Method and circuits for realizing a modular digital cross-connect network

Abstract: The method consists essentially in realizing a connection network as a set of two-stage (temporal and space) matrixes; sectioning said connection network in such a way as to be completely implemented by parts in a distributed fashion in similar or identical devices without the use of any centralized common device; realizing said two stages in such a way as to consent the switching of tributary flows organized also in a mixed way, according to any the MUX paths of the SDH standard by limiting to the lower order VCS (see ETS1 specification), and modifying the frame present at the input of the temporal stage with respect to the one described by the standard in order to allow the equalization of delay introduced on the tributaries interconnected by the temporal stage itself. The circuits including time and space division matrixes and relative connection fork, are characterized in that the temporal stage (T) consists of two time division matrixes (T1, T2) having the first one 63 inputs-63 outputs and the second one 3 inputs-3 outputs respectively, as well as of only one space stage for the frame recomposition.

Description: The present invention relates to a method of realizing a connection network for SDH (Synchronous Digital Hierarchy) signals, having particular characteristics of modularity, flexibility and protectability against failures. All synchronous tributaries of lower level standardized by ETSI, i.e. VC12, VC2 and VC3 ("Virtual Container" of level 12, 2 and 3) are switchable signals and therefore the invention assumes the form of a possible realization of the LPC (Lower Path Connection) function contemplated by CCITT G783 specification for SDH standard compatible multiplexers. The invention concerns also the devices for implementing the network conceived on the ground of said method. STATE OF THE ART The intelligence of switching and protection of information flows carried in the public and private communication networks in the last ten years has been subjected to a migration from the switching exchanges to transmission equipments at a higher and higher frequency. The network which at the beginning of the eighties was substantially based upon: switching means with function of sorting the information traffic; multiplexing means for the "gruming" of the traffic and nationalization of the transport; real transmission means with the function of remotizing information also at long distances; already with the last generations of equipments of the plesiochronous hierarchy was "de facto" updated with new elements called Drop/Insert which are located at the boundary between multiplexing and switching. The Drop/Insert elements create a static-exchange and protection level which is higher than the one of the switching exchanges. The introduction in the international standards (CCITT) of the new information transmission standard called SDH (Synchronous Digital Hierarchy) has definitely helped to change the previous idea of the network in which switching and transmission were two absolutely distinct functions into the new vision in which they are deeply interpenetrated. In fact the same format of the standard carries the functionalities which are typical of the control and observation of the transmission lines at the same time the functionalities which are typical of the exchange architecture of information flows. In particular, the introduction of the VC (Virtual Container) as a basic-information unit and also as exchange unit between several carrying flows allowed to conceive apparatuses for handling such entities which, besides the trasmission functions at the various levels of the new hierarchy, include powerful switching functions of the virtual containers themselves. These apparatuses are called Cross Connect and Drop/Insert respectively; this description is concerned with the last ones and their detailed functionalities. Said apparatuses realize the exchange and protection layer better than the one of the exchanges, in a much more versatile way and, thanks to the prescribed standard, in a more controllable way than the previous plesiochronous scenario. Being a matter of new standards and functionalities, it is difficult to define the state of the art relative to solution of switching problems of virtual containers even if in the literature and in prototypes of apparatuses presented on occasion of international exhibitions the specific switching function of virtual containers which is the special subject of this invention is realized in a "concentrated" way i.e. a particular portion of circuit is dedicated thereto. Such kind of solution implies that the failure of such portion of circuit jeopardizes irreparably the operation of the whole equipment. In the proposed solution, the functionality is realized in a "distributed" fashion as will be detailed better and with the advantages set forth in the following description of the invention. A distributed arrangement is known from EP 0 384 581 (D2) dealing with a cross-connect core switch. D2 teaches a fully accessible (squared) matrix which is