AU6019696A

AU6019696A - Demultiplexer, a protection switch unit, a telecommunication network and a method of demultiplexing

Info

Publication number: AU6019696A
Application number: AU60196/96A
Authority: AU
Inventors: Mats Bladh
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 1995-06-01
Filing date: 1996-05-24
Publication date: 1996-12-18
Anticipated expiration: 2016-05-24
Also published as: KR19990021881A; JPH11507480A; SE9502010D0; SE9502010L; EP0873609A1; CN1191642A; SE504533C2; AU705430B2; CA2222624A1; WO1996038943A1

Description

DEMULTIPLEXER, A PROTECTION SWITCH UNIT. A TELECOMMUNICAΗON NETWORK AND A METHOD OF DEMULTIPLEXING

FIELD OF INVENTION

The present invention relates to a demultiplexer, a protection switch unit which includes one such demultiplexer, a telecommunications network which includes one such protection switch unit, and a method relating to demultiplexing. More specifically, the invention relates to the aforesaid apparatus and method in respect of demultiplexing signals that are single-mode or differential-mode signals and preferably of the CMI type at transmission speeds above about 100 Mbit/s and preferably in the range of about 140-155 Mb/s.

DESCRIPTION OF THE BACKGROUND ART

In telecommunications networks, a plurality of signal lines are often connected to an exchange through the medium of a respective terminal access unit. These terminal access units are relatively expensive and complicated units, which can sometimes break down. For this reason, the network includes at least one protection switch unit which includes a demultiplexer whose outputs are connected to the signal lines and whose input is connected to the reserve terminal access unit.

When a standard terminal access unit cannot be used for some reason or another, the exchange sends to the demultiplexer of the protection switch unit output signals which are intended for the signal line connected to the unusable normal terminal access unit via the reserve terminal access unit, said demultiplexer being controlled to forward the output signal to said signal line. Good signal quality is required when switching or demulti-ple- . xing signals having transmission speeds in the region of about 140-155 Mb/s. In this regard, the pulse shape of the output signal is often required to be the same as the pulse form of the input signal.

Problems with mismatching between components and conductors can easily occur in such demultiplexers at such high transmission speeds, therewith greatly impairing signal quality.

It is also often necessary to monitor the reserve terminal access unit and also possibly the demultiplexer in the protection switch unit with regard to faults.

A plurality of standard circuits exist for demultiplexing signals having bit rates of up to about 30 Mb/s, in a relatively simple and inexpensive manner. At higher bit rates, line impedances, connections, terminations, line runs and component data place strict requirements on circuitry. In order to keep conductor impedances as constant as possible, it is necessary to take particular care in implementing the printed board assembly.

The circuitry using standard circuits in a demultiplexer is expensive and highly current consuming.

There is no technique at present for effectively demultiplexing while monitoring in the aforesaid transmission speed ranges. Neither is there known a demultiplexer which will afford monitoring of the input signal at the same time.

US-A 5,146,113 describes an integrated circuit having several narrow, elongated resistance strips on a circuit board, to provide a board-mounted circuit with a predetermined input and/or output impedance.

US-A 5,281,934 describes a microwave multiplexor which is entirely of microstrip construction.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method of selecting one of several outputs as the output of a received input signal when demultiplexing, therewith obtaining good signal quality in a simple and inexpensive manner at the same time.

This object is achieved with a method in which an input signal received on an input is first conducted to a first transmission line and thereafter selectively to one of at least two outputs.

Another object of the invention is to provide a method of selecting one of several outputs as the output of a received input signal when demultiplexing, and of enabling a signal transferred to one of the outputs to be monitored in a simple manner.

This object is achieved with a method which also includes generating a replica of the signal forwarded to a selected output by, at the same time, conducting the input signal that was passed to the selected output to a third transmission line.

A further object of the invention is to provide a method of selecting one of several outputs as the output for a received input signal when demultiplexing, where distortion in the signal obtained on the selected output and on the replica of this signal can be reduced readily in a later stage. This object is realized with a method in which the input signal- includes two parts and the two parts of said input signal are each conducted to a respective first and second transmission line and thereafter selectively conducted to a respective first and second connection point in a selected output and also to a respective third and fourth transmission line.

Still another object of the present invention is to provide a demultiplexer, a protection switch unit including one such demultiplexer, and a telecommunications network which includes one such protection switch unit, where good signal quality is obtained in a simple and inexpensive manner when selecting one of several outputs as the output for a received input signal.

