Cisco ASR 9000 Series User Manual

Synchronous Ethernet (SyncE) provides the Ethernet physical layer network level frequency distribution of
known common precision frequency references.
To maintain SyncE links, a set of operations messages are required. These messages ensure a node is always
deriving timing from the most reliable source, and transfers information about the quality of the timing source
being used to clock the SyncE link. In SONET/SDH networks, these are known as Synchronization Status
Messages (SSM). SyncE uses Ethernet Synchronization Message Channel (ESMC) to provide transport for
SSMs.
Frequency Synchronization Timing Concepts
The Cisco IOS XR frequency synchronization infrastructure is used to select between different time sources
to set the router backplane frequency and time-of-day. There are two important concepts that must be understood
with respect to the frequency synchronization implementation.
Sources
A source is a piece of hardware that inputs frequency signals into the system or transmits them out of the
system. There are four types of sources:
•
Line interfaces. This includes SyncE interfaces and SONET interfaces.
•
Clock interfaces. These are external connectors for connecting other timing signals, such as BITS, UTI
and GPS.
•
PTP clock. If IEEE 1588 version 2 is configured on the router, a PTP clock may be available to frequency
synchronization as a source of the time-of-day and frequency.
•
Internal oscillator. This is a free-running internal oscillator chip.
Each timing source has a Quality Level (QL) associated with it which gives the accuracy of the clock. This
QL information is transmitted across the network via SSMs over the Ethernet Synchronization Messaging
Channel (ESMC) or SSMs contained in the SONET/SDH frames so that devices know the best available
source to synchronize to. In order to define a preferred network synchronization flow, and to help prevent
timing loops, you can assign priority values to particular timing sources on each router. The combination of
QL information and user-assigned priority levels allows each router to choose a timing source to use to clock
its SyncE and SONET/SDH interfaces, as described in the ITU standard G.781.
Selection Points
A selection point is any point where a choice is made between several frequency signals, and possibly one or
more of them are selected. Selection points form a graph representing the flow of timing signals between the
different cards in a router running Cisco IOS XR software. For example, one or multiple selection points
select between the different Synchronous Ethernet inputs available on a single line card, and the result of these
selection points is forwarded to a selection point on the RSP to select between the selected source from each
card.
The input signals to the selection points can be:
•
Received directly from a source.
•
The output from another selection point on the same card.
Cisco ASR 9000 Series Aggregation Services Router System Management Configuration Guide, Release 5.1.x
192
Configuring Frequency Synchronization
Frequency Synchronization Timing Concepts

Cisco ASR 9000 Series Specifications

General

Brand	Cisco
Model	ASR 9000 Series
Category	Network Router
Language	English