obtained through the use of shared busses. D2 uses a decoder as space stage and thus it can process one STM-1 at a time being the space stage static. Finally, the cross-connect according to D2 is a single-stage (Space) or three-stages (Space/Time/Space) arrangement. Description of the invention A first object of the present invention is to provide a method of realizing a modular and distributed structure of a connection network for tributary flows of the Synchronous Digital Hierarchy. The method according to the invention allows now the maximum flexibility of use from the viewpoint of switching possibilities, since it is fully accessible and not blocking for point-to-point or point-to-multipoint connections, and moreover it is suitable for very efficient implementations as to the protection of the multiplexing apparatus against failure being deprived of a centralized common device. This and other objects are achieved by the method and circuit according to the invention having the features set forth in claims 1 and 6, respectively. The corresponding dependent claims indicate further advantageous features of the method and circuit according to the invention. The various aspects and advantages of the invention will be easily understood from the description of the embodiments thereof illustrated in the drawings wherein: fig. 1 (a) represents the connection network in its most general form and a possible partialization of the network itself; fig. 1 (b) represents the equivalent of a portion of the network in terms of purely spatial matrixes, in special case of frame loading; fig. 2 shows how it is possible to extend the concept illustrated in the preceding figures to networks as their complexity becomes greater and greater; fig. 3 shows the structure of the temporal stage; fig. 4 shows, in case of STM1 (Synchronous Transport Module of level 1) frame, the modified frame which must be supplied as an input to the temporal stage so that the connection network on the whole do not introduce distortions on the standardized frame format. Fig. 1 (a) highlights modularity and symmetry of the proposed solution. The three areas bordered by a broken line indicated by a1, a2 and a3, containing identical structures, represent a possible and advantageous sectioning of the structure. In one of the preferred implementations of a type Drop/Insert multiplexer, a1 and a3 contain the aggregate processing circuits, side W and side E, while a2 contains the tributary processing circuits. At any rate signals s1, s4 and s7 structured in such a way as to be able to transport any set of lower order synchronous tributaries contemplated by standard ETSI-SDH, represent the inputs of the connection network. Similarly s3, s6 and s9, corresponding to the preceding ones, rapresent its outputs. Each area a(i) is characterized by an input and an output of the network, each input "urges" on both the outputs belonging to the other areas, each output can pick up the signal from both the inputs belonging to the other areas. In more detail : inputs s1, s4 and s7 are forked in blocks b1, b4 and b7 and sent to time switching stages b2, b2'; b5, b5'; b8, b8'. The outputs of such blocks are crossed in such a way as s2 and s5, resulted from the time switching of s1 and s4 respectively, urge on space switching block b3 in order to originate the switched signal s3; similarly s5 and s8 urge on space switching block b6 originating the switched signal s6 and finally s2' and s8' urge on switching block b9, whose output is the switched signal s9. It should be noted that the space switching blocks b3, b6, b9, because of the presence of temporal stages, are actually "S matrici DT" (time division switching spatial matrixes as called e.g. in the book M. Decina - A. Roveri: "Introduzione alle Reti Telefoniche Analogiche e Numeriche", Quaderni di ingegneria, Ed. Goliardiche hereinafter referred as D1); time switching stages b1, b2', etc. on the contrary are complex connection networks, specially constructed in order to allow the specific handling of SDH frames under any load condition and they are described in more detail hereinafter. An alternative way of describing the network of fig. 1 (a) is to consider it as three identical subnetworks taken together, each of which is formed by two time switching stages, which as a whole realize a two T-S (Time-Space) stage network connection. In particular, the three subnetworks are individuable in the sets of blocks (b2, b5', b3); (b2', b8', b9); (b5, b8, b6). In fig. 1 (b) there is illustrated the subnetwork (b2, b5', b3) in the special case when signals s1 and s4, inputs to the network, are STM-1 frames loaded with sixty-three tributaries of the type VC12. The figure shows a representation of the network in purely spatial terms, quite equivalent to the one of fig. 1 (a), but more suitable to the matrix examination as far as the theory of connection is concerned (see D1). Signals s1', s4', s2, s5' and s3 are made explicit in the sixty-three separated components that actually are time multiplexed in the sole framed signal. Temporal stages T are constituted by space equivalents for the described case, i.e. by