WO1997033380A1

WO1997033380A1 - System, arrangement and method relating to protection in a communications system and a telecommunications system with a protection arrangement

Info

Publication number: WO1997033380A1
Application number: PCT/SE1997/000373
Authority: WO
Inventors: Jan Hopfinger
Original assignee: Telefonaktiebolaget Lm Ericsson
Priority date: 1996-03-04
Filing date: 1997-03-04
Publication date: 1997-09-12
Also published as: SE9600825D0; CN1217102A; BR9707927A; SE508487C2; EP0891657A1; SE9600825L; AU2110897A

Abstract

The present invention relates to a system and a method respectively for protection switching in a communications network such as e.g. a telecommunications network which comprises a number of nodes. Each node comprises a number of signal equipment units (11, ..., 1n) providing access to/from the node. At least one redundant equipment unit (1n+1) is provided and a protection switching arrangement (10) is arranged for protecting signals to/from the equipment units. The switching arrangement (10) comprises a number of first switching means (21, ..., 2n) which connect to the access units (11, ..., 1n) and in said first switching means signal input/output means (31, ..., 3n; 41, ..., 4n) are arranged which provide conversion between external traffic signals and internal traffic signals. At least a number of the first switching means (21, ..., 2n) are connected to at least one second switching means (7) which in turn connects to said at least one redundant equipment unit (1n+1). When protection switching is needed for a signal to/from an equipment unit (11, ..., 1n), protection switching is done on an internal signal in at least two steps via a first switching means and a second switching means to/from a redundant access unit (1n+1).

Description

TITLE

SYSTEM, ARRANGEMENT AND METHOD RELATING TO PROTECTION IN A COMMUNICATIONS SYSTEM AND A TELECOMMUNICATIONS SYSTEM WITH A PROTECTION ARRANGEMENT

TECHNICAL FIELD

The present invention relates to a system and a method respectively for protection switching in a communications network, for example a telecommunications network. The invention also relates to an arrangement for protection switching and to a telecommunications system in which protection switching for signals is provided.

STATE OF THE ART

In communications systems of today information or data is transported as signals at high speeds. Moreover the signals generally carry large amounts of data. The importance of protecting these signals can thus be readily understood. Therefore mostly some kind of redundancy is provided to reduce the risks of loosing information or messages, at least to some given extent depending on the actual system, the actual situation etc. As an example, the 155 Mbps SDH (Synchronous Digital Hierarchy) and the 140 Mbps PDH (Plesiochronous Digital Hierarchy) signals in a telecommunications system carry up to about 2000 telephone calls. It is of course of the utmost importance that signals of these kinds or similar have a high availability.

Communications systems, or particularly telecommunications systems, comprise a number of nodes, hardware nodes. Such nodes should to the highest possibly extent be, seen from the outside, hardware units which repair themselves which means that if a traffic carrying part of the node becomes faulty or for some reason can not handle signals in an appropriate way, the node should switch the traffic through another path of the node. This should however not be visible from the outside of the node and there are also requirements e.g. as to the interruption time to ensure that it is acceptable for such a situation.

These problems have in a number of known systems been dealt with through a multiplication of the switching matrix or the cross- connect. In some systems the switching matrix is triplicated whereas in other systems it is duplicated.

Moreover, in US-A-5 329 520 it is intended to protect long point- to-point connections via switching in a cross connect.

However, in none of the systems are equipment units such as arrangements providing access to the cross-connects or switching matrixes etc protected but a failure or similar in such equipment units might result in a loss of important signals.

SUMMARY OF THE INVENTION

What is needed is therefore a system through which protection switching is provided for equipment units or particularly access arrangements.

What is needed is therefore also an arrangement and a method respectively for providing protection for equipment units or arrangements providing access to e.g. switching matrixes or cross- connects etc. or to provide protection for equipment units or access arrangements in general in a communications system. What is needed is also to provide for protection switching for traffic carrying parts etc. for signals which have to fulfil high requirements on transmission parameters, for example as far as pulse shape, attenuation, return-loss impedance etc. are concerned and particularly to provide for protection switching for high speed electrical signals while meeting such requirements. What is needed is also a system, an arrangement and a method which provides for a fast protection switching, i.e. that the time for switching from one unit to another in case of malfunctioning etc. is short. For a number of applications the switch-over time must be so short that it is not possible to use for example electromagnetic relays.

What is needed is also a cost-effective protection switching in a communications system.

What is needed is also a cost-effective protection switching in a communications system in which the electrical line signal or the external signal meets the requirements of the G.703 standard which is a very demanding standard and sensitive for the signal changes. In an embodiment of the communications system the signals are converted independent of the signal type and can be connected via ordinary conductors, such as coaxial cables etc.

