GB2145608A

GB2145608A - Multiplex transmission systems

Info

Publication number: GB2145608A
Application number: GB08421101A
Authority: GB
Inventors: Stephen Patrick Ferguson
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1983-08-22
Filing date: 1984-08-20
Publication date: 1985-03-27
Also published as: GB8421101D0; GB8322531D0; GB2145608B

Abstract

In a TDM transmission system the multiplex and transmission system synchronisation frames are synchronised with each other. This cross synchronisation permits the extraction of individual data channels and the insertion of replacement channels at a repeater station without de-multiplexing and re-multiplexing the TDM signal.

Description

SPECIFICATION Transmission systems The present invention relates to transmission systems and in particular to transmission systems of the kind used for the transmission of digital data.

Whilst hereinafter the invention is described with particular reference to a radio transmission system it is here noted that the invention may equally be used in line transmission systems including interalia such systems using co-axial cables or optical fibres as the transmission medium.

In transmission systems it is necessary to include repeater stations which receive signals and retransmit them at periodic intervals to counteract transmission losses. At some of the repeater stations it may be desirable to extract some of the received information and transmit some new information.

Where the incoming signal at a repeater station is a signal having a large number of channels time division multiplexed thereon it is necessary to demultiplex the signal, extract the desired channel and remultiplexthe signal including new information prior to retransmission.

In a practical radio transmission system repeater station in which a 34 Megabit per second (34M) digital data frame is assembled from a plurality of eight megabit/second (8M) digital data channels which have in turn been assembled from a plurality of two megabit/second (2M) digital data channels derived from a plurality of 64 kilobit per second (64K) digital data channels, if one of the 64K channels is to be extracted, after alignment of demultiplexing apparatus with the radio signal frame (frame alignment) and extraction of bits pertaining to the radio system the "radio serviceband", the radio synchronisation and serviceband bits are deleted to leave the 34M digital data frame.Frame alignment -- the alignment of the demultiplexing equipment with the location of the digits within the frame in their nominal numerical order by identifying the nominal first digit of each data frame-of the 34M frame is carried out followed by demultiplexing to the eight megabit frames.

The demultiplexer for the eight megabit frame containing the desired 64K channel eight megabit frame is aligned and demultiplexed to two megabit frames the one of which includes the desired channel is in turn aligned and demultiplexed to obtain the 64K channel.

Prior to retransmission the signals are remultiplexed firstly by multiplexing a newly received 64K channel with the "unwanted" 64K channels to a two megabit frame remultiplexing the two megabit channels with the other unused two megabit channels and so on, finally reinserting the radio serviceband and radio frame alignment digits.

It will be appreciated that such a repeater station requires many cards of electronics circuits for demultiplexing and multiplexing each of which is liable to failure at any time giving rise to an inherent reliability problem. Thus it will be realised that reducing the number of electronics stages required to process a signal increases the overall reliability of the transmission system.

However, it is desirable to be able to extract and replace a limited number of digits of the multiplexed signal, say, representing a single 64K data channel, at a repeater station whilst reducing the amount of processing of that signal.

It is an object of the present invention to provide a digital signal transmission system transmitting time division multiplexed signals in which the transmission medium frame alignment digits are so arranged as to permit identification, extraction and replacement of a single channel of the multiplexed signal without demultiplexing.

According to the present invention in a transmission system of the kind used for transmitting digital signals carrying a plurality of channels and a transmission serviceband combined in time-division-multiplex (TDM), transmission serviceband frame alignment digits are in synchronism with multiplexor frame alignment digits of a TDM signal carried thereby, at a receiver of the system frame alignment means is arranged to determine from said transmission serviceband frame alignment digits the location of each transmission frame within a received stream of digits and to provide synchronisation signals representing the serviceband frame alignment to permit extraction and insertion of transmission serviceband digits, and said frame alignment means is also arranged to provide synchronisation signals representing the frame alignment to reading means adapted to read one or more preselectable digits from the multiplex data stream such that a preselected TDM channel transmitted by the system may be extracted at the receiver without demultiplexing of the TDM signal.

Preferably said frame alignment means also supplies a synchronisation signal to switching means adapted to select digits for onward transmission either from the TDM signal received by the receiver or from new digital data and the switching means may be arranged to cause overwriting of the same pre-selected digits of the TDM signal as those read by use of said reading means.

Further switching means responsive to said synchronisation signals may be provided by said further switching means being arranged to insert transmission serviceband digits in the signal to be transmitted.