This object is achieved with a demultiplexer, a protection switch unit and a telecommunications network wherein the demultiplexer includes an input and at least two outputs, wherein a connection point in the input is connected to a first transmission line to which a first connection point in each output is connected via a respective first controllable signal forwarding conductor means.

Yet a further object of the invention is to provide a demultiplexer, a protection switch unit which includes one such demultiplexer, and a telecommunications network which includes one such protection switch unit, wherein the function of the demultiplexer and the function of the unit connected to the input of the demultiplexer can be readily monitored.

This object is achieved with a demultiplexer, a protection switch unit and a telecommunications network wherein each first signal forwarding conductor means is connected to a third transmission line. Still yet a further object of the invention is to provide a demultiplexer, a protection switch unit that includes one such demultiplexer and a telecommunications network that includes one such protection switch unit, wherein distortion obtained in output signals from the demultiplexer can be readily reduced in later stages.

This object is achieved with a demultiplexer, a protection switch unit and a telecommunications network wherein a second connection point in the demultiplexer input is connected to a second transmission line which, in turn, is connected to a second connection terminal in each output via a respective second controllable signal forwarding conductor means, which may optionally also be connected to a fourth transmission line.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects of the invention and advantages afforded thereby will be evident from the following description of a preferred embodiment of the invention made with reference to the accompanying drawings, in which

Fig. 1 is a block schematic of an inventive telecommunications system; and

Fig. 2 is a circuit diagram of an inventive demultiplexer.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Fig. 1 is a block schematic of a telecommunications system which operates with CMI signals transmitted at transmission speeds in the range of about 140-155 Mb/s. This system includes an exchange 1 which communicates with a plurality of signal conductors. Each signal conductor is connected to a protection unit 2 (PU = Protection Unit) . Although only eight protection units are shown in Fig. 1, it will be understood that these units may be fewer or more in number. Signals incoming to and outgoing from the protection units 2 are denoted with arrows that point towards and away from respective units 2. Each protection unit 2 is connected to the exchange 1 via a respective terminal access unit 3 (TAU) , of which only eight are shown although these units will actually be sixteen in number. The modus operandi of these terminal access units 3 plays no part in the present invention and is well known to the person skilled in this art and need not therefore be explained in great detail. It should be mentioned, however, that these terminal access units 3 are relatively complicated and expensive and have such a high fault frequency as to necessitate some form of protective switch. In order to avoid a signal conductor becoming unusable, there is provided a reserve terminal access unit 5 which when one of the regular terminal access units 3 is inoperative is switched-in to replace this unit. The reserve terminal access unit 5 is connected between the exchange 1 and a protection switch unit 4 (PSU) .

This protection switch unit is connected to each protection unit 2. The main duty of the protection units 2 is to convert single-mode signals incoming to the protection switch unit 4 into differential-mode signals, and to convert differential- mode signals outgoing from the exchange 1 via the terminal access units 3 and the protection switch unit 4 to single-mode signals. The protection switch unit 4 includes a multiplexor or selector and a demultiplexer. The multiplexor functions to receive all of the signals incoming on the conductors and to selectively forward one of said signals, and the demultiplexer functions to receive a signal outgoing from the exchange 1 and to forward this signal to a selected signal conductor.

Fig. 2 illustrates an embodiment of an inventive demultiplexer, wherein only two outputs are shown for the sake of simplicity, although it will be understood that the demultiplexer will normally include many more outputs. A first of the outputs has a first and a second connection point 0UT1+ and OUT-. The first connection point 0UT1+ of the first output is connected to a first signal forwarding conductor means Ql via a resistor Rl, said means Ql being connected to an RF transistor in the illustrated case. The resistor Rl is connected to the emitter of the transistor Ql in the illustrated case. The transistor base is connected via a resistor R8 to a first transmission line TLINE_1. One end of the transmission line TLINE_1 is terminated with a resistor R5 which is connected to a voltage source VBB.

The transistor collector is connected to a third transmission line TLINE_3 included in the demultiplexer circuit board, said third transmission line being terminated at both ends with respective resistors R13 and R12 connected to a voltage source VCC, said voltage source optionally being the same as VBB. A first connection point SUP+ is provided in an output for monitoring signals at one end of the third_ transmission line TLINE_3.