Moreover a system/method etc. is needed which in a satisfactory way provides for protection switching of signals which are sensitive to being switched, i.e. without detrimentally affecting the signals in the sense that relevant parameters are destroyed or adversely affected to a non-acceptable extent. Therefore a system/an arrangement is provided through which external signals are converted into internal signals which can be switched from one equipment unit or similar which e.g. can not or should not receive a signal intended therefor, to a redundant equipment unit. Advantageously the switching of the converted internal signal is done in two steps via first and second switching means. However, it can also be done in more than two steps. Advantageously the first switching means comprises the conversion means, e.g. input/output means comprising termination circuits for external high speed signals which are to be converted into internal signals having a format less sensitive to switching. However, the invention also applies to low-speed signals, optical signals. microwave signals etc.

In an advantageous embodiment the equipment units comprise access units. The system particularly comprises a protection switching arrangement with a number of first switching means which are connected to the equipment or access units, each first switching means comprising signal input/output means. At least a number of the first switching means are connected to at least one second switching means which is connected to the at least one redundant access unit. When protection switching is needed for a signal to an equipment unit or particularly an access unit, the protection switching is done in two stages via a first switching means and a second switching means to a redundant equipment unit. An advantageous embodiment relates to a communications system wherein the external signals are high speed electrical signals. The invention can however also be applied to other signals such as low- speed signals, optical signals, microwave signals etc. The node to which the equipment units or access units are provided may e.g. comprise a switching matrix or a cross-connect. The external signals may particularly be high speed signals which have to fulfil the requirements of the CCITT (ITU-T) Rec. G.703 Standards. Examples on signals are 155 SDH or 140 PDH Mbps signals but of course the invention likewise applies to other signals. Particularly it can also be applied to signals having even higher speeds, e.g. future systems wherein the signal speeds are considerably higher.

In an advantageous embodiment each first switching means comprises switching devices, e.g. multiplexing and/or demultiplexing arrangements for switching between an equipment unit or an access unit and the second switching means thus providing a first switching step. Advantageously the switching to and/or from a redundant equipment unit or access unit is done via the secondary switching means to which each or at least a number of, first switching means are connected wherein the second switching means through a switching device, e.g. a multiplexing arrangement selects which first switching means will be connected to the, or a, redundant equipment unit and advantageously via a demultiplexing switching device selects to which first switching means a signal from a redundant access unit will be transmitted. In the case of signals from a redundant equipment unit or access unit the so called first switching step takes place in the first switching means after the second switching step has taken place in the second switching means. The signal is in the latter case converted from an internal signal to an external signal in the selected first switching means. The protection ratio 1 : N is given by the second switching means and is according to the present invention below one, i.e. N is advantageously greater than one. In advantageous embodiments N is equal to 15 or 16 wherein N corresponds to the number of equipment units or access units for which protection switching should be provided. However, the number of equipment units is unlimited for the purposes of the present invention. Advantageously the number of first switching means corresponds to the number of equipment units for which protection switching is to be enabled, i.e. one first switching means for each equipment unit. In still other alternative embodiments the protection switching can be done in more than two switching steps.

The invention applies irrespective of the redundancy or protection switching provided for the switching matrix or cross-connect. It may for example not be protected at all or it may be duplicated or triplicated or even more than triplicated. Advantageously the cables or customer cables, in a particular embodiment, which carry the external signals are directly connected to the first switching means which comprise the input/output means or conversion means for converting the external signals into internal signals (and from internal into external signals). The requirements on the switchover time may be different for different systems and the invention is not limited to a particular switch-over time period.

A protection switching arrangement is also provided which comprises a number of signal equipment or access units which provide access to/from a switch matrix or similar. As an alternative to an ordinary access unit, e.g. in case of malfunctioning or similar, via protection switching means the signals can be switched to some other arrangement, e.g. a redundant access unit. Input/output means are provided in said protection switching arrangement for conversion between external and internal signals. Protection switching is done on internal signals but particularly the external signals are signals which are sensitive to switching, e.g. high speed electrical signals and for example have to meet the requirements of CCITT Rec. G.703 whereas the internal signals to which a conversion is done (or from which a conversion is done) are less sensitive to switching. Particularly the protection switching arrangement comprises first and second switching means thus providing protection switching in two stages.

The arrangement is generally intended for protection of signals to/from traffic carrying equipment comprising a number of equipment units such as for example access units in a telecommunications system.

Moreover a telecommunications system comprising at least one node which comprises a switch, e.g. a cross-connect or a switching matrix (replicated or not) or similar and a number of equipment units, e.g. access units providing access to/from said cross- connect or similar wherein protection is provided for said equipment or access units. At least one additional access unit is provided and a protection switching arrangement for switching signals to/from said redundant access unit if an (ordinary) access unit is faulty or for some other reason should not be used. The protection switching arrangement comprises signal conversion means for converting between external signals and internal signals less sensitive to switching than said external signals. Protection switching is done on internal signals.

Particularly the switching is done in two steps by first and second switching means. Advantageously for each access unit first switching means are provided wherein said first switching means comprises said signal conversion means for converting an external signal into an internal signal and vice versa. In the first switching means the first switching step is done in which a selection is done between an (ordinary) access unit and a second switching means. In said second switching means a selection is done between a number of first switching means, i.e. from which first switching means a signal will be transmitted to the redundant access unit.

