WO1993022859A1

WO1993022859A1 - Method and device for configuration of a time-space-time cross-connection at occasions when the need of cross-connexion changes and use thereof

Info

Publication number: WO1993022859A1
Application number: PCT/FI1993/000174
Authority: WO
Inventors: Jaakko Saijonmaa; Ove Strandberg; Jarkko M. KESTILÄ; Martti Raivola
Original assignee: Nokia Telecommunications Oy
Priority date: 1992-04-24
Filing date: 1993-04-23
Publication date: 1993-11-11
Also published as: GB2281173B; GB2281173A; GB9421173D0; DE4391854T1; AU3955693A; FI921834A0; FI90707C; DE4391854C2; FI90707B

Abstract

The method according to the invention for the calculation of the cross-connection configuration in a time-space-time (T-S-T) is based on the fact that the examined data streams are divided into defined periods. Within a period the connection matrix (31) is generated by the calculator (32) for each time slot, so that first a suitable space switch output is found, and then a signal of such an input time switch is selected, which can be connected to the selected output. According to the invention the capacity of the cross connect can be effectively utilized without arranging e.g. extra capacity for the space switch.

Description

METHOD AND DEVICE FOR CONFIGURATION OF A TIME- SPACE-TIME CROSS-CONNECTION AT OCCASIONS WHEN THE NEED OF CROSS-CONNECTION CHANGES AND USE THEREOF

The invention relates to a method for the non-blocking configuration of a cross connect for digital transmission lines in situations of changing cross-connection requirements. The invention also relates to a cross connect implementing the method and to the use of this cross connect.

The synchronous digital hierarchy (SDH) comprises quite a large and far advanced entity in order to transmit time division signals in the telecommunication network, the backbone network of which is developing from separate PCM encoded links towards a remotely controlled cross connect network. The recommendation CCITT G.707 defines the signals of the first level synchronous transport module (STM-l) for SDH signals having a transmission rate of 155.520 Mbit/ε. The basic STM-l frame is composed of bytes (8 bits), of which there are 2430 including the control blocks; then an STM-l frame transmits 63 subsystem containers (e.g. TU-1, Tributary Unit, which can contain a 2 Mbit/s signal of a common 30 channel PCM system) . The STM-l frames are repeated 8000 times each second, which is the same as in the subsystem; thus each byte of a frame forms a 64 kbit/s channel. The SDH signals or transport modules are formed by interleaving the bytes of the subsystem signals.

An SDH cross connect (DXC) can transmit traffic between different SDH levels and switch traffic between different signals. A typical higher level cross connect (DXC, Digital Cross Connect, CCITT draft recommendations G.εdxc-1...-3 ) is the so called 4/1 cross connect, in which 2 Mbit/s channels are connected between the input and output ports. An important object of the cross connect is to optimize the utilization degree of the transmission network's capacity. Further is must be able to handle a flexible reconfiguration or to reroute the connections of the network. and to ensure the rapid initialization of reserve routes in network fault situations. The mentioned CCITT SDH recommenda¬ tions try to define the logical functio , i.e. the functional structure of devices, but they avoid the detailed structural description of the devices.

The digital cross connect has been substantially studied in order to find an architecture which meets the optimal conditions. A structure which readily meets the conditions regarding capacity, non-blocking properties and implementation, is the TST (Time- Space-Time) structure, or the time-space-time cross connect, schematically shown in figure 1. In the TST connection structure the task to find a non-blocking connection consumes much calculation power, even though the TST switch in principle is non-blocking. The need for extensive calculation is partly due to the large number of bytes or channels, contained e.g. in the STM-N signals. An advantageous of the TST architecture is that the size of the cross connect can be dimensioned for the required connection capacity, so that the equipment solution is more economical than with other architectures (for example S-T-S etc. ) .

