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Cable Latency Issue

Last response: in Networking
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November 8, 2010 6:11:28 PM

So I moved into this new apartment a few months ago, and have always noticed that my internet can be a POS at times. It's Comcast Cable, and I've had cable for years in many different places I've lived, but have never experienced something as bad as this.

Obviously, with cable there are high traffic times where bandwidth will bottleneck and my speeds will go down/latency will go up. However, I do not believe it should at this extent. At good times, I would get 20Mbps download/2 Mbps upload speeds...great speeds, what I'd expect from cable. During high traffic times it would drop to something like 5/1.5 or 3/0.5. Still decent speeds, definitely good enough for optimal online gaming. This is using speedtest.net

The problem lies in the fact that at all times pingtest.net reads above a 100ms ping, usually always a packet loss greater than 0% (usually around 2-4%), and a jitter about half of the ping. Speeds are now staying constantly at 7 or 8 download and 1-2 upload. This is using Denver's server, and I'm about <10 miles from downtown.



That's my pingtest that I just performed at a very low traffic time. At high traffic times it just gets worse. I play games like SC2 a lot and notice at random times I get absolutely terrible command lag. I mean 2-3 seconds delayed where I'll tell an SCV to go build something and it will respond extremely late. Micro is impossible when it does this.

Now I've tried talking to Comcast and they keep telling me the speeds going down is normal for a big apartment building in a city near a university where a lot of people live. However, they couldn't explain the latency problem, which is what mainly is causing frustration. All they said was they would come in and make sure internet is coming in through the cables good and that's all they would do. Not getting much help from them, so I was wondering if anyone who knew networking well here could help solve my problem.

Troubleshooting included not using my router and plugging straight into the wall, tried multiple computers along with my PS3, and I also tried the internet in different rooms. All result with the same issue. Since I've ruled out my router, my computer, and the room I'm in, I can't see how it could be something one of my devices is doing. Anyone have any suggestions?

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November 10, 2010 10:32:36 PM

Update:

Ran a ping test on the building manager's computer and got 0% loss with 11 ping and 2 jitter. A grade. He's the only one in the building that runs off of a separate account. That leads me to believe it's an internal wiring problem. Anyone have any knowledge on what type of wiring issue could lead to this problem?
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November 11, 2010 5:36:08 AM

Yes. I'm not sure of the specifics of the wiring of the building itself but here is the likely problem. I see this all the time and the cause can be tracked down and corrected although I am sure no one, -you, the landlord, OR Comcast- will want to pay for the time involved. The simplest solution will be to demand that they run you a dedicated modem line back to the MDU where the services enter the premesis. They know they are supposed to do that anyways, but 99% of the time when you hooked up like I am sure they did there are no problems. It's that 1%........

There are a number of things which will definately cause your problem:
-Old, sub-spec cable commonly referred to as RG-59. Nowadays, with signal frequencies regularly operating at 1ghz or higher, RG-59 in unacceptable for broadband active communications. The sheath is made of copper most often and it is not used anymore today except for perhaps very specific purposes.

- High traffic at those times will bump you out if you are farther away than most from the point of entry to building.

- In your case I am betting that they have an amplifier in the loop somewhere. Amps do not pass signal well and should be avoided. Once again, they know they are supposed to run new modem lines which bypass any splitters on active accounts.

- Loose connectors, splices, damaged insulation on the sheath of the cable, sub-spec or failing splitters (all DC and line splitters should be specced to 1Ghz or higher...and NO, Radio Shack 2Ghz splitters are not okay. I don't care if it says Phillips either. If it ain't a Comcast splitter then it should be tossed)

- Can you see if the lines to your modem are laying on anything that gets hot? A hot piece of coax will build resistance and cause loss of signal and latency issues.

- How far away from the point of entry to the building is your modem? A piece of RG-6 (which is modern coaxial ) is only rated for about 250-290 feet. If any further away than that you should have RG-11 feeding your modem. Its a MUCH better piece of cable with longer transmit distances and better signal response.

If they do not want to just show up and drop you a new modem line then talk to their tech ops manager and ask that they run a 24 or 48 hr QoS test with their DSAM signal meters on your modem line to capture any fluctuations in the line conditions. Have they run a packet loss and throughput test? Get them over there when you are experiencing the problem. Find out what their meters are showing them. I need to know three things. Stand there and look over their shoulder cuz they may try to BS you. HAve them show you three things on the meter but don't tell them you want to see until they have it hooked up and running. They can't bust out a stored file to show you because after the test finishes it automatically displays the results. If he fiddles with more than one button click he is doing something to the results. What you need to know is this:

What is your upstream signal? IDEAL is 48-49db Acceptable is 46-51
What is your downstream signal? IDEAL is 0 Acceptable is -3.5 - + 3
What is your error correction? There are two results: PRE and POST A clean line will show a 1e09 in the PRE. A POST of anything but 1e09 is going to cause problems.

Try working through all of this and let me know how it works out. I work communications in NE Colorado and am certain we can resolve this. Like I said though, if they run a new line (correctly) the problem will go away.