For an external signal incoming towards a cross-connect or similar the first switching step is performed before the second switching step whereas for a signal leaving the cross-connect via a redundant access unit, the second switching step takes place before the first switching step.

Still further a method for protecting signals going through at least one node comprising a number of surrounding equipment units, e.g. access units is provided. The method comprises the steps of: providing at least one redundant equipment unit, hereinafter simply referred to as a redundant access unit; providing a protection switching arrangement; if an access unit is defective or similar, switching signals to said redundant access unit. Particularly a signal conversion from an external to an internal signal and vice versa is done in said switching arrangement, switching only being performed on an internal signal.

Advantageously the protection switching is done in at least two steps. In advantageous embodiments the redundant access unit comprises a processor which controls the second switching means. Still further the second switching means may order the first switching means to perform a protection switching step, i.e. to switch a signal to/from the second switching means.

A method for protecting switching of high speed signals in a telecommunications network is also provided. The method comprises the steps of converting high speed external signals into internal signals, performing a first switching step at a signal which can not be transmitted to an intended first equipment unit in which the signal is transmitted to second switching means to which at least a number of the first switching means are connected, performing a second switching step in said second switching means which step comprises selecting which first switching means will be allowed to send a signal to a redundant network equipment unit. The selected signal is then transmitted to the redundant equipment unit. Advantageously the conversion between external and internal signal is done in the first switching means. Yet further the second switching means provides for 1:N switching wherein N exceeds 1. It is an advantage of the present invention that protection switching can be provided for signals sensitive to switching, e.g. high speed signals or particularly electrical high speed signals having to fulfil high requirements e.g. as far as pulse shape, return-loss, impedance etc. is concerned without risking that the signal is destroyed or will not fulfil the given requirements. Another advantage of the invention is that the switch-over time is short, particularly shorter than what can be achieved with the use of electromagnetic relays. Another advantage of the invention is that the protection arrangement is reliable and in that the availability per se is not decreased or suffering from the switching. Another advantage is that the protection switching is cost-effective and that the power dissipation is low.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will in the following be further described under reference to the accompanying drawings in which: FIG 1 illustrates a network node without equipment protection, FIG 2 schematically illustrates a number of equipment units for which a protection switching arrangement is provided,

FIG 3 illustrates a block diagram of a second switching means for a particular embodiment,

FIG 4 illustrates a block diagram of a first switching means, FIG 5 shows a flow diagram describing equipment protection switching.

DETAILED DESCRIPTION OF THE INVENTION

Fig 1 very schematically illustrates a node 100 in a telecommunications network. The node 100 comprises a triplicated switching matrix (SM) 20_A, 20_B, 20_C to/from which access is provided via a number of Terminal Access Units (TAU) 1_{1, ...,}1_N; 1 '_{1, ...,}1'_N. To the triplicated switching matrix, via the terminal access units high speed electrical signals are input and high speed electrical signals also go out from the node; the signals merely being denoted In and Out respectively. Redundancy or protection switching is thus provided for the switching matrix (or e.g. a digital cross- connect). However, for the node as illustrated in Fig 1 there is no protection whatsoever provided for the terminal access unit. Examples on terminal access units are for example Exchange Terminals (ET) in ATM switches (Asynchronous Transfer Mode switches) or in the AXE system by Ericsson.

However, the node as illustrated very schematically in Fig 1 is one example on a node for which equipment protection, (i.e. here protection of the Terminal Access Units) can be provided according to the invention. However, the switching matrix must of course not be triplicated; it can also be duplicated, not protected at all or more than triplicated. Moreover, the invention applies to other equipment units than terminal access units or exchange terminals. Fig 2 shows one embodiment of a protection switching system for protecting a number of equipment units 1_{1, ...,} 1_N wherein the number N of equipment units to be protected in principle can be an arbitrary number. Advantageous embodiments e.g. relate to the cases when 15 or 16 equipment units are to be protected. In the illustrated embodiment the equipment units 1_{1, ...,}1_N are so called Terminal Access Units (TAU_{1, ...,}TAU_N.) An additional or a redundant terminal access unit 1_N+1 is provided to which a signal can be switched in case one of the access units to be protected becomes faulty or similar. The illustrated embodiment relates to a telecommunications network in which the electrical line signal or the external signal must meet the requirements of the G.703 standard which as referred to above is a very demanding standard. If an external protection switching arrangement would be placed outside the terminal access unit the signal runs the risk of being destroyed to such an extent that the signal no more fulfils the G.703 requirements.