In traditional architectures for the TST cross connect the cross connect is totally or partly doubled, whereby there are more advantageous possibilities to calculate a non-blocking connec- tron. This is the case for example in central offices, where 2 Mbit/s lines are connected to the cross connect, and where the object is to make fast channel based connections and disconnec¬ tions between 64 kbit/s lines. In the case of a TST switch in a central office, a free route through the switch must be found within limits imposed by time restrictions, whereby also blocking can be tolerated even if the switch would have free capacity, because the blocking affects only one channel at a time. A function like that is unacceptable in cross connects handling transmission lines, when it must be possible to route all inputs to corresponding outputs. The connection times on the trans¬ mission routes are long, and the connections will not change rapidly. A basic condition when the transmission routes are connected is further that the capacity of transmission routes and also the cross connect is utilized efficiently, as opposed to central offices, in which also over-dimensioned capacity could be used to prevent blocking situations.

In conventional SDH cross connects using the TST structure the problem of finding a non-blocking route has been avoided by increasing the capacity of the space switch, e.g. by doubling the frequency of the space switch. On the left in figure 1 there are the input signals II...In (here STM-l signals) and on the right there are the output signals 01...On. The time switches Til...Tin and Tol...Ton on the input side and output side, respectively, change the byte positions (within a frame) within a signal. The central space switch S connects a signal from one time switch to a signal directed to another time switch. A time slot or a byte forms a 64 kbit/s channel. In principle the time switches are memory elements and the space switch is composed of switch elements. Usually the cross connect is implemented as a module structure. The first time switch and the space switch influence the non-blocking connection from any time slot of an incoming line to the correct outgoing line. The time switch on the output side does not influence the achieving of a non-blocking connection, but it only switches the channels or time slots into the correct order required by the outgoing line.

The object of the invention is now to present for the cross-connection of SDH signals a method and an architecture implementing the method, which can realize a non-blocking connection and avoid known shortcomings and disadvantages.

This problem is solved according to claim 1 by a configuration calculation method implemented periodically. The structure of the cross connect is further developed by the method according to claim 6 for the connection of routing backup. Other preferred embodiments of the invention are presented in the dependent claims.

The invention achieves the advantage that the whole cross connect capacity is utilized. No extra cross connect capacity is required to guarantee the non-blocking function. Further it is possible with the method according to the invention to realize a synchronized operation, or an errorless change-over of the cross connect modules operating with the same clock.

The invention is applicable in the synchronous digital -hierarchy (SDH) for the connection of standardized transport modules (STM-N) . The invention is also applicable to a cross connect according to the plesiochronic transmission hierarchy (PDH) . The method according to the invention can be used in cross connect equipment of different levels, e.g. in a 4/1 cross connect or in a 3/1 cross connect, which connects 2 Mbit/s signals interfaced on the 34 Mbit/s level.

The invention is described below by examples with reference to the enclosed figures.

Figure 1 shows the principle of the time-space-time cross-con- nection.

Figure 2 shows schematically the cross connect control according to the invention. Figure 3 is a simplified flow diagram of the periodical method according to the invention. Figure 4 shows in a simplified flow diagram the method according to the invention for selection of a time slot to be connected. Figure 5 is an embodiment of the 1:N/N:1 routing backup.

Figure 1 shows in principle the structure of a time-space-time cross connect, which is used also in the invention and was described in the general section. Figure 2 shows a TST cross connect structure 10, with which the method according to the invention can be implemented, and where the control section 20 of the cross connect is able to control simultaneously all time switches 1...N on the input side to make the mutual connections of L time slots. The space switch is composed of N*N switches, which realize the connection from N inputs to N outputs . On the output side there are again N time switches, which perform the mutual connections of L time slots.

The cross connect control 20 in figure is able to realize simultaneously (in synchronism) the change-over of the connec¬ tion matrix (CM) (or the connection map) 31, so that the time and space switches in a situation of a changing connection configuration can be totally reconfigured to correspond to the new connection configuration. Thus the processor 30 has an active connection matrix 31 and as a backup/in processing a standby connection matrix (31b, not shown). The configuration calculator 32 of the processor 30 controlling the cross-connection generates by the configuration method described further below a new configuration, which is stored into the standby connection matrix, with which the updating of the connection configuration is made. The configuration calculator has as input data the channel distribution at the inputs and the desired outgoing line of each channel. The cross connect control unit 20 uses the updated matrix 31b when it controls the time and space switches to correspond to the new connection configuration.