The second connection point OUTl- of the first input is connected to a second transmission line TLINE_2 mounted on the demultiplexer circuit board via a resistor R3, and RF transistor Q4 and a further resistor RIO, similar to the first connection point OUT1+. One end of the TLINE_2 is terminated with a resistor R6 which is connected to the voltage source VBB. The collector of the transistor Q4 is connected to a fourth transmission line TLINE_4 mounted on the demultiplexer circuit board, this fourth transmission line being terminated with a resistor R14 at one end and a resistor R7 at the other end in a manner similar to the third transmission line TLINE_3, said resistors being connected to the voltage source VCC. A second connection point in the monitoring signal output is obtained at one end of the fourth transmission line TLINE_4.

Respective emitters of the first and second transistors Ql and Q4 are connected to one end of a first externally controlled switch means SI via a resistor R29 and R31 respectively. The other end of the switch SI is connected to earth.

The connection points OUT2+ and 0UT2- of a second output are connected to the first and the second transmission lines

TLINE_1 and TLINE_2 respectively in exactly the same way, via a resistor R2, a transistor Q2 and a resistor R9, and via a resistor R4, a transistor Q3 and a resistor Rll. Respective collectors of the transistors Q2 and Q3 are connected to the third and the fourth transmission lines TLINE_3 and TLINE_4 respectively. Respective emitters of the transistors Q2 and Q3 are connected to a second externally controlled switch S2 via resistors R30 and R32 in exactly the same way as the first output, said switch S2 also being connected to earth similar to the switch SI.

The transistors Ql, Q2, Q3 and Q4 and associated components are so placed along the transmission lines TLINE_1, TLINE_2, TLINE_3 and TLINE_4 that mismatches and reflexions will be minimal. In reality, further transistors for further outputs are correspondingly placed along said transmission lines TLINE_1, TLINE_2, TLINE_3 and TLINE_ .

A differential amplifier D_AMP has one output and is connected to the first transmission line TLINE_1 at a first connection point +0 therein and to the second transmission line TLINE_2 at a second connection point -0. The amplifier D_AMP has an input having two connection points +1 and -I which are each connected to a respective connection point IN+ and IN- in the input.

Resistors that terminate the ends of respective first and second transmission lines TLINE_1 and TLINE_2 where the amplifier D_AMP is connected are included in this amplifier and are also connected to the source VBB, which is also included in the amplifier D_AMP at this end of the transmission lines and is not therefore shown in the Figure.

In the preferred embodiment, each of the transmission lines TLINE_1, TLINE_2, TLINE_3 and TLINE_4 has the form of a microstrip included in the board circuitry and to which the transistors are connected. These microstrips are preferably rectilinear, have identical lengths and are essentially of unitary width. An important feature of this embodiment, however, is that the two transistors at each output have equally long paths to the connection points +0 and -0 in the output of the amplifier D_AMP. The transmission lines TLINE_1, TLINE_2, TLINE_3 and TLINE_4 may optionally be slightly narrower at those points where the transistors are connected, so as to match the impedance of a microstrip to the additional capacitance that is supplied by a passive transistor. Alternatively, the impedances of respective transmission lines can be matched to the capacitances of the passive transistors by choosing the value of the terminating resistors R5, R6, R7, . R12, R13 and R14 to correspond to the transmission lines having the additional distributed capacitances that the transistors supply.

Another conceivable variant of the transmission lines are parallel plane lines (stripline) disposed in intermediate layers of the circuit board. This transmission line embodiment, however, would require the transistors to be connected to said lines by means of bushings or throughlets, which would make the matching problem slightly more difficult to resolve than when the transistors are connected to a microstrip. However, the inputs and outputs of the demultiplexer are often arranged in the form of parallel plane lines that extend through the interior of the circuit board.

The demultiplexer illustrated in Fig. 2 functions with differential-mode signals, which results in less distortion in the signals leaving the demultiplexer. The demultiplexer operates as follows: An input signal is received on the input IN+, IN- and amplified to an appropriate level in the amplifier D_AMP. The amplified input signal is forwarded to a selected output by the first and the second transmission lines TLINE_1, TLINE_2. The amplification of the amplifier D_AMP is chosen so as to essentially compensate for subsequent losses due to output impedance and load.

