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The Clock Recovery System is able to recover the service clock using two methods, the Adaptive Clock Recovery and Differential Clock Recovery.

When emulating TDM over PSNs, the physical layer clock is sometimes not available to both ends. Adaptive Clock Recovery (ACR) is a technique where the clock from the TDM domain is mapped through the packet domain. The sending Inter Working Function (IWF) processes outgoing packets with an internal free-running clock, and the receiving IWF creates a clock based on packet arrival. The service clock frequency is adjusted depending on fill level of the jitter buffer.

• When sending TDM digital signal over PSN, the TDM data is inserted into packets in the master IWF and sent to the desired destination (slave IWF).

• The rate at which the packets are transmitted to the PSN is constant. Due to the nature of the PSN, the packets might arrive to the destination in bursts and with varying rate.

• The long-term average of this rate is equal to the insertion rate at the master IWF. Moreover, the packets in the PSN might switch their order and even be lost.

• The IWF at the far end of the PSN (slave IWF) recovers the service clock (E1/T1) used by the master IWF.

• The recovered clock is used by the slave IWF for the transmission of the data back into the TDM lines.

• The master IWF aggregates the TDM data and creates the PWE packets; these packets are transmitted to the PSN.

• The packets are received by the slave IWF and stored in a jitter buffer designed to absorb the packet delay variation (PDV).

• The packets are extracted from the jitter buffer and the clock recovery algorithm updates the service clock based on the timing information available.

DCR (Differential Clock Recovery) is another technique used for Circuit Emulation (CEM) to recover clocks based on the difference between PE clocks. The clock from the TDM domain is mapped through the packet domain. It differs from ACR in that a PRC traceable clock is used at each end. Differential timing messages are used to tune the TDM clock frequency from the sending end to the receiving end. Both ends have a source traceable reference. Because of this, the recovered clock is not affected by PDV when using DCR.

In contrast with DCR, a PRC traceable clock source is available at each end. ACR is used when a traceable source is not available at both ends of the PSN link.

The recreated service clock accuracy is dependent on the accuracy between the sending and receiving PRC frequencies.

To configure a network clock, use the following commands:

enable
configure terminal
controller T1  0/5/0
clock source  line
cem-group  0 unframed
exit
enable
configure terminal
network-clock input-source 1 controller T1 0/5/0
exit