The effective implementation of the cross connect configuration calculation according to the invention is based on the property of the TST cross connect that the new configuration can be solved one time slot at a time. In other words, when we find a non-blocking state for an arbitrary time slot K through the N time switches on the input side and the N*N space switch, we can be sure that also the other time slots K+l, K+2, ..., L can be solved by the same principle. Thus the configuration for the cross connect can be calculated one time slot at a time, so that the non-blocking connection configuration is found through the time switches on the input side and the space switch for the rest of the time slots K, K+l, K+2, ..., L.

The space switching of a certain time slot is non-blocking when all outputs of the space switch are in use. In other words, the time switch on the input side must connect all time slots or channels so that all multiplexed time slots at the inputs of the space switch are directed to different outputs. If there is no time slot at the input to be routed to a certain output, then of course it is not necessary to use this output. Thus the function of the input side time switch in front of the space switch is to distribute the channels evenly by time slots so that the space switch can switch them to the correct, desired outputs. More specifically, in the space switch such a blocking situation must be avoided, in which the same time slot would have more than one channel to be routed to the same output time slot.

With the method according to the invention the configuration calculator generates the connection connection matrix so that the cross connect according to figure 2 functions without blocking.

Responding to a connection request the calculator 32 builds as background processing a new connection matrix 31b (not shown) , which is stored in the memory of the processor 30. When the calculation of the connection matrix 31b is finished the control unit 20 updates totally the states of the cross connect modules according to the new connection matrix. Thus new connections are added to the connection matrix or old connections are removed from it, on the basis that all cross-connections are reconnected according to the new connection situation. Thus also old connections, which are maintained also after the change of the connection configuration, can get a new connection through the cross connect. The updating is accurately made with the aid of a synchronizing signal, which is supplied from a predetermined clock just before the new configuration is becoming effective. With the aid of this synchronizing signal we ensure that the old connections, which have to be maintained from the incoming line to the outgoing line, will not be disturbed when the connection matrix is updated.

In the connection method the cross connect configuration is calculated one time slot at a time. First the input channels or the bytes of the frame are divided into as many groups as there are signals or time slots to be cross-connected in the transport module. For example, an SDH STM-l line connected to the time switch on the input side has at the transmission rate 155 Mbit/s 63 periods or subsystem containers of 2 Mbit/s signals (e.g. TU-1, Tributary Unit, which can contain a 2 Mbit/s signal of a common 30 channel PCM system) . The position in the frame of each period selected according to the invention is obtained directly by a standard pointer, or it is calculated with the aid of the pointer. Accordingly a line of the plesiochronic transmission hierarchy (PDH) at the rate of 140 Mbit/s contains 64 periods of 2 Mbit/s. Within each group defined in this way the cross-connection routes are solved so that the connection configuration thus realized also means that the still uncalcu- lated time slots also can be connected. Thus the solution for the whole cross connect switch is obtained by the periodical method according to figure 3.

In this method, when one examined connection time slot is calculated, we first select those time switches which are connected to fully occupied inputs and outputs of the space switch, or which have as many unconnected channels as there are time slots left. The connection decisions are first made for this group, which is called the priority group. Those outputs which are not fully occupied must not necessarily be used in each time slot. This means that those inputs and outputs, which are in full use, must be used in this time slot; other free connections may be left unconnected, if no suitable channels are found. According to figure 3 the operation begins at the point 'start'. The first step is to realize a non-blocking connection through the time switches and the space switch in this time slot being processed. After this it is examined whether all time slots are processed or connected. If there are time slots left to be connected, then the process returns to the start and the non-blocking connection is calculated for the next time slot. After the last time slot the process goes to the 'end' . Figure 4 shows as a flow diagram the selection process of one time slot. After the start it continues to the checking block SI, where it is checked which inputs and outputs can be selected, when a route is searched for between the input and the output. Then the mentioned priority group always is first selected. If no priority connections are found, then the other inputs and outputs are placed for selection.

In the second step S2 of figure 3 the solution for one time slot is found in the calculator 30 so that out of each unconnected byte first that output is processed which has least selection possibilities in the time slots left, i.e. which has the lowest number of channels coming from different inputs. The selection is of course made from those space switch outputs, which were not earlier selected during the solving of this time slot, i.e. which are free. The outputs free for selection are checked already at the starting step SI, as is seen in figure 4, in other words we check first which outputs can be selected, and after that we find the output which has the lowest number of channels co ing from different inputs. When the output has been selected we can in principle continue to select the input.

