GB2032732A - Apparatus for and method of switching delta-modulated connections in a PCM telecommunications exchange - Google Patents

Abstract

To enable switching of delta- modulated connection in an exchange arranged for the switching of PCM frames, an input channel delta-modulated signal a) has its synchronising signal gated out and is then written into an intermediate store b) in equal-sized groups in respective PCM frames which are filled either by adding stuffing bits or repeating the delta bits. After switching a storage output c) distributes the delta bits in an output sequence into which the synchronizing signal is gated to form an output channel delta-modulated signal d).

Description

SPECIFICATION Apparatus for and method of switching delta-modulated connections in a PCM telecommunications exchange This invention relates to apparatus for and a method of switching delta-modulated connections in a PCM telecommunications exchange, in particular a telephone exchange.

A PCM exchange of this kind which is also able to switch delta-modulated signals, is already known, for example from German Specification No. 2,264,179 and in which a frame code word, switching characteristics, and delta-modulation information is fed via deltamodulation connection lines to a full store and recorded in the latter. During an interval of time in which no transmission of delta-modulation information takes place, in a second step these items of information are transferrred to a second store of the same size from where they are forwarded in the manner of a PCM word. As one is concerned here with connecting PCM connection lines to PCM connection lines, it is necessary to match the delta-modulated signals, which are to be interposed, to the frame conditions including frame code and synchronisation signals of the PCM frame.The transfer of signals from one store into the other produces a delay which affects the signals when they later leave the exchange. A further disadvantage is that a complete second store is required in which not only the information bits but also the codes must be stored. Further stores are in any case required for converting the PCM signals back into delta-modulated signals. Finally PCM is subject to different standards from delta-modulation.

It is an aim of the invention to provide a system which reduces the outlay in exchanges, reduces the delays to information passing through the exchange, and also maintains standards prescribed or recommended for delta-modulation upon all lines incoming to or outgoing from the PCM exchange.

According to a first aspect of this invention there is provided apparatus for switching delta-modulated connections in a PCM telecommunications exchange, said apparatus including means for gating out the synchronising signal from a received delta-modulated input channel signal, means for writing the delta bits consecutively into an intermediate store to be formed in equal-sized groups in PCM frames, means for switching PCM frames in input channels to PCM frames in output channels, means for intermediate output storage to distribute bits from the PCM frames in a delta bit sequence and means for gating in the synchronising signal to form a delta-modulated output signal.

The sampling rate of analogue signals in delta-modulation is generally higher than in pulse code modulation (PCM). However, the coding of the PCM sampling signal means that, in dependance on the number of quantisation stages or levels, the number of bits is greater than in the case of delta-modulation.

Therefore a PCM frame also generally contains more bits than there are delta-modulation bits in each corresponding time unit. In order to fill completely the PCM frame used for delta bits in the exchange, either stuffing bits which have no information content are used to make up the number of existing bits to the required number in known manner, or, in a further embodiment of the invention, the existing information bits are repeated appropriately frequently within the frame. This last measure has the advantage that the items of information occur several times consecutively, so that in the event of any errors it is possible to have recourse to the repeated bits (bit safeguarding). Furthermore comparison of the first bits in the frame with the repeated bits can provide greater clarity and enhance processing in the exchange.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing in which the sole figure is a schematic flow diagram of a switching operation in a PCM exchange embodying this invention.

It is assumed that the PCM exchange operates in t.d.m. i.e. each incoming connection is linked to the corresponding outgoing connection for a short period of time during one time slot. In a clearly defined time slot pattern, the PCM exchange switches through so-called bit words, e.g. 8 bits with 64 kbit/s bandwidth.

Normally delta-modulation has no word structure, but an individual bit structure. A delta-modulated item of information is generally composed of a sequence of individual bits. The signalling between subscribers and exchange generally likewise takes place with a corresponding bit sequence within the channel bit rate (inband signalling).

Between the exchange and the subscriber apparatus there is arranged a subscriber circuit which can either be accommodated at the subscriber station or in the exchange. The subscriber circuit in the exchange contains a delta-codec which carries out the digitalisation of the speech signals and the d.c. loop signalling in the outgoing direction from the subscriber. In the incoming connection direction, speech signals are produced from the delta modulated signals and the 25 Hz ringing current is transmitted to the subscriber appa ratus. Delta-codecs of this kind are known-and will not be described here in detail, and neither will the function of a PCM exchange.

Thus delta-modulated connections are to be interconnected with likewise delta-modulated connections. However, if a connection is to be established not between delta-modulated sub scribers but between one delta-modulated connection and one PCM subscriber, the delta-modulated signals can be recorded into PCM signals and vice versa with the aid of a known digital converter. In the following, however, only a switching operation between two delta-modulated connections will be explained making reference to the drawing.

In stage a) a series of delta-bits with equal spacing (32,kHz) arrive in the PCM exchange.

The synchronising signal is gated out at the input to the exchange in order to synchronise the associated pulse generators. At the output of the exchange the synchronising signal is reinserted to precede the associated deltamodulated items of information.

As the PCM frame size is 1 25 micro-seconds (8kHz) a frame contains four delta-bits with a bit spacing of 31.25 micro-seconds (32kHz). Three such four-element groups are represented in stages a) and b) of the drawing and characterised by the Roman numerals 1,11 and Ill.

The four bits in each frame are fed to an intermediate store b) whereupon they are switched, in close succession, in the required direction by the switching equipment.

Since, when a frame of 1 25 micro-seconds is to be input into the intermediate store b) the delta-bits must all be present before the input storage procedure has terminated, here a delay, relative to the start of the delta bit reception by the exchange, of one frame, thus of 1 25 micro-seconds, occurs. The time for the switching step consisting of the transfer from the input b) to the store to a storage output c) of the store is dependent upon the exchange. In the drawing a half a frame length has been assumed in this latter respect.

