Excavating
Patent
1997-11-11
1998-12-15
Canney, Vincent P.
Excavating
G06F 1160
Patent
active
058504060
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method for detecting and compensating for missing and/or incorrectly inserted cells on the reception side in the transmission of ATM cells (Asynchronous Transfer Mode cells) with an adaptation layer AAL=ATM Adaptation Layer) of type 1, the cells being assigned a sequence number (with for example a maximum of eight sequence positions) and an error detection and correction code which indicates whether or not the sequence number, and additional information dependent thereon, have been transmitted correctly, and it is decided on the basis of these data whether a cell in rejected or transmitted on and whether there are missing or incorrectly inserted cells.
The asynchronous transfer method (ATM) has been selected by ITU-T an a transport mechanism for B-ISDN broadband communications networks since 1988. Here, the user information in transmitted in cells with a fixed length of 48 bytes, preceded by a head of a length of 5 bytes.
The packaging of the user information into cells in the transmission direction and the extraction of the user information in the reception direction takes place in the ATM adaptation layer (AAL). If the user information comprises a continuous bitstream, for example a PCM voice signal, the adaptation layer of the type 1 is used.
The adaptation layer is divided into two sublayers, namely into a convergence or adaptation sublayer (CS-sublayer) and a segmentation and recomposition sublayer (SAR-sublayer)
On the transmission side, the CS-sublayer collects the user information until it has received 47 bytes. These 47 bytes are transmitted on, together with the sequence number (3 bits) and possibly other information (1 bit) to the SAR sublayer. The SAR sublayer adds an error detection and correction code (4 bits) to the sequence number and to the other information, so that a cell of 48 bytes is produced, which can be transmitted with the aid of the ATM network. Whenever a cell of 47 bytes is transmitted on from the SC sublayer to the SAR sublayer, the sequence number is increased by 1 and may have a value between 0 and 7. The error detection and correction code makes it possible to check on the reception side whether the sequence number and the additional information have been correctly received.
On the receiver side, the SAR sublayer transmits on to the CS sublayer the 47 bytes of user information together with the sequence number (composed of 3 bits), the bit with the additional information dependent on the sequence number, and the state value of the sequence number and the aforesaid bit which indicates whether or not this number and this bit have been correctly transmitted. The SAR sublayer uses the error detection and correction code to determine the state value.
By processing the sequence number of the cells and of the state value, missing or incorrectly inserted cells can be detected. This information can then be further processed in the part of the CS sublayer in which the unequal delay time of the cells in the ATM network is processed.
Such a method for detecting and compensating for missing and/or incorrectly inserted cells is described in an annex of the publication ETS DE/NA-52617 "B-ISDN ATM Adaptation Layer UAAL) specification, type 1" issued by the European Telecommunications Standards Institute (ETSI).
In this known method, two successive cells are analyzed and a decision as to whether a cell is missing or incorrectly inserted is not made until after this analysis has been carried out on the basis of the sequence number of the cell and of the state value. This means that a received cell is initially stored until the following cell is received, and is only afterwards either rejected or further processed. If it is determined that one or more cells have been lost, one or more dummy cells are added for the first of the aforesaid two successive cells, in order to maintain the correct bit number. If the cell in incorrectly inserted, it is destroyed.
The processing thus takes place on a stored cell. This known method is essentially
REFERENCES:
patent: 5163057 (1992-11-01), Grupp
Patent Abstracts of Japan -JP 6085834 -Mar. 25, 1994 vol. 18, No. 342 (E-1570).
Canney Vincent P.
Siemens Aktiengesellschaft
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