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Concept

Concept. Use strong CRC in FO packets prevents damage propagation detects errors (wrong decompression) Use keyword bit and large SN in SO packets robust against bit errors robust against long loss events Similar to the concept in bi-directional reliable mode!.

Roberta
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Concept

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  1. Concept • Use strong CRC in FO packets • prevents damage propagation • detects errors (wrong decompression) • Use keyword bit and large SN in SO packets • robust against bit errors • robust against long loss events Similar to the concept in bi-directional reliable mode!

  2. Short Introduction to Keyword Approach • Update and Non-Update packets • Update packets change the context of the decompressor • Non-Update packets do not update the context • Mapping to FO and SO packets • Update packets are FO packets with NKW-flag=1 • Non-Update packets are SO packets or FO packets with NKW-flag=0 • Strategy of sending the packets • send a sequence of n update packets to transmit an irregularity in the packet stream • send non-update packets • after running out of the SN-window (e.g. after 60 SO packets with 6 bit SN): • Option 1: Switch to FO state and update the context • Option 2: Stay in SO state

  3. Short Introduction to Keyword Approach • Option 1: • after 60 packets the context is updated by a sequence of FO packets • robust against residual bit errors (no implicit context update with SO packets) • additional FO packets are sent, which means additional overhead and a increase of the context loss probability • Option 2: • compressor stays in SO state • robust against packet losses (accepts up to 60 consecutive packet losses without loosing the context) • update of the sliding window with every packet, which means susceptible against residual bit errors

  4. Short Introduction to Keyword Approach • Example use for a talk spurt Option 1 Start of a talkspurt n Packets FO FO SO FO FO FO SO SO SO n Packets 60 Packets Option 2

  5. Two Test Cases • Evaluation of CRC and Keyword in different scenarios 1. Long loss events 2. Residual bit errors

  6. 1. Long Loss Events • Answer from 3GPP: • up to 200ms loss due to hard handover • longer losses without loosing the connection are possible due to other events (e.g. “elevator event”) • 200 ms of packet loss means for a speech service a loss of 10 consecutive packets • additional loss of e.g. 5 packets on this link or any other link before the compressor is not a rare event (especially in bad channel conditions, shortly before or after a handover) • loss of 15 or more consecutive packets is not a rare event, in fact in mobile environments handover occurs frequently

  7. 1. Long Loss Events (2) • Keyword SO packets (6 bit SN): • up to 60 packets may be lost, before the context has to be invalidated • CRC SO packets (4 bit SN): • a loss of more than 12 consecutive packets leads to context invalidation • What does context invalidation mean • Loss of all correctly received packets until the context is refreshed • Context refresh time in bi-directional optimistic mode: • one round trip time • Context refresh time in uni-directional mode: • time until next spontaneous refresh, might be seconds!

  8. 1. Long Loss Events - Conclusions • CRC SO packets lead to context invalidation for reasonable loss events • In uni-directional mode this might lead to seconds of additional packet loss

  9. 2. Residual Bit Errors • Answer from 3GPP: • range of residual bit error rates: 10-6 .. 10-2 • Undetected bit errors in the compressed header are possible and could occur frequently

  10. 2. Residual Bit Errors (2) • Undetected bit errors in CRC SO packets can lead to context invalidation! • Example: • Compressor sends compressed SN: 1 - 2 - 3 - ... • Due to a bit error in the compressed SN the decompressor receives: 1 - 10 - 3 - ... • The probability that this error is not detected by the 3-bit CRC over the 320 bit decompressed header is about 1/8 • In the case that the error is not detected, the decompressor updates the context to SN=10 • the next packet would be decompressed with SN=3+modulo, CRC will detect the wrong decompression • context has to be invalidated, all following packets have to be discarded

  11. 2. Residual Bit Errors (3) • Keyword Approach • bit error in SN: • packet is decompressed wrong • next packet will be correct again • bit error in Keyword-bit • the first packet with a wrong keyword is assumed to have a bit error and is therefore discarded • if the next packet also has a wrong keyword, a packet loss is assumed

  12. 2. Residual Bit Errors - Conclusion • Keyword SO packets are robust against bit errors • wrong decompressed packets might be given away (as it would be without header compression) • no damage propagation • CRC SO packets might invalidate the context due to residual bit errors • loss propagation due to a bit error • especially in uni-directional mode this is not acceptable

  13. Summary / Conclusion • 4 bit SN and 3 bit CRC: • CRC to weak in case of residual bit errors • SN to short in case of consecutive packet loss • Both leads to context invalidation! 0 1 2 3 4 5 6 7 PTI SN CRC

  14. Proposal for SO packet format • 2 byte SO header with strong CRC and long SN: 0 1 2 3 4 5 6 7 PTI SN CRC • 1 byte SO header with Keyword 0 1 2 3 4 5 6 7 PTI KW SN • or both?

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