IL46447A - Pilot controlled data transmission system - Google Patents

Description

nina »"y mpiaon o»3in3 in»» nanya Pilot controlled data transmission system SIEMENS AKTIENG-ESELLSCHAFT C. 44230 The invention relates to pilot-controlled data-transmission systems for the transmission of a data transmission band which is accompanied by a pilot with a frequency which may be switched over, the system having an analysis device for the . recognition of the pilot frequency and a band stop filter in the transmission path, with the aid of which the pilot frequency is blocked, in particular for a telecommunication system providing multiple connections which are individually pilot-monitored and may be switched over, at least in one transmission section, at least at one end of that transmission section being provided with means for determining what is the operating state of the connected station and/or of the incoming transmission section, this state being indicated by a signal transmitted below the frequency band provided for data transmission to another station, the local station containing at least one pilot signal generator with a frequency below the data transmission band which is to be tranmitted, and means being provided to produce control and/or display criteria in the other station in dependence upon the received pilot tones, wherein an individual frequency is transmitted for each respective operating state which is to be distinguished, and the noise spectrum is analysed in the part of the transmission band below the data transmission band.

A data transmission system of this kind is described, for example in the German Patent Specification No. 2,044,001.

The pilot is transmitted together with the data transmission band j and has a frequency which may be switched-over between discrete values below the frequency band provided for the data transmission, so as to serve to monitor the operating state of low frequency transmission devices for the data transmission. At the receiving end, in dependence upon the received pilot frequency, control-and/or display criteria are formed.

It can be expedient to simultaneously carry out an analysis of the noise spectrum in the part of the transmission band below the data transmission band or useful band in order to effect a substitute connection in the event of detection of a disturbed transmission channel. Therefore it can be necessary to block the pilot received on one transmission path if a pilot is newly fed-in via another transmission section, since otherwise beats can occur which may simulate noise in any ■ noise-analysis equipment at the end of the next transmission section.

The pilot frequency and the data transmission band can be very close to one another, and may therefore be relatively difficult to separate. For example, a pilot frequency of 340 Hz may be used with a data transmission band commencing at 360 Hz., in a typical system.

The demands made on the band stop " filter, in particular in terms of blocking attenuation and the steep flanks required for its characteristic can lead to a relatively large outlay in, equipment and space requirements, in particular when the band stop filter in the data transmission system is not an .monitored active element that is7 but is designed as a passive circuit.

One object of the present invention is to provide a data- transmission system of the type described in detail above, which operates in such manner that pilot frequencies received in a receiving device can be blocked in a manner which may be realised as simply as possible, and that the transit time distortions normally caused in the data transmission band by the pilot filter lie within the limits permissable for the relevant operatin state of the data transmission system.

The invention consists in a pilot controlled data transmission system in which an analysis device is provide'd for the recognition of the pilot frequency, and a band stop filter is placed in each transmission path to block, the received pilot frequency, and in which means are provided at at least one end of a transmission section to provide a criterion for the operating state of this station and/or of the incoming transmission section, and -transmitted back -at a frequency below the frequency band provided fo.r data transmission to the opposite station, wherein the station at said one end of said transmission section contains at least one pilot signal generator with a frequency below the data transmission band which is to be transmitted, and control - and/or display criteria are formed for transmission, in the opposite station in dependence upon the received pilot frequency, an individual frequency being transmitted for each operating state which is to be distinguished, and the noise spectrum being analysed in the part of the transmission band below the data transmission band, said band stop filter being provided with means by which its blocking range is controlled from the analysis device for the recognition of the pilot frequency.

. These measure produce the advantage that the blocking range of the band stop filter of the particular frequency which is to be blocked can automatically be adapted to the particular requirements .in the event of the additional use of devices already provided for other purposes in the data transmission system. In the case of one or more switching sta*tes, the bond stop filter requires only to be designed for a part of the total range provided for the pilot frequencies, so that a high blocking attenuation may be acnieved with a particularly low outlay in equipment.

If the pilot frequencies are transmitted in the direct vicinity of the data transmission band, and in particular closely below a LF signal band, then the construction of the band stop filter is desirably such that the transit time distortions at the edge of the data transmission band do not exceed the particular required maximum values. Considerations have shown that in data transmission systems in which the frequency of the pilot transmitted from a station serves as a criterion for the state of the station or of lines incoming to said station, by means of a special assignment of the pilot frequencies to the message criteria, it is possible to reduce the demands on the band stop filter located in the opposite station.

' Advantageously the data transmission system is designed to employ a pilot frequency lying closest to the data transmission band to serve as a message criterion for a state of the dat transmission system in which the opposite direction is disturbed. In particular in the case of pilot-monitored multiple connections which may be switched over, and in which the received pilot is used to indicate the state of the outgoing lines reported back from the opposite station that member of the pilot frequencies lying closest to the data transmission band can be used as a message criterion for the state "both outgoing lines disturbed".

