Method and devices for testing ATM connections

Abstract: The information part of the test cells has twice the same contents, including time indications, sequence number within a connection and connection number. The repetition of the contents occurs octet per octet in inverse form.

Description: The present invention relates to a method for testing the characteristics of an ATM connection between a first and a second point of an ATM network, to a test cell generator with devices for generating the data sequence of predetermined ATM test cells at said first point and to an evaluation circuit for testing said predetermined ATM test cells at said second point. In ATM systems errors of various kinds may occur and must all be detected in whatever way. For instance, an error is certainly present when no bit or cell synchronisation can be performed. Also the absence of response to whatever questions should be recognized as an error. It is also known to include in the information part of an ATM cell an error protection code to detect transmission errors. Not all kinds of errors can be detected. This is the case with errors which occur only spuriously, although such errors adversely affect the transmission quality of an ATM connection between two points of an ATM network. A solution to this problem is given by a method according to claim 1. This method is preferably performed by means of a test cell generator according to claim 4 and of an evaluation circuit according to claim 10. The basic idea of this solution is the realization of test connections with a sequence of test cells suitably built up. Preferred embodiments are apparent from the subclaims. A suitably built up test cell , just as every ATM cell, comprises a cell header and an information part, the information part according to the invention having twice the same contents, amongst which a time indication, a connection number and a sequence number of the cell in a connection. The repetition of the contents happens in inverse form. Preferably each octet should be followed by its inverse. In this way substantially all characteristics of an ATM connection can be tested. This method is also advantageous in that in the information part the numbers of zeroes and ones are equal so that the parity is not affected. Also the method can be executed by simple means. It is to be noted that the European Patent Application EP-A2-0 402 741 discloses a method and a device for detecting and correcting errors in the header of any incoming ATM cell. However, this known method is not adapted to test all the above characteristics of an ATM connection. Moreover, the known method does not disclose nor teach the use of specific test cells having a particular layout by which the information part of the test cell is adapted to test these characteristics. It further do also not disclose a test cell generator nor an evaluation circuit adapted to handle such particular test cells. The above mentioned and other objects and features of the invention will become more apparent and the invention itself will be best understood by referring to the following description of an embodiment taken in conjunction with the accompanying drawings wherein : Fig. 1 shows the structure of an ATM test cell according to the invention; Fig. 2 represents a block diagram of a test cell generator according to the invention; Fig. 3 shows a block diagram of an evaluation circuit according to the invention. In a schematic way the first line of Fig. 1 shows the structure of a complete ATM test cell. The cell begins with a header H comprising five octets or bytes. This cell header H is used in a known way to transfer the cell to a predetermined destination within an ATM network. The structure of this cell header is not characteristic for the invention and the cell header may be changed on purpose within the course of a connection. It can therefore not be included in the test. For the present invention it is also without importance which kind of connection to be tested is concerned, i.e. if a connection between two points within an exchange is concerned or if a connection between two distant points is tested or if a loop which is built up and tested is concerned. The header H for instance contains a pair of bits serving to distinguish test and maintenance cells from data cells within a connection, whereas the first octet MCT of the information part following the cell header H is used to distinguish test cells from maintenance cells. For MCT a predetermined code has to be reserved. The following octet, MCT , represents the inverse of the preceeding octet MCT. This inversion is performed in such a way that every bit is inverted. MCT and MCT are followed by 18 octets which alternately contain either only ones or zeroes. Each pair of such octets constitutes a flag FL. These 18 octets have three purposes. Errors which maintain a connection continuously on zero or on one should be recognized. Two types of ATM test cells should be distinguished from each other, namely a TYPE-1 cell the data of which are transmitted in a non-inverted way and is followed by their inverse form and a TYPE-0 cell the data of which are inverted, the following inverted octet then containing the non inverted data. Each pair of octets is finally also used to detect bit errors. Measurement data MD follow these 18 octets. Three different measurement data are transmitted, namely a time indication TS, a sequence number SN of the cell in the connection, and a connection number ID. These three measurement data are each time transmitted as 16-bit words. For a TYPE-1 cell T1C the transfer is performed in the order T1, T1 , T2, T2 , etc. as represented in Fig. 1. The time indication TS is used to take the variation in transfer time (delay time) into account. The sequence number SN is used to detect the disappearance or repetition of a cell. A faulty connection number ID indicates that a cell of another connection has erroneously appeared in this connection and that the cell header thus has been modified erroneously. The rest of the test cell is completed with octets which again alternately contain only ones or only zeroes. By inverting octet per octet inside the whole information part, no part of the cell is left out of consideration. Inside a cell errors could be produced resulting from a parallel processing with a 16-bit width. Therefore in a test connection the two kinds of test cells TYPE-1 and TYPE-0 are used alternately and in predetermined or arbitrary