Abstract:
The present invention relates to a protection switch in a telecommunications system. The protection switch includes at least one line driver connected to an outgoing line, at least one line receiver connected to an incoming line, a protection ring interface including an incoming link and an outgoing link to a protection ring, a line interface including said incoming and outgoing lines, a connection interface including said incoming and outgoing lines connected to a connection board, and three switches. A first switch is connected to the incoming line and to the protection ring, a second switch is connected to the outgoing line and to the protection ring, and a third switch is connected to the protection ring and to the incoming and to the outgoing lines, for re-routing traffic. The invention also relates to a method in which the protective switch is used for line protection and for equipment protection.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and to a device for protecting a transmission system for protecting the system against line and board failures. 
     BACKGROUND OF THE INVENTION 
     It is necessary to ensure in transmission systems, such as telecommunications systems, that the signals transmitted in the system will arrive at the receiver, even though a standard line may be broken or a receiving or transmitting unit is out of operation. A system operator accepts no losses of signals in the transmission system, and consequently it is necessary to take precautionary measures to ensure that the system will function faultlessly even when such faults occur A number of devices and methods are known to this end. 
     Patent Specifications U.S. Pat. No. 5,365,510, U.S. Pat. No. 5,179,548 and EP-A1 677 936 teach different methods of handling cable breakages in optical transmission systems. Briefly, the protective systems include an additional line in the form of an optical fibre which is connected between the different nodes in the system, in the same manner as the standard line but separated physically therefrom. Communication on the spare line takes place when the standard line has broken at some place or other. In an acute case, when the transmitter unit or receiver unit is out of function, a so-called head is formed on one side of the node and a so-called tail is formed on the other side of the node, therewith disconnecting the node from the network, such that communication will continue between the remaining nodes in the network. 
     SUMMARY OF THE INVENTION 
     It is necessary to provide in transmission systems, and then particularly in telecommunications systems, some form of protective device and some form of protective procedure that will ensure that signals transmitted in the system will always reach their intended receiver, irrespective of whether a line breakage occurs or if a transmitter/receiver unit stops functioning. 
     The present invention addresses a problem as to how this protective device can be implemented in the transmission system in the cheapest and simplest possible manner, and how the combined protection against a line fault and against a board fault can be incorporated in one and the same device. 
     The aforesaid problem is solved by means of the present invention in that the boards which form the interface against outgoing and incoming lines, the so-called line boards, include a protective switch and are mounted in a protective ring together with a stand-by unit. 
     Each line board includes a line driver, a line receiver and a number of switches. Each line board includes an associated incoming and outgoing line. These boards include the following interfaces: 
     an interface against its corresponding connection board, a so-called connection interface; 
     the interface against incoming and outgoing lines, the so-called line interface; and 
     an interface against the protective ring, a so-called protective ring interface. 
     As before mentioned, the protective ring comprises said collection of line boards to be protected, and a stand-by unit which is actuated in the event of a line fault or in the event of a connection board fault. Switching to the stand-by unit can be controlled by a central monitoring unit mounted in a magazine or in the stand-by unit. 
     The stand-by unit includes a stand-by line board and a stand-by connection board. A connection board forms the interface against the switching centre or telephone exchange unit in the transmission system and thus includes an interface against this switching centre, the so-called switching centre interface, and a further interface against the line board, the aforementioned connection interface. 
     When a fault occurs in the transmission system, the line driver will detect the loss of signal, LOS, and can change the states of the switches in the protection switch in coaction with the central unit, such as to switch an appropriate part of the traffic to the stand-by unit. 
     The protection ring may include means for transmitting a pilot tone or some equivalent signal from the stand-by connection board, for monitoring the protection ring. 
     Thus, the intention of the present invention is to combine protection against line faults and board faults in one single device that includes the protection ring and the protection switches. 
     The present invention has the advantage of being user-friendly and flexible. 
     Another advantage afforded by the present invention is that it does not require the use of one card to remedy a line fault and another card to remedy a board fault in one and the same transmission system. 
     Another advantage is that the present invention does not require a spare line for each line board. 
     Still another advantage is that each line board in the protection ring can be a stand-by line board. 
     Yet another advantage is that the protection ring can be monitored via a so-called pilot tone that will not interfere with traffic in the system. 
    
    
     The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an inventive protection switch. 
     FIG. 2 illustrates a line switching centre or exchange unit interface module that includes a protection ring that includes a number of line boards in accordance with the invention. 
     FIG. 3 shows the line/exchange interface module with the protection ring in normal traffic. 
     FIG. 4 shows the line/exchange interface module with the protection ring in the case of a line fault. 
