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billhamel .net
Hinsdale, IL
·Verizon Broadban..
·AT&T U-Verse

1 edit

2 recommendations

reply to guidotti

Re: [Speed] Problem with Downstream on MACH20 in Chicago

I consider myself to possess "above average" intelligence when it comes to cabling, cabling issues, modems, connectors, taps and splitters, signal levels, etc etc...

1.) Are you running your speed test over a wireless connection? If so, the fact that you receive 5.5 mbits/sec is indicative of someone trying to get their "full connection speed" in spite of the fact that the wireless connection is the sole reason they receive such levels.

2.)If you switched to a fully wired connection, or already using a wired connection, then I feel your pain...why?
WELLL...I was reanting the SB5100 from RCN until recently when I decided to use my own cable modem, the SB5120, just like you use.

Many problems exist as everyday occurances and cannot necessarily be "corrected". These things include: Ingress (noise inserted into a cable "system" from the outside), temperature, another user's cabling, etc etc...
BUT one thing is for sure, the RCN installer more likely than not made sure the connectors and cabling were up to par prior to installation, checked signal levels, made sure you were online etc...
Back to why i'm posting....I am having not a full "speed" isse, but more of a "intermittent" issue. For instance, when online gaming, I continually get dropped from games, online in general, and experience a TON of lag. When did this happen?
I installed the SB5120 when it was at least still 45 degrees at night, made sure all connectors were good, all cabling was RG-6, and that I had no unterminated outlets or unnecessary splitters. I came to find out that while my internet use was not affected, my signal levels worsened significantly, but not enough to cause great concern.

Then came the 20 degree nights here in Chicago in the past week or so. THIS is when my cable internet started to really affect alot of downloading, connectivity, and lastly the gaming issue. I googled this phenomena and found that temperature affects signal levels very frequently as well as the fact that Chicago is a large metropolitan noise black hole, and thus we have 2 major reasons WHY my mach20 speeds dropped to machCRA* speeds. Read this post to find out more on the temp. issue:
»SNR correlates with temperature. Cool day -> bad signal.
and to read up on the ingress issue:
»Cable Modems and Wiring Issues »What is Ingress?
WHILE we are at it, my cable line monitor ping & packet loss graph:
» ··· e=151200

So combined with the above conditions, I come to find that the Motorola SB5120 is the worst of the modems in terms of isolating the noise, or ingress, that the cable modem would normally experience. This issue can be read up by reading this post:
»Re: [Connectivity] Motorola 5100 vs. 5120

Long story short...since this has JUST really become an issue for me, I am going to go back and try the SB5100 that I originally rented from RCN and see if that solves my issue. I have already talked with tech support regarding this phenomena and they agree there is a problem, but since nothing has changed except the modem im using, and the temperature, either or both of the above have to be culprits. I will update, but good luck!


PS: I have RCN Mach20 using the Motorola SB5120 near the corner of Racine and Belmont in Chicago. My signal levels are as follows (read by going to »

Your Modem Mac 00:15:xx:xx:xx:xx
Your IP 24.148.xx.xx
Forward Frequency 735000000 mhz
Return Frequency 33000000 mhz
Forward Modulation qam256(4)
Modem TX Power 41.7 dBmv
Foward Signal to Noise 34.2 dBmv
Downstream Ratelimit 22000000
Forward Power Level 6.8 dBmv
Upstream Ratelimit 2200000

Poway, CA
So far as I know, the issues with noise tolerance with the TI chipset used in the SB5120 have been largely addressed by firmware changes along the way. I don't know if the 5120 has been made as tolerant as the Broadcom-based models, but many people (myself included) are getting excellent results from this model.

Perhaps RCN is not using the most-current 5120 firmware, and is thus not benefiting from the latest improvements.

