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.
２．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.
|
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
|
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
|
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