     FIG. 5 shows the line/exchange interface module with the protection ring in the event of a board fault. 
     FIG. 6 is a flowchart illustrating the start-up of the inventive protection ring. 
     FIG. 7 is a flowchart illustrating activation of the protection ring in conjunction with different types of fault. 
     FIG. 8 is a flowchart illustrating deactivation of the protection ring in respect of different types of fault. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates one embodiment of an inventive protection switch. The protection switch includes three switches  1 ,  2  and  3 , a line driver  100 , a line receiver  200  and a pair of amplifiers  20  and  30 . The protection switch also includes an interface against a connection board, a so-called connection interface, an interface against a so-called protection ring, a so-called protection ring interface, and an interface against an incoming line  10  and an outgoing line  11 , a so-called line interface. The protection ring interface includes a link incoming to the line board and a link outgoing from said line board. The connection interface and the line interface include an incoming line  10  and an outgoing line  11 . 
     The switches  1  and  2  are able to take two different states or positions, i.e. state a and state b. Switch  3  can take three different states or positions, state a, state b and state c. Switch  1  is connected to the incoming line  10  and to the protection ring. Switch No.  2  is connected to the outgoing line  11  and to the protection ring. Switch No.  3  is connected to the protection ring, to the incoming line  10  and to the outgoing line  11 . 
     When switch  1  is in state a, switch  2  is in state a and switch  3  is in state a, the incoming and outgoing links in the protection ring are mutually connected, i.e. the protection ring will not influence other traffic. 
     When switch  1  is in state b, switch  2  is in state a and switch  3  is in state c, the incoming link of the protection ring is connected to the incoming line  10 , and the outgoing line  11  is connected to the outgoing link of the protection ring. 
     When switch  1  is in an arbitrary state, switch  2  is in state b and switch  3  is in state b, the incoming link on the protection ring will be connected to the outgoing line  11  and the incoming line  10  will be connected to the outgoing link on the protection ring. 
     The line driver  100  can either transmit on the link incoming from the protection ring interface or on the line  11  outgoing from the connection interface. The line receiver  200  can either transmit on the line  10  incoming to the connection interface or on the link outgoing to the protection ring interface. 
     The line receiver circuit  200  is adapted to detect faults in the network, Loss Of Signal, LOS. In the event of a network malfunction, for instance a cable breakage or transmitter/receiver fault, a signal is sent to the central unit and also to the line driver. 
     Each of the protection switches is mounted on a respective line board belonging to a specific incoming line  10  and a specific outgoing line  11 . 
     FIG. 2 illustrates a line/exchange interface module that includes three connection boards and one of the line boards from a so-called stand-by unit. The coupling boards include a so-called exchange interface. Each of the connection boards is connected to corresponding line boards via a respective incoming line  10  and outgoing line  11 . As before mentioned, the interface between the line board and the connection board is called the connection interface. All line boards including the stand-by unit are connected together in the protection ring. When a line breaks, either the incoming line  10  or the outgoing line  11 , or a transmitter/receiver unit is out of function, this is detected by the line receiver  200  in the protection switch, wherewith said line receiver sends a signal to the central unit which, in turn, orders switching from either the standard line board to the stand-by line board or from the standard connection board to the stand-by connection board, depending on the nature of the fault, ile. Whether the fault is a line fault or a transceiver fault. 
     FIG. 3 illustrates the line/ex change inter face module in normal traffic, i.e. in the so-called normal state of the protection ring. In this state of the module, a so-called pilot tone can be sent from the connection board in the stand-by un it so as to monitor the protection ring. When there is no fault on the protection ring, the pilot tone will return to the stand-by unit, whereas if there is a fault on the protection ring, the pilot tone will not return to the stand-by unit. A fault message can be sent to the central unit to the effect that a fault has occurred in the protection ring and that remedial action should therefore be taken. The pilot tone monitoring the protection ring can be sent simultaneously with other traffic on standard lines  10 ,  11 , without disturbing the traffic. The stand-by unit is constantly available as a substitute for either the line board or the connection board. Each connection board and line board may be designed as a stand-by unit. 
     FIG. 4 shows the line/exchange interface module with a line fault at the line interface adjacent a line board, which is designated x. The line board x belonging to the broken line is here replaced with the stand-by line board. Traffic outgoing on the broken line then passes from the standard connection board through the standard line board to the stand-by line board via the protection ring, and from there onto the stand-by line. Incoming traffic on the broken line is switched to the stand-by line and to the stand-by line board and through the standard line board to the standard connection board, via the protection ring. 
     This switching process is controlled via a central unit, which receives signals from the line driver and line receiver of the line board carrying information as to which switches  1 ,  2 ,  3  shall be switched and the states or positions to which said switches shall be switched. 
