T1 connection to pstn and glare issue

[sarahr202]

Hi everybody.

I understand the how loop start has problem of glare and how we came the idea of ground start to overcome this issue.

Please consider the following scenario:

phones---pbx----t1-------pstn.

We should not have any glare issue on t1 connection between pbx and pstn because this is not a local loop but rather a digital connection to pstn.

Am I correct ?

thanks and have a great day.

[Chris Deren · Accepted solution]

Even with T1 there is a small possibility for a race condition, however very unlikely. In either case it is a good practice to send calls out in descending order if inbound are arriving as ascending to limit the possibility.  If the T1 is a PRI circuit rather than T1-CAS this never happens in my experience.

HTH,

Chris

[Harmit Singh · Accepted solution]

Hi Sarah,

Adding to what Chris rightfully said, here is a doc which talks about glare conditions in T1 CAS configs:

http://www.cisco.com/en/US/tech/tk652/tk653/technologies_tech_note09186a00800e2560.shtml#topic1

Note:

It is possible for answer supervision to be provided with loopstart           connections if the network equipment can handle line-side answer supervision.           Also, loopstart provides no incoming call channel seizure. Therefore a           condition known as glare can arise, where both parties (Foreign Exchange Office           [FXO] and FXS ) try to simultaneously place calls. Glare can be avoided when           you configure the T1-CAS gateway's port           selection order in such a way that the inbound and outbound calls are in           reverse order. For example, if the inbound calls are sent by the provider on           the FXO ports in the order of port 1, port 2, port 3 and port 4, then configure           the Cisco CallManager Route Group to route outbound calls on those same ports           in the order port 4, port 3, port 2 and port 1.

HTH.

Regards,

Harmit.