Service-module t1 fdl att

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May 10th, 2002
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We have a large installed frame relay network and typically use 2610's at the access layer. AT&T is our f/r carrier and on a site which had been going up and down for over 3 weeks we were having trouble getting them to acknowledge a problem.

We finally stumbled across a tech who had 'heard' of cisco and asked us to enter the command:

Service-module t1 fdl att

This allowed him to see information from our csu according to him. He was able to determine that a lec needed to be dispatched...

Could someone explain what this command actually does and if it is a good/bad/indifferent thing? Also, does it present a security concern?



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mfaust Fri, 05/24/2002 - 12:19
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FDL stands for Facility Data Link. It is part of the overhead bit of a T1 ESF circuit. The FDL reports line and alarm conditions accross the link to the other CSU/DSU. This information can be retrieved from either CSU/DSU and used for trouble shooting. There are two common type of FDLs, ANSI and ATT. It doesn't matter which one you use. In fact you don't need to use either. If neither one is enabled the only downside is that the information is not available. If you choose to make use of the FDL feature You must configure it the same at both ends. They are very similar, but not compatable. By setting the FDL to ATT you matched the setting that was used at the other end, thus allowing the tech to see the line condition. As far as security goes - if someone were to bridge on to the T1 they would be able to see the same information in the FDL as you would see at your end. But that wouldn't worry me, considering that they would also have aaccess to the entire data stream!!!

klouthan Sat, 05/25/2002 - 01:05
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FDL is what the carriers use to manage their networks.

If you consider that a ds0 is 64k and there are 24 of them that comes up to 1536k. Ahh, but a raw T1 is 1544k. The FDL makes use of the extra 8k unused when the carrier "channelizes" the T1. Its basically an unused 8k channel. The ANSI T1.403 and AT&T TR54016 settings are just a way to make use of that channel.

I don't see anything wrong with it, unless you don't want your carrier getting CSU stats off of you.


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