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November 11, 2010 6:19:53 AM

Wow, that sure is a lot of helpful information. I'll be sure to give all of these a go and I'll report back on the outcome. Thank you!
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November 12, 2010 3:30:25 AM

Hey my pleasure! Hope it helps!
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November 12, 2010 5:21:23 PM

Problem fixed!

I went down to the MDU with the head maintenance guy and checked out everything you had on the list. The cables were RG-6, which was expected from a building that is only 3 years old, and my apartment is definitely within the range you mentioned for that cable. Additionally no amps were used in the loop.

The connection starts at the modem and a single line goes into a 22 port splitter, which goes off to each apartment. There are 9 apartments on each floor, one telecommunications room per 2 floors; 4 floors in total (5 floors minus the first floor being commercial business). Once the connection gets to my apartment it goes into another port which splits it off into the separate rooms of my apartment. Does this seem pretty normal? Nonetheless, we couldn't find a problem within any of those specs.

One of the managers at Comcast Business then came out to take a look. We ran readings off of the modem on my laptop, which gave a upstream of 47.5, a downstream of 5.9 (this is out of the range you mentioned, I spoke to him about it and he said he likes to have a range of -5 to 10, not sure how you feel about that), and another variance measure called SNR of 34.757; that value is what he was concerned about, and a ping test showed a lot of packet loss and ping spikes.

We ran over to the modem and he started one by one unplugging apartments from the loop. We landed on a specific line that when unplugged, EVERY SINGLE problem disappeared. Whoever is living in this either is running some type of server or has some type of jack problem. We left him unplugged and management will probably be getting a call from whoever that is about no internet and that's where we will be able to find out what he's doing in there.

However, for now, everything's great! Here's what my pingtest now reads:



Hopefully things stay this way and, whatever that guy was doing in his apartment, hopefully he learns to cut it out. Thanks for your help again!
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November 14, 2010 5:31:25 AM

Glad to hear it was that easy to track it down, Chicagohawk!

By the way you described the installation I would say it is well laid out. That 22port block is actually a tap straight off a mainline and is exactly how I would have expected it to be done. The way your unit is fed is fine too- it is actually hooked up no different than a house would be. Ideally your modem would be on the very first splitter entering your unit (the splitter would be called a dc_. dc4, dc6, dc9, etc....that provides signal isolation between the two outgoing ports and would prevent interefering signals from other digital equipment further hooked up in other locations throughout the home. (Digital set top boxes)
EVEN IF your modem was situated on that 'tv loop' off the DC and a tv was on that other port there would be isolation between them to some degree and they all head back out together toward the tap anyways. Looking at your original jitter it was definately showing some interference and you did an excellant job of troubleshooting it. If the technicians looked at the signal constellation it sounds like they may have been able to see some signal ingress. Regardless, those Comcast guys are good if you can get em really digging into a problem. I never had any problems when I was a consumer and they were absolutely spot on again when he targeted the SnR. 34.7 is a poor SnR. I would be curious to see if it was the upstream or the downstream measure he was looking at. SNR is measured two ways:
We measure signal to noise ratio on the upstream and the downstream. In a shared environment where you were feeding a modem off a shared line with another apartment, or the line feeding your apartment was damaged, you would likely see a poor downstream (or inbound) SnR. This would obviously bring signal noise and interference into your lines. Measuring the upstream (outgoing) SnR is useful for a technichian because he can attenuate the outgoing signal strength of your modem to a point where your outgoing signals are transmitting loudly enough for the supplying node and CMTS to see and the background noise has been reduced to a degree where it only negligibly effects your neighboring services.
When a technician goes to install a modem, as you know, they are installing to a target level. 'tuning' the frequencies. Ideally (I say ideally because there are times due to certain line conditions where it is not possible) the final step in balancing that modem line will be to put a mitigation filter (attenuator) on the tap-side of your line. These filters trap and dampen the outgoing signal and its accompanying noise levels and force your modem and often your digital TV box to increase transmit power internally, thereby making your return data more visible to the ISP's equipment. I disagree with the tech about the +10 on downstream signal. It should never be any higher than +5 if you want a clean signal. It is high enough to still be strong during environmental and network fluctuations, but it is not overdriving your modem (picking up any quiet but latent interference in the lines and essentially amplifying it so your modem will pick it up). This could lead to a poor downstream SnR.
If you have a poor downstream SnR, and your neighbor with the noisy apt. has a noisy upstream SnR you will definately be picking up interference from his unit. Even running a vacuum cleaner near cut/damaged lines will throw impulse noise into the system. He most likely has no modem in his apartment or he has no attenuator on his lines to quiet down his cable box. He may even just have a old poorly shielded tv, broken jack or a scraped cable.

Basically, as far as SnR goes, if it was your downstream SnR hitting yer modem at 35-36 you should be fine. Thats pretty avg. If your upstream is leaving at 37 or lower though, it would definately explain your latency issues. When I install a customer I will leave them at a 38-39 SnR. If lines are undamaged and downstream is good you should be able to hit that target every time without too much work. 38-39 is a very quiet line.

Glad everything worked out okay for you and hopefully you are a little more informed in case someone else tanks your service again from another apartment.

Cheers!


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November 14, 2010 7:00:03 AM

Best answer selected by chicagohawk.
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