Explicit Pointer Adjustment Relay (Epar) - Cisco ASR 900 Series Configuration Manual

Clock Recovery System for SAToP

Explicit Pointer Adjustment Relay (EPAR)

A pointer management system is defined as part of the definition of SONET. If there is a frequency offset
between the frame rate of the transport overhead and that of the SONET Synchronous Payload Envelope
(SPE), the alignment of the SPE slips back periodically or advances in time through positive or negative
stuffing. Similarly, if there is a frequency offset between the SPE rate and the VT rate it carries, the alignment
of the VT slips back periodically or advances in time through positive or negative stuffing within the SPE.
The emulation of this aspect of SONET network in pseudowire emulation network may be accomplished
using EPAR feature.
EPAR uses N and P bits in CEP header to signal negative or positive pointer justification event. EPAR is
supported on STS-1, STS-3C, STS-12C, STS-48C and VT-1.5 levels. N and P counters are added to
communicate the signaling of the pointer events over CEP pseudowire.
Figure 10: EPAR
Effective Cisco IOS-XE Release 3.18 SP, EPAR is enabled by default.
Restrictions for EPAR
• EPAR is applicable only for circuit emulation for SONET LO & HO paths and is not applicable for
PDH.
• EPAR is effective only when both ends of the pseudowire have access to a common timing reference.
1-Port OC-192 or 8-Port Low Rate CEM Interface Module Configuration Guide, Cisco IOS XE Everest 16.7.x (Cisco
Configuring ACR for OCn
ASR 900 Series)
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