Fluctuating Transmit Attenuation

Hi,

My router is encountering fluctuating transmit/upstream attenuation. It is connecting using ADSL2+ (BisPlus). Does anyone know what could cause this, and what I can do to try fix it? BT have tested the line and claim it is fine. I just find it strange that the Tx attenuation is the one fluctuating, yet the Rx remains identical. Any ideas as to what could this indicate?

First time it connects:

Downstream Speed: 6859 kbps
Upstream Speed: 937 kbps
Receive SNR: 5.0
Rx Attenuation: 44 db
Tx Attenuation: 26 db
Power: 12.5

Second time it connects:

Downstream Speed: 140 kbps
Upstream Speed: 777 kbps
Receive SNR: 7.0
Rx Attenuation: 44 db
Tx Attenuation: 88 db
Power: 12.5

Third time it connects:

Downstream Speed: 175 kbps
Upstream Speed: 695 kbps
Receive SNR: 8.5
Rx Attenuation: 44 db
Tx Attenuation: 86 db
Power: 12.5

The only significant difference I can see is the Tx Attenuation. The SNR goes up slightly, but nothing much. Rx Attenuation remains the same. BT claim the line is fine.

Cheers,

Mike

p.s. If anyone is interested, here is the full line stats for the ADSL device:

PortClassATM = true
Version = 1.01
LineRate = 403
TxCellTransmitted = 0
RxCellReceived = 0
cbr_CPS = 0
rvbrSCR_CPS = 0
vbrSCR_CPS = 0
rvbrPCR_CPS = 0
vbrPCR_CPS = 0
ubr_CPS = 8000
ubrMCR_CPS = 0
CACMode = Simple
CACFunction = 0x20029448
PortSpeedHook = 0x00000000
vpiRange = 8
vciRange = 16
defaultPcr = 0
trafficShaping = false
NiType = uni
isDslDmaUp = 1
enabledChannels = 5
SnmpIfIndex = 1
SnmpIfType = 37
SnmpVcListHead = 0x2071e89c
DriverVersion = 1.75
APIVersion = GS_API_626
FirmwareVersion = E.37.9.54
DspVersion = 0x00000090
Connected = true
OperationalMode = BisPlus
State = Showtime
Watchdog = 0x00000053
OperationProgress = 0x00000000
LastFailed = 0x00000063
TxBitRate = 712000
RxBitRate = 171000
RxATTNDR = 180000
TxATTNDR = 712000
TxCellRate = 1679
RxCellRate = 403
PhyTXCellCount = 84782
PhyRXCellCount = 917395
PhyCellDropCount = 1
OverallFailure = 0
InterleaveDpDn = 1
InterleaveDpUp = 1
RSCorrectedErrorsDn = 0
RSUnCorrectedErrorsDn = 10
RSCorrectedErrorsUp = 0
RSUnCorrectedErrorsUp = 0
SuperFramesDn = 158000
SuperFramesUp = 158000
InterleaveRDn = 0
InterleaveRUp = 0
InterleaveSDn = 0
InterleaveSUp = 0
FastRDn = 0
FastRUp = 0
BisMDn = 1
BisMUp = 1
BisBDn = 4
BisBUp = 21
BisTDn = 4
BisTUp = 4
BisLDn = 45
BisLUp = 180
BisRDn = 0
BisRUp = 0
BisSDn = 0.88
BisSUp = 0.97
BisDelayDn = 0.25
BisDelayUp = 0.25
ShowtimeStart = 2686
ATURANSIRev = 7
ATURANSISTD = 2
ATUCANSIRev = 3195076608
ATUCANSIId = IFX
ATUCANSISTD = 2
DataBoost = Disable
LocalITUCountryCode = 46591
LocalSEF = 0
LocalEndLOS = 0
LocalSNRMargin = 8.5 dB
LocalLineAttn = 44.0 dB
INPup = 0.0
INPdown = 0.0
PMstatus = L0
RawAttn = 0.0 dB
LocalTxPower = 12.5 dB
LocalFastChannelRxRate = 171000
LocalFastChannelTxRate = 711800
LocalFastChannelFEC = 0
LocalFastChannelCRC = 0
LocalFastChannelHEC = 0
LocalFastChannelNCD = 0
LocalFastChannelOCD = 0
LocalFastChannelLCD = 0
LocalInterleavedChannelRxRate = 0
LocalInterleavedChannelTxRate = 0
LocalInterleavedChannelFEC = 0
LocalInterleavedChannelCRC = 0
LocalInterleavedChannelHEC = 0
LocalInterleavedChannelNCD = 0
LocalInterleavedChannelOCD = 0
LocalInterleavedChannelLCD = 0
RemoteTxPower = 14.2 dB
RemoteSEF = 0
RemoteLOS = 0
RemoteLineAttn = 86.0 dB
RemoteSNRMargin = 8.5 dB
RemoteFastChannelFEC = 0
RemoteFastChannelCRC = 0
RemoteFastChannelHEC = 0
RemoteFastChannelNCD = 0
RemoteFastChannelLCD = 0
RemoteInterleavedChannelFEC = 0
RemoteInterleavedChannelCRC = 0
RemoteInterleavedChannelHEC = 0
RemoteInterleavedChannelNCD = 0
RemoteInterleavedChannelLCD = 0
LocalMgmtFEC0 = 0
LocalMgmtCRC0 = 10
LocalMgmtFECErroredSec = 0
LocalMgmtErroredSec = 4
LocalMgmtSeverelyErroredSec = 0
LocalMgmtLOSErroredSec = 0
LocalMgmtUnavailErroredSec = 0
LocalMgmtHEC0 = 2
RemoteMgmtFEC0 = 0
RemoteMgmtCRC0 = 0
RemoteMgmtFECErroredSec = 0
RemoteMgmtErroredSec = 0
RemoteMgmtSeverelyErroredSec = 0
RemoteMgmtLOSErroredSec = 0
RemoteMgmtUnavailErroredSec = 0
RemoteMgmtHEC0 = 0
LocalTPSTCCell0 = 1082815
LocalTPSTCCell0UpperLayer = 917123
LocalTPSTCBitError = 0
RemoteTPSTCCell0 = 121351
RemoteTPSTCCell0UpperLayer = 3302
RemoteTPSTCBitError = 0
PSDMaskStatus = Standard
StatusFailCount = 0
LineUpCount = 3
ProfileStatus = MAIN
Action = Startup
ActivateLine = None
LineStatus = true
HostControl = Enable
AutoStart = true
failsafe = false
ShowtimeLed = 2
Retrain = EnableOverallOnly
Defaults = AnnexM2PlusAuto
LineMode = AnnexM2PlusAuto
Whip = Disable
WhipActive = Inactive
PMmode = L2L3NotAllowed
DyingGasp = Disable
UtopiaInterface = Level2
PhysicalPort = 0
ClockType = Oscillator
Profile = MAIN
DetectNoise = Disable
MaxInterleaveD = 64
SHalf = Disable
CabinetMode = Disable
PSDMask = AnnexM-ALL
SRA = Disable
MaxSNRMargin = Disable
EcFdmMode = EC
MaxBitsPerBin = 15
TxStartBin = 6
TxEndBin = 31
RxStartBin = 6
RxEndBin = 511
RxAutoBinAdjust = Disable
TxAttenuation = Bis_0DB
BitSwap = Enable
BitSwapUp = Enable
MaxDownRate = 511
Capability = MA-ALL
CodingGain = auto
FramerType = Type3
FastRetrain = Enable
resetDefaults = false
portSnmpIfIndex = 1
portSnmpIfType = 37

