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which: 
     FIG. 1 is a block diagram illustrating interconnection of telephone networks; 
     FIG. 2 is a block diagram illustrating some embodiments of the present invention; and 
     FIG. 3 is a flow chart illustrating an embodiment of the method of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a block diagram illustrating interconnection of telephone networks. A local telephone network, denoted network  1 , is connected to remote telephone networks, denoted network  2  and network  3 . The terms “remote” and “local” should not be interpreted literally; the networks may actually, and often do cover the same geographical region. 
     Typically each network has a different operator. Usually telephone calls originate and terminate in the same network, and each network establishes its own general line level. However, sometimes a remote subscriber  10  (A-subscriber) in network  2  calls a local subscriber  12  (B-subscriber) in network  1 . In this case the call is directed from switch SW 2  in network  2  to SW 1  in network  1  on a trunk TR 12 . Since the line levels may be different in the two networks, such a connection may result in an annoying higher or lower speech level than normal at subscribers  10  and  12 . Furthermore, line level sensitive equipment, such as echo cancellers, may also have performance problems due to unusually high or low signal amplitudes. 
     A similar problem occurs when a remote subscriber  14  (A-subscriber) calls a local cellular phone MS (B-subscriber). In this case performance problems may occur in the speech coder that is located in either base station BS or switch SW 1 . 
     The same problems also occur when a local subscriber calls (either on a regular or cellular phone) a remote subscriber, as illustrated by local subscriber  16  (A-subscriber) in network  1  calling remote subscriber  18  (B-subscriber) in network  3  over route SW 1 -TR 13 -SW 3 . The problems also occur between local and remote cellular phones. 
     FIG. 2 is a block diagram illustrating some embodiments of the present invention. This figure is an expanded version of the right part of FIG. 1, in which blocks essential to the present invention have been included. 
     The system in FIG. 2 includes a pool  20  of echo cancellers and a pool  22  of speech coders/decoders connected to switch SW 1  of local network  1 . When a call originating/terminating in a remote network and terminating/originating at a PSTN subscriber in the local network is established, this call is allocated one of the echo cancellers in pool  20 . During the call the echo canceller measures the line level of the incoming signals from the remote network, for example by using the algorithms described in [1], and reports this line level to a processing unit  24 . Processing unit  24  may comprise the control unit that controls switch SW 1 . Processing unit  24  also receives information from switch SW 1  regarding the trunk that is associated with the measured line level and whether the call is terminating/originating in the local network. The trunk information identifies the calling/called remote network. The measured line level and associated trunk are recorded by processing unit  24  for each remote call. Such measurements are accumulated in a histogram, and an average line level L associated with the remote network is calculated in accordance with:        L   =         ∑   i                       h   i          c   i             ∑   i          c   i                                
     where c i  denotes the number of recorded calls to/from the remote network that have a measured line level in a line level interval around a center level h i . Although not necessary, these intervals typically are of equal length. The length h i+1 −h i  of an interval should be chosen in accordance with the line level adjustment step size to be used. A preferred length is 3 dB. The intervals should cover all possible line levels, i.e. from −68 dBm 0  to 3 dBm 0 . 
     The calculated average line level L may be compared to a desired line level D (typically the general line level of the local network  1 ) to perform an appropriate adjustment. 
     To avoid degradation of speech quality of individual connections when line levels differ much from the average L, the level adjustment is preferably done taking into account the variability of the line levels on a given route. A measure of this variability may be expressed as:        V   =           ∑   i              (       h   i     -   L     )     2          c   i             ∑   i          c   i                                  
     The resulting adjustment may be expressed as:        K   =     {             max        (     L   ,     min        (       L   +   Δ     ,       D   MAX     -   rV       )         )       ,     Δ   &gt;   0                   min        (     L   ,     max        (       L   +   Δ     ,       D   MIN     +   rV       )         )       ,     Δ   &lt;   0                                      
     where Δ=D−L, and D MAX , D MIN  are the upper and lower limits for acceptable line level limits, respectively, for example D MAX =−6 dBm 0 , D MIN =−27 dBm 0 . A preferred value of the constant r is r=3. 
     At least 100 measurements (Σc i =100) should be made to build up the histogram before any adjustments are done. Preferably more measurements, for example 1000, should be used. 
     Sliding average and variability computations are also possible, but the presently preferred method is to update the histogram with every measurement and to recompute L and V from time to time. 
     To cope with possible significant changes in the average level of a remote network, it is also possible to regularly start a new histogram, for example once a month. In this case the adjustment computed from the old histogram should be used until enough measurements have been collected to make the new histogram reliable. 
     The above principles are of primary interest in exchanges having echo cancellers, such as Mobile Services Switching Centres and international exchanges. Furthermore, only calls that originate or terminate in the local network are of interest. Transit calls are ignored. In this regard a call is usually associated with its remote network of origin or termination, since levels of pure transit calls are typically not changed by intermediate networks. 
     The calculated adjustment K is transferred from processing unit  24  to an echo canceller in pool  20  each time an echo canceller is allocated to a call that originates or terminates in the remote network that is associated with this adjustment value (each remote network is associated with its own adjustment). This may be done by “virtual trunks”, which is a method of assigning route specific parameters to an echo canceller. It will set up the echo canceller to fit the route to which it will be connected. See [3] for more information on virtual trunks. The adjustment itself is typically performed by multiplying each incoming (to the local network) sample in the echo canceller by a scale factor. 
     The above discussion related to echo cancellers in pool  20 . However, the same measurements and adjustments may also be performed in speech coders in pool  22 . In fact they may be performed in any equipment having signal processing capabilities to perform the measurement algorithms and adjustment. 
     FIG. 3 is a flow chart illustrating an embodiment of the method of the present invention. In step S 1  a call to or from a remote network is setup. Step S 2  measures the received line level of this call. Step S 3  determines the trunk on which the call was setup to identify the remote network. Step S 4  records the information. Step S 5  tests whether enough data for calculating an average has been collected. If not, the procedure returns to step S 1  and repeats steps S 1 -S 4  for the next call to or from the same remote network. If enough data has been collected, measures of the average and variance of the received line levels are calculated in steps S 6  and S 7 . Step S 8  calculates the adjustment from these parameters. Finally, step S 9  uses the determined adjustment for adjusting the line level of future calls to and from this remote network. Steps S 1 -S 8  may be repeated at a later time to determine a new adjustment. Similar procedures may be performed for each remote network. 
     In the above description it has been assumed that adjustments are made in signals directed to the local network. However, if desired adjustments in the opposite direction may also be performed on outgoing signals. 
     The present invention has several attractive features: 
     1. Both the measurements and adjustments are automatic. 
     2. The invention may be implemented in already existing equipment simply by reprogramming. 
     3. The method is robust against errors, since a large amount of measurements are averaged before actual adjustments are made. 
     4. The proposed method is capable of adjusting levels immediately at the start of a call, thereby avoiding abrupt level changes. 
     It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims. 
     REFERENCES 
     [1] “Objective Measurement of Active Speech Level”, ITU-T Recommendation P.56 
     [2] Ying Tao, “Automatic Level Control”, G.ALC DRAFT 3, ITU-T Recommendation G.ALC 
     [3] A. Eriksson et. al., “Ericsson echo cancellers—a key to improved speech quality”, Ericsson Review No. 1, 1996, pp. 25-33.