SYNCHRONIZATION OF SDH SIGNALS

Abstract: In an SDH cross connect all switch elements are synchronized to the same multiframe reference clock (AU/TU "master"). According to the invention the synchronizing modules (AU/TU) calculate such pointer information that the time switch (T) on the input side can connect the signals to the space switch (S) so that the incoming signal's phase difference to the reference is observed. The time and space switches do not require a multiframe synchronization, and the incoming signal is not buffered.

Description: Synchronization of SDH signals The invention relates to a synchronization method according to the preamble of claim 1 in a node of the synchronous digital hierarchy. The synchronous digital hierarchy (SDH) comprises quite a large entity to be very far advanced 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-1) for SDH signals having a transmission rate of 155.520 Mbit/s. The SDH signals or transport modules are formed by interleaving the bytes of the subsystem signals. The frames are combined into multiframes . In the synchronous digital hierarchy each frame contains at its beginning a synchronization signal for the synchronization of an the signal, such as an STM-1 signal. The frames are combined into multiframe signals, and for instance an AU-4 multiframe signal contains four frames or STM-1 signals. In the cross connect interface the signals coming from the line are synchronized with the cross connect clock. The incoming signals thus have the same frequency and are in synchronism, but at the cross connect input they still have different phases, whereby phase differences may occur relating both to a byte and to a frame. In order to realize the cross-connection the signals must be further processed in some way. A simple and known idea is to decompose in a demultiplexer the signals on the incoming lines into signal elements, as is shown in figure 1. The signal elements are connected through the cross connect, which in this case could simply be a large space switch, and then the cross-connected signals are combined by a multiplexer to be sent on the outgoing lines. A solution according to prior art is also to delay the incoming signals with buffers, so that the signals offered to the cross connect have the same phase. Such a solution entails an inconvenient construction and requires much components needing too much space. Due to the speeds and the large frame structures of the SDH systems the buffering would require a large amount of memories, which in practice does not appear as a sensible solution. However, this is the case for example in central offices, where 2 Mbit/s lines are connected to the cross connect. A speed like this is low compared to e.g. the SDH speeds, at which the distances between subracks and racks cause considerable delays of the transmitted signals. Due to the buffering the frame beginning of each incoming line is temporally at the same point when they are input to the time switch on the input side, which ensures the correct operation of the space switch. An operation like that of the central offices is unacceptable in SDH cross connects. The connection times on the transmission 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. Due to the high operating speed it is also necessary to observe in the SDH cross connect the changing distances, which affect the coincidence of the bytes, i.e. the phase of a signal can change when it is connected to a space switch located in another rack. When SDH signals are handled it is also conceivable that we examine in each stage of the cross connect the pointer information in the frame of the signal input to the stage, i.e. in a TST switch both in the input and output time switches and in the space switch between them. This is however a task requiring much equipment, and further both the control logic and the algorithms must be extremely fast, when we take.into account the high speeds of the SDH signals and the comprehensive routing. examination for large numbers of time slots. The object of the invention is now to devise such a way to synchronize the cross-connection matrix with which it is possible to avoid the disadvantages of prior art, i.e. the large number of components and the space requirements. This problem is solved by the characteristics of claim 1. An essential prerequisite is that the whole cross connect or all cross connect modules always are synchronized to a defined reference clock, e.g. to the multiframe clock received on a line. Let us further state here that it is known from e.g. the publication DE-A1-4012762 to apply the so called ATM (Asynch- ronous Transport Module) within an SDH frame. This means a synchronization at the frame level, not at the multiframe level as in the present invention. In the present invention all logical connections or the pointer operations are made in accordance with this time reference. The input signals are not as such delayed to have the same phase, nor is any buffering implemented. Instead we utilize the information included in a frame concerning the addresses of the TU-12 signals to be connected, which addresses are shown by the AU-4 pointers. The realization of the invention is that the interface modules calculate such pointer information that the time switch on the input side of the cross