When the input is selected in step S9 according to figure 4 we use the number of connections to be routed from one input time switch to a single output time switch, in other words we select that input time switch which has the highest number of channels for the already selected (S2) output. Even in this case the inputs already processed are not considered, and this is ensured in the checking step SI of figure 4 immediately at the beginning of the pair selection, as was also made in step SI when the output was selected.

The checking block SI thus first ensures that the output can be selected, and then that the input can be selected. The selections are performed after the check. The criterion used in selecting the output is based on the improvement of finding a non-blocking connection for the last routing selections of the time slot. The basis for this is that it the output containing the lowest number of inputs is easier to connect at the beginning than at the last selection of the time slot. On the other hand, the criterion for the input selection is based on the fact that the selection aims at keeping at least one channel at the input for each output, as long as it is possible. In this way the selection freedom is maintained, and the solutions for the next time slots can be found more easily.

If we for the input selected in step S9 find more than one alternative in step S10 having the same high number of connections for the selected outputs, then in step S13 according to figure 4 we select that input, which has the lowest number of channels going to different outputs.

If the steps S2 and S13 of figure 4 above result in that we find according to the selection criterion in the steps S2, S14 an equally valued time switch in several inputs or outputs, then the first available alternative is selected in step S4 and in step Sll, respectively. If this selected alternative according to the check in step S7 does not provide a non-blocking connection, then a recursive search for the solution must be made for the present time slot. Now the search is made so that the step S12 removes the selections until the previous, still untested equally valued alternative, and then the process returns to step S4 or Sll, which selects the next equally valued alternative. The new selections are checked in phase S7, and if the selections did not succeed with any of the alternatives, then the process returns to the previous selection situations, in which the equally valued alternatives are valid. If required, this is repeated until the a solution is obtained for the time slot. Thus, in the worst case, all equally valued alternatives are processed, whereby the process first tries to move the selection to different input time switches, until all input alternatives have been processed. If even this does not produce any successful result, then the selection is moved to different outputs. Thus all alternatives for the input side are processed for each alternative on the output side. In the worst case the search for the time slot solution can comprise a maximum of (N*N)-1 search processes.

The above described method also includes, according to figure 4, the processing of an exceptional situation in step S5, before the process continues to select the input in step S9. An exceptional situation can occur when the selection is made from a group with a lower priority input or output interface, in spite of the fact that no channel is found for each corresponding input or output of a higher priority group, which also has to be connected in the present time slot. Such an exceptional situation is detected in step S5, the process continuing in step S6 where the selection is made according to the higher priority group of the input or output, when this is the last chance for the input or output which must be connected to it in the present time slot. In the order which is used according to the invention, first the output and then the input, an exceptional situation occurs after the selection of an output, but before the selection of an input, as is shown in figure 4. According to the above the reason is that a priority group has more inputs than there are different outputs. Then the selection could be made for the input providing the highest number of channels, although the only connection possibility for a single channel in another input would be lost. Therefore the usual input selection criteria, or the steps S9 - Sll, are bypassed, and the mentioned single channel is selected in step S6. The selection is again checked in step S7, after which the process continues in the normal way described above.

Above we described the .basic implementation of the method according to the invention. The detailed algorithms used in the configuration calculator 32 (figure 2) can be freely chosen, e.g. according to any method known per se, if they only can realize the periodical calculation principle according to the invention.

The invention can be applied in cross connects, which use simplex or bothway traffic switching. In bothway cross connects the connection configuration in the other direction can be generated in a complimentary way, e.g. as a mirror image. However, in a cross connect embodying the inventive method it not possible as such to realize broadcasting, or to copy a selected signal into several output channels. To accomplish this the cross connect must be provided with more capacity. This is made so that one input and one output line are reserved to be used by a limited broadcasting function. Selection blocks SE (not shown) and time switch blocks TE (not shown) for each output line are further arranged after the time switches on the output side of the TST cross connect. ' An input channel assigned for broadcasting is connected through the space switch to the mentioned reserved output line, in which the TST output time switch arranges the channels. The channel arrangement is then adapted so that the respective output lines TE of the cross connect can receive the broadcasting channels through the selectors SE. Then the output time switch TE arranges the time slots connected to it into the order required by the transmission. Alternatively, for a complete broadcasting operation, it is possible to arrange a space switch of the double capacity, whereby the first half of the space switch capacity is used to route the time slots connected according to the inventive method, and whereby the second half of the capacity is used for broadcasting operations . The broadcasting operation can then use straightforward copying of a time slot to the desired time slots of the second half.

