Synchronizing method for SDH systems as well as method of and circuit arrangement for identifying different data structures
TECHNICAL FIELD 
     The present invention relates to the transmission of digital information over a telecommunications network and more particularly to the synchronization of received &#34;Synchronous Digital Hierarchy&#34; (SDH) data payload frames. 
     CLAIM FOR PRIORITY 
     This application is based on and claims priority from German Patent Application No. 40 12 762.1 dated Apr. 21, 1990, To the extent such prior application may contain any additional information that might be of any assistance in the use and understanding of the invention claimed herein, it is hereby incorporated by reference. 
     BACKGROUND ART 
     The introduction of the synchronous digital hierarchy SDH by the CCITT (Comite Consultatif International Telephonique et Telegraphique) permits the implementation of a worldwide uniform transmission network. 
     The bit rates of the SDH are specified in CCITT Recommendation G.707. The signal structure at the network node interface is contained in CCITT Recommendation G.708, and the multiplexing structure in CCITT Recommendation G.709. 
     An SDH frame according to CCITT G.708 consists of octet-oriented synchronous transport modules STM-1 with a bit rate of 155,520 kbits/s. 
     The frame consists of 270 columns containing 9 rows each. 9×9 octets are dedicated to a so-called section overhead SOH, and 261×9 octets to a useful-information part, called &#34;payload&#34;. 
     Row 1 of the section overhead SOH contains a frame sync word with a Length of 6 octets. Row 4 of the section overhead contains a pointer AU-4 PTR which addresses a path overhead POH of a virtual container VC4 of the payload. The path overhead POH contains an additional pointer H4 which addresses cell headers of a 260×9-octet cell-structured container C4. The cell structure of the container allows the asynchronous transmission of data in the form of ATM cells (asynchronous transport modules) within a synchronous frame (SDH). 
     To establish fast frame synchronization, it is known to continuously scan n-bit bit patterns of the received bit stream at a local clock rate, namely at 1-bit intervals, with the aid of a predetermined n-bit frame sync word, to compare the n-bit bit patterns with the frame sync word, and to output frame sync information in case of agreement (see published German Patent Specification 28 55 676 A1). Before it can be inferred with a high-degree of reliability that the transmission link is synchronized, the periodically occurring sync word must be detected repeatedly at predetermined intervals. This has the disadvantage that it takes very long, at least two frame lenghts, until synchronization is achieved. 
     In systems where ATM cells have to be transmitted, the synchronization time should be very short since service quality is greatly impaired by synchronization losses. 
     It is also known to transmit cells of constant length without external frames (asynchronous transport modules). 
     In case of purely cell-structured transmission, cells of equal length are transmitted with a periodicity of 53 octets, each cell containing 5 octets for a cell header and 48 octets for the payload. The cell header is encoded in a polynomial code, so that the uncoded payload can be distingushed from the cell header by decoding the cell header at the receiving end. 
     DISCLOSURE OF INVENTION 
     It is an object of the invention to provide a synchronizing method for systems frame-structured in accordance with a synchronous digital hierarchy SDH which ensures high reliability and a short synchronization time. 
     Starting from the two noncompatible data structures, further objects of the invention are to provide a method of identifying the correct data structure with a high degree of reliability and at no appreciable extra cost, and a circuit arrangement for carrying out the method. 
     For synchronization, a frame sync word detected only once is not confirmed by being repeatedly and periodically detected again as is done in the prior art, but confirmation is obtained by decoding headers of the ATM cells transmitted in the SDH frame, so that synchronization can be accomplished within one frame length. 
     According to the invention, a criterion independent of the sync word, namely the correct decoding of the cell headers in the payload, is used to confirm the frame sync word, since use is made of the fact that the payload, i.e., the ATM cells, is addressed via pointers spaced a predetermined distance from the sync word. 
     This synchronizing method is suitable for all frame-structured systems which transmit a periodic sync word and simultaneously address, via at least one pointer, a data area containing the payload with separate code words. 