One of the outputs, such as the first output for instance, is activated by the first externally controlled switch SI, which activates the transistors Ql and Q4 connected to the input when said input is closed. The second input is passive, i.e. the second switch S2 is off. The switches may comprise any one of a number of different designs known to the person skilled in this art, for instance transistors. The switches SI and S2 are controlled by a control circuit arranged in the protection switch unit and functioning to close a switch depending on which of the aforesaid terminal access units is inoperative. This switching function may also be accomplished directly from the aforesaid exchange. Closing of the switch SI results in biassing of the transistors Ql and Q4 and the amplified input signal is conducted from the transmission lines TLINE_1 and TLINE_2 through the transistors Ql and Q4 and to the connection points OUT1+ and OUT1- in the first output.

The demultiplexer is adapted to conduct an output signal also to the third and the fourth transmission lines TLINE_3 and TLINE_4 immediately when the output signal is obtained on an output. This means that a replica of the input signal is also obtained. This replica is obtained on the connection points SUP+, SUP- of the monitoring output and is processed in a monitoring circuit, which may be included in the protection switch unit. The demultiplexer may also include a second differential amplifier for amplifying the monitoring signal. It can be established whether or not the reserve access unit is broken or not, by analyzing this signal. An analysis of the signal will also reveal whether or not the^'' demultiplexer is working in the manner intended. This represents a saving in the additional monitoring circuits that would otherwise be required.

The connection of the transmission lines means that any reflexions therein will be quickly extinguished. The trans- mission lines are arranged so that the third and the fourth transmission lines TLINE_3 and TLINE_4 have roughly half the impedance of the first and the second transmission lines TLINE_1 and TLINE_2. This is achieved by virtue of selecting the width of the transmission lines and their respective distances to the earth plane. As a result of such selection, the collector voltage on a transistor will be lower than the emitter and the base voltage. Crosstalk from collector to base will therefore have only a very slight influence on signal quality. A certain degree of crosstalk may occur in the reverse direction, from base to collector, but the quality of the monitoring signal is not equally as important as the quality of the output signal. Since the collector voltage is low in comparison with the base and emitter voltage, the transistor can be considered as an emitter follower. This gives a high input impedance and therewith has little influence on the first and the second transmission lines TLINE_1 and TLINE_2, which has only a slight effect on the pulse shape.

The amplifier D_AMP may alternatively be connected to the opposite end of the first and the second transmission lines TLINE_1 and TLINE_2. The monitoring signal output may also be placed at the opposite end of the third and fourth transmission lines TLINE 3 and TLINE 4.

Claims

1. In demultiplexing processes, a method of selecting one of several outputs as the output for an input signal received on an input (IN+, IN-) , characterized by conducting the input signal or at least a first part thereof from a first connection point (IN+) in the input to a first transmission line (TLINE__1) ; conducting the input signal or the first part thereof selectively from said first transmission line to a first connection point (0UT1+) in one of at least two outputs (OUT1+, OUT1-, OUT2+, OUT2-) on the demultiplexer via a first controllable signal forwarding device (Ql) as an output signal or a first part of an output signal of high signal quality.

2. A method according to Claim 1, characterized by conducting the input signal or the first part thereof to a third transmission line (TLINE_3) at the same time as said input signal or said first part thereof is conducted to said first connection point (OUT1+) in the output, such as to create a first replica of the output signal or the first part thereof.

3. A method according to Claim 1 or Claim 2, characterized in that the input signal includes a first and a second part, wherein the second part of the input signal is conducted, simultaneously with the first signal part, from a second connection point (IN-) in the input to a second transmission line (TLINE_2) and the second part is thereafter conducted from said second transmission line to a second connection point

(OUT1-) in said one output, via a second controllable signal forwarding device (Q4) .

4. A method according to Claim 3, characterized by conducting said second part of the input signal to a fourth transmission line (TLINE_4) at the same time as said second part is conducted to said second connection point (0UT1-) , such as to generate a second replica of the second part of said output signal.

5. A method according to any one of the preceding Claims, characterized by amplifying the input signal prior to conducting said signal to the transmission lines (TLINE_1, TLINE_2) which together bind the input to said one output.

