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Digital-video-transport-error-handling-in-cell-based-communications-systems

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Details

Inventors: Mao, Weidong;
Assignee: Next Level Communications (Rohnert Park, CA)
Primary Examiner: Moise; Emmanuel L.
Assistant Examiner:
Attorney, Agent or Firm: J. P. Blasko Professional Corp., Eldering; Charles A., Blasko; John P.

In a communications system packets containing compressed digital signals are transported in multiple cells, typically in an Asynchronous Transfer Mode format. Upon reception of these cells, a determination is made as to whether cells have been lost, additional cells inserted, or whether there were bit errors in the transmission of the cells. If so, null packets are substituted for the erred packets and are sent to the video decompression layer.

DETAILED DESCRIPTION The present invention is directed to a method and apparatus for handling errors in a digital video transport system.
Packets which arrive over an Asynchronous Transfer Mode (ATM) cell based network and containing Motion Picture Expert Group (MPEG) packets of video data are inspected to determine if there are errors in the MPEG packets.
If so, null MPEG packets are substituted for the erred packets.
This prevents unwanted freezing and blocking of the video image which can occur if faulty MPEG packets are passed from the transmission system to the MPEG decoder and video display system.
In a preferred embodiment, MPEG packets are placed in a Convergence Sublayer Protocol Data Unit (CS-PDU) and information regarding the length of the MPEG packet or packets is stored in a trailer field in the CS-PDU.
The CS-PDU is mapped into ATM cells.
When two MPEG packets are placed in a CS-PDU, the CS-PDU is mapped into eight ATM cells.
When a single MPEG packet is mapped into a CS-PDU, the CS-PDU is mapped into five ATM cells.
The mapping of a single MPEG packet into a CS-PDU occurs when a MPEG packet contains a Program Clock Reference (PCR) timestamp, and that MPEG packet is the first of two packets.
The PCR is time sensitive and should not be delayed by waiting for an additional MPEG packet.
When the ATM cells arrive, they are inspected to determine if the CS-PDU is contained in five or eight cells.
In a preferred embodiment the end of an ATM Adaptation Layer 5 (AAL5) packet is determined using from the last bit of Payload Type Indicator (PTI) field in the ATM header.
A comparison is made between the payload length based on the number of ATM cells received in the CS-PDU payload and the length as described in the CS-PDU trailer.
In the event that the two measurements do not agree a determination is made that the number of cells received is incorrect, and that there will be errors in the MPEG packets.
Null MPEG packets are substituted for the erred packets.
Error checking is also done on the received CS-PDU using the Cyclical Redundancy Check (CRC) present in the trailer field

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