EP0872055A1

EP0872055A1 - Multiplexer, protection switch unit, telecommunications network and a multiplexing related method

Info

Publication number: EP0872055A1
Application number: EP96917756A
Authority: EP
Inventors: Mats Bladh
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 1995-06-01
Filing date: 1996-05-22
Publication date: 1998-10-21
Also published as: CN1185877A; SE9502011L; JPH11507479A; AU705474B2; AU6019496A; KR19990022025A; CA2220966A1; SE504521C2; SE9502011D0; WO1996038939A1

Abstract

There is provided a multiplexer which in a multiplexing process selects an output signal from a plurality of input signals, a protection switch unit which includes one such demultiplexer and a telecommunications network which includes one such protection switch unit, the multiplexer has an output (OUT+, OUT-) and at least two inputs (IN1+, IN1-, IN2+, IN2-) where two input signals corresponding to the inputs are received. A first connection point (IN1+, IN2+) in each input is connected to a first transmission line (TLINE_1) via a respective first controllable signal forwarding device (Q1, Q2). An externally controllable switch means (S2, S1) is also connected to each output for forwarding a signal or at least a part thereof delivered to said input to a first transmission line (TLINE_1). The first transmission line (TLINE_1) is connected to a first connection point (OUT+) in the output, where a signal of good quality is obtained.

Description

MULTIPLEXER, PROTECTION SWITCH UNIT, TELECOMMUNICATIONS NETWORK AND A MULTIPLEXING RELATED METHOD

FIELD OF INVENTION

The present invention relates to a multiplexer, a protection switch unit which includes one such multiplexer, a telecommunications network which includes one such protec¬ tion switch unit, and a multiplexing related method. More specifically, the invention relates to the aforesaid devices and method in respect of single or differential multiplexing 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 and can sometimes break down. For this reason, the network includes at least one protection switch unit which is connected between the signal lines and a reserve terminal access unit connected to the exchange. The unit includes a multiplexer or selector whose inputs are connected to the signal lines and whose output is connected to the reserve terminal access unit. When a regular terminal access unit cannot be used for some reason or another, the multiplexor in the protection switch unit is controlled to forward to the reserve access unit the signal corresponding to the normal terminal access unit.

Good signal quality and controlled amplification is needed when selecting or multiple¬ 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 and the level of the output signal required to be the same as it would have been if the signal had not passed through the multiplexer. Problems with mismatching between components and conductors can easily occur in such multiplexers at such high transmission speeds, therewith greatly impairing signal quality.

A number of standard circuits exist for multiplexing signals having bit rates of up to about 30 Mb/s in a relatively simple and inexpensive manner exist. At higher bit rates, line impedances, connections, terminations, line runs and component data place strict requirements on circuitry. In order to keep the imdepedances as constant as possible, special care has to be put into the realisation of the board circuitry. The work involved is made more difficult when a large number of input signals shall be multiplexed. The circuitry involving standard circuits in a multiplexer therewith becomes expensive and highly current consuming.

Various ECL-type multiplexers are known to the art; see for instance WO93/17500. ECL-multiplexers have a relatively high amplification (in the region of 15 - 30 Db) which must be attenuated in one or more additional circuits so that signal levels will remain unchanged. This attenuation creates a number of problems. When more than ten signals are to be multiplexed it is necessary to connect together more ECL- multiplexers, which leads to still higher amplification and still more attenuation prob¬ lems. The pin positioning of these circuits also makes the placement of conductors much more difficult if constant impedance and delay is to be achieved.

There is no technique at present for effectively multiplexing in the aforesaid transmission speed ranges.

US-A 5,146,113 describes an integrated circuit having several narrow, circuit-board mounted elongated resistance strips to provide a board-mounted circuit with a predetermined input and/or output impedance. US-A 5,281,934 describes a microwave multiplexer which is entirely of microstrip construction.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method of selecting an output signal from a plurality of input signals when multiplexing, therewith to obtain good signal quality in a simple and inexpensive manner.

