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Hiroshi Ishiyama
Cisco Employee
Cisco Employee

Introduction

 

When using digital voice interfaces like T1 CAS and PRI on Voice GW, attention needs to be paid to clock synchronization.  In an event the clock is not accurately synchronized, line slips might occur and cause issues with audio quality and faxing.

In this document, we will explain how to approach clock synchronization on the ISR 4K.

 

1.Digital Voice Interface module on the ISR4K

 

Voice is processed by the DSP(PVDM4)that is inserted in the NIM-xMFT-T1/E1 module used in ISR 4K.
The Backplane Clock Mux can be used to synchronize the clock between modules, but it is not necessary to synchronize the clock with the backplane.
If you design the clock such that every module belongs to a different clock domain, the module can be connected to different devices providing different clocks.

 

.Command to synchronize clock on ISR4K

 

On ISR4K, clock can be synchronized with System level or Controller level commands.

 

System level command

 

network-clock synchronization automatic

 

  Enables network clock synchronization (required command)

 

network-clock synchronization participate slot / subslot 

 

  Decides whether or not you want to synchronize the module with the backplane clock. Synchronization is enabled by default

 

network-clock input-source priority controller [t1|e1] slot/bay/port 

 

 Configures the clock source of the backplane

  

Controller level command)

 

clock source line [primary|secondary]

  Configures the clock source of the backplane

 

clock source network

 

  Uses the backplane clock or the oscillator clock as the module clock source

  

Note: The command 'clock source internal' is used for data transmission, not for voice transmission.

 

3. Configuration examples for clock synchronization 

 

Below we will provide examples for clock synchronization on ISR4K

Example 1: You want to use the line clock as the master clock and synchronize with the backplane.

isr4k-clock1

 

network-clock synchronization automatic

network-clock synchronization participate 0/1

network-clock input-source 1 controller T1 0/1/0

 

controller T1 0/1/0

     clock source line primary

 

Example 2: You want to use the line clock as master, but not synchronize with the backplane

isr4k-clock2

network-clock synchronization automatic

no network-clock synchronization participate 0/1

network-clock input-source 1 controller T1 0/1/0

 

controller T1 0/1/0

     clock source line primary

 

Example 3: You want to use the backplane clock as the module clock source

isr4k-clock3

network-clock synchronization automatic

network-clock synchronization participate 0/1

network-clock input-source 1 controller T1 0/1/0

 

controller T1 0/1/0

     clock source network

 

Conclusion



When using digital voice interfaces, clock synchronization need to be designed correctly. Audio quality issues or fax failures can happen as a result of wrong clock design.

Please understand the impact of the clock related commands on ISR4K and correctly configure the clock.

 

References

Network Synchronization for the Cisco 4000 Series Integrated Services Routers

Comments
donaliforb
Level 1
Level 1

Hi hishiyam,

Very nicely explained articale. We are in the position of using the ISR4321 as the network side. So we want to provide clocking to PBXs.

We do not want to take clocking from the line (end customer's PBX). In this scenarion the most logical approach is that in Example 3. 

But in this case we get about 4 slips / 15 min. 

Only if we use the example1 with "network-clock input-source 1 controller E1 0/2/0" can we stabalise the line in terms of slip and CRC errors. But we do not like Example 1 as it means that as a service provider we are relying on the PBXs to provide clocking and in certain cases this might be fine but in others with unreliable clocks this is not.

 

In the current constallation the NIM has two E1 going to the same PBX and slips happen on both E1 simultaniously.

Any thoughts on or advice. Many thanks, Donal

Hiroshi Ishiyama
Cisco Employee
Cisco Employee

Hi Donal,

If you configure your ISR4321 to provide T1 clock (as clock master),
PBX or Telco should be configured to receive T1 clock from ISR4321.
Otherwise slip error will be observed on the T1 line.

You will need to consult with PBX vendor on the clock configuration
of the PBX.

If the PBX is connected with both ISR4321 and Telco, you need to take
care the clock synchronization design.
ISR4321 can support multiple clock domains (by using different NIMs),
but most of PBX can support single clock domain (by single TDM bus)
Possibe clock synchronization design are as follows.

Example 1: configure telco as clock master.

   Telco -----------> PBX ----------> ISR4321
       master   slave     master   slave

Example 2: configure ISR4321 as clock master.

   Telco <---------- PBX <---------- ISR4321
        slave   master     slave  master

I am not sure if Telco can receive clock from PBX.

Hiroshi

donaliforb
Level 1
Level 1
Hi Hiroshi,
We are the Telco and connect into the core with SIP over either Fiber or VDSL

Core-SBC -------(SIP over Fiber or VDSL)----> ISR4321-----> PBX (two E1s on same NIM)
master -> slave

Our only way of getting to a No-Slips situation is by taking the clocking from the line. If we configure “Clock source Network” then we always have slips.

Regards
Donal
Hiroshi Ishiyama
Cisco Employee
Cisco Employee
Hi Donal,

If the PBX is only connected with ISR4321 (via two E1s on the
same NIM), the PBX need to receive the clock from the line and
send E1 frame using the same clock.
Would it be possible for you to check the PBX's configuration.
(Is the PBX's line is configured as "slave clock" or "internal clock"?)

