Clock Recovery System

The Clock Recovery System recovers the service clock using Adaptive Clock Recovery (ACR) and Differential Clock Recovery (DCR).

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Information About Clock Recovery

Adaptive Clock Recovery (ACR)

Adaptive Clock Recovery is most commonly used for Circuit Emulation (CEM). ACR is an averaging process that negates the effect of random packet delay variation and captures the average rate of transmission of the original bit stream. ACR recovers the original clock for a synchronous data stream from the actual payload of the data stream. In other words, a synchronous clock is derived from an asynchronous packet stream. ACR (Adaptive Clock Recovery) is a technique where the clock from the TDM domain is mapped through the packet domain.

Differential Clock Recovery (DCR)

Differential Clock Recovery (DCR) is another technique used for Circuit Emulation (CEM) to recover clocks based on the difference between PE clocks. TDM clock frequency are tuned to receive differential timing messages from the sending end to the receiving end. A traceable clock is used at each end, which ensures the recovered clock is not affected by packet transfer.

Benefits of Clock Recovery

  • Customer-edge devices (CEs) can have different clock from that of the Provide-edge devices (PEs).

Scaling Information

IM Card

Pseudowires Supported (Number of Clocks Derived)

DS1

48

DS3

1344

OC-192 Interface Module or 8-port Low Rate CEM Interface Module (10G HO / 10G LO)

2000

Prerequisites for Clock Recovery

  • Ensure that the DS1 and DS3 IMs (CPE) have their own clock, used as a service clock.

  • Ensure that CEM is configured before configuring the global clock recovery.

  • Enable RTP for DCR in CEM, as the differential clock information is transferred in the RTP header.

Restrictions for Clock Recovery

  • The reference clock source is used and locked to a single clock.

  • The clock ID should be unique for a particular interface module for ACR/DCR configuration.

How to Configure ACR and DCR

Configuring ACR for T3

Configuring Adaptive Clock Recovery of T3 Interfaces for SAToP

Before You Begin

  • The node (router) on which the DS3 is configured for ACR, must have its own clock derived from BITS/GPS/Stratum clock.

  • The minimum packet size of CEM pseudowires on the network that delivers robust clock recovery is 256 bytes.

To configure the clock on T3 interfaces for SAToP in controller mode, use the following commands: 
 
enable
configure terminal
controller t3 0/0/1
cem-group 0 unframed
clock source recovered 1
exit
To configure the clock recovery on T3 interfaces in global configuration mode, use the following commands: 
 
recovered-clock 0 0
clock recovered 1 adaptive cem 1 0
exit

Note

The clock configuration on controller must be done before configuring the clock recovery on global configuration mode.

Verifying Adaptive Clock Recovery Configuration of T3 Interfaces for SAToP

Use the show recovered-clock command to verify the adaptive clock recovery of T3 interfaces for SAToP: 
Router# show recovered-clock 

Recovered clock status for subslot 0/4
---------------------------------------
Clock    Type     Mode        CEM    Status       Frequency Offset(ppb)  Circuit-No

0        DS3      ADAPTIVE     0    	ACQUIRED   		n/a                    0 (Port)
Use the show running-config | section command to verify the configuration of adaptive clock of T3 interfaces: 
Router# show running-config | section 0/0/1
controller MediaType 0/0/1
 mode t3
controller T3 0/0/1
  cem-group 0 unframed
 clock source recovered 1
 cablelength 224
interface CEM0/0/1
 no ip address
 cem 0
Use the show running-config | section recovered-clock command to verify the recovery of adaptive clock of T3 interfaces: 
Router# show running-config | section recovered-clock
recovered-clock 0 0
 clock recovered 1 adaptive cem 1 0

Configuring DCR for T3

Configuring Differential Clock Recovery of T3 Interfaces for SAToP

Before You Begin

  • Before you start configuring DCR, RTP must be enabled on the CEM interface. The RTP is used to carry the differential time.

  • The minimum packet size of CEM pseudowires on the network that delivers robust clock recovery is 256 bytes.

To configure Differential clock recovery on T3 interfaces for SAToP in controller mode, use the following commands: 

 
enable
configure terminal
controller t3 0/0/1
cem-group 0 unframed
clock source recovered 1
exit

To configure RTP header under interfaces, use the following commands: 

 
interface cem 0/0/1
cem 0
rtp-present

To configure Differential clock recovery of T3 interfaces in global configuration mode, use the following commands: 

 
recovered-clock 0 0
clock recovered 1 differential cem 1 0
exit

Note

The clock configuration on controller must be done before configuring the clock recovery on global configuration mode.