This object is achieved with a method in which at least two signals are received on inputs which correspond to said signals, and in which one signal is selectively conducted from one of the inputs to a transmission line from which an output signal is obtained.

Another object of the invention is to provide a method of selecting an output signal from a plurality of input signals when multiplexing wherein distortion in the output signal can be reduced in later stages in a simple manner.

This object is achieved with a method in which each input signal includes two parts and in which each of said two input signal parts is conducted from one of the inputs to a respective transmission line.

A further object of the invention is to provide a multiplexing related method for selecting an output signal from a plurality of input signals which also offer an output signal that has essentially the same signal level as the input signals or is only slightly amplified in comparison therewith.

This object is achieved with a method which also includes amplification of the output signal from said transmission line. Still another object of the present invention is to provide a multiplexer, a protection switch unit including one such multiplexer, 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 an output signal from a plurality of input signals.

This object is achieved with a multiplexer, a protection switch unit and a telecommunications network wherein the multiplexer includes two or more inputs and an output and has a first connection point in each input connected to a first transmission line via a respective first externally controllable signal forwarding device, said first transmission line being connected to a first connection point in the output.

Yet a further object of the invention is to provide a multiplexer, a protection switch unit which includes one such multiplexer, and a telecommunications network which includes one such protection switch unit wherein distortion in output signals from the multiplexer can be readily reduced in later stages.

This object is achieved with a multiplexer, a protection switch unit and a telecommunications network wherein a second connection point in each multiplexer input is also connected to a second transmission line via a respective second externally controllable signal forwarding device, said second transmission line being connected to a second connection point in said output.

Still yet a further object of the invention is to provide a multiplexer, a protection switch unit that includes one such multiplexer and a telecommunications network that includes one such protection switch unit wherein multiplexer output signals have essentially the same signal level as the input signals or only a slightly amplified variable signal level in comparison therewith. This object is achieved with a multiplexer, a protection switch unit and a telecommunications network in which an amplifier is connected between the multiplexer input and said transmission line.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects of the invention and advantages afforded thereby will be evident from the following description of preferred embodiments of the invention made with reference to the accompanying drawings, in which 0

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

Fig. 2 is a circuit diagram of an inventive multiplexer; and

5 Fig. 3 is a circuit diagram of an alternative embodiment of the multiplexer shown in Fig. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

0 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 or lines. Each signal line is connected to a protection unit 2 (PU = Protection Unit). Although only eight protection units are shown in Fig. 1 , it will be 5 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 even though these units will actually be sixteen in number. The modus operandi of these o 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 signals incoming to the terminal access units 3 and to the protection switch unit 4 into differential signals, and to convert differential signals outgoing from the exchange 1 via the terminal access units 3 and the protection switch unit 4 to single signals. The protection switch unit 4 includes a multiplexer or selector and a demultiplexer. The multiplexer 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 multiplexer, wherein only two inputs are shown for the sake of simplicity, although it will be understood that the multiplexer will normally have sixteen inputs. A first of the inputs has a first and a second connection point IN1+ and IN1-. The first connection point IN1+ of the first input is connected to a first signal forwarding conductor device Q2 via a resistor R2, said device Q2 being an RF transistor in the illustrated case. The resistor R2 is connected to the emitter of the transistor Q2 in the illustrated case. The base of the transistor Q2 is connected via a resistor R9 to a voltage source VBB which has an internal impedance such that the base is earthed with respect to the signal delivered to the connection point IN1+. The collector of the transistor Q2 is connected via a resistor R41 to a transmission line or conductor TLINE_1 provided in a multiplexer circuit board. The transmission line TLINE_1 is terminated at both ends with a respective resistor R12 and R44, which in turn are connected to a voltage source VCC. The voltage source VCC may have the same voltage as the voltage source VBB.

