Clock Recovery System for SAToP

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 (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 is a technique where the clock from the TDM domain is mapped through the packet domain, but is most commonly used for Circuit Emulation (CEM). ACR is supported on unframed and framed modes of SAToP.

Note

Framing type should be maintained same in all routers end to end.

Effective Cisco IOS XE Everest 16.5.1, ACR is supported on the 8-port T1/E1 interface module.

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. DCR is supported on unframed and framed modes of SAToP.

Note

Framing type should be maintained same in all routers end to end.

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)

T1 CEM Interface Module

1476

Prerequisites for Clock Recovery

  • The clock of interface modules must be used as service clock.

  • CEM must be configured before configuring the global clock recovery.

  • RTP must be enabled 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 or DCR configuration.

  • When CEM group is configured, dynamic change in clock source is not allowed.

  • ACR clock configuration under each controller should be performed before configuring CEM group.

How to Configure ACR and DCR

Configuring Adaptive Clock Recovery of T1 Interfaces for SAToP

Before configuring Adaptive Clock Recovery, CEM must be configured. Below are the guidelines to configure clock recovery:

  • The node (chassis) on which the DS1 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 64 bytes.

To configure the clock on T1 interfaces for SAToP in controller mode: 
 
enable
configure terminal
controller t1 0/4/0
cem-group 0 unframed
clock source recovered 1
exit
To configure the clock recovery on T1 interfaces in global configuration mode, use the following commands: 
 
recovered-clock 0 1
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.

Configuring Adaptive Clock Recovery of T3 Interfaces for SAToP

Before configuring Adaptive Clock Recovery, CEM must be configured. Below are the guidelines to configure clock recovery:

  • The node (router) on which the interface module 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: 
 
enable
configure terminal
controller t3 0/4/12
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 1
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 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 0/1
---------------------------------------
Clock     Type   Mode         CEM    Status    Frequency Offset(ppb) Circuit-No
0          DS3    ADAPTIVE      0    ACQUIRED      n/a                 12 (Port)

Use the show running-config | section command to verify the configuration of adaptive clock of T3 interfaces: 

Router# show running-config | section 0/4/12
controller MediaType 0/4/12
mode t3
controller T3 0/4/12
cem-group 0 unframed
clock source recovered 1
cablelength 224
interface CEM 0/4/12
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 12 0

Configuring Differential Clock Recovery of T3 Interfaces for SAToP

  • 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 interface for SAToP in controller mode: 

 
enable
configure terminal
controller t3 0/4/12
cem-group 0 unframed
clock source recovered 1
exit

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

 
interface cem 0/4/12
cem 0
rtp-present

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

 
recovered-clock 0 1
clock recovered 1 differential cem 12 0
exit

Note

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

Associated Commands

The commands used to configure adaptive clock recovery and differential clock recovery are:

Commands

URL

clock recovered adaptive cem

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/interface/command/ir-cr-book/ir-c2.html#wp8894393830

clock recovered differential cem

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/interface/command/ir-cr-book/ir-c2.html#wp8894393830

cem-group

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/interface/command/ir-cr-book/ir-c1.html#wp2440628600

recovered-clock

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/interface/command/ir-cr-book/ir-o1.html#wp8262293900

controller t1/e1

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/interface/command/ir-cr-book/ir-c2.html#wp1472647421

clock-source

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/interface/command/ir-cr-book/ir-c2.html#wp6081785140

network-clock input-source

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

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

MIBs

MIB

MIBs Link

—

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http://www.cisco.com/go/mibs

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