1-11
PA-A2 ATM CES Port Adapter Installation and Configuration
OL-3460-01
Chapter 1 Overview: PA-A2 ATM CES Port Adapter
Circuit Emulation Services Overview
Figure 1-8
Typical Configuration of Network Clock Source
Clocking Modes for CES Operations
For CES operations, three clocking modes can be used in conjunction with any ATM CES port adapter.
These clocking modes are described in the following sections in the recommended order of consideration
and use:
•
Synchronous Clocking Mode, page 1-11
•
Synchronous Residual Time Stamp Clocking Mode, page 1-12
•
Adaptive Clocking Mode, page 1-12
•
Summary of Clocking Modes, page 1-13
Synchronous Clocking Mode
Synchronous clocking mode requires a PRS and network clock synchronization services. When
equipped with an ATM CES port adapter and appropriate software, any Cisco 7200 series router or
Cisco uBR7200 series can serve as a means for synchronizing the flow of user CBR traffic through the
network.
Synchronous clocking can be used for unstructured service (clear channel) and is the only clocking mode
for structured (N x 64 kbps) CES services. It is the recommended option for three reasons:
•
First, this clocking mode is the only one that supports full CES functionality. SRTS and adaptive
clocking do not support structured CES services.
•
Second, synchronous clocking exhibits superior stability, reliability, and wander/jitter
characteristics.
•
Third, synchronous clocking is typically used in public telephone systems, making a precision
reference signal readily and widely available for the synchronizing of CBR data transport.
For these reasons, synchronous clocking is the default clocking mode for all CES services.
PBX
T1/E1
T1/E1
OC3
ATM
WAN
uplink
OC3
ATM
WAN
uplink
Network clock source
set to ATM WAN uplink
Primary Reference
Source (PRS)
Network clock source
set to ATM WAN uplink
T1/E1 port Tx
clock set to network
derived, synchronized
clocking type used
T1/E1 port Tx
clock set to network
derived, synchronized
clocking type used
PBX Tx clock
source is
derived from
the Rx clock
PBX Tx clock
source is
derived from
the Rx clock
T1/E1
T1/E1
Network clock synchronization
supported in the ATM and ports
Tx clock uses network clock
as reference
S5992
Router
Router
PBX
ATM
network