order. Fig. 2 shows an embodiment of a test cell generator by which the above described ATM cells may be generated. The test cell generator TG includes a control circuit TCGC, a read and write memory MEM, a connection number generator ID, a clock LT, a three-state bus TSB having a width of 8 bits, an inverter INV and an output register REG. The clock circuit LT, the connection number generator ID and the memory MEM are provided with three-state outputs which are able to be activated via activation inputs OE1, OE2, OE3 with the help of the like named activation signals OE1, OE2, OE3. The leads of the three-state bus TSB are connected to the supply voltage VCC via resistances R. The clock circuit LT and the connection number generator ID are provided with an inverting input SeUL via which and by means of a like named inversion signal SeUL an inversion from high value to low value data bytes and vice-versa may be realized. The clock circuit LT is controlled by the cell clock circuit CCL, whilst the output register REG is controlled by the byte clock BCL. The inverter INV may be switched between the non inverting and the inverting condition by means of a signal INVC. The memory MEM may transmit data via the bus TSB as well as to the control circuit TCGC or receive data from this circuit. To this end an 8-bit wide bidirectional data connection MDA is provided between both circuits and between the control circuit TCGC and the memory MEM also a three bit wide address connection, a write lead MW and a read lead MR are foreseen. Next to the connections to the memory already mentioned the control circuit TCGC is also provided with outputs OE1, OE2, OE3 for the activating signals OE1, OE2 and OE3 and for the activation of the three-state outputs, with an output SeUL for the like named inversion signal SeUL, with outputs CCL and BCL for the cell clock CCL and the byte clock BCL, and with the output INVC for the signal INVC controlling the inverter INV. The octets of the cell header H are generated by the memory MEM and fed through the inverter INV without inversion. The two following octets, MCT and MCT , are generated by the memory MEM two times after each other in the non-inverse form and are fed through the inverter once without inversion and once with inversion. The flags FL are formed with the help of the resistances R when the three-state outputs are not activated, and then by alternate non-inversion and inversion in the inverter INV. The time indication is provided by the clock circuit LT, the sequence number is generated by the memory MEM and the connection number is provided by the connection number generator ID. Hereby each time the high value data byte is generated twice and the low value data byte is generated twice, and fed through the inverter once without inversion and once with inversion. The sequence number stored in the memory MEM is registered in the control circuit TCGC after each complete transmission, it is incremented therein by one and then reloaded in memory MEM. The subsequent flags are obtained in the same way as the first flags. The control circuit TCGC essentially operates in the same way as a known programming circuit, requiring a counter and a programme memory. To these circuits an adder circuit for incrementing the sequence number as well as known devices for providing the timing have to be added. Fig. 3 shows a possible embodiment of an evaluation circuit EC for testing ATM test cells. This evaluation circuit EC includes a circuit ERRC for fault recognition, a circuit HIDC for e.g. cell header recognition, a circuit TYPD for cell type recognition, a clock LCTI, a circuit SEQE for determining the sequence number, and a circuit DELC for determining a relative or absolute transfer time. The fault recognition circuit ERRC investigates if octets occur in associated pairs which are mutually inverted. To this end a memory, an inverter and a comparison circuit are required for each of the eight bits. The errors occurring within an ATM test cell are counted, the counter position of this counter constituting the signal ERR PAT. Further signals are provided: ERR SEQ when an error occurs in the connection number; ERR TIME when an error occurs in the time indication, and ERR ID when an error occurs in the sequence number. The signals ERR PAT and ERR ID are also transmitted to the circuit for cell header recognition HIDC. The so called cell header recognition circuit HIDC compares the five bytes of the cell header of an incoming cell with the five bytes of a reference cell, the header bytes each including a pair of above mentioned bits, hereafter called PT, which are activated when test or maintenance cells are concerned and which are not activated in the case of data cells. Hereby the output CELL REC is activated when : PT is activated and when not only the five bytes of the header unit also but also the MCT and MCT bytes are equal; or when PT is not activated and the five bytes of the header are equal. When now CELL REC is activated then the sequence number of the incoming cell is compared with that of the reference cell and the output CELL INS is thereby activated when : these connection numbers are different and there are no errors in the connection number of the incoming cell; the last mentioned connection number is erroneous and the signal ERR PAT moreover exceeds a predetermined threshold. The circuit for cell type recognition TYPD distinguishes between cells of type TYPE-1 and cells of TYPE-0 and produces a signal TYPE indicating the cell type. The circuit SEQE for determining the sequence number extracts the sequence number as a 16-bit signal SEQ NR in function of the signal TYPE. The circuit DELC for determining the relative or absolute transfer time DERLC extracts the time indication TS from the cell in function of the signal TYPE, subtracts it from the local time LTI of the clock LCTI and produces at its output the difference as a 16-bit signal DL. When the clock circuit LT in the test cell generator T6 and the clock LCTI in the evaluation circuit are not running in synchronism the absolute transfer time cannot be obtained. However, for a sequence of test calls the relative transfer time differences may be obtained in this way and these transfer time differences are in general of greater importance than the absolute values of the transfer times. By the above invention all means are provided for testing the characteristics of an ATM connection. While the principles of the invention have been described above in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