The second connection point IN1- of the first input is connected to a second transmission line TLINE_2 mounted on the multiplexer circuit board via a resistor R4, an RF-transistor Q3 and a further resistor R43, similar to the first connection point IN1-. The base of the transistor Q3 is connected to the voltage source VBB via a resistor R11. The other end of the TLINE_2 is terminated with a resistor R7 and a resistor 45, which are connected to the voltage source VCC. Respective emitters of the first and second transistors Q2 and Q3 are connected to one end of a first externally controlled switch means S2 via a respective resistor R31 and R33. The other end of the switch S2 is connected to earth.

The connection points IN2+ and IN2- of a second input are connected to the first and the second transmission lines TLINE_1 and TLINE_2 respectively in exactly the same way, via a resistor R1 , an RF-transistor Q1 and a resistor R40, and via a resistor R3, an RF-transistor Q4 and a resistor R42. Respective bases of the transistors Q1 and

Q4 are connected to the voltage source VBB via a respective resistor R8 and R10.

Respective emitters of the transistors Q1 and Q4 are connected to a second externally controlled switch S1 via respective resistors R30 and R32 in exactly the same way as with the first input, said switch S1 also being connected to earth similar to the switch

S1.

The transistors 0.1, Q2, Q3 and Q4 and associated components are so placed along the transmission lines TLINE_1 and TLINE_2 that mismatches and reflections will be minimal. In reality, further inputs are correspondingly placed along said transmission lines TLINE_1 and, TLINE_2.

A differential amplifier D_AMP has one input and is connected to the first transmission line TLINE_1 at a first connection point +| therein adjacent the resistor R44 and to the second transmission line TLINE_2 at a second connection point -I adjacent the resistor R45. The amplifier D_AMP has an output which has two connection points +0 and -0 which are each connected to a respective connection point OUT+ and OUT- in the output. The amplifier D_AMP may also include the resistors R44 and R45.

In the preferred embodiment, each of the transmission lines TLINE_1 and TLINE_2 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 input have equally long paths to the connection points +1 and -I in the input of the amplifier D_AMP. The transmission lines TLINE_1 and TLINE_2 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 values of the terminating resistors R7, R12, R44 and R55 so as to correspond to the transmission lines having the additional distributed capacitances that the inputs 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 multiplexer are often arranged in the form of parallel plane lines that extend through the interior of the circuit board.

The multiplexer illustrated in Fig. 2 functions with differential signals, which results in less distortion in the signals leaving the multiplexer. The multiplexer operates as follows: Two input signals are received on a respective input IN1+, IN1- and IN2+ IN2- (only two inputs are shown for the first circuit, although in reality sixteen input signals are normally received via sixteen inputs. Other numbers of signals may be received however, such as four or eight for instance.)

One of the inputs, such as the first input for instance, is activated by the first externally controlled switch S2, which activates the transistors Q2 and Q3 connected to the input when said input is closed. The second input is passive, i.e. the second switch S1 is off. The switches may comprise any one of a number of different designs known to the person skilled in this art, and may have the form of transistors for instance. The switches S1 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. When the switch S2 is closed, the transistors Q2 and Q3 are biassed and the input signal is forwarded through the transistors Q2 and Q3 to the transmission lines TLINE_1 and TLINE_2. The signal propagates in both directions on the transmission lines TLINE_1 and TLINE_2 and one half of the signal, which has essentially one half of the amplitude of the input signal, is received in the amplifier D_AMP. For the reason mentioned above, this amplifier could equally as well be placed in the other end of the transmission lines TLINE_1 and TLINE_2. Because of the terminating resistors R7, R12, R44 and R55, mismatching at the active input connection points to the transmission lines TLINE_1 and TLINE_2 will not result in reflections. The amplifier D_AMP then amplifies the signal such that the output signal will obtain the same signal level as the input signal. Amplification of the amplifier D_AMP can also be adapted to take into account losses in cables and electrical contacts.