The protection switching arrangement 10 according to this embodiment of the invention comprises a number of first switching means 2_{1, ...,} 2_N (also denoted protection units PU), one for each equipment unit 1_{1, ...,}1_N, and second switching means 7 also denoted protection switching unit (PSU) 1 : N wherein 1:N is the protection ratio. In the first switching means 2_{1, ...,}2_N a first protection switching step is done whereas in the second switching means 7 a second protection switching step is done which will be more thoroughly explained hereinafter. "First" and "second" is here merely taken as denotations since the order of the switching steps depends on the direction of the signal, i.e. if it is incoming towards an access unit or similar the first switching step is done before the second, but if a signal leaves a redundant access unit, the second switching step is performed before the first step. External signals 11_{e1, ...,}11_eN are thus e.g. input to the first protection switching means 2_{1, ...,}2_N. Particularly the external signals are high speed electrical signals which fulfil the G.703 requirements. External signals 12_e1,_{, ...,}12_eN are also output from the first switching means 2_{1, ...,}2_N which however will be discussed later on. The first switching means 2_{1, ...,}2_N each comprises input/output means 3₁, 4₁;_...,3_N,4_N which are also denoted conversion means. In the signal input means 3_{1, ...,} 3_N the corresponding external signals are terminated and converted into internal signals 11_{i1, ...,}H_iN. Thus, in the signal input means 3_{1, ...,} 3_N the external signals which are more sensitive to switching are converted into internal signals which better stand being switched. This means that a format conversion is done to a format which better can be switched. The signal input means 3_{1, ...,} 3_N therefore comprise e.g. termination circuits for pulse conversion, impedance conversion etc. In a similar way, an internal signal 12_{i1, ...,}12_iN is converted to an external signal in the signal output means 4_{1, ...,}4_N which external signal e.g. fulfils the G.703 requirements. The dashed line 22 in Fig 2 indicates in a schematic way the termination of G.703 signals. When thus the conversion from an external signal into an internal signal has been done, a first switching step is done in a first switching device 5₁. In this first switching step a selection is done for an incident signal between a corresponding equipment or access unit 1_{1, ...,}1_N and the redundant equipment or access unit 1_H+1. The other way round, for an internal signal 12_{i1, ...,}12_lN is selected in a second switching device 6_{1, ...,}6_N of the respective first switching means 2_{1, ...,}2_N if the signal from an "ordinary" access unit 1_{1, ...,}1_N is to be switched or if the signal from the redundant access unit 1_N+1 is to be switched. The first switching devices 5_{1, ...,}5_N here comprise demultiplexing arrangements whereas the second switching devices 6_{1, ...,}6_N comprise multiplexing arrangement. The first switching means are advantageously arranged in the connection field for customer cables, i.e. customer cables are directly connected thereto. The first switching means are particularly also denoted Protection Units PU_{1, ...,}PU_N.

If thus an ordinary access unit 1_{1, ...,}1_N can be used, i.e. there is no failure, the internal signal 11_{i1, ...,}11_iN is switched to the concerned ordinary access unit 1_{1, ...,}1_N. If on the other hand the first access unit 1₁ (for example) is faulty or does not operate properly, the first switching device 5₁ of the first switching means 2₁ takes the position of the dashed line and the internal signal 11_i1 is switched to the second switching means 7. The second switching means 7 comprises an input switching device 8 in the form of a multiplexing arrangement to which signals can be sent from the first switching means in case of malfunctioning of an access unit to the redundant access unit 1_N+1. The second switching means 7 also comprises an output switching device 9 in the form of a demultiplexing arrangement which is connected to the redundant access unit 1_N+1 for transmitting signals from that unit to a first switching means 2_{1, ...,}2_N. The second switching step carried out in the second switching means 7 comprises a selection of which of N first switching means that will transmit/receive a signal to/from the redundant equipment unit 1_N+1. The external signals (e.g. G.703) are as referred to above terminated already in the first switching means or in the Protection Units (PU) which is as close to customer cables as possible and the protection switching, the first as well as the second steps, are performed on an internal signal.

Advantageously electronic circuits are used for the protection switching of the internal signals. In figure 2 SIS indicates switch interface signals to/from the switching matrix which can be provided with redundancy in any appropriate way which is however not the concern of the present invention. The secondary switching means 7, PSU 1:N provides a protection ratio of 1:N wherein N in principle can be an arbitrary integer number. Advantageously the switch-over time is below 10ms. The switch overtime depends on error detection, processors and so on. The protection switching itself according to the invention introduces generally almost no delay at all, e.g. a delay in the order of μs. However, the switch over time can be shorter, or even considerably shorter than 10 ms, but also longer - it depends on a number of factors, the needs and requirements etc. Advantageously the equipment protection switching arrangements operate in duplex, i.e. it is performed at transmitted and received signals at the same time. The invention is however not limited thereto. Still further the protection ratio can be defined according to the particular needs of a user. In one particular embodiment there is no microprocessor in the second switching means ( PSU ) but the second switching means (PSU) is controlled by a processor at the redundant equipment unit, i.e. TAU_N+1. In alternate embodiments may however the second switching means PSU be provided with a processing arrangement, or it can even be arranged in other ways. According to one embodiment however the redundant access unit 1_N+1 processor is responsible for taking decisions. From a software point of view (in a particular embodiment), the second switching means can be treated as a distributed part of the redundant access unit. In one embodiment there are one way communication channels from all equipment or access units, i.e. all "active" equipment unit such as TAUs to the second switching means, i.e. PSU. The protocol is an on/off protocol which is active when an equipment unit requests protection switching. Once protection switching is required, the equipment unit sends a protection switching request to the processor of the switching matrix through turning off the Switch Interface SI (see Fig 2). However, this is only one way of doing it; a number of other alternatives also being possible which however will not be further discussed herein. Protection switching at the first switching means is advantageously ordered by the second switching means. In an advantageous embodiment the equipment units or particularly terminal access units can be provided with the ability of turning off the switch interface when there is an equipment protection switching request. Alternatively bits can be set in the interface. However also other alternatives are possible. The equipment units may also be responsible for power feeding of the first switching units as well as they may be provided with a hardware alarm channel to the second switching means which however merely relate to particular embodiments, the power feeding can in principle be provided in any convenient manner. Furthermore, protection switching does not have to be provided for the power feeding, this merely relates to an advantageous embodiment. A redundant terminal access unit may be substantially of the same kind as an ordinary or active terminal access unit. Advantageously a control channel is provided to the second switching means which is active only in stand-by position. The software may also be so structured that the redundant terminal access unit (or equipment unit in general) takes the protection switching decisions as referred to above. Advantageously it is also responsible for controlling of the second switching means. However this merely relates to a particular advantageous embodiment.