6. A demultiplexer having an input (IN+, IN-) and at least two outputs (OUT1+, OUT1-, OUT2+, OUT2-) , wherein a first connection point (0UT1+, 0UT2+) in each output is connected to a common line (TLINE_1) via a respective first controllable signal forwarding device (Ql, Q2) , wherein said first connec¬ tion point in each output is also connected to an externally controlled switch (SI, S2) which functions to control said first signal forwarding device to forward a signal or a first part of a signal applied through the common line to the first connection point in said output, characterized in that the common line is a first transmission line (TLINE_1) which is also connected to a first connection point (IN+) in the input.

7. A demultiplexer according to Claim 6, characterized in that each first signal forwarding device (Ql, Q2) is also connected to a third transmission line (TLINE_3) for generating a monitoring signal or a first part of a monitoring signal.

8. A demultiplexer according to Claim 6 or Claim 7, characterized in that a second connection point (OUT1-, 0UT2-) in each output is connected to a second transmission line (TLINE_2) via a respective second controllable signal forwarding device (Q4, Q3) , wherein said second transmission line is connected to a second connection point (IN-) in the input, and wherein said switch (SI, S2) also controls said second signal forwarding device.

9. A demultiplexer according to Claim 8, characterized in that each second signal forwarding device (Q3, Q4) is also connected to a fourth transmission line (TLINE_4) for generating a second part of a monitoring signal.

10. A demultiplexer according to any one of Claims 6-9, characterized in that each of said transmission lines (TLINE_1, TLINE_2, TLINE_3, TLINE_4) is a microstrip mounted in the demultiplexer circuit board.

11. A demultiplexer according to any one of Claims 6-9, characterized in that each of the transmission lines (TLINE_1, TLINE_2, TLINE_3, TLINE_4) is a parallel plane conductor mounted in the demultiplexer circuit board.

12. A demultiplexer according to any one of Claims 6-11, characterized by an amplifier connected between the input

(IN1+, IN1-) and said transmission lines (TLINE_1, TLINE_2) that connect the input with each output (OUT1+, OUT1-, OUT2+, OUT2-) .

13. A demultiplexer according to Claim 7 or Claim 9, characterized in that the signal forwarding devices (Ql, Q4, Q2, Q3) are transistors, wherein the emitter is connected to the connection point in the output (OUT1+, OUT1-, OUT2+, OUT2-) via a resistor (Rl, R3, R2, R4), the base is connected to the transmission lines (TLINE_1, TLINE_2) that connect the input (IN+, IN-) with each output, and the collector is connected to the transmission lines (TLINE_3, TLINE_4) so as to generate the. monitoring signal.

14. A demultiplexer according to any one of Claims 6-13, characterized in that the transmission lines (TLINE_1, TLINE_2, TLINE_3, TLINE_4) are terminated with a respective resistor (R5, R6, R12, R13, R7, R14) .

15. A protection switch unit (4) including a demultiplexer, which has an input (IN+, IN-) and at least two outputs (0UT1+, 0UT1-, 0UT2+, 0UT2-) , wherein a first connection point (0UT1+, OUT2+) in each output on the demultiplexer is connected to a common line (TLINE_1) via a respective first controllable signal forwarding device (Ql, Q2) wherein also connected to each output is an externally controllable switch means (SI, S2) which functions to control said first signal forwarding device to forward to the first connection point in the output a signal or a first part of a signal applied over the common line or conductor, characterized in that the common line (TLINE_1) is a first transmission line which is also connected to a first connection point (IN+) in the input of the demultiplexer.

16. A unit (4) according to Claim 15, characterized in that each first signal forwarding device (Ql, Q2) is also connected to a third transmission line (TLINE_3) so as to generate a monitoring signal or a first part of a monitoring signal.

17. A unit (4) according to Claim 15 or Claim 16, character- ized in that a second connection point (OUT1-, OUT2-) in each demultiplexer output is connected to a second transmission line (TLINE_2) via a respective second controllable signal forwarding device (Q4, Q3) , wherein the second transmission line is connected to a second connection point (IN-) in the input, and wherein said switch means (Si, S2) also control said second signal forwarding devices.

18. A unit (4) according to Claim 17, characterized in that each second signal forwarding device (Q3, Q4) is also connected to a fourth transmission line (TLINE_4) so as to generate a second part of a monitoring signal.

19. A unit (4) according to any one of Claims 15-18, characterized by an amplifier connected between the input (IN1+, IN1-) and the transmission lines (TLINE_1, TLINE_2) that connect the input with each demultiplexer output (0UT1+, OUT1-, OUT2+, OUT2-) .