connect can switch the signals to the space switch in a correct phase, i.e. so that the phase difference between each input signal and the reference is taken into account when the bytes are arranged into time slots in the input time switches. The signals connected to the space switch have then a correct byte order, or the signals have a correct phase regarding both the bytes and the frames. Now it is not necessary to separately synchronize the signals in the space switch. In the time switches on the output side the bytes coming from the space switch are arranged into the correct time slots for the transmission, but according to the invention not even this switching stage requires separate synchronization/pha¬ sing. According to claim 8 the method according to the invention is used for the errorless change of the connection matrix controlling the cross-connection. The cross connect architecture according to the invention is presented in claim 9. Other preferred embodiments of the invention are presented in the dependent claims. Below the invention is described by examples with reference to the enclosed figures. Figure 1 shows schematically the embodiment of a known cross connect. Figure 2 shows the principle of the time-space-time cross-con¬ nection. * Figure 3 is an illustration of the implementation of the cross connect according to the invention. Figure 4 is a more detailed block diagram of the cross connect according to the invention. Figure 5 shows the contents of the section overhead block of the STM-N frame structure. Figure 6 shows the contents of the connection matrix change-over byte, whereby figure 6a contains a change-over instruction and figure 6b is a situation without the change-over instruction. Figure 7 shows the synchronization signals on the synchronization bus r whereby figure 7a is a normal multiframe synchroniza¬ tion signal, to which a matrix change-over instruction according to the invention is added in figure 7b. Figure 8 shows schematically an expanded space switch with the capacity of 128 x 128 STM-1 signals. The design and function of the time-space-time cross connect used also in the invention is explained with the aid of figure 2. On the left there are the input signals II...In (here STM-1 signals) and on the right there are the output signals 01...On. There is a time switch for each line. 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 transmits 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. Generally the cross connect is implemented as a module structure with two-way operation. The cross connect according to the invention is symbolically shown in figure 3. Figure 4 shows a cross connect architecture, with which the TU-12 signal connection is realized at the VC-12 (VC, Virtual Container) level, and in which the synchronization required by the invention is taken into account. In the same way as in figure 2 we have here time switches (T switch) and a space switch (S switch) . The space switch in this example is a space switch which connects in total 16 STM-1 input lines to 16 outputs. In front of the time switches on the input side there is connected an interface module or synchronization circuit AU/TU (AU/TU pointer proc. ) which receives from the transmission line an STM-1 signal connected via interface modules (not shown) in AU-4 form containing 4 frames. The cross connect clock (CLOCK) W operating at the frequency 38,8 MHz is connected via the selector means SEL to the synchronization circuits AU/TU and to the space switch S. The space switch is doubled (not shown). All AU-4 signal pointers are in this cross connect locked by the AU/TU circuits by the AU-4 pointer value 522. The frame synchronization of the cross connect is supplied by the bus DXC Sync- to the AU/TU circuits, which in the same way as the time switches and the space switch are also preferable realized as ASIC circuits. The synchronization reference for the cross connect is preferably supplied by a selected circuit AU/TU (AU/TU 'master' ) , and further distributed to all other AU/TU circuits via the DXC Sync bus interconnecting, them, whereby all logical connections are made on the basis of this time reference (figure 7a) by manipulating the signal pointers in the AU/TU circuits. The DXC Sync pulses represent 2 kHz multiframe synchronization pulses. The AU-4 signal in figure 7a represents a synchroniza- tion, to which the cross connect is locked (value 522 of the AU-4 pointer). Alternatively some other suitable clock or reference source (not shown) could be used for the synchroniza¬ tion. In principle the multiframe synchronization pulses will be needed when all Aϋ-4 signal of the cross connect must be synchronized regarding the multiframe, this situation occurring e.g. when the power is switched on, or if the DXC Sync synchronization is lost. Then the switches T and S have to be resynchronized. The processor controlling the cross-connection triggers the synchronization, whereby the interface modules AU/TU (SDHSYN) realize the start and updating of the output counters on the bases of the DXC Sync pulses. After the start all interface modules AU/TU monitor the DXC Sync signals in order to protect the bus and to detect any connection matrix change-over instructions, which is briefly described below with reference to figures 6 and 7. The required