I will admit to not having "above-average intelligence", but my opinion is that the signal levels you posted are quite good, so pretty much any modem should have no problems working correctly in your location.

billhamel .net
Hinsdale, IL
·Verizon Broadban..
·AT&T U-Verse

1 edit
Yea. While I will admit I have good signal levels there I failed to provide my previous signal levels before doing a COMPLETE overhaul of the wiring situation. While the firmware upgrade issue and subsequent noise related issues may be solved with new firmware, I have done some research and have found that my firmware is quite outdated compared to many others using the SB5120. While this is RCN's responsibility to monitor and upgrade users of the SB5120, I doubt they have kept on top of it. My firmwares' version is as follows:
Software Version: SB5120-
Hardware Version: 4
MIB Version: II
GUI Version: 1.0
VxWorks Version: 5.4

Now my cable signal seems quite stable at those numbers stated above. In essence, to correct the UDP packet loss and restore my speeds to the FULL Mach20 specifications I did the following: ran a separate RG-6 Quad Shield line (using high-quality F-type compression connectors and an expensive COAX cable stripper) to my modem, installed a 1-way passive tap (one port of a 1-way tap has -1dB of loss while the other port takes on a full -6dB insertion loss), and finally I installed a 4-way Scientific Atlanta tap MODEL SAT4G-11.
Link to SAT4G-11 here:
» ··· 7ab0.pdf
Link to 1-way tap description, use of, and more here:
»Cable Modems and Wiring Issues
My NEW setup configuration to my modem is as follows (where ---> indicates a RG-6 COAX cable run):
cable plant--->1-way 'lossless' tap--->Motorola SB5120

Connected to the 2nd port of the 1-way 'lossless' tap is the SAT4G-11 and off of the SAT4G-11 is one more 2-way splitter to accommodate for the extra cable run now used by my modem.

I have to say I have been working all night on the internet and monitoring signal levels, UDP packet loss (mainly the intermittent dropouts when my SNR began to fluctuate), and the temperature... and I believe I have really solved all my problems.
Prior to solving such problems, My "Forward Signal to Noise" levels were between 32-33.6dBmv, the "Modem TX Power" was at 52dBmv, and my "Forward Power Level" read at 8.2dBmv.

My OLD setup to my cable modem was as follows (where ---> indicates a cable run using RG-6 COAX):
cable plant--->2-way splitter -3.5dB/port & -130EMI--->ANOTHER 2-way splitter--->Motorola SB5120

Off of the FIRST 2-way split I had one port running to the SECOND splitter and the OTHER port off of the FIRST splitter ran to a 4-way -3.5dB/port passive splitter connecting 4 televisions. So then off of the SECOND 2-way splitter, I had my modem connected to one port while the OTHER port of the SECOND splitter was connected to a HD-DVR.

It seems complicated, and this took me more than an hour to complete, but is quite easy if you have the resources.
Feel free to PM me and I can hopefully answer any questions any of you have.


HERE is another link that is a good step-by-step discovery into RCN, speed drops during the winter, and the associated temperature:
» ··· 679.html


Download #1
File size transferred : 29.5 MB (30937448 bytes)
Total time taken : 11.56 seconds (11563 milliseconds)
Throughput : 2675.0 KB/sec [Kilobyte-per-second]
= 2.67 MB/sec [Megabyte-per-second]
= 21400.0 Kbps [Kilobit-per-second]
= 21.4 Mbps [Megabit-per-second]
Upload #1
File size transferred : 29.5 MB (30937448 bytes)
Total time taken : 117.69 seconds (117687 milliseconds)
Throughput : 262.0 KB/sec [Kilobyte-per-second]
= 0.26 MB/sec [Megabyte-per-second]
= 2096.0 Kbps [Kilobit-per-second]
= 2.1 Mbps [Megabit-per-second]


Chicago, IL
Bill and friends,

Thanks for the reply.
Your post was very informative.
I am glad your levels are back to normal.
While RCN does seem to be using an older firmware, my speeds have been back to normal for the most part. I do agree that the cold weather is probably the culprit.

Again thanks for the replies.

Happy Holidays,