Claims: Method for testing the characteristics of an ATM connection between a first and a second point of an ATM network, characterized in that said method includes the step of transmitting a sequence of predetermined test cells at said first point, and the step of receiving and testing said test cells at said second point, each of said test cells including a header and an information part, said information part comprises two portions carrying the same contents, the second portion being the inverse of the first one. Method according to claim 1, characterized in that both portions are interwoven in such a way that each octet of the first portion is immediately followed by the associated inversed octet of the second portion. Method according to claim 1, characterized in that each information part includes at least a time indication of the transmission time (TS), a sequence number (SN) in the ATM connection and a connection number (ID), these information being in inverse form or not and that the rest of the information including none of these data contains an indication (FL) that said data are in inverse form or not. Test cell generator with devices for generating the data sequence of a predetermined ATM test cell at a first point of an ATM network and according to the method of claim 1, characterized in that the devices are executed in such a way that the data sequence may selectively, and at least piecewise, be generated in not-inverse form or in inverse form. Test cell generator according to claim 4, characterized in that the data sequence is realized under the form of octets and is possibly inverted per octet. Test cell generator according to claim 5, characterized in that the devices include : a clock circuit (LT) controlled by the cell clock (CCL), a connection number generator (ID), a write and read memory (MEM), a control circuit (TCGC), a switcheable inverter (INV) and an output register (REG). Test cell generator according to claim 6, characterized in that the clock circuit (LT), the connection number generator (ID) and the write and read memory (MEM) are all coupled with the output register (REG) via an inverter (INV) and that all these units are controlled by the control circuit (TCGC). Test cell generator according to claim 7, characterized in that it produces test cells the information part of which at least includes: a time indication (TS) of the sending time, a sequence number (SN) in an ATM connection and a connection number (ID) provided by the clock circuit (LT), the memory (MEM) and the connection number generator (ID) respectively. Test cell generator according to claim 7, characterized in that said memory (MEM) also generates the headers of the test cells. Evaluation circuit for testing predetermined ATM test cells at a second point of an ATM network and according to the method of claim 1, characterized in that said evaluation circuit includes a cell header recognition circuit (HIDC) which by comparing an incoming cell with a reference cell is able to find out if said incoming cell is destined to the evaluation circuit (CELC REC) or not. Evaluation circuit according to claim 10, characterized in that the data of said test cell are inverted octet per octet and that there is provided a circuit (TYPD) to find out if the data received are first non inverted and then inverted or vice-versa. Evaluation circuit according to claim 10, characterized in that said test cell includes a time indication (TS) of the sending time and that there is provided a circuit (DELC) to extract said time indication (TS) from said test cell and to calculate the difference thereof with a local time indication (LCTI). Evaluation circuit according to claim 10, characterized in that the incoming cells at said second point are provided with a header (H) with a first cell indication (PT) which is activated when a said test cell or a maintenance cell and not a normal data cell is concerned, and that these cells moreover contain an information part including a second cell indication (MCT, MCT ) which is activated when a said test cell is concerned, a time indication (TS) of the sending time of said test cell, a sequence number (SN) in the ATM connection and a connection number (ID). Evaluation circuit according to claim 13, characterized in that said cell header recognition circuit (HIDC) compares the header of an incoming cell with that of said reference cell and that, when these headers are equal and the first cell indication (PT) is activated, it moreover compares the second cell indications (MCT, MCT ) and in case they are equal provides a first output signal (CELL REC) which indicates that the incoming cell is destined to the evaluation circuit. Evaluation circuit according to claim 14, characterized in that said first output signal (CELL REC) is also activated when the compared headers are equal, but when the first cell indications (PT) are not activated. Evaluation circuit according to claim 14 or 15, characterized in that the cell header recognition circuit when the first output signal (CELL REC) is activated also compares the connection number (ID) of the incoming cell with that of the reference cell and generates an activated second output signal (CELL INS) when these connection numbers are different and there are no errors in the connection number of the incoming cell, said activated second output signal (CELL INS) indicating an error. Evaluation circuit according to claim 10, characterized in that it includes an error recognition circuit (ERRC) to detect and correct bit errors, the detected errors being counted and the result of this count being indicated by a third output signal (ERR PAT). Evaluation circuit according to claims 13 and 17, characterized in that said error recognition circuit (ERRC) also checks the bit errors in an incoming connection number and, upon detection of an error, activates a fourth output signal (ERR ID). Evaluation circuit according to claims 16 and 18, characterized in that the second output signal (CELL INS) is activated when the first output signal (CELL REC) and the fourth output signal (ERR ID) are activated and when the third output signal (ERR PAT) exceeds a predetermined threshold.

Inventors: DENISSEN FRANK LODEWIJK, VAN TETERING JOHANNES ANTHONIU, DENISSEN, FRANK LODEWIJK, VAN TETERING, JOHANNES ANTHONIUS MARIA
Applicants: ALSTHOM CGE ALCATEL, ALCATEL
Application year: 1992
Main classification: H04L1
IPC codes: H04L1