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FAQ-Why the actual downstream bitrate seems to be decreasing while the net downstream bitrate is constant

Publication Date:  2012-07-24 Views:  31 Downloads:  0
Issue Description
The customer has collected some VDSL2 performance information(see attachments). The actual downstream bitrate seems to be decreasing little by little (~100 kbps) each day, concluded from the last 5 days,while the net downstream bitrate is constant.The customer wants to know if there is some problem in the sytem that caused this case happen.
Alarm Information
Handling Process
After confirming the phenomenon with R&D we provide the follwoing explaination:
1.What the user saw is line rate ,the actual rate of bearer service is channel rate ,which didn't change.
2.Line rate is calculated out by the amount of bit bearer multiply 4khz ,including  rate after trellis code cost. After bitswap,amount of bit bearer on every tone will change,and result costing of trellis change, finally result amount of bit change.This is an explanation from BRCM:
If tones are being dropped during bitswap, the trellis overhead goes down, so the line rate reduces, but the bearer rate should stay constant.
In 992.1 standard, we can find with trellis coding, SUM(bi)=8*(K+R/S)+roundup(ncloaded/2)+4(G.993.2 Chapter 10.3.1), or SUM(bi)=8*(K+R/S) without trellis coding
Root Cause