     FIG. 5 shows the line/exchange interface module with a transceiver fault in a connection board, here designated connection board x. The stand-by connection board has replaced the malfunctioning standard connection board in the illustrated case. Traffic outgoing from the stand-by connection board passes through the stand-by line board to the standard line board via the protection ring, and then passes out on the standard line. Incoming traffic from the standard incoming line  10  the standard line board then passes the stand-by line board via the protection ring, and from there to the stand-by connection board. 
     The switching process is effected via a central unit also in this case, said central unit receiving from the own monitoring of the connection board information signals that inform the central unit which switches  1 ,  2 ,  3  shall be switched and the positions or states to which said switches shall be switched. 
     FIG. 6 is a flowchart that illustrates the protection ring start-up procedure. The first stage of the start-up procedure is to place all switches in the protection switch on the line board to their respective so-called normal states. Thus, all switches, i.e. switch  1 ,  2  and  3  in the protection switch in FIG. 1, will take position a. With the switches in this state, only the line board is connected to the protection ring. The next stage is therefore to connect the stand-by connection board to the protection ring. This is achieved by switching switch  1  to state b, switch  2  to state a and switch  3  to state c on the stand-by line board. 
     Finally, monitoring of the ring is commenced by the stand-by connection board generating a pilot tone, or signal, having similar properties and sending the pilot tone in the closed protection ring. When no breakages are present in the protection ring, the pilot tone will return to the connection board. On the other hand, if the pilot tone fails to return to the connection board, this indicates a break somewhere in the ring. 
     In this mode, the protection ring adopts a normal state in which the stand-by unit is constantly arranged to replace a corresponding standard board and line, wherein monitoring of the protection ring takes place at the same time as traffic flows on the incoming standard lines  10  and the outgoing standard lines  11 . 
     The procedure illustrated in FIG. 7 begins from the terminating stage in FIG. 6, i.e. the stage in which no faults were discovered and in which the protection ring operates in its normal state. 
     In the event of a fault in the protection ring, the stand-by connection board senses the loss of the pilot tone transmitted in the protection ring. The stand-by connection board sends to the central unit a signal to the effect that a fault has occurred in the protection ring. The central unit brings to the attention of an operator the fact that a fault has occurred on the protection ring and that some form of repair must be made. 
     In the event of a line fault at arbitrary standard line board, which is again designated x, that is to say a standard incoming or outgoing line is faulty after the line board x, the line interface of the stand-by line board is connected to the protection ring. This is effected by setting sw 1  to state or position y, sw 2  to state or position b and sw 3  to state or position b on the stand-by line board. State or position y indicates that the switch is able to take an arbitrary position or state. The standard connection board is then connected to the protection ring, by setting sw 1  in state or position b, sw 2  in state or position y and sw 3  in state or position c on the standard line board x. Finally, a signal to the effect that a line fault has occurred is sent from the standard line board x to the operator, via the central unit. 
     In the event of a fault on the transceiver unit in any one of the standard connection boards, which is also designated connection board x, the first step in the process is to stop monitoring of the protection ring, by switching-off the pilot tone. This is followed by updating the configuration data of the stand-by connection board, in other words in the event of a fault, the central unit or some other unit having the same function must select another traffic route. Line board x is then connected to the protection ring, by setting switch  1  to position or state y, setting switch  2  to position or state b and setting switch  3  to position or state b. Finally, there is sent to the operator an alarm signal indicating that a fault has occurred on the connection board x in the line/exchange interface module. 
     FIG. 8 is a flowchart which describes restoration to a normal state subsequent to having repaired a ring fault, line fault or a board fault. 
     Once having repaired the protection ring, all that is required is for a message to be sent to the operator to the effect that the protection ring has been restored, whereupon the operator can erase the alarm. 
     After having repaired a line fault adjacent the line board x or after having repaired the connection board x, the operator is informed that the line or the connection board has been repaired. The same procedures are then carried out, irrespective of whether the fault was a line fault or a connection board fault. The connection board x is first connected to a standard line. The protection ring is then connected to the line board x, by setting switch  1  to its state or position a, setting switch  2  to its state or position a and setting switch  3  to its state or position a, i.e. setting all switches  1 ,  2 ,  3  to their respective normal states. The stand-by connection board is then connected to the protection ring, by setting switch  1  to its state or position b, setting switch  2  to its state or position y and setting switch  3  to its state or position c by the stand-by line board. Finally, the alarm to the operator is erased. 
     It will be understood that the invention is not restricted to the aforedescribed and illustrated embodiments thereof, and that modifications can be made within the scope of the following claims.