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Author Topic: Upstream CRC counter in stats  (Read 484 times)

Weaver

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Upstream CRC counter in stats
« on: September 22, 2021, 08:22:15 PM »

[This may be a stupid question: Iím tired and my brain isnít working.]

In my modem stats an upstream CRC count is reported. I just had a thought: downstream CRC counting is straightforward, you just evaluate the CRC value across the received PDU content and then compare that computed value with the CRC field that is appended to the received PDU. But how can a modem know about upstream data corruption? It is the remote end that sees the corrupt upstream PDUs arriving. Is there some mechanism then where the one modem can ask the far end what its error count is?
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burakkucat

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Re: Upstream CRC counter in stats
« Reply #1 on: September 22, 2021, 08:54:25 PM »

Here follows a snippet of the statistics from my CPE --

Code: [Select]
> xdslctl info --state
xdslctl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:    0
Last initialization procedure status:   0
Max:    Upstream rate = 15843 Kbps, Downstream rate = 44601 Kbps
Bearer: 0, Upstream rate = 9997 Kbps, Downstream rate = 40000 Kbps
Bearer: 1, Upstream rate = 0 Kbps, Downstream rate = 0 Kbps

The "Bearer: 0" line shows the current synchronisation speed, xTU-C <-> xTU-R, of the channel over which the end-user's data is carried.

The "Bearer: 1" line shows the current speed of the channel used by the xTU-C and the xTU-R to exchange information.

I seem to recall ejs mentioning that the latter channel is present for all xDSL modems but is not necessarily reported by all such devices. (Hopefully I have remembered that correctly . . . otherwise I expect to be corrected.  ;)  )
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Chrysalis

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Re: Upstream CRC counter in stats
« Reply #2 on: September 22, 2021, 09:42:18 PM »

[This may be a stupid question: Iím tired and my brain isnít working.]

In my modem stats an upstream CRC count is reported. I just had a thought: downstream CRC counting is straightforward, you just evaluate the CRC value across the received PDU content and then compare that computed value with the CRC field that is appended to the received PDU. But how can a modem know about upstream data corruption? It is the remote end that sees the corrupt upstream PDUs arriving. Is there some mechanism then where the one modem can ask the far end what its error count is?

The dslam and yes.
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kitz

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Re: Upstream CRC counter in stats
« Reply #3 on: September 26, 2021, 08:05:41 PM »

As chrys says - its the DSLAM.

The modem and router "talk" to each other and shares information with the other end to be able to provide the full set of stats.
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