Third switching means also responsive to said synchronisation signals may be used to re-write said transmission frame alignment digits and a fourth switching means may be used to overwrite error check digits of the signal with error check digits calculated from the received signal as modified by the operation of at least one of the other switching means.

Said error check digits may be parity check digits determined by a parity generator and the receiver may have second reading means responsive to said synchronisation signals adapted to extract the parity check digits for use by a parity check circuit arranged to provide a parity error signal. Control means may be provided between said parity generator and the fourth switching means, said control means being responsive to said parity error signal to insert a parity error in the signal to be transmitted in respect of each parity error received in the received signal.

Alternatively corrected parity may be inserted in the signal to be transmitted and data representing received parity errors may be inserted in a prespecified channel of the signal to be transmitted or in the transmission serviceband.

The receiver may include third reading means responsive to a synchronisation signal supplied by said frame alignment means to extract the transmission serviceband.

Two or more of said switching means may be combined in a data selector having a plurality of selectable inputs and two or all of said reading means may be combined in a single reading circuit having a plurality of selectable outputs.

A transmission system in accordance with the invention will now be described by way of example only with reference to the accompanying drawings of which: Figure lisa schematic diagram showing a multiplexor signal frame and a radio signal frame; Figure 2 is a block schematic diagram of a base station transmitter of the transmission system; Figure 3 is a block schematic diagram of a repeater station of the transmission system; Figure 4 is a block schematic diagram showing the "readout" of Figure 3 in greater detail; and Figure 5 is a block schematic diagram showing the "overwrite" of Figure 3 in greater detail.

Referring to Figures 1 and 2 the transmission system to be described is a radio transmission system which is supplied by conventional multiplexors (not shown) with a multiplexed signal shown schematically as a 'MUX' frame in Figure 1.

The multiplexed signal may be for example a thirty-four megabit/second signal comprising a number of signal channels, say sixty-four kilobit channels, which have been conventionally multiplexed by way of two megabit/second and eight megabit/second multiplexing to form the thirty-four megabit/second signal.

Within the MUX frame, frame alignment digits 1 are are provided which on input to the base station transmitter of Figure 2 on an input 2 are detected by a conventional frame alignment circuit 3.

The frame aligment digits 1 which occur at prespecified intervals in the frame form a unique pattern which may be used to detect the location of other lower order (e.g. the eight megabit/second) bits within the frame.

In conventional radio transmission systems a radio service band (in effect an additional channei) is added to the MUX frame together with radio frame alignment digits. In such conventional systems there is no specified relationship between the radio frame and the MUX frame, the radio serviceband and radio frame alignment digits being handled separately from the MUX frame.

In other words, the addition of the radio digits and rate changing (and their subsequent extraction or replacement) is carried out asynchronously. In the radio transmitter of Figure 2 a bit insertion and rate changing circuit 4 could be used in a similar manner to a conventional circuit of that kind provided that a sync switch 5 is in the open state.

However, when the sync switch 5 is in the closed state, the bit insertion and rate change circuit 4 is arranged to superimpose the radio serviceband and radio frame alignment digits in synchronism with the MUX frame. Thus, in the present example, in the radio frame shown in Figure 1 radio frame alignment digits 6 are inserted adjacent the MUX frame alignment digits 1 and the radio serviceband digits are similarly inserted in prespecified locations within the radio frame.

Although in the present system the radio frame alignment digits 6 are inserted adjacent the MUX frame alignment digits it will be appreciated that the radio frame alignment digits may be inserted anywhere within the radio frame provided that there is a synchronous relationship between the radio frame and the MUX frame. Alternatively the radio serviceband digits may be inserted in a synchronous relationship with the MUX frame alignment digits, the MUX frame alignment digits 1 being used also as the radio frame alignment digits.

Referring now to Figures 1 and 3 at a repeater station of the radio transmission system the demodulated radio signal is passed to a frame recognition circuit 7 thence to a readout circuit 8 and an overwrite circuit 9 by way of a connection 10.

Synchronisation of the radio frame by use of the frame alignment digits 6 as detected by the frame recognition circuit7 determines a synchronisation signal passed by way of a connection 11 to the readout circuit 8 and the overwrite circuit 9.

The readout circuit 8 extracts as hereinafter described from the incoming radio frame the radio serviceband digits, parity digits and customer channel information which are output respectively on connections 12,13 and 14.

In the overwrite circuit 9 some digits of the incoming signal on the connection 10 are overwritten with new radio serviceband digits received on a connection 15 and new customer channel digits received on a connection 16. The modified radio frame is output on a connection 17 for onward transmission.