Hiroshi
donaliforb
Level 1
Level 1
Good point. I will check how the customer’s PBX is set up. They claim to take the clocking from the line but it really looks like that they use an internal clock. Hence the fact that when the ISR is set to Network there are slips.

Hi,

I have configured 3 E1 isdn PRI located at one NIM-1MFT-T1/E1 and one NIM-2MFT-T1/E1, like the required is that the ISR4331 acts as isdn network side I have configured clock as you indicate in example 3 but I have a lot of sleeps in the 3 controllers.

Wich can be the problem.

Thank you very much in advance for your attention.

Best regards.

Hi all,


I have found this note in the chapter 'Network Synchronization for the Cisco 4000 Series ISRs' into the document 'Configuring the Cisco Fourth-Generation T1/E1 Voice and WAN Network Interface Module'

 

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/interface/command/ir-cr-book/ir-l2.html#wp3547345570

 
'Since the T1/E1 NIM on Cisco 4000 Series ISR Platforms only perform clocking locally on the module and do not participate in backplane clocking it is not recommended to use these new commands for clocking configuration. Instead it is recommended to disable clock participation in the backplane with the command no network-clock synchronization participate [slot | subslot]'
 

That seams different that you is suggesting on example 3, in fact, I have applied the 'no network clock participate' command for the two slots where I have the NIM cards an 'sleeps` have been reduced drastically, but still exists, now, I get a couple of 'sleeps' per hour.


I do not understand why, if router is providing clock, I getting 'sleeps' in the controllers.

Thank you in advanced for your answer.

 

 

 

 

Neeragu
Cisco Employee
Cisco Employee

Hey Everyone,

Can anybody tells what would be the appropriate solution for the below customer query 

"We have 2 routers, each with 2 back-to-back PRIs, these routers are configured as the network side and is supposed to provide clocking to the remote PBX.

 

PRIGW-VG01-003006-01SE#sh run | s 0/1/0|0/2/0

controller T1 0/1/0

framing esf

clock source network

linecode b8zs

cablelength long 0db

pri-group timeslots 1-24

 

controller T1 0/2/0

framing esf

clock source network

linecode b8zs

cablelength long 0db

pri-group timeslots 1-24

 

interface Serial0/1/0:23

no ip address

encapsulation hdlc

isdn switch-type primary-ni

isdn protocol-emulate network

isdn incoming-voice voice

isdn supp-service name calling

isdn outgoing display-ie

trunk-group VESTA

 

interface Serial0/2/0:23

no ip address

encapsulation hdlc

isdn switch-type primary-ni

isdn protocol-emulate network

isdn incoming-voice voice

isdn supp-service name calling

isdn outgoing display-ie

trunk-group VESTA

 

The first PRI doesn’t have clocking issues, the other one does.

 

PRIGW-VG01-003006-01SE#sh controller t1

T1 0/1/0 is up

  Applique type is Channelized T1

  Cablelength is long gain36 0db

  No alarms detected.

  alarm-trigger is not set

  Soaking time: 3, Clearance time: 10

  AIS State:Clear  LOS State:Clear  LOF State:Clear

  Framing is ESF, Line Code is B8ZS, Clock Source is Network.

  BER thresholds:  SF = 10e-3  SD = 10e-6

  Data in current interval (599 seconds elapsed):

     0 Line Code Violations, 0 Path Code Violations

     0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins

     0 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,

     0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins,

     0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

 

T1 0/2/0 is up

  Applique type is Channelized T1

  Cablelength is long gain36 0db

  No alarms detected.

  alarm-trigger is not set

  Soaking time: 3, Clearance time: 10

  AIS State:Clear  LOS State:Clear  LOF State:Clear

  Framing is ESF, Line Code is B8ZS, Clock Source is Network.

  BER thresholds:  SF = 10e-3  SD = 10e-6

  Data in current interval (134 seconds elapsed):

     0 Line Code Violations, 0 Path Code Violations

     1 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins

     1 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,

     749 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins,

     749 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

 

What we want is to verify how the clocking is supposed to be configured in this scenario.  These “circuits” are on different NIMs and hence are consider their own clocking domain, but since they connect to the same remote device, they need to be in sync.

 

NAME: "NIM subslot 0/1", DESCR: "NIM-4MFT-T1/E1 - T1/E1 Serial Module"

PID: NIM-4MFT-T1/E1    , VID: V04  , SN: FOC213550N8

 

NAME: "NIM subslot 0/2", DESCR: "NIM-1MFT-T1/E1 - T1/E1 Serial Module"

PID: NIM-1MFT-T1/E1    , VID: V04  , SN: FOC21374Q0E

 

rrighi
Level 1
Level 1

@Neeragu  Good morning Neeragu, i've got the same issue.

1 router with 2 E1 port to connect 2 legacy PBX. I need to configure both E1 port to generate same clock for PBX ( that take clock from line) but on one of this i've got slip error.

If i use a Cisco 2911 all works like a charm.

Did you solve the issue? Could yo

Michael Mertens
Level 1
Level 1

Hiroshi

Beautiful, beautiful diagrams, document and explanation on the ISR clocking. I want/need to use the back-plane clock as the module clock source (example 3), however, that diagram did not load. Could you please add it again? I want to comprehend as much of the ISR4xxx as possible. THANK YOU!!!
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