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From: (Floyd Davidson)
Subject: Re: CSU/DSU Purpose???
Date: 26 Oct 1999 08:05:56 GMT

Louis Guillama <> wrote:
>I've been given several contradictory reasons for employing the use of the
>CSU/DSU.  Can someone please provide closure to this question?  Some say it
>is to provide performance monitoring, while others insist it's not that at
>all, that the device is used instead to convert AMI into a uni-polar
>bitstream.  So who's right?

The original purpose was to provide an interface at the demarc
between telco and customer premise equipment (each of which
might have very different data formatting).

That interface may include these functions:

       1.  Format conversion.
       2.  Signal regeneration and/or conditioning.
       3.  Loopback facilities.
       4.  Maintaining 1's density.
       5.  Alarm functions.
       6.  Keep alive signals.
       7.  Sealing current on dry spans.

Hence the only incorrect part of what you were told was the
indication that a CSU/DSU does any of the above to the exclusion
of the other listed functions.


Floyd L. Davidson                
Ukpeagvik (Barrow, Alaska)

From: (Floyd Davidson)
Subject: Re: CSU/DSU Purpose???
Date: 26 Oct 1999 10:46:02 GMT

Brad Reese  <> wrote:
>CSU/DSU is short for Channel Service Unit/Data Service Unit.  ...
>Such a device is required for both ends of a T-1 or T-3
>connection, and the units at both ends must be from the same

Such a device is not necessarily required at both ends, and when
used in pairs they do not necessarily have to be from the same

CSU/DSU devices interface telco to customer equipment, and if
only one end of a T1 terminates in customer equipment, then only
one end requires such a device.  (The other end may terminate in
the telco's frame relay equipment, a switch, a digital
crossconnect, etc.)

Each CSU/DSU must be compatible with the equipment and data, and
there are several manufacturers whose CSU/DSU's will work with
each other's.  For example, I have several satellite circuits
here that have Motorola FT100's on one end and GDC 552A units at
the opposite end.  (Perhaps such mix and match provisioning is
not recommended for everyone, but it does work with the right


Floyd L. Davidson                
Ukpeagvik (Barrow, Alaska)

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