Verifying the Differential Clock Recovery Configuration of T3 Interfaces for SAToP

Use the show recovered-clock command to verify the differential clock recovery of T3 interfaces for SAToP: 
Router# show recovered-clock 

Recovered clock status for subslot 0/4
----------------------------------------
Clock    Type     Mode        				CEM    Status       Frequency Offset(ppb)  Circuit-No

0        DS3      DIFFERENTIAL     0    	ACQUIRED   		n/a                    0 (Port)
Use the show running-config | section command to verify the configuration of differential clock of T3 interfaces for SAToP: 
Router# show running-config | section 0/0/1
controller MediaType 0/0/1
 mode t3
controller T3 0/0/1
  cem-group 0 unframed
 clock source recovered 1
 cablelength 224
interface CEM0/0/1
 no ip address
 cem 0
  rtp-present
Use the show running-config | section recovered-clock command to verify the recovery of differential clock of T3 interfaces: 
Router# show running-config | section recovered-clock
recovered-clock 0 0
 clock recovered 1 differential cem 1 0

Configuring ACR for Channelized T3/T1

Configuring Adaptive Clock Recovery of Channelized T3/T1 Interfaces for SAToP

Before You Begin

  • The node (chassis) on which the T3/T1 is configured for ACR, has to have its own clock derived from BITS/GPS/Stratum clock.

  • The minimum packet size of CEM pseudowires on the network that delivers robust clock recovery is 256 bytes.

To configure adaptive clock on a channelized T3/T1 interfaces under controller for SAToP in controller mode, use the following commands: 
 
enable
configure terminal
controller t3 0/0/1
t1 1 cem-group 0 unframed
t1 1 clock source recovered 1
exit
To configure recovery of adaptive clock of channelized T3/T1 interfaces in global configuration mode, use the following commands: 
 
recovered-clock 0 0
clock recovered 1 adaptive cem 1 0
exit

Note

The clock configuration on controller must be done before configuring the clock recovery on global configuration mode.

To remove the clock configuration in ACR and DCR, you must remove the recovery clock configuration in global configuration mode and then remove the controller configuration.

Verifying the Adaptive Clock Recovery Configuration of Channelized T3/T1 Interfaces for SAToP

Use the show recovered-clock command to verify the adaptive clock recovery of T3/T1 interfaces for SAToP: 
Router# show recovered-clock

Recovered clock status for subslot 0/4
----------------------------------------

Clock    Type      Mode        CEM    Status      Frequency Offset(ppb)  Circuit-No
0        DS3-ds1  	ADAPTIVE     0     ACQUIRED 			n/a                    0/1 (Port/t1)
Use the show running-config | section command to verify the configuration of adaptive clock of channelized T3/T1 interfaces: 
Router# show running-config | section 0/0/1
controller MediaType 0/0/1
 mode t3
controller T3 0/0/1
 framing c-bit
 cablelength 224
 t1 1 cem-group 0 unframed
 t1 1 clock source recovered 1
interface CEM0/0/1
 no ip address
 cem 0
Use the show running-config | section recovered-clock command to verify the recovery of adaptive clock of channelized T3/T1 interfaces: 
Router# show running-config | section recovered-clock
recovered-clock 0 0
 clock recovered 1 adaptive cem 1 0

Configuring DCR for Channelized T3/T1

Configuring Differential Clock Recovery of Channelized T3/T1 Interfaces for SAToP

Before You Begin

  • Before you start configuring DCR, RTP must be enabled on the CEM interface. The RTP is used to carry the differential time.

  • The minimum packet size of CEM pseudowires on the network that delivers robust clock recovery is 256 bytes.

To configure differential clock on a channelized T3/T1 interfaces under controller in controller mode, use the following commands: 
 
enable
configure terminal
controller t3 0/0/1
t1 1 cem-group 0 unframed
t1 1 clock source recovered 1
exit
To configure RTP header under channelized T3/T1 interfaces, use the following commands: 
 
interface cem 0/0/1
cem 0
rtp-present
To configure recovery of differential clock of channelized T3/T1 interfaces in global configuration mode, use the following commands: 
 
recovered-clock 0 0
clock recovered 1 differential cem 1 0
exit

Note

The clock configuration on controller must be done before configuring the clock recovery on global configuration mode.

Verifying the Differential Clock Recovery Configuration of Channelized T3/T1 Interfaces for SAToP

Use the show recovered-clock command to verify the differential clock recovery of T3/T1 interfaces for SAToP: 
Router# show recovered-clock 

Recovered clock status for subslot 0/4
---------------------------------------- 

Clock    Type         Mode        			CEM   	 Status     Frequency Offset(ppb)	Circuit-No

0        DS3-ds1      DIFFERENTIAL   0       ACQUIRED    n/a                  0/1 (Port/t1)
Use the show running-config | section command to verify the configuration of differential clock of channelized T3/T1 interfaces: 
Router# show running-config | section 0/0/1
controller MediaType 0/0/1
 mode t3
controller T3 0/0/1
 framing c-bit
 cablelength 224
 t1 1 cem-group 0 unframed
 t1 1 clock source recovered 1
interface CEM0/0/1
 no ip address
 cem 0
  rtp-present
Use the show running-config | section recovered-clock command to verify the recovery of differential clock of channelized T3/T1 interfaces: 
Router# show running-config | section recovered-clock
recovered-clock 0 0
 clock recovered 1 differential cem 1 0

Additional References for Clock Recovery

Related Documents

Related Topic

Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

Standards and RFCs

Standard/RFC

Title

ITU -T G.8261

Timing and synchronization aspects in packet networks

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MIB

MIBs Link

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http:/​/​www.cisco.com/​go/​mibs

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