What modem is it?

I would suggest trying a different modem to rule out an issue with the device itself.

Generally upstream attenuation is half of what the downstream attenuation is. And if it was a dodgy wiring connection I'd expect both to vary.

Does it ever return to the 26dB figure? Is it random as to what it does?

Three possibilities initially come to mind.

1. Output stage of the modem (within the router) is failing and whereas it believes it is transmitting at a specific level but actually is is lower.

2. The received power measurement at the exchange is incorrect due to a card fault.

3. The filter is faulty in such a way that the low frequency of the upstream signal is seeing a low impedance at the filter (rather than the normal high impedance) and the signal is being attenuated because of that.


Try a different router/modem, try plugging into the test socket (with no other connections) and try changing filters.

It's a Conexant Viking PCI card - http://www.rocksolidelectronics.com/pages/products/v... - running on a linux fedora box.

I'll try another modem first, and if that fails, get some new filters.

Shouldn't you be worrying more about the abysmal Downstream Speed even with an apparent constant Attenuation?

Also I would guess that your router may be reporting reversed Up & Down Attenuations, as some routers tend to do. That would make 2 out of 3 readings look realistic, as well as account for the low Downstream Speed.

Or you may be misinterpreting "Tx" & "Rx"!

For reference - yes it was a bad modem. Thanks all for the advice.