Replacing Adtran and CSU/DSU with cisco router

Unanswered Question
Jul 10th, 2009

Hello All,

I am trying to configure a cisco router to be the interface between a PBX and the WAN link. I have a PBX connected to a Adtran. The Adtran then goes to the DSX1 of a CSU/DSU and the NI lead on the CSU/DSU is connected to an ethernet which serves as the WAN link to carry the voice from the PBX to a remote site. The CSu/DSU has a serial interface that goes to a serial card on an old cisco router. I am trying to replace the CSU/DSU and Adtran with a C2821. Is this possible? If so what module do I use on the router and how do I go about with it?



I have this problem too.
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Paolo Bevilacqua Sat, 07/11/2009 - 01:58

Yes, that should be possible, but I'm confused about the CSU/DSU having an ethernet when it should not.

Try collecting all the device exact part no in detail, to understand how things are currently arranged.

Once you confirm it's in fact a static mux configuration, you either do the same with the router, or carry PBX voice over ip IP so makeing the full T1 BW available for data.

George Thomas Sat, 07/11/2009 - 07:04

Hi Paolo,

Sorry for the confusion but by ethernet what I meant is a T1. The T1 is connected to the NI side of the CSU/DSU and the CSU/DSU is a verilink C150. The DSX1 side is connected to the PBX. Do I use a VWIC card for the T1 PRI from the PBX?

Also, will you please explain more as to what you meant by "carry PBX voice over ip IP so making the full T1 BW available for data." Many Thanks.



Paolo Bevilacqua Sat, 07/11/2009 - 12:21

Yes, you use a vwic2-2mft-t1/e1 to connect circuit and pbx.

Then you have two choices: static sharing of T1 for voice and data like now, or carry voice over IP, save BW and all get the other advantages of voip.

In either case I recommend you don't try this alone because it's easily source of frustration and customers get nervous when voice is down. Hire a reputable professional for the job.

pcameron Mon, 07/13/2009 - 15:22

It's quite common to replace the CSU/DSU with a router. You need to use the T1 Drop and Insert feature on the T1 VWIC's. This allows a TDM connection between specified timeslots on the card. Normally the service provider will split the T1 timeslots into a voice group and a data group. The data can be terminated a channel-group, and the voice channels can be cross connected to the other T1 port. The PBX thinks it is directly connected to the SP, but the T1 is actually being shared.

Refer here for some more detail on the Drop and Insert feature (this is more voice related but should give you an idea) -

in your case you should be OK to use a config similar to this -

This example shows a VWIC-2MFT-T1-DI where the first T1 controller (T1 0/0/0) has timeslots 1-12 configured for serial data, and timeslots 13-24 are drop and insert timeslots from the second T1 controller (T1 0/0/1). Notice the connect command to connect the tdm-group timeslots together.


network-clock-participate wic 0

network-clock-select 1 t1 0/0/0


controller T1 0/0/0

description - connection to service provider

framing esf

linecode b8zs

channel-group 0 timeslots 1-12 speed 64

tdm-group 1 timeslot 13-24


controller T1 0/0/1

description - trunk to PBX

framing esf

clock source internal

linecode b8zs

tdm-group 1 timeslot 13-24


interface Serial0/0/0:0

description - data conection

ip address


connect 1 T1 0/0 1 T1 0/1 1


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