The bits are read out one-by-one and consecutively from the storage output c), resulting in a continuous bit sequence as shown in stage d) of the drawing, the bit sequence being identical to the item of delta-modulated information (and making up the sequence l,ll,lll) which was received by the exchange. A period of time amounting to one frame is again provided for this store output step so that here a further time delay of 1 25 microseconds occurs.

As can be seen from the drawing, the four bits in stage a) fill only half the 8 bit PCM frame in the stores b), c) and further four bit time slots remain free in the PCM frame.

These free bit time slots can be filled with stuffing bits which have no information content. However it is also possible to repeat in the free bit time slots the first four bits of the frame. This provides the possibility of later comparing these repeated bits with the first bits and correcting any errors which have occurred during the switching operation.

For the four bits of each frame, a rate of sampling of the analogue information of 32 kHz has been assumed so that four delta-bits, originally spaced by 31.25 micro-seconds occupy one 8 bit PCM frame. In the case of a sampling rate of 1 6 kHz, only two bits, originally spaced by 62.5 micro-seconds, would occupy one frame so that 6 stuffing bits or a triple repetition of the two information bits would be necessary in order to complete the PCM frame. In the case of a sampling rate of 64 kHz (8 delta bits per frame) the frame would be full without additional bits, whereas in the case of a sampling rate of 8 kHz 7 stuffing bits would have to be added or 7 repetitions made.

The frames formed in this way provide that delta-modulated items of information are switched through the PCM exchange in the same way as PCM-modulated items of information; however, they retain their individual bit structure in spite of word interleaving. It is nevertheless possible to carry out a multiplex signal formation of a plurality of channels.

Due to the fact that the delta-modulation information does not require to be transferred to a second store from which it must be read out, no store is required for this purpose. The transfer time is also avoided which means that the information leaves the exchange in a short period of time.

A further reduction in outlay is achieved in that the synchronising signal is gated out at the input to the exchange and is reinserted at the output. As a result fewer storage positions are required in the stores, i.e. only those for the information.

The individual bit structure for the connection of digital transmission links for deltamodulation information is subject to international requirements. These requirements are fulfilled since the delta-bits arrive at the exchange in the correct structure and leave the exchange in the same form.

Claims (14)

1. Apparatus for switching delta-modulated connections in a PCM telecommunications exchange, said apparatus including means for gating out the synchronising signal from a received delta-modulated input channel signal, means for writing the delta bits consecutively into an intermediate store to be formed in equal-sized groups in PCM frames, means for switching PCM frames in input channels to PCM frames in output channels, means for intermediate output storage to distribute bits from the PCM frames in a delta bit sequence and means for gating in the synchronising signal to form a delta-modulated output channel signal.

2. Apparatus according to claim 1 wherein the equal-sized groups each comprise less than the full number of bits which can be contained in one of said PCM frames and means are provided for ensuring that each said PCM frame contains its full number of bits.

3. Apparatus according to claim 2 wherein means are provided for adding stuffing bits to the group of delta bits in each said PCM frame to fill the latter with bits.

4. Apparatus according to claim 2 wherein the number of delta bits in each said group is an integral sub-multiple of the total number of bits which can be contained in each said PCM frame and means are provided for repeating the delta bits in each said PCM frame so as to fill the latter.

5. Apparatus according to claim 4 wherein each said PCM frame contains 4 delta bits repeated once to fill the PCM frame.

6. Apparatus according to claim 4 or claim 5 wherein means are provided for using the repeated bits for bit safeguarding.

7. Apparatus for switching delta-modulated connections in a PCM telecommunications exchange, said apparatus being substantially as described herein with reference to the accompanying drawing.

8. A PCM telecommunications exchange including apparatus according to any one of the preceding claims.

9. A method of switching delta-modulated connections in a PCM telecommunications exchange, said method including the steps of gating out the synchronising signal from a received delta-modulated input channel signal, writing the delta bits consecutively into an intermediate store so as to form the delta bits in equal-sized groups in PCM frames, switching PCM channels in input channels to PCM frames in output channels, storing the output channel PCM frames and distributing bits from the latter PCM frames in a delta bit sequence and gating in the synchronising signal to form a delta-modulated output channel signal.

1 0. A method according to claim 9 wherein the equal-sized groups each comprise less than the full number of bits which can be contained in one of said PCM frames and each said PCM frame is provided with its full number of bits.

11. A method according to claim 10 wherein stuffing bits are added to the group of delta bits in each said PCM frame to fill the latter with bits.

1 2. A method according to claim 10 wherein the number of delta bits in each said group is an integral sub-multiple of the total number of bits which can be contained in each said PCM frame and the group of delta bits is repeated in its relevant PCM frame to fill the latter.

13. A method according to claim 12 wherein each said PCM frame contains a group of 4 delta bits and the group is repeated once to fill the PCM frame.

14. A method according to claim 1 2 or claim 1 3 wherein bit safeguarding is carried out using the repeated bits.

1 5. A method of switching delta-modulated connections in a PCM telecommunications exchange, in which the delta-modulated signals can be switched from input channels to output channels when they have been arranged into PCM pulse frames each having the same number of bits, said method comprising gating out the synchronising signal at the input to the exchange and gating in the synchronising signal again at the output, assembling delta bits consecutively in the sequence of their arrival in an intermediate store for the counting of the required number of bits and for the frame formation, whereupon, following a switching operation, they are intermediately stored at the output in order to be distributed to form a delta bit sequence.

1 6. A method of switching delta-modulated connections in a PCM telecommunications exchange, said method being substantially as described herein with reference to the accompanying drawing.