The earthed base stages in the transistors Q2 and Q3 in the active input give a precise terminating impedance, since this is determined in the most part by the emitter series resistance when the biassing current is sufficiently high. This gives good termination of incoming signals and therewith prevents reflections. The collector output impedance will be high in relation to the load to be driven, in other words the transmission line. Any variation that may occur in the collector impedance will have very little affect on signal quality and the level of the output signal.

Crosstalk from passive inputs will be low, since the current to the transistors of the passive inputs is cut off and because the bases of said transistors function as cut-offs. Furthermore, when the voltage source is switched off, only noise and crosstalk from the lines need be blocked.

The transistors Q1, Q2, Q3 and Q4 may alternatively be coupled differently to that described. In this variant, each connection point in each input is connected to a respective base via a respective resistor and to the voltage source VBB via a respec¬ tive resistor. In this regard, the emitters of the transistors at respective inputs must be mutually connected by further resistors. This solution thus requires more components in the multiplexor than was earlier the case and also requires the provision of extra components such as capacitors and chokes in order to be able to activate the multiplexor from the remote end, ie the end from which the signal arrives. Moreover, the capacitance between the base and the emitter of the transistors results in greater crosstalk than in the earlier described case. It is also difficult to obtain good signal quality.

In an alternative embodiment, shown in Fig. 3, the transistors are replaced with PIN- diodes D2, D1, D4 and D3, each of which has its cathode connected to a respective connection point IN1+, IN2+, IN1- and IN2- in the input via a respective resistor R2, R1 , R4 and R3, while the anodes of said diodes are connected directly to a respective transmission line TLINE_1 and TLINE_2. This circuit lacks the voltage source VBB and the resistors connected between said voltage source and respective transistor bases in the Fig. 2 embodiment. Furthermore, resistors R31, R30, R33 and R32 connected between the switches S1 , S2 and the connection points are replaced with impedances Z31 , Z30, Z33 and Z32. The circuits are identical in all other respects and all remaining reference signs have the earlier significance. In the case of the Figure 3 embodiment, the terminating impedance is determined in an active input, for instance the first input of the series resistors R2 and R4, the impedances Z31, Z33 and the series resistance in the diodes D2 and D4 and all following transmission lines with terminations. The impedances may be in the form of just a resistor or as a resistor in series with an inductor. Resistors in series with an inductor give a high impedance, which means that all current to the diodes D1, D2, D3 and D4 will also go out to the transmission lines TLINE_1 and TLINE_2. Mismatching between diode and transmission line will be much greater in this embodiment than in the Figure 2 embodiment. However, no reflec- tions will be generated provided that matching in the remaining connection points of the transmission lines is good. Variation in diode resistance from diode to diode does not affect the input impedance to the transmission lines to any appreciable extent.

Claims

1. In multiplexing processes, a method of selecting an output from a plurality of input signals, wherein at least two input signals are received on inputs (IN1+, IN1-, IN2+, IN2-) corresponding to said signals, characterized by conducting selectively one of the input signals or at least a first part of one of said signals from a first connection point (IN1+) in one of the inputs to a first transmission line (TLINE_1) via a first controllable signal forwarding device (Q2; D2), wherewith said input signal or said first part of said input signal is obtained from said first transmission line as an output signal or a first part of an output signal of good signal quality.

2. A method according to Claim 1 , characterized in that the input signals each includes a first and a second part and the output signal also includes a first and a second part, wherein the second part of said input signal is conducted, at the same time as the corresponding first part, from a second connection point (IN1-) in said one input to a second transmission line (TLINE_2) via a second controllable signal forwarding device (Q3; D4); and in that the second part of the output signal is obtained from said second transmission line.