longest delay which has to be taken into account in the multiframe synchronization is 9719 bytes (4 x 2430 bytes/STM-1) . A comparison with the multiframe synchronization mark provides a delay value required by the control byte, the value being calculated in the interface modules AU/TU Pointer Proc. With this operation according to the invention all interface modules or synchronization circuits AU/TU are synchronized regarding the multiframe. The output signals of the-> AU/TU circuits have the same clock frequency, which is supplied to the first time switch T. The relative delay of the time switches may be ignored, and the AU-4 signal multiframe synchronization obtained by the AU/TU circuits is maintained still when supplied to the space switch. In principle the AU/TU circuits also realize the byte synch¬ ronization. In practice however, e.g. in the cross connect according to figure 4, the signals at the inputs of the space switch S have mutual delay differences of about +4 bytes or clock periods, which can be due to the different distances between the respective time switches and the space switch. In order to compensate for these delay differences the inputs of the space switch are provided with short buffers having a filling length arranged to be such that the respective signal is synchronized to the selected 'master' multiframe clock. Then the bytes and frames have a correct phase before the actual space switching. A synchronization of this kind is an absolute requirement for the errorless operation of the cross connect. The above mentioned compensating within +4 clock periods can be tuned when the cross connect is taken into operation or when physical changes are made to its cabling, because the different delay differences are due to different distances . Delay information obtained during the tuning can be stored into a memory of the control logic and used to change the input buffer length, e.g. in connection with a change-over of the connection matrix. Alternatively it could be arranged so that the respective delay difference at the space switch input is measured with the aid of a suitable reference signal, after which the buffer length is automatically set by the control logic. The cross connect in figure 2 is able to realize in synchronism an errorless change-over of the connection matrix (CM) in the time switches on the input and output sides and in the space switch, so that when the connection configuration changes, the time and space switches can be completely reconfigurated to reflect the required new connection configuration. With the cross connect architecture according to the invention this can be made without having doubled time switches. In the configuration we preferably utilize the division of the signal frame into time periods, corresponding e.g.to TU-12 containers, and the connec¬ tion routes are selected for each time slot. The connection matrix change-over signal "special" is also transmitted on the DXC Sync bus, as is shown in figure 7b. When a new connection configuration has to be initialized, the pulse "special" is generated on the DXC Sync bus at the beginning of the third frame in the multiframe, the pulse "special" triggering the transmis¬ sion of the change-over actuation instruction CM (in figures 5 and 6a, 6b) from each AU/TU circuit. From the AU/TU circuits the instruction CM is sent in the signal stream to the time and space switches, whereby the receiving switch realizes an errorless change-over of the connection matrix during the section overhead block. Figure 8 shows an example of a large space switch (128 x 128 STM-1; or 32 x 32 STM-4) , in which 32 x 32 switches in pairs are arranged in subracks. Then a rack contains four subracks. The input lines and the output lines are optical (optical intraoffice interface) ; the input lines are branched. Between the subracks there are optical STM-4 interfaces, and between the space switches electrical AU-4 interfaces. The above mentioned +4 clock periods buffer of the space switch suffices normally quite well e.g. in a 16 x 16 STM-1 cross connect, but due to the long distances the case of figure 8 calls for further actions regarding the synchronization. In principle it would be possible to delay the AU-4 signal frames with buffers. However, according to the invention we use delay calculation in the AU/TU circuits. In the case of figure 8 the control logic monitors the buffer filling level at the input of each space switch. If the fiLling level exceeds a predetermined limit the control logic reports this to the synchronizing AU/TU circuit, which then makes the corresponding change in the synchronization according to a predetermined algorithm. A programmable frame synchronization of this kind mainly compensates for fixed delays, which can be determined by the configuration of the cross connect. The input buffers of the space switch, on the other hand, will handle any other, 'non-fixed' or 'changing' delays. Although we have here studied a cross connect, a person skilled in the art understands that the method according to the invention also can be applied to other SDH nodes, containing a cross-con¬ nection function. The cross-connection can have many levels (e.g. S-T-S) and an arbitrary size. The same method according to the invention can also be applied at different SDH levels.

