Abstract:
Given the prerequisite of a division of a subscriber line circuit into a high-voltage portion and a signal processor that defines the transmission-oriented properties, voltages arising at significant locations of the receiving branch and of the transmitting branch of the signal processor as a result of supplied and reflected sine signals are taken via a digital interface and specific transmission properties such as reflection attenuation and hybrid transfer attenuation are calculated therefrom on the basis of a correlation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for checking the transmission properties of a subscriber line circuit together with subscriber line and subscriber terminal equipment connected thereto in a digital time-division multiplex (TDM) telecommunication system, and more particularly in which the subscriber line circuit is composed of a high-voltage module that predominantly contains line drivers as well as elements for indicating subscriber line state and a signal processor module wherein, among other things, the analog-to-digital conversion and the digital-to-analog conversion, the realization of a two-wire-to-four-wire conversion, the setting of the two-wire impedance and the transmission and receiving signal amplification occur. 
     2. Description of the Prior Art 
     Heretofore, such tests were implemented with a separate testing device that had access to specific circuit points of the subscriber line circuit or of the subscriber line via operated relays. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a method for checking the transmission properties of a subscriber line circuit wherein the conditions initially set forth are established and for which such a test access via relays can be forgone in the implementation thereof. 
     The above object is achieved, according to the present invention, in that various locations of the receiving branch and of the transmitting branch of the signal processor module are accessed via a digital interface and a correlation product is formed from different combinations of, respectively, two voltages from those voltages that occur at these locations during the operation of the subscriber line circuit, or due to separately-supplied and reflected signals, the evaluation of this correlation product respectively yielding a particular information about a specific transmission-oriented property. 
     In accordance with a further development and feature of the invention, an access and a correlation processing also occur for the result of the evaluation of the direct-access component of current that is obtained in the high-voltage portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the invention, its organization, construction and operation will be best understood from the following detailed description, taken in conjunction with the accompanying drawings, on which: 
     FIG. 1 is a schematic representation of a test access via relays as practiced heretofore; and 
     FIG. 2 is a schematic illustration of a subscriber line circuit divided into a high-voltage portion and a signal processor module and having the properties required for the implementation of the method of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a subscriber terminal equipment TE is illustrated as connected to a subscriber line circuit SLC via a subscriber line TL. A testing device P whose properties shall not be discussed in detail has access to the subscriber line TL and to the subscriber line circuit SLC via relays Re1 and Re2 (symbolically illustrated by switch contacts). 
     Referring to FIG. 2, the subscriber terminal equipment TE is connected to the high-voltage portion SLIC-H of a subscriber line circuit via a subscriber line TL. A pair of line sensors S1 and S2 that serve the purpose of identifying the state of the subscriber line loop are indicated as critical components of the high-voltage portion. A transverse current signal is acquired via the resistors R1-R4 and via a differential amplifier VD1, the transverse current signal serving as a transmission signal. Receiving signals arriving from the opposite direction are amplified via a pair of line drivers T1 and T2 and are forwarded to the leads of the subscriber line. The high-voltage portion also comprises a plurality of resistors R5-R8 that are connected between the leads of the subscriber line and at whose junctions voltages are taken that are supplied to the inputs of a differential amplifier VD2 for acquiring a signal corresponding to the direct-access component current. 
     As a further component of the subscriber line circuit, FIG. 2 also symbolically illustrates a signal processor module SLIC-SP. The components of the signal processor module connected to the receiving branch Rx or, respectively, to the transmitting branch Tx of this module or, respectively, inserted into these branches are to be understood as functional blocks. In detail, an analog-to-digital converter A/D or, respectively, a digital-to-analog converter D/A are provided as well as a feedback circuit Z that, via an adder Ad1 (further connected to receive an input signal via the receiving branch input Rx) supplies a portion of the signal arising in the analog-to-digital converter A/D to the digital-to-analog converter D/A that serves for setting the two-wire impedance. A coupler B that lies between the receiving branch Rx and at the transmitting branch Tx via an adder Ad2 and that serves for two-wire-to-four-wire conversion, is shown along with gain setting circuits GR and GT which set the gain in the corresponding branches Rx and Tx, and a control P in which a digital interface is also realized. Also illustrated is a correlator K. The numerals 2-5 in the receiving branch and in the transmitting branch or, respectively, at the output of the differential amplifier VD2 connected thereto indicate access points which the signal processor accesses via the digital interface, for which reason these numerals are also marked on the control P. 
     In accordance with the invention, a respective pair of voltages is selected from the voltages that appear at the indicator points during operation of the subscriber line circuit and this pair of voltages is subjected to the formation of a correlation product from which a specific transmission property is then respectively calculated. The following table thereby illustrates the various possible combinations or, respectively, various transmission properties to be tested. 
     
         ______________________________________          Measured Value                       Measured Value          at the Measuring                       at the MeasuringTypes of Measurement          Point a      Point b______________________________________reflection attenuation          2            1gain Rx        0            2gain Tx        3            6hybrid transfer attenuation          2            3transmission level on line          2            4balance measurement          5            4balance measurement          2            5______________________________________ 
    
     By forming a correlation product of two signal quantities, a particular information concerning the degree of dependency of these signal quantities is obtained and this can lie between 0, i.e. complete independence, and 1, i.e. complete dependence. 
     When, therefore, a signal that is the amplified signal received via the input Rx appears at the measuring point 2 is compared to the signal at the measuring point 1 that corresponds to the signal reduced by the portion of the reception signal reflected at the two-wire input and returned via the feedback circuit Z, then the correlation formation provides information concerning the reflection attenuation. 
     When the signal quantities at the input side and the output side of the gain setting circuits G R  and G T , i.e. the signal quantities at the measuring points 0 and 2 or, respectively, the measuring points 3 and 6 are placed in relationship to one another by forming a correlation product, then the result of the formation of the correlation product supplies information concerning the gain in the receiving branch or, respectively, in the transmitting branch. 
     When the signal at the measuring point 3 that represents the signal proceeding onto the transmitting branch Tx and that is composed of the signal coming from the two-wire line, i.e. from the subscriber line circuit portion SLIC-H and a portion of the reception signal incoming via the reception branch Rx that corresponds to the reflection at the two-wire-to-four-wire junction, (whereby this portion is defined by the setting of the coupling at the coupler B) is correlated with the reception signal at the measuring point 2 then a value for the hybrid transfer attenuation results from the correlation product. 
     When the signals at the measuring point 5 at which as presented, a signal corresponding to the longitudinal current is acquired is subjected to a correlation with signals measurable either at the measuring point 2 or at the measuring point 4, information concerning the balance conditions of the subscriber line circuit is obtained. 
     Instead of a check during active operation of the subscriber line circuit, the reaction to separately-supplied signals that are returned via a test loop L can also occur during the passive operation thereof. 
     Although I have described my invention by reference to particular illustrative embodiments thereof, many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention. I therefore intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly be included within the scope of my contribution to the art.