According to one further modification of the method the connection required by the routing backup is arranged so that the contents of the input channel to have a backup is copied to two output channels, i.e. to output channels forming the backup of each other. First it is examined whether there is a free time slot for the backup channel in the input time switch. When such a free time slot is found the contents of the time slot to have a backup is copied to this free channel. Thereafter both channels are connected through the space switch to different output time switches and then out to the lines. If no free time slot is found in the input time switch, then the backup connection in the space switch is made so that the contents of the time slot at some input of the space switch, which time slot shall have a backup, is copied to two output channels, or to two different output time switches and out to the lines.

When the routing backup is made according to the 1+1 -principle by doubling the bothway signal to have a backup, then each cross connect with routing backup will have both an 1:N broadcasting function and an N: 1 function to select the corresponding signal from N signals in the return direction. Then the TST cross connect according to the invention in the space switch can copy the signals to have a backup onto the corresponding signals to have a backup in the return direction, and thus a non-blocking operation is obtained.

According to figure 5 the time slots of the connections lrN/N:l having a backup are first connected, and then the others. If all input/output time switches don't have them, then the priority time slots are connected in a normal way through them.

In bothway routing backup there are always N+l input time slots, one signal to have a backup, which thus has to be copied into N outputs, and N backup signals, of which one must be selected to 1 output. Thus there are also N+l outputs.

The connection is made by selecting all the N+l time slots in the input time switch for the same period, the space switch selects one of the backup time slots to be the selected output channel and copies the input time slot to have backup to all N backup time slots. The output time switches make normal time switching to the outputs. No connection configuration has to be calculated when a rapid routing backup is made.

The method according to the invention provides an effective calculation method for the connection configuration. Then a new configuration can rapidly be calculated in the processor 30 (figure 2). This enables the implementation of a non-blocking and synchronous cross-connection, which can handle an errorless connection matrix change-over, without the need for double time switches in the cross connect. The method according to the invention can be applied also in such cross connects where byte parts arranged in parallel are switched through the space switch.

The invention was described above on the basis that we first select the input and then the output. A person skilled in the art understands that the method can be applied in the opposite way by calculating non-blocking connections by periods, so that first the incoming lines are selected and then the outgoing lines. The above mentioned period was 63 or 64 time slots, whereby the number of bytes to be processed during a selected period is 63 or 64 times the number of the input time switches. However, it is conceivable that some other number of bytes is selected as the period.

Claims

1. A method for configuration a time-space-time cross connect in situations of changing cross-connection needs, whereby the cross connect is provided with time switches (T) and space switches (S), and whereby the signals coming to and going from the cross connect (10) are high speed serial mode data streams (1...N) having a logical frame structure, within which the logical units to be transmitted and cross-connected are bytes, and whereby the respective configuration of the cross connect is stored in a connection matrix (31), characterized in that

- the new connection configuration for the cross connect is generated periodically, whereby each period comprises a prede¬ termined number of time slots, and whereby the connection configuration generated for each time slot is stored into a standby connection matrix (3lb) , and that

- during each period from the still unselected bytes the time switch on the input side selects such a byte to be connected through the space switch, that this byte can be connected to the desired output of the space switch during the examined period, whereby

- the time switch on the output side arranges the bytes connected to the respective outputs of the space switch into an order required by the outgoing transmission line, and that

- the standby connection matrix (31b) is marked as the active connection matrix (31) at a desired moment when all periods of the cross connect have been processed.

2. Method according to claim 1, characterized in that, during the generation of the connection configuration of the examined period, the bytes to be processed first are selected (SI) in order from those time switches, in which the number of unprocessed bytes is equal to the number of unprocessed time slots.