20. A unit (4) according to Claim 16 or Claim 18, character¬ ized in that the signal forwarding devices (Ql, Q , Q2, Q3) are transistors, wherein the emitter is connected to the connection point in respective outputs (OUT1+, OUT1-, OUT2+, 0UT2-) via a respective resistor (Rl, R3, R2, R4), wherein the base is connected to the transmission line (TLINE_1, TLINE_2) that connects the input (IN+, IN-) with each output, and wherein the collector is connected to the transmission line (TLINE_3, TLINE_4) so as to generate the monitoring signal.

21. A unit (4) according to any one of Claims 15-20, characterized in that the transmission lines (TLINE_1, TLINE_2, TLINE_3, TLINE_4) in the demultiplexer are terminated with a respective resistor (R5, R6, R12, R13, R7, R14) .

22. A telecommunications network that includes two or more lines connected to an exchange (1) via a respective terminal access unit (3) , wherein each line is also connected to a protection switch unit (4) which, in turn, is connected to the exchange via a reserve terminal access unit (5) , wherein the protection switch unit includes a demultiplexer that has an input (IN+, IN-) and at least two outputs (0UT1+, 0UT1-, 0UT2+, 0UT2-), wherein a first connection point (0UT1+, 0UT2+) in each demultiplexer output is connected to a common line (TLINE_1) via a respective controllable signal forwarding device (Ql, Q2) , wherein each output is also connected to an externally controlled switch means (SI, S2) for controlling said first signal forwarding device to forward to the first connection point in the output a signal or a first part of a signal which is applied over the common line, characterized in that the common line (TLINE_1) is a first transmission line which is also connected to a first connection point (IN+) in the demultiplexer input.

23. A telecommunications network according to Claim 22, characterized in that each first signal forwarding device (Ql, Q2) is also connected to a third transmission line (TLINE_3) so as to generat-e a monitoring signal or a first part of a monitoring signal.

24. A telecommunications network according Claim 22 or Claim 23, characterized in that a second connection point (OUT1-, OUT2-) in each demultiplexer output is connected to a second transmission line (TLINE_2) via a respective second control¬ lable signal forwarding device (Q4, Q3) which is connected to a second connection point (IN-) in the input, wherein said.switch means (SI, S2) also controls said second signal forwarding device.

25. A telecommunications network according to Claim 24, characterized in that each second signal forwarding device (Q3, Q4) is also connected to a fourth transmission line (TLINE_4) so as to generate a second part of a monitoring signal.

26. A method relating to demultiplexing for selecting one of a plurality of outputs as an output for an input signal received on an input, comprising the steps of:

- conducting the input signal or at least a first part of said signal from a first connection point in the input to a first transmission line; and

- conducting the input signal for the first part of said signal from the first transmission line to a first connection point in one of at least two demultiplexer outputs via a first controllable signal forwarding device as an output signal for a first part of an output signal that has good signal quality.

27. A demultiplexer that has an input and at least two outputs, wherein a first connection point in each output is connected to a common line via a respective first controllable signal forwarding device; wherein there is also connected to each output an externally controlled switch means which functions to control the first signal forwarding device to forward to the first connection point in said output a signal or a first part of a signal that is applied over the common line; and wherein the common line is a first transmission line which is also connected to a first connection point in the input.

28. A protection switch unit comprising a demultiplexer which has an input and at least two outputs, wherein a first connection point in each demultiplexer output is connected to a common line via a respective first controllable signal forwarding device; wherein there is also connected to each output an externally controlled switch means which functions to- control said first signal forwarding device to forward to the first connection point in the output a signal or a first part of a signal applied over the common line; and wherein the common line is a first transmission line which is also connected to a first connection point in the demultiplexer input.

29. A telecommunications network comprising two or more lines which are connected to an exchange via a respective terminal access unit, wherein each line is also connected to a protection switch unit which, in turn, is connected to the exchange via a reserve terminal access unit, wherein the protection switch unit includes a demultiplexer which has an input and at least two outputs, wherein a first connection point in each demultiplexer output is connected to a common line via a respective first controllable signal forwarding device; wherein there is also connected to each output an externally controlled switch means which functions to control said first signal forwarding device to forward to the first connection point in said output a signal or a first part of a signal applied over the common line; and wherein the common line is a first transmission line which is also connected to a first connection point in the demultiplexer input.