complete space matrix with sixty-three inputs and sixty-three outputs. Space stage S has been replaced by an equivalent structure constituted by sixty-three space matrixes having two inputs and one output (for the equivalence between time matrixes and space matrixes reference can be made to D1). This representation highlights the two-stage structure of the connection network and put its properties in light from the viewpoint of the switching theory. The network is quite accessible since: the number of outputs of the first stage matrixes is equal to the number of matrixes forming the second stage; the number of inputs of the second stage matrixes is equal to the number of the matrixes constituting the first stage; each matrix of the first stage has a connection path towards each matrix of the second stage; moreover the network is not blocking both for point-to-point connection and for point-to-multipoint connections since the multiplicity of connections between the matrixes of the first stage and the second stage ones are equal to the number of outputs of the matrixes of the second stage. In fig. 2 there is shown how it is possible to extend asymmetrically the previously described network where in one of the three areas it is desired to elaborate a greater number of input-output signals. In the emphasized case the functions contemplated for the area a2 have been doubled by adding an area a2' identical to the preceding one, and the functions contained in areas a1 and a3 have been changed accordingly in order to maintain the characteristics of complete accessibility and non-blocking of the network. In particular it is necessary to increase the number of type stages and the number of the inputs of the type S stages. Further increases of network can be realized by iterating the described method. Fig. 3 illustrates the structure of the temporal stage (T-blocks in the preceding figures) which allows to process different types of tributaries present in the SDH frames standardized by ETSI, also in case of mixed paths (e.g. type-VC12 tributaries mixed with type-VC2 and/or VC3 tributaries). It should be noted that what had been previously indicated as purely temporal stage for simplicity's sake it has been in turn realized as a two-stage cascade, a temporal one composed of two temporal matrixes T1 and T2 of different type, and a space one, composed of an "S matrice DT". Signal s1, a synchronous frame containing structures of type TUG3 (Tributary Unit Group of level 3), loaded in turn with synchronous tributaries VC3 or a mixture of tributaries type VC2 and VC12, is forked in block b1 and, at the same time, to time division matrixes T1 and T2, the first one a sixty-three input and sixty-three output matrix, the second one a three input/three output matrix. Indeed matrix T1 is used for processing that part of load structured with tributaries of type VC12 and VC2, characterized by a recurrence of sixty-three bytes, while matrix T2 in the processing of that part of load structured with type-VC3 tributaries, characterized by a recurrence of three bytes. The space b4 re-forms the outgoing frame s4 by picking up the signal s2 for the position of switched load composed of VC12 and VC2 and by picking up the signal s3 for the portion of switched load composed of VC3. All the operations are controlled through memories containing information relative to the structure of the frame and to the desired switching. A characteristic feature of the proposed structure consists in that the two temporal matrixes T1 and T2 have effect upon different portions of the processed synchronous frame. It should be observed now that time division matrixes cause an average delay on the switched signal which is equal to the recurrence of the switching, unless alterations in the sequetiality of the input tributaries (see D1) can be accepted, which is intolerable for the present application. Consequently the frame portions processed by the two matrixes, which have different recurrence of operation, are subjected to different delays and at the moment of recomposition give size to a frame no longer complying with the standard. In order to overcome this drawback a reconfiguration of the frame entering the temporal stage of the connection network is operated in the invention according to the scheme illustrated in fig. 4 for the specific case of an STM1 frame. As it can be seen in the figure, the reconfiguration consists in anticipating the part of load structured with VC12 and VC2 of sixty-three bytes, excluding the portions of frame not processed (and hence not delayed) by matrix T1, and in anticipating the part of load structured with VC3 of three bytes, still excluding the portion of frame not processed by matrix T2. By operating as described above a frame fully in conformity with the synchronous standard is obtained at the output of space recomposition block b4. Even if the invention has been described with reference to embodiments thereof illustrated in the drawings for simplicity, it is obvious that these are not limiting but they are susceptible to variations and modifications apparent to those skilled in the art.

