DSU/CSU?

Tue Jun 19 10:16:02 PDT 2007

On Jun 19, 2007, at 7:52 AM, David Alban wrote:

> Do these acronyms mean anything to you?

The DSU/CSU as I understand it is kinda like a modem in that it  
translates the signaling from the high capacity T1 line to serial  
signaling. It sounds like the "Kentrox"[1] is the hardware in  
question. The first thing I'd suggest is performing a loopback test  
on the local line. See:

http://www.cisco.com/warp/public/471/hard_loopback.html

If the loopback tests work, then the problem might be internal  
building wiring. If the line is in an office building the source of  
the problem might be between the office and the Main Point Of Entry.  
That usually happens when phone techs muck about and dislodge things.  
AT&T/your ISP have probably already ruled this out with their own  
loop-back test.

If AT&T/the ISP did a loop-back test successfully, and the local  
loopback test works, then the issue is likely the cable connecting  
the DSU/CSU to the smart jack on the wall. Ideally that cable will be  
as short as possible. I've had problems when using cables longer than  
20 feet, and there is a recommended maximum length that I  
disremember. That might be loopback testable by using an RJ-45 to  
RJ-45 connector and loopback plug at the end of the cable that  
normally plugs into the wall jack.

There is a handy book from O'Reilly on the subject that might suggest  
avenues of inquiry. It does go into the nitty gritty of how T1 and  
other WAN lines work.

http://www.oreilly.com/catalog/t1survival/

And of course, the first avenue of inquiry is always "what changed?".  
Was there a router software upgrade on either end; a hardware change,  
etc?

Disclaimer:

IANACCAnything; the above is just from personal experience dealing  
with T1 lines in our office.

HTH,

Tony Del Porto
SysAdmin
USENIX Association
2560 9th Street, Suite 215, Berkeley CA 94710
tony at usenix.org | www.usenix.org | www.sage.org
http://www.usenix.org/about/tonyd.gpgkey

[1] http://www.kentrox.com/products/dserv%5Ft1/