3. A method according to any one of the preceding Claims, characterized by controlling the signal forwarding devices (Q2, Q3, Q1, Q4; D2, D4, D1, D3) connected to the inputs (IN1+, IN1-, IN2+, IN2-) to forward the input signals from the inputs to said transmission lines (TLINE_1 and TLINE_2) by an externally controlled switch (S2, S1) corresponding to each input.

4. A method according to any one of the preceding Claims, characterized by amplifying the output signal obtained from said transmission lines (TLINE_1 and TLINE_2).

5. A multiplexer having at least two inputs (IN1+, IN1-, IN2+, IN2-) and an output (OUT1+, OUT1-), wherein a first connection point (IN1+, IN2+) in each input is connected to a common line (TLINE_1) via a respective first controllable signal forwarding device (Q2, Q1; D2, D1), wherein there is connected to each input an externally controlled switch (S2, S1) which functions to control said first signal forwarding device to forward a signal or a first part of a signal delivered to said input to said common line, characterized in that the common line (TLINE_1) is a first transmis¬ sion line which is also connected to a first connection point (OUT+) in the output.

6. A multiplexer according to Claim 5, characterized in that a second connection point (IN1-, IN2-) in each input is connected to a second transmission line (TLINE_2) via a respective second controllable signal forwarding device (Q3, Q4; D4, D3), wherein said second transmission line is connected to a second connection point (OUT-) in the output, and wherein said switch (S2, S1) also controls said second signal forwarding device.

7. A multiplexer according to Claim 5 or Claim 6, characterized in that each of said transmission lines (TLINE_1, TLINE_2,) is a microstrip mounted in the multiplexer circuit board.

8. A multiplexer according to any one of Claims 5-7, characterized in that each of the transmission lines (TLINE_1, TLINE_2,) is a parallel plane conductor mounted in the multiplexer circuit board.

9. A multiplexer according to any one of Claims 5-8, characterized by an amplifier (D_AMP) connected between one end of the transmission lines (TLINE_1 , TLINE_2) and the output (OUT+, OUT-) for amplifying the signals obtained from said transmission lines.

10. A multiplexer according to anyone of Claims 5 - 9, characterized in that the signal forwarding devices (Q1, Q2, Q3, Q4) are transistors, wherein the emitter is connected to the connection points (IN2+, IN1+, IN1-, IN2-) in corresponding inputs via a resistor (R1, R2, R4, R3), the collector is connected to the transmission lines (TLINE_1, TLINE_2), and the base is earthed with respect to the signal delivered to the connection point in a corresponding input.

11. A multiplexer according to any one of Claims 5-9, characterized in that the signal forwarding devices (D1 , D2, D3, D4) are diodes.

12. A multiplexer according to any one of Claims 5-11 , characterized in that the transmission lines (TLINE_1, TLINE_2) are terminated with a respective resistor (R12,

R44, R7, R45).

13. A protection switch unit (4) including a multiplexer, which has at least two inputs (IN1+, IN1-, IN2+, IN2-) and an output (OUT+, OUT-), wherein a first connection point (IN1+, IN2+) in each multiplexer input is connected to a common line (TLINE_1) via a respective first controllable signal forwarding device (Q2, Q1 ; D2, D1), wherein also connected to each input is an externally controllable switch means (S2, S1) which functions to control said first signal forwarding device to forward to the common line a signal or at least a part thereof applied to the input, characterized in that the common line (TLINE_1) is a first transmission line which is also connected to a first connection point (OUT+) in the multiplexer output.

14. A unit (4) according to Claim 13, characterized in that a second connection point (IN1-, IN2-) in each multiplexer input is connected to a second transmission line (TLINE_2) via a respective second controllable signal forwarding device (Q3, Q4; D4, D3), wherein second transmission lines are connected to a second connection point (OUT-) in the multiplexer output, and wherein said switch means (S2, S1) also function to control said second signal forwarding devices.