In the following a more detailed description of the second switching means 7 (PSU) will be given under reference to Fig 3. The second switching means 7, hereinafter denoted Protection Switching Unit PSU executes the protection switching orders. When a protection switching request PS req is received from an equipment unit, here TAU the PSU 7 via the control part 15 sends protection commands to the first switching means, hereinafter denoted PU. The control part 15 of the PSU 7 further sends loop commands to the first switching means PU as well as PSU turn-on(off) commands. When protection switching will be done (see Fig. 4) the signal received from PU to PSU is turn on or activated (A), the loop command (if loops are used) (B) is turn off and a protection switching order (C) is given. This means that the PSU 7 controls the first switching means via controlling the protection switching function, the loop function and the turning on/off of the signal from first switching means PU to second switching means PSU. The second switching means 7 comprises a number (here 16) of signal input means 13_{1, ...,}13₁₆ to which signals 11_{i1, ...,}11_i16 can be input to the multiplexing arrangement 8 which here is a multiplexer 17:1 which means that there are 17 inputs and 1 output. The multiplexers can be analogue as well as digital. In this case 1:16 redundancy is provided. There are 17 inputs because of the internal loop function and thus loop input means 13 are also provided. The looping function as such is however not substantial for the protection switching as such and does of course not have to be included; it merely relates to a particular embodiment allowing testing etc. The multiplexing arrangement 8 selects which signal input connection that is to be allowed to transmit a signal (11_i1) (via input port 23) to the redundant Terminal Access Unit TAU₁₇. In a similar way the demultiplexer which is a demultiplexer 1:17 selects to which first switching means an input signal from the redundant terminal access unit TAU₁₇ (via an input port 24 and signal regeneration means 16) is to be sent. Switching is done in the demultiplexer 9 and a first switching means is selected and the signal is sent via one of the output ports 14_{1, ...,}14₁₆, the output internal signals being denoted 12_{i1, ...,}12_i16. In this case also an extra output port 14_L is provided for the internal loop to the loop input port 13_L of the multiplexing arrangement 8 (which however is not necessary for the invention). Supervision logic circuits 17 are provided e.g. assisting in looping signals and supervising of the redundant terminal access unit TAU₁₇. The signal is looped from TAU₁₇ back to TAU₁₇. Via the signal regeneration circuit 16 line signals are regenerated in transmit direction. However, the supervision logic and the signal generation are here merely given for illustrative purposes, alternative solutions are also possible or the functions may even be left out.

In one embodiment, although this is not essential for the protection switching as such, the second switching means 7 comprises at least one DC/DC converter. In the illustrated embodiment the PSU is responsible for power feeding of the first switching means. However, this is merely one example on power feeding; of course the first switching means does not have to be fed by the second switching means; the feeding can be done in any other appropriate way. Thus the power feeding is in this case duplicated which also is advantageous. Normally a PROCESSING UNIT 20 is fed by its ordinary TAU, but if this TAU is faulty or gone, the power feeding is taken over by PSU.

Advantageously the second switching means 7 communicates with the redundant access unit TAU₁₇ and still further it receives fault messages from all "active" or ordinary TAUs. The second switching means 7 is advantageously controlled by a processor of the redundant access unit TAU₁₇. The logics generating protection switching commands to the PUs are advantageously so designed that the most possible fault scenario will put PUs in a "non protection switching state" i.e. no incorrect protection switching is done if the PROCESSING UNIT 20 controlling logic is faulty. In the shown embodiment it is assumed that the external signals are 140 Mbps signals. In that case the processor at TAU₁₇ controls 16 incoming 140 Mbps links corresponding e.g. to 32000 telephone calls. If this processor "brakes down" all the incoming traffic may be interrupted and therefore a safe procedure is advantageously used when the processor sends orders to the second switching means. This is however not part of the present invention. However, of course the invention also to applies to other signals such as for example 155 Mbps signals and signals with others bit rates as well. Moreover the invention is likewise applicable to other signals than electrical signals (high speed or low speed) such as for example microwave signals, optical signals etc. However, depending on the bit rate of the signal analogue parts are advantageously optimized for the relevant clock frequencies.