As a result of these measures even the pilot frequency directly adjacent to the data transmission band can be 'safely blocked with simple measures without the transit time distortions in the data transmission band which have been accepted leading to an inacceptable impairment of the operation. As long as disturbance-f ee operation is possible, the required blocking attenuation can be realised in a simple, fashion without the transit time and attenuation distortions in the useful range of the transmission channel exceeding the desired values.

The invention will now be described with reference to the drawing, which is a schematic circuit diagram of one exemplary embodiment.

In the embodiment illustrated, a data transmission network is shown which uses a conference line or common signal channel to connect a plurality of stations U , U2 etc., to one another via four-wire lines. To give increased operating reliability for the individual channels, two mutually independent paths are provided from one end. station to the other or between one station and a central control unit Z, via a common junction K, nnd the connections produced in this way are constantly monitored at their receiving ends with the aid of pilot signals which are transmitted in addition to the items of data. If .an operating channel breaks down,. and thus the associated pilot frequency ceases to be received, a switch-over device automatically switches over to the second channel which is ready for operation as a substitute line.

In this case the junction K lies in the transmission path between the central control Z, which is not shown in detail in the drawing, and the two sub-stations U and U2, also not shown in detail are also connected via the juration K. The junction K is connected to the central control Z via two outgoing lines D and D2, and two incoming lines Al and A2. The connection to the sub-station Ul is established by two outgoing lines B and B2 and two incoming lines CI and C2, whilst the connection to the sub-station U2 is established by two outgoing lines E an E2 and two incoming lines Fl and F2.

In the junction K of the .central control 2, and in the substations Ul and U2 , reception is carried out via a change-over switch and transmission is effected in parallel via a hybrid circuit.

The lines Al and A2 coming from the central station Z may be selectively connected via a change-over switch 1 to a band stop filter 21, whose output is connected via a resistor 3 to a busbar b of the 26-dB junction . The busbar b is connected via the series combinations of a resistor 4 and a band stop filter 6 to a hybrid circuit 9, and is also connected via the series combination of a resistor 15 and a band stop filter 17 to a hybrid circuit 18.

The hybrid circuit 9 serves to transmit a data transmission band in parallel on the lines Bl and B2 , and to feed the pilot frequency supplied by a pilot generator 8 into the lines Bl and B2. The hybrid circuit 18 serves to transmit a data transmission band in parallel on the lines El and E2 and to feed the pilot frequency supplied by a pilot generator 19 into the lines El and E2.

The lines CI and C2 from the sub-station Ul are selectively connected with the aid of a change-over switch 10 to a band stop filter 22 whose output is conducted via a resistor 5 to a busbar a_ of the junction . The lines Fl and F2 emanating, from the sub-station U2 are selectively connected with the aid of a change-over switch 20 to a band stop filter 23 whose output is conducted via a resistor 16 to the busbar a_ of the junction K.

The busbar a_ is also conducted via a resistor 14 to a band stop filter 13 whose output is connected to a hybrid circuit 11 which serves to emit the signals received from the sub-stations Ul or U2 in parallel to the lines Dl and D2,, and the pilot signal received from a pilot generator 12 is fed into the lines Dl and D2.

In the stations Z, Ul and U2 , transmission and reception is again effected in parallel fashion on two equivalent lines.

The pilot signal not only serves for line monitoring, breakdown being indicated by absence of the pilot signal, but also serves to signal back data in the opposite station. For this purpose its frequency can be selectively switched between one of four values, 300 Hz, 315 Hz, 330 Hz or 345 Hz.

In the band stop filters 21, 22 and 23, located in the respective transmission paths at the output of the particular switch-over point., the blocking range can be switched over with the aid of control signals applied via inputs fl'to f3, which are in each case formed in a device which serves to detect the particular pilot frequency (not illustrated). Such a device serves, for example, to analyse the pilot frequency received from the station Ul via the lines CI and C2 and to obtain display criteria for the operating state and a switchover criterion for the operation of the change-over switch 10. The device supplies the control signal f2, with the aid of which the blocking range of the band stop filter 22 is controlle Correspondingly, the control signals to inputs fl and f3 -are obtained in devices (not shown) which analyse the respective pilot frequency signal incoming on the lines Al, A2 and Fl, F2, preceding the relevant band stop filter 21 or 23.

The junction K shown in the Figure serves as a distribution and collection point for a meshed network composed of LF lines. In this junction K, the output side of each receiving connection has the respective change-over switch, 1, 10 or 20 followed by it's associated band-stop filter 21, 22 and 23, which each attenuates the incoming pilot voltage by 40 dB. Together with action of the transinitting-end filters 6, 13 and 17, which each possess an attenuation of 20 dB, it is thus ensured that the received pilots signals do not form disturbing beats with the outgoing, pilot signals, which could lead to mis-alarms when the noise in the channel is analysed.

