Monday, August 28, 2006

Testing Bandwidth Speed For A Private Point-To-Point T1 Line


You have a private point to point T1 (not connected to internet) at work which connects 2 offices. Supposedly, this is a full T1 (not fractional), but simple file copy testing seems to indicate a slower speed. What is a simple software test that will allow you to test the true connection speed from both ends?


If you have a Windows Server at one end then setup the FTP service, then on the other end use the command line FTP client in windows to copy over a file. That will give you a numeric rate of speed.

A full T1 is 1536Mbts, which is equivalent to 192Kbytes/sec. A 10Mb file would take 53 seconds to copy over the link. (10MB x 1024 / 192)

If a private T1 is not running at the correct speed then a common problem is clocking. Traditionally in a private span, one of the DSU's is set to obtain clock from the span, the DSU on the other end is set to provide clock to the span. (the setting is often labeled 'internal' clocking which is a misnomer) If both sides are set to obtain clock from the span and the span has no clocking on it, then they will both freewheel and you will get massive clock slips. (any decent DSU has a way to query it's stats and you will see this)

Sometimes you'll find with private spans that the Telco is multiplexing the span with other spans, and they put clock onto the span even though they are not supposed to do so. Then if you source clock from one of the DSU's you get clock slips again, since the other side flops between sources trying to sync. With these spans both sides should be set to obtain clock from the span.

Other times you may find that due to some setting of the span that only 1 side can source clock, it is like the telco equipment will sync to the clock on one side only, you will have problems if you try to force it the other way.

Another thing that is important to know is that with private spans it is best to use the same make/model/manufacturer of DSU on each side. Don't put a Kentrox on one end of the span and a Lucent on the other. Sometimes DSU's of different manufacturers will not sync with each other properly over a private T span.

If you have clocking problems call your phone company and ask the tech to look at the span FREQUENCY (which takes special equipment). If the tech knows what he is doing he can see if the span is running off frequency, if so that's a clocking problem. A, B, or A+B (round robin) testing with a T1 test set is useless in this instance because each DSU will immediately sync to the test set and clocking problems will vanish.

A T1 monitor (you can sometimes find them on Ebay for a song) can be helpful for private T1 spans. These are devices that are rack mounted and are permanently inserted in the span, one at each end. The better DSU's have this circuitry inside of them, look for manageable DSUs if you don't have them.

And of course, both DSUs must be setup the same for the other parameters. ESF, B8ZS are what you want. Do NOT use AMI for data applications!! AMI does not have sufficient ones density.

Note that there are other important things to be aware of when setting up T1s. If the tech who is setting up the private T1 is very careful, patient, and reads all of the literature on T spans then he will probably do OK. But the garden variety Windows desktop tech....who is lost without a a terrible choice for setting up a T1. And unfortunately, even many telco techs are totally out of their element with problems with T1s.

If you need to call in someone, call a private company that sells phone systems, particularly larger ones (e.g. Cisco, Avaya, Lucent, Mitel, NEC, Nortel, Toshiba, Vodavi, Artisoft, ShoreTel). Ask if they have ever solved clocking problems on a T span. You see, private T1 spans used for voice are extremely sensitive to misconfiguration, the voice will garble badly long before data problems show up. Tell them you have a T1 span that is running data, and the data over it is slow. If they don't know what your talking about, or they start telling you some baloney about viruses or some such, take a hike.

To have confidence you're getting a qualified and certified expert looking at your issue....I suggest using this free service which searchs for qualified and certified dealers by geographic location (US only)........

Telecom Equipment Dealers


Blogger Buzz Buzzard said...

Also SpeedTest.Net is made in Flash and is pretty cute, more accurate speed tests can be completed on MondoSpeed Worldwide Speed Test (

You can choose from a list of more than 40 high speed servers spread all around the world.

This server keeps in history all tests results ever.

3:57 AM  

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