Fig 4 schematically illustrates a block diagram of a first switching means 2 here denoted PROCESSING UNIT 20 1. The PUs are advantageously designed for protection switching purposes and each access level needs a PROCESSING UNIT 20 of its own. The protection switching at each PROCESSING UNIT 20 is advantageously controlled by PSU as referred to above. In the shown embodiment there are three control signals to each PROCESSING UNIT 20 from PSU. However, the PROCESSING UNIT 20 can of course be designed differently, this merely giving one example. As referred to above no looping need to be provided. The PROCESSING UNIT 20 may e.g. take the form as schematically illustrated in Fig. 2 just including two switches, just one signal, e.g. protection switching active or not.

The first signal is called a protection switching order and when this is active, the transmit signal towards G.703 will be sent from TAU_1+N via PSU instead of from the ordinary TAU. The second control signal relates to turning on/off the signal to PSU. When this is active, the traffic signal is sent from PROCESSING UNIT 20 to PSU. Finally when the loop command is active, the transmit signal is sent to PSU. When none of the control signals is active, it means that the traffic is normal, thus neither protection nor testing is active. If there is a protection command and the signal to the PSU is turn on, protection switching will be carried out. According to one embodiment the loop command may be active whereas there is no switching order and no turning on signal to the PSU. In this case the transmit signal is looped from an active TAU to PSU. This is a test and advantageously it is allowed in traffic. Another test that advantageously can be performed in traffic is monitoring of the received line signal at PSU/TAU_N+1. In this case the signal to PSU is in turning on state. Advantageously however it is not allowed in traffic to loop a signal from PSU back to PSU. The signal from PSU is then sent into line. In this case protection and loop commands are active whereas signal to PSU is off.

In Fig 4 a received line signal ll_el is received at input port 3₁ of first switching means PU₁. If it is supposed that it is a G.703 signal, this is terminated in said input port 3,. The signal 11_i1 (i.e. the converted internal signal) is sent to TAU₁. If on the other hand there is a protection switching order and a turning on of the signal to PSU, the signal is via the first switching device 5₁ switched also to PSU instead, i.e. it is sent both to TAU₁ and to PSU. A transmit signal from an ordinary TAU can be switched through the multiplexer 6₁ to the output port₁4 wherein the conversion to a G.703 signal 12_i1 is done. If on the other hand there is a protection order and a signal from PSU is switched, i.e. there is a signal transmitted from PSU, the signal is switched through the multiplexer 6j to the signal output means or the output port 4₁ for conversion to an external transmit signal 12_e1. A loop demultiplexer 5_L ( for testing purposes only; not required for protection) is also provided which is activated in the case of a looping command or a looping command and a protection switching command. This is however not permitted in traffic as discussed above. The termination of the external signals in the input and output ports respectively i.e. the conversion, requires power feeding to the first switching means. As referred to above this can be done via the second switching means. In the shown embodiment it is +12V duplicated, both from a TAU and the PSU. Thus, in this case PROCESSING UNIT 20 power feeding is also protected. +12V is of course merely given for exemplifying reasons, any other value also being possible.

In Fig. 5 a flow diagram relating to an advantageous embodiment is disclosed. It is supposed that a HW-fault occurs at TAU_i, 201. The

HW fault is detected 202. HW-fault detection information is then sent to PSU 203A and/or to the switch processors) 203B.

Advantageously a protection switching decision is taken by the switch processor(s), (SP(s)), 205. Then there are two alternatives. The first alternative relates to sending a protection order to the redundant access unit TAU_N+1, 206. The protection switching order is then processed at TAU_N+1, 207 and a protection switching order is sent to the PSU control part, 208. From the PSU 203A a protection switching order can however in an alternative embodiment be sent to the PSU control part, 208. Thus, going via PSU 203A or via 203B relate to two different embodiments.

Moreover the redundant TAU_N+1 is initialized, 209. The second possibility is that there is no protection switching order, 206B. This may e.g. be the case if protection switching already is active for another TAU. Then information is given to the central processor, CP, 207B.

However, when a protection switching order has been given to the PSU control part, 208, a protection switching (PS) order is given to PU_i, 210. Then the loop command is to be off (if such functionality is applied), the protection command is to be on and signal to PSU command is to be on.

A protection switching command is also given to MUX/DEMUX at PSU which comprises selecting of input/output no "i", 211.

Protection switching is then executed at PU_i, 212 and at PSU, 213. After that PS in performed, which here means that the system is ready for taking over the traffic via the redundant TAU_N+1, 214. TAU_N+1 then takes over the traffic, 215, and the control processor ( CP ) of the system is informed, 216.