The practical design of the pilot filters 21, 22, and 23 does not present particular difficulties as long as the pilot frequency is 300 Hz. In this case the distance from the useful band is such that the transit time requirements and the requisite attentuation curve can be fulfilled with relatively simple means.

With regard to the signalling-back which is provided in the data transmission device, and in which in the stations of the network utilise a change in the pilot frequency transmitted to the opposite direction to transmit additional items of inform-ation from a free station to the engaged station, the blocking band must be capable of being extended in particular to 345 Hz, which is the highest pilot frequency in the assumed example. However, the useful band commences at only a short interval from this value, in particular at 360 Hz, it can present difficulties if given spatial and component cost conditions are imposed, to safely avoid the occurrence of impermissable high attenuation and transit time distortions in the transmission range. There are four alternative items of data v/hich are to be transmitted with the aid of the pilot frequencies from one station to another, possessing the following significance:- (a) Both outgoing lines in order; (b) Outgoing line 1 disturbed; (c) Outgoing line 2 disturbed; (d) Both outgoing lines disturbed.

These four separate criteria are assigned the respective pilot frequencies 300 Hz, 315 Hz, 330 Hz and 345 Hz.

The frequency 345 Hz is especially assigned to case (d) meaning "both lines disturbed". The pilot filters 21, 22 and 23 can each be switched over between two switching states. In one switching state, each pilot filter, 21, 22 or 23 is such that it fulfills the blocking requirements for the frequencies 300 Hz, 315 Hz and 330 Hz, which in view of the distance of the uppermost pilot frequency from the LF band can be effected with relatively simple means. Each pilot filter, 21, 22 or 23 may be switched over in such manner that the upper cut-off frequency or blocking range of the pilot filter can by an automatic disconnection of impedances be shifted by 15 Hz towards higher frequencies when the pilot frequency 345 is received, so that the blocking requirements are again fulfilled. The impairment in the transmission quality then introduced into in the receiving path by the reduction of the useful band is of no significance as the received pilot frequency indicates that both transmitting lines are disturbed, and thus normal operation is not possible in any case.

The frequency recognition device (not shown), provides a digital signal when the frequency 345 Hz is received, which signal operates, to control the band stop filter, by control of three transistors which themselves modify a plurality of capacitances in such manner that the pilot signal is again attenuated by 40 dB.

The frequency recognition device can be provided with oscillating circuits which are tuned to the individual pilot frequencies which are provided. The pilot voltage of each provided frequency can be conducted to an oscillating circuit which is tuned to this frequency, wherein d.c. voltages obtained by rectification cf the oscillating circuit voltages control signal emission in dependence upon their value. If the pilot frequencies 'are narrowly spaced, each frequency producing a signal transmission will expediently be checked by comparison of the obtained d.c. voltages to determine whether the d.c. voltage obtained from the oscillating circuit with a resonance frequency in the order of this frequency is the maximum of the d.c. voltages, wherein a positve result to this test serves as -triggering criterion for the requisite signal emission.

Claims (5)

WHAT WE CLAIM IS:

1. A pilot controlled data transmission system in which an analysis device is provided for the recognition of the pilot frequency, and a band stop filter is placed in each transmission path to block the received pilot frequency, and in which means are provided at at least one end of a tranmission section to provide a criterion for the operating state of this station and/or of the incoming transmission section, and transmitted back at a frequency below the frequency band provided for data transmission to the opposite station, wherein the station at said one end of said transmission section contains at least one pilot signal generator with a frequency below the data transmission band which is to be transmitted, and control -and/or display criteria are formed for transmission in the opposite station in dependence upon the received pilot frequency, an individual frequency being transmitted for each operating state which is to be distinguished, and the noise spectrum being analysed in the part of the transmission band below the data transmission band, said band stop filter being provided with means by which its blocking range is controlled from the analysis device for the recognitioriof the pilot frequency.

2. A system as claimed in Claim 1, in which that one of, the pilot frequencies which is closest to the data transmission band serves as a message criterion for a state of the system in which operation in the opposite direction is disturbed.

3. A system as claimed in "Claim 2 in which multiple connection paths are provided and the received. pilot frequency s signifies the state of the outgoing lines as reported back from the opposite station, and that value of the available pilot frequencies which lies closest to the data transmission band serves as the message criterion for the state "both outgoing lines disturbed". * .

4. A system as claimed in any preceding Claims, in which each said band stop filter provided in a junction of a data transmission network between a respective change-over switch provided for incoming operating-and substitute line and the junction incorporates frequency switch-over means.

5. A pilot controlled data transmission system substantiall as described with reference to the drawing."