Network Jitter and Latency issues

[geoffhellyer]

Folks,

Great forum, well done for setting it up!

Maybe there is someone out there who can help me out with a problem I'm having.
I've just set up a point to point VC link between London & New York, we have a VS4000 in London and VSX7000 in New York. We have a 1Meg SDSL connection at either end and the bandwidth is consistently over 900kb with an average ping time of app 100ms. Yet I cannot get a steady connection at any speed, for longer than 20 mins. The audio is constantly being interupted and video regularly freezes for a second or two. My suspicion is it's just too much traffic within the internet that is causing the problem and there is no way of controlling that. There is app 20 hops between the two end points so I know that isn't helping either. I've tried virtually all the hardware adjustments I can think of and have tried various different routers. but that doesn't seem to help. I don't think this is a bandwidth issue, more a flow control problem. I don't know enough about the subject but, if there is some way of queing the packets before they are processed that may solve the problem. Any ideas?

Yours in exasperation.

Geoff

[Glen Sykes]

The contention of your 1mb will have a big part to play in this, as will the amount of traffic passing across the routers in between you and the far end. If pings are consistently within 100ms, the problem looks more likely to be jitter.

A rule of thumb measurement is keep latency below 300ms and jitter below 50ms in order to ensure good quality video, however different vendors have different guidelines to these times.

The only way of applying 'flow control' is to implement Quality of Service (QoS). However (unfortunately) in order to do this, QoS must be enabled on the entire network, meaning those 20 or so routers your traffic is crossing. If both of your systems are on the same providers network, you stand a chance of obtaining a higher service level from them which includes QoS, however if this is across 'the internet', this is probably the best you are going to get.

The other thing worth checking though is the port duplex settings at each end. Quite often if systems are set to auto negotiate port speeds, a mismatch occurs and this results in terrible packet loss. Better to fix the speed on both the codec and the switch port.

[Sean Lessman]

Quite often if systems are set to auto negotiate port speeds, a mismatch occurs and this results in terrible packet loss. Better to fix the speed on both the codec and the switch port.


Even worse, auto negotiate at one end and forced at the other...big no no. This will result in the wrong duplex setting.

Sean

[Gary Miyakawa]

Several things that I'd be interested in...

Does the call fall back to PVEC video ?

Have you tried the call at, say, 768kb or 512kb ? Does it still fail ?

Have you tried a 1Kb Ping to see what kind of latency you get with it ?

Good luck!

Gary Miyakawa

[tom9933]

Unfortunately PVEC which might help a bit in this situation only works between like pairs due to a generational difference. I.E. VSX->VSX and 4000/EX/FX->4000/EX/FX. If you have a set of units to try then PVEC may definitely help out in this situation. Now having said that you should still be seeing dynamic bandwidth kick in, if so what do the numbers look like? My guess is that the situation is similar to what Glen said about the problem most likely being a jitter issue.
Two other things to try are switching the modems (I had a lot of fits with a bad cable modem at my house that only sustained video calls found) and getting the DSL service through providers that directly peer with each other. This will generally cut the number of hops by a large number and even without QOS low speed (512 and below) calls are generally very usable.
BTW have you tried to call both sites from a third party? I ask this because you may be able to isolate some of the issues to one side of the call and help with the troubleshooting.
Good luck,

[geoffhellyer]

Thanks for the responses, I really appreciate it.

The providers are BT in the UK and Earthlink in the US. I have the LAN set at 10Mb - duplex. I've tried the call at 386/512/768. The call stats don't really vary too much between them and neither does the actual call quality, the overall call quality just goes down in line with the call speed. Contention is 10:1. The only difference apart from the endpoints that I can think of, is in London we are using NAT where as we don't in NY.

[Kevin]

Hi Geoff

I have found in the past that the last mile (at least here in the UK) caused the most problems. Alternatively it may be just one of 20(!) the hops in particular causing the problem. Unfortunately its kinda hard to influence the route taken on the internet. How many of the hops are within your ISP? If it is a quite a few you may well find that having a chat with the ISP, or even changing ISP helps.

Just out of interest you could try calling via our demo MCU.

This is based in San Jose - so this may well make the poblem worse, though if it does avoid one of the problem hops you may find things look better. (Of course our clever codecs may well help too :))

This will also allow you to test each endpoint in some isolation (as you can loop video back to your self with one endpoint).

Details of how to use it are on www.codian.com/demo and I created a web login for VTCtalk users:

username: vtctalk
password: vtctalk

Good luck!
Kevin

[mbtmike]

I have not tried this product but have read some spec sheets it may be worth trailing at both ends
http://picturephone.com/products/packetmaster.htm

1>have you played with the MTU sizes on the codecs...?

Good luck

[David Fourie]

Hi Geoff,

Are you still having problems with your UK to NY calls?

Something worth checking with your ISP's is if any other 20 hops are Cisco 7200 series routers, running old IOS. I've seen these forward duplicate packets causing sporadic packet loss. An upgrade to 12.3 fixes the problem.

If there is no way of setting up a true VPN with QoS (DiffServ) then an EASY-VC E107 should sort you out. The vendor I use has put these on DSL links for other clients and had excellent results.

http://www.bulldoginfo.com/vc/easyvc.php (or follow mbtmike's link)

Cheers
dave

[geoffhellyer]

Thanks Dave,

Yes we are, I've fallen back on ISDN for the moment and asked our network boys to take a look at the problem. These may be useful in the mean time though.

Best

Geoff