Clock slips on connection to legacy PBX

[johnnylingo]

Our 2811 Voice Gateway is configured as follows:

Controller T1 0/0/0 = PRI Connected to PSTN. Clock source is line.

Controller T1 0/1/0 = PRI Connected to Legacy PBX. Clock source is internal.

The connection to the PSTN shows no slips over a 24 hour period, but the connection to the PBX shows a slip once about every 30 seconds. Fax calls off the PBX are frequently dropped.

Clocking configured as follows:

network-clock-participate wic 0

network-clock-participate wic 1

network-clock-select 1 T1 0/0/0

Anyone seen this before, and have suggestions? All of the clocking slip examples I've seen before dealth with connections to the PSTN, where clock source was line. We essentially have no support for the Legacy PBX, so I'm unable to even verify what clock source they're using.

[Tommer Catlin]

IN CCM, change the clock source to the PBX. Let the it be the source.

[dgahm]

Johmny,

The PBX is likely set for internal, so you really need to get someone to change it. If you change the router clock settings to slave off the PBX you will then be slipping to the PSTN, as the carrier will not slave off of you.

If you really can't change the PBX, you could move the PBX T1 to another router that is set to line. The calls would then be VOIP between the PBX and PSTN, and you wouldn't need to have a common TDM clock.

Please rate helpful posts.

Dave

[johnnylingo]

That is my guess. Unfortunately the PBX is essentially a hand-me-town from the corporate company, and no longer supported by them. Getting another router isn't possible, due to budget constraints (this is a 25 user office).

I probably should have mentioned that the clock slips were initially appearing on the PSTN connection. That was easily fixed with:

network-clock-select 1 T1 0/0/0

If I change the clocking on PBX connection to Line, must I add it to the network-clock-select config?

[dgahm]

On TDM capable routers like the 2800 there absolutely has to be a single clock used by all voice T1s. Calls that tandem through the router between T1s will use the network clock.

Here are a couple documents that might be helpful

http://www.cisco.com/en/US/products/hw/routers/ps259/products_tech_note09186a008031a072.shtml

http://www.cisco.com/en/US/products/hw/modules/ps2617/products_tech_note09186a008039c316.shtml

Please rate helpful posts.

[johnnylingo]

I tried setting the clock to Line for the connection to the Legacy PBX. It actually increased the frequency of the slips (about one ever 15 seconds), so I undid that and we're back to one every 30 seconds or so.

I also just found out the clock slips started happening about the same time the phone room experienced a power outage, so am wondering if we're looking at a peice of hardware that went bad, mostly likely on the PBX end.

Really, the PBX is only being used for fax machines, so they will probably want a migration plan. The only problem is the highest density module for the 2811 is a VIC-4FXS/DID, correct?

[dgahm]

You can use a NM-HDA-4FX with the EM-HDA-8FX to give you 12 ports, plus use one VIC-4FXS for another 4, so you could get up to 16.