Claims: Claims 1. A method to synchronize the SDH signals in an SDH node containing a cross connect, whereby the incoming signal streams (AU4 #1...#16) containing a logical multiframe structure (AU-4) are connected to the cross connect through interface modules (AU/TU) , characterized in that the interface modules (AU/TU) of the cross connect are logically synchronized by a multiframe reference clock (DXC Sync), on the basis of which the logical connections are made, and that the interface modules (AU/TU) for the incoming signals (AU-4) calculate such pointer information that the time switches (T) on the input side of the cross connect can connect the signals supplied to the space switch (S) synchronized to the same phase regarding the multiframe reference clock (DXC Sync). 2. Method according to claim 1, characterized in that the control logic transmits said synchronization pulse (DXC Sync) at the beginning of the first frame in the multiframe structure. 3. Method according to claim 1 or 2, characterized in that the interface modules (AU/TU) are synchronized via said synchroniza¬ tion bus (DXC Sync) with the aid of a reference clock provided by a reference source (AU/TU 'master' ) . 4. Method according to any previous claim, characterized in that each input of the space switch is provided with a buffer, the length of which is controlled to compensate for the effects of the delay differences on the connections between the time switches and the space switch. 5. Method according to claim 4, characterized in that the length of the buffer at each input of the space switch is +4 clock periods. 6. Method according to claim 4 or 5, characterized in that the filling level of the space switch buffers is monitored, and that, if the filling level of an input buffer exceeds a predetermined limit, the control logic controls with an instruction repre¬ senting this case the synchronization calculation of the corresponding interface module into such a direction, that the buffer filling level decreases below said limit. 7. Method according to any previous claim, characterized in that, for an errorless change-over of the connection matrix, - as a preparation for the connection matrix change-over the control logic sends via the synchronization bus (DXC Sync) to the switching means (T, S) a preparatory synchronization pulse (DXC Sync 'special') when calculation of the new connection matrix is finished, and that - triggered by this synchronization pulse ('special') the interface modules (AU/TU) add a connection matrix change-over instruction (CM) at a predetermined position in the signal stream (AU-4) output from the interface module (AU/TU), whereby - the change-over instruction (CM) propagates in the signal (AU-4) through the time switches (T) and the space switches (S), in which the change-over instruction (CM) causes a respective change-over of the connection matrix synchronized to the signal (AU-4) frame structure. 8. Method according to claim 7, characterized in that the control logic transmits said synchronization pulse ('special') at the beginning of the third frame in a multiframe comprising four frames. 9. A cross connect architecture for the synchronization of SDH signals in an SDH node, the cross connect architecture comprising in succession arranged time switches, space switches and time switches, whereby the incoming signal streams (AU4 #1...#16) comprising a logical frame structure (AU-4) are arranged to be connected to the cross connect through interface modules (AU/TU) , characterized in that the cross connect interface modules (AU/TU) are arranged to be logically synchronized by by a multiframe reference clock (DXC Sync), on the basis of which the logical connections are made, and that the interface modules (AU/TU) calculate for the incoming signals (AU-4) such pointer informa¬ tion that the time switches (T) on the input side of the cross connect can connect the signals supplied to the space switch (S) synchronized tothe same phase regardingthe multiframe reference clock (DXC Sync) . 10. Cross connect architecture according to claim 9, charac¬ terized in that the cross connect comprises a control logic controlling the time and space switches via an interconnecting bus, whereby the control logic selects the reference source (AU/TU 'master' ) at the beginning of the synchronization operation.

Inventors: ALATALO HANNU, KLINE ERIC, NIEMINEN JUHANI, SUVITAIVAL PEKKA, ALATALO, HANNU, KLINE, ERIC, NIEMINEN, JUHANI, SUVITAIVAL, PEKKA
Applicants: ALATALO HANNU, KLINE ERIC, NIEMINEN JUHANI, NOKIA TELECOMMUNICATIONS OY, SUVITAIVAL PEKKA
Application year: 1993
Main classification: H04L7
IPC codes: H04J3, H04L12, H04Q11