Referring now to Figure 4 in which corresponding connections shown in Figure 3 are similarly designated the readout circuit is fed by a clock signal from a stable clock generator (not shown) which signals are used by a control pulse generator 20 to generate appropriate digit synchronisation signals for use by a number of blanking circuits 21 -23.

It is here noted that the blanking circuits 22, 23 make direct use of the digit synchronisation pulses respectively to blank out all except the radio serviceband and parity digits of the demodulated signal received on the connector 10 for further use at the repeater station.

The parity digits are checked by a parity error detection circuit (not shown) against the received radio frame to determine the number of parity errors therein which, if the number of errors is greater than a predetermined number indicates a fault in a preceding part of the radio transmission system. It is also noted that the data defining received errors may be inserted in the radio serviceband or in a channel of the MUX frame reserved for error monitoring purposes so that when the parity digits are recalculated (as hereinafter described) and inserted in the data stream, the output of the repeater station is parity correct but an indication of a preceding transmitter failure is given.

However, in conventional radio transmission systems, once a parity error is present it remains in the frame until detection attheterminl station.

Facilities of the present system are described hereinafter which permits the present radio transmission system to mimic a conventional system by maintaining an equal number of parity errors in the transmitted signal as are present in the received signal such that if no alternative error indication is provided the location of the first station at which errors are occurring may still be determined at the terminal station.

To enabie a wanted data channel to be extracted from the radio frame for, say, use by a customer, the blanking circuit 21 is responsive to control signals from a time slot selector 24 which is responsive to the digit synchronisation signals from the control pulse generator 20 and to signals received on a connection 25 to provide the control signals on a connection 26.

Thus a particular channel or channels may be remotely selected and the blanking circuit 21 will blank out all but the digits representing that selected channel.

The output of the blanking circuit 21, that is the digits making up the wanted channel signal, is fed to an elastic store for example a first-in-first-out (FIFO) store 27 the output of which is controlled by pulses derived from the clock signal by a divider circuit 28 to provide a stream of digits to a data selector 30 which is controlled by a rate change circuit 29 which similarly is responsive to a signal supplied from the divider circuit 28. The data selector 30 and rate change circuit 29 which may be a phase locked loop circuit provide respectively timing and relatively jitter-free data signals to an interface coder 31 wherein the signals are combined in a manner suitable for use by a customer. The interface reader may, for example, provide an output in the form known as HDB3 coded.It is here noted that the divider 28, the rate change circuit 29, FIFO 27, the data selector 30 and the interface coder 31 are the component parts of a conventional multiplexor circuit as indicated by the chain dash line 32.

Referring now to Figure 5 in which connections to the overwrite circuit 9 of Figure 3 are similarly designated, the overwrite circuit is responsive to the synchronisation signal on the lead 11 to overwrite specified digits of the radio frame with new customer data received on the connection 16 and new radio serviceband digits received on the connection 15. To this end the stable clock pulse signal referred to in the description of the readout circuit of Figure 4 is passed to a control pulse generator 40 (which may be the control pulse generator 20 of Figure 4) which provides pulses to control a number of data selectors 41 -44 each of which is in effect a high speed switch capable of switching between one of two inputs and an output.

The data selector 41 is controlled by a time slot selector 45 which is responsive to the control pulses supplied by the control pulse generator 40 and a remote channel select signal 25 and which may be the time slot selector 24 of Figure 4.

The data selector responds to the control signal from the time slot selector either by passing the demodulated data stream (the radio frame) received on the input 10 or new customer data received by way of the input 16 and a conventional multiplexor circuit 46 to its output 47.

The output signal from the data selector 41 is fed to a parity generator 48 which calculates the value of parity digits now required to be inserted so that the radio frame transmitted is parity correct. However, as has been previously mentioned, it may be desirable for the same number of parity errors as the received radio frame.

Accordingly, the calculated parity stream is passed to an error insertion circuit 49 which is responsive to an error counter 50 to pass the parity digits unamended or to modify one of the parity digits.

The error counter 50 is in practice a one bit up/ down counter (of a bistable circuit) which is ciocked up by the parity check circuit 49 and which provides an error signal to the error insertion circuit 49 whenever it is not set at zero.

The parity digit stream as modified if necessary is inserted at appropriate points in the radio frame by the data selector 42. It is here noted that the parity data relates only to the multiplexor frame although it will be appreciated that the radio frame parity could be calculated if required by repositioning the parity deriving elements (48 to 50) and the data selector 42 after the data selector 44. Data defining the digits of the radio frame alignment word is held in a frame word store 51 and the digits are selected from that store and overwritten in the data stream in turn by the data selector 43 to ensure that the radio frame alignment digits are transmitted from each repeater station uncorrupted.

