Slips on E1

Answered Question
Sep 15th, 2010

Hi to all,

i have a 2811 Voice Gateway and it is configured as follows:

Controller E1 0/0/0 = PRI Connected to Legacy PBX.  Clock source is internal.

Controller E1 0/1/0 = PRI Connected to PSTN.  Clock source is line

.

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 300 seconds and i think it cause me some problem with fax-machine of PBX.

Clocking configured as follows:

network-clock-participate wic 0
network-clock-participate wic 1
network-clock-select 1 E1 0/1/0

There is a way to troubleshooting these sleeps? If i set the PBX as the source clock and i configure E1 0/0/0 with clock source line is a good test? or it can cause slips on E1 0/1/0?

The output is the follow:

MGCP_GW#show controllers e1 0/0/0 brief
E1 0/0/0 is up.
  Applique type is Channelized E1 - balanced
  Cablelength is Unknown
  No alarms detected.
  alarm-trigger is not set
  Version info Firmware: 20090408, FPGA: 13, spm_count = 0
  Framing is CRC4, Line Code is HDB3, Clock Source is Internal.
  Data in current interval (708 seconds elapsed):
     0 Line Code Violations, 0 Path Code Violations
     2 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins
     2 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs
  Total Data (last 24 hours)
     0 Line Code Violations, 0 Path Code Violations,
     204 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins,
     204 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 4 Unavail Secs

Thanks a lot.

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Correct Answer by Steven Holl about 6 years 2 months ago

That test confirms that the clocking on the PBX isn't set right.  You need to set the PBX to receive clock from the Cisco router.

Use the second configuration (which is the one I recommended earlier in my post) and troubleshoot your clocking issues from the PBX side.

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Steven Holl Wed, 09/15/2010 - 09:19

Short answer:

Configure the PBX circuit to send clock, and configure the PBX to receive clock.  Then use this config:

network-clock-participate wic 0
network-clock-participate wic 1
network-clock-select 1 E1

That will do it.

Longer generic answer:

Internal DSPs on the 2800 and 3800 platforms run off of the router's  internal clock. Since we route voice packets to the DSP, we need to make  sure that the source for these packets (the voice PRIs) are also in the  same clocking domain as the internal router.  The easy way to do this is  to either send clock out the PRI (which you usually cannot do to a  provider), or tell the router to synchronize the internal clock with the  clocking that is being received from the provider PRI.  This is done  with the following two commands:

network-clock-participate wic X <----This tells the WIC for voice to  participate in the internal router clock.
network-clock-select 1 t1 <---This tells the router to  synchronize the internal clock with the clocking received off of the  specified port.

Generally, anytime you do voice over a TDM circuit, you need to  participate the WIC card, and make sure the source for the internal  clock is the same clock that the PRI is running off of.  When a data  circuit is on the same box, you normally don't want to participate that  card.

Also, if you have a scenario where you have multiple voice TDM circuits  terminating on the same gateway, and the circuits are to different  providers or are on different clocking domains, you will run into  issues.  The ways to resolve that are to accept clock in one provider,  and send  clock out to the other provider/PBX.

Here's some documents regarding the above commands:

http://www.cisco.com/en/US/customer/products/hw/routers/ps259/products_tech_note09186a008031a072.shtml
http://www.ciscotaccc.com/kaidara-advisor/voice/showcase?case=K20760878

There's also a good document on the general basics of clocking from  multiple sources here:

http://www.cisco.com/en/US/customer/products/sw/iosswrel/ps1834/products_feature_guide09186a008007fe8b.html#1019562

cmaiolani Thu, 09/16/2010 - 00:31

Hi Steven,

Thanks for the answer.

Maybe "Configure the PBX circuit to send clock, and configure the PBX to receive clock" is not correct or not?

But, If i have understand right, my configuration is already correct? But i still see some slips.

I don't understand one thing: my E1 0/1/0 is connected to Telco and the clock source is line, it gives clock to the router and my E1 0/0/0 connected to PBX in "emulate network" has clock source internal and so i think it takes the same clock of E1 0/1/0, right?

But in this way i have slips on E1 0/0/0.

The only way to not have slips on E1 0/0/0 is to set this with clock source line and set:

network-clock-participate wic 0
network-clock-participate wic 1
network-clock-select 1 E1 0/0/0

but in this way i see some slips in E1 0/1/0.

Thanks a lot.

Steven Holl Thu, 09/16/2010 - 06:23
Maybe "Configure the PBX circuit to send clock, and configure the PBX to receive clock" is not correct or not?

Yeah I phrased that poorly.  Have the Cisco side of the PBX circuit send the clock, and have the PBX receive the clock.

Because you are accepting clock from the provider, you need to select the provider clock to sync the DSP clock with it.  Currently, you have the PBX clock selected.

network-clock-participate wic 0
network-clock-participate wic 1
network-clock-select 1 E1 0/1/0

controller e1 0/1/0

clock source line

controller e1 0/0/0

clock source internal

If you still see slips on the PBX side after this, the PBX's clocking isn't set correctly.

cmaiolani Thu, 09/16/2010 - 08:47

Thanks a lot,

it's all clear so the initial configuration is OK.

I don't know if in the past there were these slips but i have put the attention in this direction when one day faxs didn't work well as in the past.

In addition in the last period fax is worse and people say that hear some kind of noise during a call when the call is directed to the PBX.

It seems like the PBX Clock or PBX's PRI interface doesn't work well. I think that a power off/on of the PBX can solve the problem.

Did you have similar issue in the past?

Steven Holl Thu, 09/16/2010 - 09:13

Yes, slips will cause faxing issues.

If you see slips and aren't seeing other types of layer 1 errors, it's always a clocking misconfiguration or design issue.

From your config, it isn't right as per your description.  You said the PBX was on 0/0/0, and you had 0/0/0 selected.  You need to select the provider clock, not the PBX clock.

If it runs clean for days and then all of a sudden you seen some slips and it recovers, then someone's clock is bad.  I've never encounted a 'bug' regarding clocking on an ISR in my time here troubleshooting TDM interfaces.

cmaiolani Thu, 09/16/2010 - 09:50

Hi,

Sorry...i hope in this way it is clear..

This config. was a personal test

network-clock-participate wic 0

network-clock-participate wic 1

network-clock-select 1 E1 0/0/0

controller e1 0/1/0

clock source line

controller e1 0/0/0

clock source line

and i see that in this way, e1 0/0/0 (PBX connection) doesn't have slips while e1 0/1/0 (Telco) has slips. It seems like the PBX gives me the clock.

But the current config that i have in the Voice Gateway is:

network-clock-participate wic 0
network-clock-participate wic 1
network-clock-select 1 E1 0/1/0

controller e1 0/1/0

clock source line

controller e1 0/0/0

clock source internal

and in this way i have slips on e1 0/0/0 (PBX conn.)  (one slip every 7 minutes)

Correct Answer
Steven Holl Thu, 09/16/2010 - 11:10

That test confirms that the clocking on the PBX isn't set right.  You need to set the PBX to receive clock from the Cisco router.

Use the second configuration (which is the one I recommended earlier in my post) and troubleshoot your clocking issues from the PBX side.

cmaiolani Wed, 09/29/2010 - 03:18

Hi Steven,

finally my customer has changed the PRI interface of PBX and everything now works fine, even if i still see 200 slips every 24 hours, maybe they are so few to create problems.

Thanks a lot for the support.

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