3. Method according to claim 1 or 2; characterized in that, during the generation of the connection configuration of the examined period, the bytes to be processed are selected (S2) in order from those respective input time switches, which have requirements for connections to the other side of the space switch to that output time switch having the lowest number of connections from different inputs (S2).

4. Method according to claim 1 or 2, characterized in that during the generation of the connection configuration of the examined period the bytes to be processed are selected for such an input switch, which has the highest number of time slots to be connected to a certain single output.

5. Method according to claim 1 or 2, characterized in that, during the generation of the connection configuration of the examined period, the bytes to be processed are selected so that from equally valued selection alternatives that input is selected, which has the lowest number of connections to different outputs (S13) .

6. Method according to any previous claim 3 - 5, characterized in that, during the generation of the connection configuration of the examined period, the respective byte to be processed is selected so that it first meets the criterion of the time switch on the output side (S2) and then also the criterion of the time switch on the input side (S9) regarding the selected output side time switch.

7. Method according o claim 1 or 2, characterized in that, during the generation of the connection configuration of the examined period, the bytes to be processed are selected (S2) in order from those respective output time switches, which have requirements for connections to the other side of the space switch to that input time switch having the lowest number of connections from different outputs.

8. Method according to claim 1 or 2 , characterized in that, during the generation of the connection configuration of the examined period, the bytes to be processed are selected for such an output switch, which has the highest number of time slots to be connected to a certain single input.

9. Method according to claim 1 or 2, characterized in that, during the generation of the connection configuration of the examined period, the bytes to be processed are selected so that from equally valued selection alternatives that output is selected, which has the lowest number of connections to different inputs (S13) .

10. A method to implement the connection required by routing backup in a digital cross connect, whereby a method according to any previous claim is used to calculate the connections, characterized in that the input byte to have a backup is copied to two output bytes, whereby

- first a free time slot for the backup byte is searched for in the time switch on the input side, and when a free time slot is found the contents of the byte to have backup is copied to different time switches through the space switch, or - if no free time slot is found in the time switch on the input side the backup connection is made in the space switch so that the contents of the time slot to have backup in some of its outputs is copied to two outputs.

11. A method to implement the connection required by routing backup in a digital cross connect, in which the routing backup realizes in one cross connect both an 1:N broadcast function and an N:l selection function, whereby a method according to any previous claim or any other method is used to calculate the connections, characterized in that in the time switches on the input side both the time slot to be 1:N broadcasted and all N time slots of the selection function, all of them always connected to different input time switches, are connected to the same time slot, and that in the space switch is performed both the 1:N broadcasting onto all N selection function time slots, which in the space switch have been connected to the outgoing lines of the broadcasting time slots, and the selection of the desired time slot, out of the N selection function time slots, to the selected output time switch.

12. A time-space-time cross connect for observing the cross-con- nection requirements in changing situations, whereby the cross connect is provided with time switches (T) and space switches (S), and whereby the signals coming to and going from the cross connect (10) are high speed serial mode data streams (1...N) having a logical frame structure, within which the logical units to be transmitted and cross-connected are bytes, and whereby the respective configuration of the cross connect is stored in a connection matrix (31), characterized in that the new connection configuration is adapted to be calculated in the configuration calculator (32) of the processor controlling the cross connect, and to be stored in its standby connection matrix (31b), whereby the new connection configuration is activated in synchronism, controlled by a signal propagating through the cross connect.

13. A cross connect according to claim 12, characterized in that one incoming line and one outgoing line are reserved in the space switch to be used by a limited broadcasting operation, whereby the input byte assigned for broadcasting is copied through the space switch to selected time slots of said reserved outgoing line, which time slots are selected with a selector group (SE)> added after the cross connect and connected to respective outgoing lines by a time switch group (TE) added to the cross connect.

14. A cross connect according to claim 12, characterized in that the space switch is provided with the double capacity for a complete broadcasting function, whereby the first half of the capacity of the space switch is used to route the bytes selected according to the method of any,previous claim 1 - 6, and whereby the second half of the capacity of the space switch is used for broadcasting connections according to the method of any previous claim 1 - 6.

15. The use of a cross connect according to any previous claim for the cross-connection of transport modules (STM-N) of the synchronous digital hierarchy, or for the cross-connection of plesiochronic transmission systems.