It is an advantage of the present invention that thus the protection switching of an internal signal can be performed by electronic circuits which have considerably better long term reliability than for example electromagnetic relays. It is also an advantage that 1:N protection, wherein N is greater than 1, gives a minimum redundancy which means that the extra cost to obtain a much higher availability is comparatively low, as an example less than 10% for 1:16 protection which can be compared to more than 100% for 1:1 protection. Moreover market adaptations for different cables/contact can be done by simply modifying in the first switching means or in the particular embodiments described PROCESSING UNIT 20 which means that there can be one PROCESSING UNIT 20 for e.g. every market whereas the sub-racks, backplanes, TAU-boards and TSU-boards will be the same for all users. Market adaptation is thus easy to handle and very cost effective. Another advantage is that power dissipation is low. The embodiments described particularly refer to terminal access units, particularly terminal units, i.e. the circuit boards taking care of the line signals to/from for example a cross-connect can be protected. However, protection switching for other kinds of equipment units as well is naturally also provided for through the present invention. Also in other aspects the invention can be varied in a number of ways without departing from the scope of the claims.

Claims

1. System for protection switching in a communications network, e.g. a telecommunications network, comprising a number of nodes, each of which comprises a number of signal equipment units (1_1,...,1_N;TAU_x;x=1,...,N) providing access to/from said nodes

c h a r a c t e r i z e d i n ,

that the system comprises at least one redundant equipment unit (1_N+1; TAU_N+1) and a protection switching arrangement (10) for protecting signals to/from said equipment units comprising a number of first switching means (2_{1, ...,}2_N) connecting to the access units (1_{1,.. .,} 1_N;TAU_X;x=1,...,N) each comprising signal input/output means (3_{1, ...,}3_N;4_{1, ...,}4_N) providing conversion between external traffic signals and internal traffic signals and in that at least a number of the first switching means (2_{1, ...}2_N) are connected to at least one second switching means ( 7; PSU 1 :N ) connecting to said at least one redundant equipment unit (1_N+1;TAU_N+1) so that when protection switching is needed for a signal to/from an equipment unit (1_{1, ...,}1_N;TAU_X;X=1,...,N) protection switching is done on an internal signal in at least two steps via one of said first switching means and said second switching means to/from a redundant access unit ( TAU_N+1).

2. System according to claim 1,

c h a r a c t e r i z e d i n ,

that the external signals (11_{e1, ...,}11_en;12_{e1, ...,}12_en) are high speed signals.

3. System according to claim 1,

c h a r a c t e r i z e d i n ,

that the external signals are low-speed electrical signals, microwave signals or optical signals.

4. System according to claim 1 or 2,

c h a r a c t e r i z e d i n ,

that the signals are electrical signals.

5. System according to anyone of the preceding claims, c h a r a c t e r i z e d i n ,

that the node comprises a switching matrix (SM) or a cross-connect.

6. Arrangement according to claim 4 or 5,

c h a r a c t e r i z e d i n ,

that the external signals should fulfil the requirements of the standard according to CCITT Rec. G.703.

7. A system according to claim 6,

c h a r a c t e r i z e d i n ,

that the signals are 155 Mbps STM signals.

8. A system according to claim 6,

c h a r a c t e r i z e d i n ,

that the signals are 140 Mbps signals.

9. System according to anyone of the preceding claims,

c h a r a c t e r i z e d i n ,

that each first switching means (2_{1, ...,}2_N;PU_{x;1, ...,N}) comprises a first switching device (5_{1, ...,}5_N) e.g. comprising a demultiplexer and a second switching device (6_{1, ...,}6_N) e.g. comprising a multiplexer for switching to/from an equipment unit or the second switching means (7), thus providing a first switching step.

10. System according to claim 9,

c h a r a c t e r i z e d i n ,

that switching to/from a redundant access unit (1_M+1; TAU_N+1) is done via the second switching means (7; PSU 1:N) to which each or at least a number of the first switching means (PU_x;x=_{1, ...,N}) are connected thus providing a second switching step.

11. System according to claim 9,

c h a r a c t e r i z e d i n ,

that the second switching means (7; PSU) comprises an input switching device (8), e.g. a multiplexer, for selecting which first switching means (2_{1, ...,}2_N;PU₁) will be connected to the redundant equipment unit (1_N+1;TAU_N+1), i.e from which first switching means a signal will be transmitted to the redundant unit.

12. System according to anyone of the preceding claims,

c h a r a c t e r i z e d i n ,

that the second switching (7;PSU) means comprises an output switching device (9), e.g. a demultiplexing arrangement for selecting to which of a number of first switching means (2_{1, ...,}2_N;PU_x) a signal from a redundant access unit (1 _N+iTAU_N+) will be transmitted and in that the first switching step is carried out before converting the internal signal into an external signal.

13. System according to anyone of the preceding claims,

c h a r a c t e r i z e d i n ,

that the protection ratio (1:N) given by the second switching means (PSU 1:N) and corresponding to the number of equipment units to be protected is lower than one, i.e. that N is greater than 1.

14. System according to claim 13,

c h a r a c t e r i z e d i n ,

that N = 15 or 16.

15. System according anyone of claims 4-14,

c h a r a c t e r i z e d i n ,

that the switching matrix (SM) is unprotected or duplicated.

