Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Patent
1998-05-11
2000-06-20
Baker, Stephen M.
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
714776, H03M 1300
Patent
active
060790423
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
A major concern in any communication network is the control of transmission errors. Redundancy is the basis of error control. Automatic Repeat Request (ARQ) schemes are often preferred to Forward Error Correction (FEC) schemes because they are easy to implement and show superior performance at low bit-error rates. The throughput rate of an ARQ scheme strongly depends on the number of requested retransmissions and this falls rapidly with increasing bit error rate (BER). For applications involving real-time video and voice over very high speed networks, ARQ-based schemes impose an unacceptably long transmission delay. The problem is more acute if the data paths are fairly long, as in wide area networks (WANs). On the other hand, since FEC has no retransmissions, the throughput rate of FEC schemes is more or less constant, set by the code rate and to a large extent, independent of the channel error probability. Recovering lost data reduces the need for data retransmissions and enhances the quality of real-time application (video and voice) that cannot rely on acknowledgments (ACKs) and retransmissions because of the large delays involved.
High speed networks, such as Asynchronous Transfer Mode (ATM) and other WANs, are based on the fiber-optic links with very low bit error rates, typically from 10.sup.-10 to 10.sup.-14, over long distance fiber cable. The main cause of data loss in these networks is buffer overflow during congestion, rather than bit errors. In ATM networks, low priority cells are dropped first in the event of congestion. Since a lost cell is just a block of bits in error, essentially some form of FEC is needed.
At present, apart from the techniques of error concealment, there is only the recommendation of the International Telecommunications Union (ITU-T) for error recovery in ATM transmission. ITU-T recommendation I.363 `B-ISDN ATM Adaptation Layer Specification` Rev 1 Geneva 1993. This technique uses either a (128,124) Reed-Solomon (R-S) Forward Error Correction (FEC) method with interleaving or a (94,88) R-S code with interleaving. The first method recovers 4 lost cells in a group of 128 with a 3.1% overhead, while the second can recover 1 lost cell in a group of 16 with a 6.8% overhead. These techniques are specifically for end-end recovery.
SUMMARY OF THE INVENTION
Most high speed networks use or will be using the Asynchronous Transfer Mode (ATM) technique. In this technique, all digital data are segmented into groups of 44 bytes. Each of these segments has 2 bytes each, appended before and after, resulting in a 48 octet payload. The data link protocol adds an additional 5 byte header containing the routing information to form the final ATM cell. While the links are relatively error free, there is always potential for cells to be dropped by the network due to congestion. This may occur at switches or elsewhere.
In the present invention, a method is provided for recovering lost cells whereby the ATM adaptation layer is capable of selectively implementing an error recovery scheme if required. In this method, the ATM layer may either insert or delete the recovery cells, depending upon the data being transmitted. For example, if voice data is being transmitted, one may choose not to insert the cells since occasional lost cells do not cause significant loss of quality. However, for more sensitive data such as video data, the ATM layer can insert the recovery cells. The present invention also allows for the recovery of lost cells along the network itself without having to wait until the cells reach their destination.
The implementation of the cell loss recovery method of the present invention is applicable to both random cell losses and burst cell losses which occur in any telecommunication infrastructure including, but not limited to, all packet networks, ATM networks, wireless networks and ISDN and other conventional networks. The cells are recovered if the cells are lost in the network due to congestion or resource failures. The cells are also recovered when
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Chakravarthy Cadathur V.
Hong Kicheon
Vaman Dhadesugoor R.
Baker Stephen M.
The Trustees of the Stevens Institute of Technology
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