     If two different data structures (SDH system, ATM cells) are to be identified, the method preferably provides for a change to two steady states, and the circuit arrangement for carrying out this method, is preferably capable of changing the sequence of functional blocks by means of the switching units in such a way that not just a bypass arrangement (either frame structure or cell structure) but flexible utilization of all functional blocks is made possible and only a minimum amount of circuitry is required. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     An embodiment will now be explained with reference to the accompanying drawings, in which: 
     FIG. 1 shows a structure of a frame arranged in a synchronous digital hierarchy SDH; 
     FIG. 2 is a state diagram describing the synchronizing method according to the invention; 
     FIG. 3 is a block diagram of the circuit arrangement according to the invention, and 
     FIG. 4 is a state diagram describing the method according to the invention for identifying different data structures. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     For a synchronous digital hierarchy SDH introduced by the CCITT, a frame structure as specified in CCITT Recommendation G.708 is used. An SDH frame consists of octet-oriented synchronous transport modules STM-1 for 155,520-kb/s signals. 
     As shown in FIG. 1, the frame consists of 270 columns and 9 rows for 1 octet each. 9×9 octets are dedicated to a section overhead SOH, and 261×9 octets to the useful information, called &#34;payload&#34;. 
     The first row of the section overhead SOH contains a sync word SYNC which has a length of 6 octets and is transmitted periodically (cf. FIG. 1) for frame synchronization. 
     The payload includes a cell-structured container C4 which can transmit an asynchronous cell-structured signal, ATM cells, with a bit rate of 149.76 Mb/s. 
     The ATM cells have a periodicity of 53 octets, with 5 octets for a cell header and 48 octets for the payload. The cell header is encoded in a polynomial code, so that the uncoded payload can be distinguished from the cell header by decoding the cell header at the receiving end. 
     Associated with the container C4 is a path overhead POH, and the path overhead and the container C4 form a virtual container VC4. 
     Since the frame of the virtual container VC4 and the frame of the higher-level synchronous transport module STM-1 have no fixed phase relationship to each other, the so-called pointer technique is employed. 
     A first pointer AU-4 is contained in row 4 of the section overhead SOH and addresses the virtual container VC4, i.e., it points to the path overhead POH. 
     Row 6 of the path overhead contains a second pointer H4 which addresses the beginning of an ATM cell, i.e., the cell header (cf. FIG. 1). 
     The frame structure of FIG. 1 shows only the information essential to the invention. A more detailed description is contained in CCITT Recommendations G.707, G.708 and G.709. 
     Starting from a frame structure as shown in FIG. 1, the synchronizing method according to the invention will now be explained with the aid of the state diagram of FIG. 2. 
     To establish synchronization SYNC, the frame sync word is first detected in a hunt mode HUNT by comparing an incoming bit sequence bit by bit with a predetermined reference bit pattern representing the sync word. 
     Since the frame is divided into octets, immediately after the detection of a frame sync word all following octets are aligned. 
     In the following presync state PRESYNC, the first pointer AU-4, contained in row 4 of the section overhead SOH and spaced a constant distance from the sync word, is read. The AU-4 pointer points to the first octet in the virtual container VC4, i.e., to the path overhead. Row 6 of the path overhead POH contains the second pointer H4, which addresses the beginning of an ATM cell, i.e., the cell header. 
     According to the polynomial code in which the cell header is encoded, decoding is now performed. If a predetermined number x of successive correct cell headers are detected, a transition to the sync state SYNC takes place. The value of x may be 6, for example. 
     The method according to the invention--detecting a periodic frame sync word, reading the predetermined pointers in the frame, and looking for code words (cell headers)--permits very fast synchronization even before a complete frame was received. 
     As long as the frame sync word and the cell headers are now found to be in the correct position, the system remains in the sync state SYNC. 
     In response to an error message, a transition to a check state TROUBLE takes place. If a number f of successive wrong cell headers are now detected within a number z of successive cells, a change to the hunt mode HUNT takes place, which is followed by the synchronization just described. 
     If, for example, only cell-header errors caused by single bit errors were reported, and the number x of successive correct cell headers are detected again, the system returns to the sync state SYNC. Thus, resynchronization is avoided if only minor errors occur. 
     The synchronization method according to the invention is universally applicable to those systems which transmit a periodic sync word and address, via at least one pointer, a data area in which data is transmitted in a separate code. 