The new radio serviceband digits are written into the radio frame by the data selector 44 and the modified signal is output by way of a differential coder 52 together with the stable clock signal to the modulator of the transmitter of the repeater station.

Whilst as herein described with reference to Figures 4 and 5 the blanking circuits 21-23 and the data selectors 41-44 are defined as individual circuits it will be realised that a single multi-output blanking circuit and a single multi-input blanking circuit and a single multi-input data selector circuit could be used therefor.

It will also be appreciated that many of the functions attributed to individual circuit elements could be accomplished by use of high speed microprocessors.

Many advantages of a transmission system of the kind hereinbefore described in comparison with a conventional transmission system will be apparent.

For example, whether in a radio transmission system or in a line transmission system using coaxial cables or optical fibres, the reduction in the number of circuits required to extract and overwrite a single data channel contributes to overall cost reduction, space saving and improved reliability. It will be appreciated also that without a multiplicity of multiplexors and demultiplexors frame realignment times are reduced considerably.

Claims

1. transmission system of the kind used for transmitting digital signals carrying a plurality of channels and a transmission service band combined in time-division multiplex (TDM) wherein transmission serviceband frame alignment digits are in synchronism with multiplexorframe alignment digits of a TDM signal carried thereby, at a receiver of the system frame alignment means is arranged to determine from said transmission serviceband frame alignment digits the location of each transmission frame within a received stream of digits and to provide synchronisation signals representing the serviceband frame alignment to permit extraction and insertion of transmission serviceband digits, and said frame alignment means is also arranged to provide synchronisation signals representing the frame alignment to reading means adapted to read one or more preselectable digits from the multiplex data stream such that a preselected TDM channel transmitted by the system may be extracted at the receiver without demultiplexing of the TDM signal.

2. A transmission system as claimed in Claim 1 wherein said frame alignment means also supplies a synchronisation signal to switching means adapted to select digits for onward transmission either from the TDM signal received by the receiver or from new digital data presented at another input of the receiver.

3. Atransmission system as claimed in Claim 2 wherein said switching means is arranged to use said new digital data to overwrite the same preselected digits of the TDM signal as those read by use of said reading means.

4. A transmission system as claimed in Claim 2 or Claim 3 wherein a second switching means is responsive to said synchronisation signal to insert transmission serviceband digits in the signal to be transmitted.

5. Atransmission system as claimed in any preceding claim wherein third switching means is responsive to synchronisation signals from said frame alignment means to rewrite said transmission from alignment digits in the signal to be transmitted.

6. A transmission system as claimed in any one of Claims 2 to 5 wherein fourth switching means is responsive to synchronisation signals from said frame alignment means to overwrite error checking digits of the TDM signal to be transmitted with error checking digits calculated from the received signal as modified by at least one other of said switching means.

7. A transmission system as claimed in Claim 6 wherein said error check digits are parity check digits derived from a TDM signal by a parity generator.

8. Atransmission system as claimed in Claim 7 wherein the receiver has second reading means responsive to said synchronisation signals to deduce the parity check digits which are used by a parity check circuit, said parity check circuit providing error signals indicative of errors (if any) in the received TDM signal.

9. Atransmission system as claimed in Claim 8 wherein control means are provided, said control means being responsive to error signals from said parity generator to insert an error in the signal to be transmitted in respect of each error present in the received TDM signal.

10. A transmission system as claimed in Claim 8 wherein data representing received parity errors as determined by said parity check circuit is inserted in a prescribed channel of the signal to be transmitted or in the transmission service band.

11. A transmission system as claimed in any preceding claim wherein the receiver includes third reading means responsive to said synchronisation signals to extract the transmission service band from the received TDM signal.

12. A transmission system as claimed in any one of Claims 4 to 10 wherein a plurality of said switching means are combined in a data selector having a plurality of selectable inputs, said data selector being responsive to control means to select signals from appropriate ones of the inputs to form the TDM signal to be transmitted.

13. A transmission system as claimed in any one of Claims 8 to 11 wherein a plurality of said reading means are combined in a signal reading device having a plurality of selectable outputs.

14. A transmission system substantially as hereinbefore described with reference to the accompanying drawings.

15. A repeater station for a transmission system and substantially as hereinbefore described with reference to Figures 3 to 5 of the accompanying drawings.