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7.8

Poor performance on our cable modem testbed

Asked by sburriss23 in DSL Lines / Cable Internet

Tags: cmts

Hi all.

Our cable network testbed at work has been having some performance issues. It is set up with a CMTS, and about 6 cable modems (only about three of which are being used at the moment). The Upstream speeds we regularly achieve are about 230-240 Kbit/s and Downstream speeds are around 2.7-2.8 Mbit/s. Im sure this is well below what we should be getting.

On the CMTS, downstream frequency is set to 550MHz and upstream 38MHz. Here are a couple of commands i have run to try and get some more info...

CMTS#show controllers cable 1/0 downstream
freq 550MHz, channel width 6MHz, 64-QAM, symbol rate 5.056941 Msps
fec ITU-T J.83 Annex B, R/S interleave I=32, J=4

CMTS#show controllers cable 1/0 upstream
freq 38MHz, channel width 0.200MHz, QPSK symbol rate 0.160 Msps
SNR = 26.2dB

On one of the cable modems ive had a look at what frequency has been locked for both DS and US, the DS is equal to that at the CMTS exactly, but the US is slightly out. Here is the output...

Rx: tuner_freq 550000000, symbol_rate 5057000, local_freq 11520000
    snr_estimate 366(TenthdB), ber_estimate 0, lock_threshold 23000
    QAM in lock, FEC in lock, qam_mode QAM_64   (Annex B)
Tx: tx_freq 38001276, symbol rate 1 (160000 sym/sec)
     power_level: 12.50 dBmV (current)
                  13  (gain in US AMP units)
                  4   (BCM3300 attenuation in .4 dB units)


Im thinking that the US symbol rate and bandwidth is too low, but dont know enough about it to work out whats wrong. Is there anything i can do or check that may give me answers as to why we are getting such slow rates.

EDIT: I should also mention that previously we had been having trouble with cable modems locking onto a signal, when trying to connect three and using some splitters. We adjusted the attentuators (lowered the dB level) connected on the CMTS and this seems to have solved that problem.

Thanks for any help.

Shaun.
[+][-]02/04/06 08:11 PM, ID: 15874945Accepted Solution

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About this solution

Zone: DSL Lines / Cable Internet
Tags: cmts
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Solution Provided By: Mrclean0325
Participating Experts: 1
Solution Grade: A
 
[+][-]02/04/06 10:11 AM, ID: 15873037Expert Comment

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