15. A unit (4) according to any one of Claim 13 or Claim 14, characterized by an amplifier (D_AMP) connected between one end of the transmission lines (TLINE_1, TLINE*_2) and the multiplexer output (OUT+.OUT-) for amplifying signals obtained from said transmission lines.

16. A unit (4) according to any one of Claims 13 - 15, characterized in that the signal forwarding devices (Q1, Q2, Q3, Q4) at each multiplexer input are transistors, wherein the emitter is connected to the connection point (IN2+, IN1+, IN1-, IN2-) in respective inputs via a respective resistor (R1, R2, R4, R3), wherein the collector is connected to the transmission line (TLINE l, TLINE_2), and wherein the base is earthed with respect to the signal delivered to the connection point in a corresponding input.

17. A unit (4) according to any one of Claims 13 - 15, characterized in that the multiplexer signal forwarding devices (D1, D2, D3, D4) are diodes.

18. A unit (4) according to any one of Claims 13 - 17, characterized in that the multiplexer transmission lines (TLINE_1 , TLINE_2.) are mounted on a multiplexer circuit board and terminated with a respective resistor (R12, R44, R7, R45).

19. 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 multiplexer that has an input (IN1+, IN1-, IN2+, IN2-) for each line and an output (OUT+, OUT-), wherein a first connection point (IN1+, IN2+) in each multiplexer input is connected to a common line (TLINE_1) via a respective controllable signal forwarding device (Q2, Q1; D2, D1), wherein each input is also connected to an externally controlled switch means (S2, S1) for controlling said first signal forwarding device to forward to the common line a signal or at least a part of said signal delivered to said input, characterized in that the common line (TLINE_1) is a first transmission line which is also connected to a first connection point (OUT+) in the multiplexer output.

20. A telecommunications network according to Claim 19, characterized in that a second connection point (IN1-, IN2-) in each input of the multiplexer included in the protection switch unit (4) is connected to a second transmission line (TLINE_2) via a respective second controllable signal forwarding device (Q3, Q4; D4, D3), wherein said second transmission line is connected to a second connection point (OUT-) in the multiplexer output, and wherein said switch (S2, S1) also controls said second signal forwarding device.

21. A telecommunications network according to any one of Claims 19 or 20, characterized by an amplifier (D_AMP) connected between one end of the transmission lines (TLINE_1, TLINE_2) and the multiplexer output (OUT+, OUT-) for amplifying signals obtained from said transmission lines.

22. In a multiplexing process, a method of selecting an output from a plurality of input signals, wherein at least two input signals are received on inputs corresponding to said signals, wherein one of the input signals or at least a first part of one of said signals is conducting selectively from a first connection point in one of the inputs to a first transmission line via a first controllable signal forwarding device, wherewith said input signal or said first part of said input signal is obtained from said first transmission line as an output signal or a part of an output signal of good signal quality.

23. A multiplexer having at least two inputs and one output, 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 connected to each input an externally controlled switch which functions to control said first signal forwarding device to forward to said common line a signal or a first part of a signal delivered said to input, and wherein the common line is a first transmission line which is also connected to a first connection point in the output.

24. A protection switch unit including:

5 a multiplexer which has at least two inputs and one output, wherein a first connection point in each multiplexer input is connected to a common line via a respective first controllable signal forwarding device, wherein also connected to each input is an externally controllable switch means which functions to control said first signal forwarding device to forward to the common line a l o signal or at least a part of a signal delivered to the input, and wherein the common line is a first transmission line which is also connected to a first connection point in the multiplexer output.

25. A telecommunications network that includes:

15 two or more lines 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 multiplexer that has an input for each line and an output, 20 wherein a first connection point in each multiplexer input is connected to a common line via a respective controllable signal forwarding device, wherein each input is also connected to an externally controlled switch means for controlling said first signal forwarding device to forward to the common line a signal or at least a part of a signal delivered to said input, and 25 wherein the common line is a first transmission line which is also connected to a first connection point in the multiplexer output.