12 channels of a PRI

Unanswered Question
Sep 11th, 2007

Hello, we're looking to hook up a PRI but don't intend on using all 24 channels of it.

Our current setup is a Cisco 1751 router with 2 DSP's, enough to cover 12 channels. (It also as a WIC-1DSU for data)

My question is, if I conifigure the VWIC to support only 12 channels of the PRI, this won't hurt our service will it? Like dropped calls because the rest of the channels aren't in service.. The telco assumes we're using all 24 channels, but we won't (unless we up our DSP's).

Also, if I want to configure it to only use the 12 channels, how does the D channel work with that? Would that mean I'd really only have 11? Is there an extra command on the controller that I'd have to specify the D channel, or just configure the "pri-group timeslots 1-12"??

Thanks for any help!

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Overall Rating: 3 (2 ratings)
Paolo Bevilacqua Tue, 09/11/2007 - 07:11


I think that is correct, the switch will place calls to you on low channels. You include D-channel with:

pri-group timeslots 1-2,24

On the the other hand, if you see incoming calls on the hight channels, configure these instead.

Hope this helps, please rate post if it does!

snooter Tue, 09/11/2007 - 07:21

One more question.. Do you think having the WIC and a VWIC in the same router is an overload for it? I wouldn't think so since it obcviously supports it, but for some reason whovever setup this network put both a router and a vg200 gateway in this really small office (VG200 has 2DID ports/1751 has 2 FXO's and a WIC). Suppose they (some vendor) were just trying to milk it for every penny.

Majdi Harb Tue, 09/11/2007 - 08:24

HI snooter,

when ISDN is implemented in PRI way. it supports 23 channels (for T1). any implementation supports a data (D) channel, (i.e., channel 24) and used to carry signaling information (CCS).

so configuration will look like:

controller t1 slot/port

pri-group timeslots 1-12,24

obviously, You can configure the PRI group to include all available timeslots, or you can configure a select group of timeslots for the PRI group.

for load and capacity issue, i don't think your 1751 won't be overloaded based on what you mentioned in your post.



snooter Mon, 09/24/2007 - 10:33


Looking at the product description thought of the VWIC-1MFT-T1 card I need. It says it uses channel 16 as the trunk signaling channel. Does that mean I have to include 16 in the controller configuration?

Paolo Bevilacqua Mon, 09/24/2007 - 11:19

No, TS 16 is used for E1 only. If referenced in T1 documentation, is a mistake.

Hope this helps, please rate post if it does!

Paolo Bevilacqua Tue, 09/11/2007 - 08:32

12 channels on the 1750 should be fine> Both devices are quite aged already so you can look to replace them both in a single unit sometime.


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