Data and Voice on VWIC-2MFT-T1

Unanswered Question
Mar 25th, 2007


I have a 2821 router with 2 VWIC-2MFT-T1 cards that have 3 point-to-point circuits and a CAS voice T1 connected to them. I would like the 3 data Ts to get their timing from the line. However when this is done I get timing slips. I would really like to understand what effect the network-clock-participate command has on these data T1's and if it is possible to have them get their timing off the line?


Rob Walk

I have this problem too.
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Paolo Bevilacqua Sun, 03/25/2007 - 13:24

It's strange, the T1's if regular circuits, should use clock line. Check with telco, if these are ad-hoc HDSL spans in fact router clock on one side may be needed. But if everything is OK on all ports, I would not worry and change anything.

The network-clock-participate defines if a port participates to the list of preferential clock sources in a router. The highest priority clock is used as source on ports with internal clocking.

Hope this helps, please rate all useful posts!

robertlwalk Sun, 03/25/2007 - 17:58

Thank you for your reply.

The T1s are "regular" p2p circuits that are multilink PPP. I would like the "core" router to provide timing for the remote sites. This 2821 is a remote site router so I would like it to get it's timing from the line for consistency.

In this case the CAS T1 is in wic 2 so I I entered the command network-clock-participate wic 2. Now unless I enter network-clock participate wic 1 the controller in wic slot 1 will not come up. I was then seeing slips on the CAS T1 so I entered network-clock-select specifying the voice T1 port. This is when I started seeing slips on my 3 p2p circuits.

I'm ok with supplying clock on the data circuits but I am still curious to know if this is how it really works. Also, my other question is what if I had a different T1 like an Internet T1 that a need to get clocking off of? According to what I have experienced this wouldn't be possible in the same chassis.

Paolo Bevilacqua Sun, 03/25/2007 - 18:12

Hi, you might be running the situation described in the following doc, it mentions E1 but I think is the same for T1:

What you should also consider, who is providing clock to the Voice T1? is there a PBX? Unless this is configured for internal clock, the router should provide clock.

Note: The 2-port E1 Multiflex Trunk VWICs has only one shared clocking domain between the two E1 controllers. So, if both E1 controllers are configured to derive clocking from the line, the clock references received must be synchronous. Otherwise, timing slips will occur on at least one of the controllers. If the clocking sources are pleisochronous and one controller is configured for "clock source line primary" and the other for "clock source line", it is likely the second controller will exhibit controlled slips in the output of the "show controllers E1" command. This behaviour is a hardware limitation of the 2-port E1 Multiflex Trunk VWIC product and is by design. However, independent clocking sources can be supported on the 2-port T1/E1 VWIC2 product when both controllers are used for data-only purposes.

Again, if you are unsure no how your data T1 should be clocked, ask a competent person at your telco. Normally, you never provide clock, the circuit provides clock at both ends.

robertlwalk Sun, 03/25/2007 - 18:36

Thanks again for the reply.

So this explains why the data T1 on the same physical controller card is seeing slips. What I still don't understand is why am I seeing slips on the 2 data T1s on the 2nd VWIC2-2MFT-T1 card?


Rob Walk


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