Question on ISDN B channel order

[ciscoroyzhang]

Hi there,       I am configuring an ISDN fractional 10 for one of our branch office.  my understanding is that  for B channel, Cisco IOS default is in descending order, same as the Telstra the provider, to avoid confliction with Telco,  we should place our end in ascending order. But in my new router, I accidently leave the B channel in descending order, it still working fine.  I get a bit confused now, theoretically, the router will use the channel 31 to start sending first call, but because it is a fractional, the call should dropped. I am right here?  Or the IOS can automatically select the channel 9 to send the call instead of the 31 and for Telco, they still send call in channel 31, so there is no conflict.   

Can someone confirm to me on that one?  thanks... Roy

[Harmit Singh]

Hi Roy,

I'm guessing that you are referring to a H.323 gateway, not MGCP, right? Your understanding of channel order is correct.

Configuring B Channel Outgoing Call Order

You can configure the router to select the first available B channel in  ascending order (channel B1) or descending order (channel B23 for a T1  and channel B30 for an E1). To configure the optional task of selecting B  channel order for outgoing calls for PRI interface types, use the  following command in interface configuration mode:

Command	Purpose
Router(config-if)# isdn bchan-number-order {ascending | descending}	Enables B channel selection for outgoing calls on a PRI interface (optional).

Before configuring the ISDN PRI on your router, check with your service  vendor to determine if the ISDN trunk call selection is configured for  ascending or descending order. If there is a mismatch between the router  and switch with regard to channel availability, the switch will send  back an error message stating the channel is not available. By default,  the router will select outgoing calls in descending order.

HTH.

Regards,

Harmit.

[paolo bevilacqua]

It is normal that is working anyway. As long the B-channel is provisioned in switch, telco will accept calls on any channel.

The cisco documentation posted above is wrong in that regard.

[ciscoroyzhang]

Hi Paolo,   thanks for the comments,

Let say both side uses the Descending method, what happens, if the outbound call send via channel 31 in client router and at same time,  Telco also try to send the inbound to the channel 31?  or this will never happened, since there is some kind of mechanism in Telco side that if it find that channel is used, it will move to the second available, which is 30.

[paolo bevilacqua · Accepted solution]

Yas, as soon a channel is requested it will not be used for another call, so chances of glare are virtually non-existant.

Thank you for the nice rating and godd luck!