16. System according to anyone of claims 4-14,

c h a r a c t e r i z e d i n ,

that the switching matrix is triplicated or more than triplicated.

17. System according to anyone of the preceding claims,

c h a r a c t e r i z e d i n ,

that (customer) cables carrying external signals are directly connected to the first switching means (2_{1, ...,}2_N;PU_x;_{x=1, ...,N}).

18. System according to anyone of the preceding claims, c h a r a c t e r i z e d i n ,

that the switch overtime is lower than approximately 10 ms.

19. Protection switching arrangement (10) for e.g. a communications system comprising at least one node comprising a switch and a number of signal equipment units (1_{1, ...,}1_N;TAU_{1, ...,}TAU_N) providing access to/from the switch,

c h a r a c t e r i z e d i n ,

that the protection switching arrangement (10) comprises signal input/output means (3_{1, ...,}3_N;4_{1, ...,}4_N) converting an external signal into an internal signal and vice versa and in that it further comprises switching means (2_{1, ...,}2_N,7) for switching signals to/from a redundant equipment unit (1_N+1;TAU_N+1) or similar in case a signal can not or should not be sent to an ordinary equipment unit (1_{1, ...,}1_N;TAU_{1, ...,}TAU_N) and in that protection switching only is done on internal signals.

20. Protection switching arrangement according to claims 19, c h a r a c t e r i z e d i n ,

that at least some of the external signals (11_{e1, ...,}11_eN;12_{e1, ...,}12_eN) are high speed electrical signals and in that said signals are converted in said input/output means to/from internal signals (11_{i1, ...,}11_iN;12_{i1, ...,}12_iN) which are less sensitive to switching.

21. Protection switching arrangement according to claim 19 or 20, h a r a c t e r i z e d i n ,

that the protection switching means comprises first and second switching means (2_{1, ...,}2_N;7) thus providing protection switching in two steps.

22. Protection switching arrangement according to claim 21, c h a r a c t e r i z e d i n ,

that in the first switching step a signal is switched to/from an ordinary equipment unit (1_{1, ...,}1_N;TAU_{1, ...,}TAU_N) if such should be used, otherwise to/from the second switching means (7).

23. Protection switching arrangement according to claim 22, c h a r a c t e r i z e d i n ,

that in the second switching means (7; PSU) is selected which of a number of first switching means will be connected to the redundant equipment unit (1_N+1;TAU_N+1) or similar and in that the number of first switching means corresponds to the number of equipment units.

24. Arrangement according to claim 23,

c h a r a c t e r i z e d i n ,

that the second switching means (7; PSU) is connected to each of said first switching means (2_{1, ...,}2_N;PU_{1, ...,}PU_N) and to a redundant equipment unit (1_N+1;TAU_N+1) or similar.

25. Method for protecting external signals incoming to a number of network equipment units providing access to a switch or similar in a communications network comprising the steps of: converting the external signals into internal signals, performing a first switching step if a signal cannot be transmitted to an intended first equipment unit in which the signal is transmitted to second switching means to which at least a given number of the first switching means are connected,

performing a second switching step in said second switching means, which step comprises switching said signal to a redundant equipment unit,

transmitting the selected signal to the redundant equipment unit.

26. Method according to claim 25,

c h a r a c t e r i z e d i n ,

that conversion between external and internal signals is done in the first switching means.

27. Method according to claim 25 or 26,

c h a r a c t e r i z e d i n , that the second switching means provides for 1:N switching, wherein N exceeds and correspond to the number of equipment units.

28. Method for protection of signals going through at least one node in a communications system, said node comprising a central switching arrangement and a number of equipment units, e.g. access units providing access to/from said node, the method comprising the steps of:

- providing at least one redundant unit to/from which signals are transmitted if an equipment unit can not/should not be used, - providing a protection switching arrangement;

- if an equipment unit is defective or similar, switching signals via said redundant unit wherein the number of equipment units is higher than the number of redundant units,

- performing a signal conversion from an external signal to an internal signal or vice versa before/after protection switching.

29. Telecommunications system comprising at least one node which comprises a central switch (SM) and a number of equipment units providing access to/from said central switch,

c h a r a c t e r i z e d i n ,

that for at least a number of nodes at least one redundant equipment unit (1_N+1;TAU_N+1) and an equipment protection arrangement are provided and in that said protection arrangement comprises signal input/output means (3_{1, ...,}3_N;4_{1, ...,}4_N) providing conversion between external/internal signals and in that said protection arrangement further comprises first and second switching means (2_{1, ...,}2_N;7) for switching signals to/from said redundant equipment unit (1_N+1;TAU_N+1), switching only being done on internal signals.

30. Telecommunications system according to claim 29,

c h a r a c t e r i z e d i n ,

that the signal input/output means (3_{1, ...,}3_N; 4_,1..., 4_N) are arranged in said first switching means (2_{1, ...,}2_N) in which a signal is switched to/from an equipment unit (1_{1, ...,}l_N,TAU_{1, ...,}TAU_N) or the second switching means (7) in which the signal is switched to/from said redundant equipment unit (1_N+1;TAU_N+1).