Claims: Method of realizing a connection network for Synchronous Digital Hierarchy (SDH) signals comprising SDH frames, in which network low-order virtual containers are processed, characterized in that it comprises the following steps: providing aggregate (a1, a3) and tributary (a2, a2') subnetworks, each subnetwork (a1; a2, a2'; a3) having a subnetwork input and a subnetwork output for input signals (s1; s4, s10; s7) and output signals (s6; s9, s11; s3), respectively, each subnetwork (a1; a2, a2'; a3) further comprising time switching blocks (b2, b2', b2"; b5, b5', b11', b11"; b8, b8', b8") and space switching blocks (b6; b9, b12; b3); forking (b1; b4, b10; b7) the input signals (s1; s4, s10; s7) thus obtaining forked signals (s1', s1", s1'''; s4', s4", s10', s10"; s7', s7", s7"'); sending the forked (s1', s1", s1'''; s4', s4", s10', s10"; s7', s7", s7"') to the respective time switching blocks (b2, b2', b2"; b5, b5', b11', b11"; b8, b8', b8") so as to obtain processed signals; and sending each of the processed signals of one subnetwork (a1; a2, a2'; a3) to the space switching blocks (b3, b6, b9, b12) of the other subnetworks (a3, a2, a2'; a1, a3; a1, a2, a2') thus obtaining the output signals (s6, s9, s11; s3) from each subnetwork (a1; a2, a2'; a3). Method according to claim 1, characterized in that it further comprises the step of reconfiguring the SDH frames entering the time switching blocks (b2, b2', b2"; b5, b5', b11', b11"; b8, b8', b8") in order to allow the equalization of the delay introduced on the tributary subnetworks interconnected by the same time switching blocks (b2, b2', b2"; b5, b5', b11', b11"; b8, b8', b8"). Method according to claim 2, characterized in that the step of reconfiguring the SDH frames entering the time switching blocks comprises the step of advancing by sixty-three bytes that part of load structured with VC12 and VC2 and by advancing by three bytes the part of load structured with VC3. Method according to any of claims 1-3, characterized in that the step of providing tributary subnetworks comprises the step of providing a first and a second tributary subnetworks (a2; a2') and in that the step of providing aggregate (a1; a3) subnetworks comprises the step of providing two aggregate subnetworks. Method according to any of claims 1-4, characterized in that the step of sending the forked signals (s1', s1"; s4', s4"; s7', s7") to the respective time switching blocks (b2, b2'; b5, b5'; b8, b8') comprises the step of sending the forked signals (s1', s1"; s4', s4"; s7', s7") to time switching blocks (T1, T2) and a space switching block (S). Circuit for implementing a connection network for Synchronous Digital Hierarchy (SDH) signals comprising SDH frames, in which network low-order virtual containers are processed, characterized in that it comprises: aggregate (a1; a3) and tributary (a2, a2') subnetworks, each subnetwork (a1; a2, a2'; a3) having a subnetwork input and a subnetwork output for input signals (s1; s4, s10; s7) and output signals (s6, s9, s11; s3), respectively, each subnetwork (a1; a2, a2'; a3) further comprising time switching blocks (b2, b2', b2"; b5, b5',b11', b11"; b8, b8', b8") and space switching blocks (b6; b9, b12; b3); fork means (b1; b4, b10; b7) for forking the input signals (s1; s4, s10; s7); means for connecting the fork means (b1; b4, b10; b7) and the time switching (b2, b2', b2"; b5, b5',b11', b11"; b8, b8', b8"); and means for connecting the time switching blocks (b2, b2', b2"; b5, b5',b11', b11"; b8, b8', b8") of one subnetwork (a1; a2, a2'; a3) to the space switching blocks (b3, b6, b9, b12) of the other subnetworks (a3, a2, a2'; a1, a3; a1, a2, a2'). Circuit according to claim 6, characterized in that the tributary subnetwork comprises a first and a second tributary subnetworks (a2; a2'). Circuit according to claim 7, characterized in that the time switching blocks (b2, b2'; b5, b5'; b8, b8') comprise time switching blocks (T1, T2) and a space switching block (S).

Inventors: LOMETTI ALBERTO, VALUSSI ROMANO, LOMETTI, ALBERTO, VALUSSI, ROMANO
Applicants: CIT ALCATEL, ALCATEL
Application year: 1993
Main classification: H04Q11
IPC codes: H04Q11, H04J3