Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
New Member

G.703 circuit


I've got a G.703 (75 ohm BNC type) connected to a NM-1CE1U on a Cisco 3660 router. It's a 2mbps link. The controller keeps going down and them up again over and over. I contacted the telecom provider and they say everything is fine with the link itself. Here's a sample of the error message (I used 'term mon' for this):

Here is a log:

19:11:35 %CONTROLLER-5-UPDOWN: Controller E1 2/0, changed state to down

19:11:36 %CONTROLLER-5-UPDOWN: Controller E1 2/0, changed state to up

19:11:47 %CONTROLLER-5-UPDOWN: Controller E1 2/0, changed state to down

19:11:48 %CONTROLLER-5-UPDOWN: Controller E1 2/0, changed state to up

Notice that the controller only stays down only for a sec at a time.

Here is the running-config:

controller E1 2/0

framing NO-CRC4

channel-group 1 timeslots 1-31

interface Serial2/0:1

ip address

ip access-group 100 in

no cdp enable

When the controller goes down the serial stays up. (Unless the controller is down for a long period of time.) But when this gets worse (the controller going down more frequently) I completely lose the WAN link.

Am I missing something? Thank's for the help.

Jorge B.


Re: G.703 circuit

Hello Jorge,

What type of circuit is it exactly? You configured it to use all timeslots for a single serial link but with an E1, channels 0 and 15 are used for control signalling. Your setup includes channel 15 for datatransmission.

To use all available bandwidth you should configure chanel group 1 to contain channels 1-14 and 16-31. Syntax:

channel-group 1 timeslots 1-14, 16-31

This will result in a bandwidth of 1920 kb. If this is supposed to be an unstructured G703, you might have the wrong module or the wrong IOS version.




Re: G.703 circuit

Hi Leo,

could it be you're confusing channelised E1 with ISDN PRI?

For ISDN PRI, slot 15 represents the D channel used for signalling, but I see no reason why you couldn't use 31 channels in a framed (fractional or channelised) E1 (where slot 0 is used for framing and there is no signalling).

All the framed E1's we use are 1984 kbps, so 31 x 64k.

I agree that if the line is unstructured G703 you need a different interface on the router, e.g. the VWIC-nMFT-G703 (n=1,2)



Re: G.703 circuit


I suppose that you are correct. I have indeed assumed that a PRI always uses two B-channels for framing and signalling. This was probably not correct.

It is however still possible that this problem is caused by a mismatch between the PRI-config at the provider and the endusers site.

The enduser has set it up as a channelized E1. The provider might have configured a standard ISDN30 link. The constant restarting of the link can be caused by this mismatch.



New Member

Re: G.703 circuit

Hello you all!

First off all, thank you for your time while trying to solve my problem…

Well, I’m sure the provider has installed a unstructured G703 link. In fact, I even know it is multiplexed and put on a SDH fiber ring to the destination. I have got other G703 with analogous configuration settings (also using the whole 31 time-slots) working perfectly well.

Another thing I have noticed, is that the controller doesn’t just go down and up always. It only does this for a random period of time and them stops in an up status. Example: it works fine from 0:00 to 09:30, then from 9:30 until 12:30 it continuously changes state. After that it stays up for a long period of time again.

You’re probably wondering if the failures coincide with traffic peaks. Well, no it doesn’t . I’ve seen the link doing fine in busy hours.

I just have to be sure that there is no configuration problem with the router. By the way, the IOS version is 12.2(5a).

Thank’s again!

Jorge B.


Re: G.703 circuit


if you are really sure it is unstructured G.703 (also called unframed, clear channel or full 2Mb) then you have the wrong interface, the NM-nCE1 can only do structured E1 (also called framed E1, fractional E1, n x 64K, G.703 with G.704 structuring, or channelised E1 (when you have multiple links multiplexed on one E1)) or ISDN PRI, see

Anyway, assuming it is a framed E1, my next thought is that either the settings (also check crc4) do not match with those of the provider as Leo suggested, or that you may have a faulty network module. Any chance that you could try the same E1 on a different module, or this module on a different E1?

This link might also be useful:

Troubleshoot Channelized E1 and Channelized T1



CreatePlease to create content