     Starting from, on the one hand, an SDH system as described above and, on the other hand, a purely cell-structured transmission of ATM cells having a periodicity of 53 octets, the circuit arrangement according to the invention will now be described with the aid of FIG. 3 and the method according to the invention for identifying different data structures will be described with the aid of the state diagram of FIG. 4. 
     A bit sequence incoming on an input line EL is fed to a frame sync word detection unit FRAME in which the incoming bit sequence is compared bit by bit with a predetermined reference bit pattern representing the frame sync word. 
     The frame sync word detection unit FRAME is connected via a first switching unit S1, in a first switch position, to a cell header decoder unit CELL connected to an output line AL, so that the bit sequence passing transparently through the frame sync word detection unit FRAME can be checked quasi-simultaneously for regularly inserted code words which represent cell headers. 
     In FIG. 3, the respective first switch position of a switching unit is indicated by a solid line and the second switch position by a broken line. 
     After detection of a frame sync word, the frame sync word detection unit FRAME is connected via a second switching unit S2, in the second switch position, to means ALIG for aligning the received bit sequence in octets, i.e., for octet-boundary correction, and to succeeding means SOH, POH for reading a pointer spaced a predetermined distance from the frame sync word, with the reading means SOH, POH connected via the first switching unit S1, in the second switch position, to the cell header decoder unit CELL. The output line AL is connected via a third switching unit S3, in the second switch position, to the cell header decoder unit CELL (cf. FIG. 3). 
     The reading means consist of first means for reading a first pointer AU-4, which is contained in the section overhead SOH of the SDH frame, row 4, columns 1 through 9, and addresses the path overhead POH of the cell-structured virtual container VC4, and succeeding second means for reading a second pointer H4, which is contained in the path overhead POH, row 6, and indicates the beginning of an ATM cell, i.e., of the cell header, so that the code words can be decoded in the cell header decoder unit CELL (cf. FIG. 1). 
     If no frame sync word is detected but one correct cell header was decoded, the first switch positions of the three switching units S1, S2 and S3 are effective, so that the incoming bit sequence passes through the frame sync word detection unit FRAME and the first switching unit S1 to the cell header decoder unit CELL, then through the second switching unit S2 to the means ALIG for aligning the octets, and then through the third switching unit S3 to the output line AL (cf. FIG. 3). To control the functional blocks FRAME, ALIG, SOH, POH, and CELL, a connection with a control unit C is provided, as indicated in FIG. 3. 
     The method according to the invention for identifying different data structures will now be explained with the aid of the state diagram of FIG. 4. 
     At the beginning, the incoming bit sequence is checked, in a hunt mode HUNT, for the predetermined frame sync word and then for regularly occurring cell headers. 
     If a correct frame sync word or a correct cell header is detected, a change to a frame sync mode or a cell sync mode, respectively, takes place. 
     If one correct cell header is detected, CD1 (no frame sync word detected), a transition to a cell presync state PRE CELL takes place and if at least six successive correct cell headers are detected, CDX (X=6), a transition to a steady cell sync state CELL SYNC takes place. 
     If a frame sync word is detected, FSD1, in a cell presync state PRE CELL, a transition to the frame presync state PRE FRAME takes place. 
     Analogously, if in the frame presync state PRE FRAME the frame sync word is not confirmed, but the predetermined number of correct cell headers are detected, CDX, a transition to the cell presync state PRE CELL takes place. 
     Thus, according to the invention, starting from a hunt mode HUNT, the two possible steady states FRAME SYNC and CELL SYNC are reachable at any time. 
     If in the cell sync state CELL SYNC a number y of successive, wrong cell headers are detected, NPCDy, the system returns to the cell presync state PRE CELL, and if another wrong cell header is detected NPCD1, the system returns to the hunt mode. 
     Analogously, a transition occurs from the frame sync state FRAME SYNC to the frame presync state PRE FRAME if either y successive wrong cell headers are detected, NPCDy, or m successive frame sync words are not detected, NFSDm, with the system returning to the hunt mode if another frame sync word is not detected, NFSD1. 
     The method according to the invention requires no additional information to distinguish the different data structures, and the circuit arrangement according to the invention can reliably identify two functionally basically different data structures with a minimum amount of circuitry.