This chapter describes
the Synchronous Ethernet features, standards, and limitations in the
Cisco ME 1200 Series Carrier Ethernet
Access Device. This chapter also describes procedures
to configure Synchronous Ethernet.
Synchronous Ethernet Overview
A separate external
time-division multiplexing (TDM) circuit is required to provide synchronized
timing to multiple remote network elements (NEs) for packet transport networks
like Cisco Carrier Packet Transport system. The Synchronous Ethernet (SycnE)
feature addresses this requirement by providing effective timing to the remote
NEs through a packet network without using an external circuit for timing.
With Ethernet equipment
gradually replacing existing Synchronous Optical Networking (SONET) and
Synchronous Digital Hierarchy (SDH) equipment in service-provider networks,
frequency synchronization is required to provide high-quality clock
synchronization over Ethernet ports. The SyncE feature provides the required
synchronization at the physical level. Operation messages maintain SyncE links
and ensure that a node always derives timing from the most reliable source.
SyncE uses the Ethernet Synchronization Message Channel (ESMC) to enable
traceability of the best clock source to correctly define the timing source and
prevent a timing loop.
The Cisco ME 1200 Series
Carrier Ethernet Access Device supports Synchronous Ethernet (SyncE), which is
the physical layer frequency-synchronization solution for IEEE 802.3 links.
SyncE is defined by the ITU-T standards such as G.8261, G.8262, G.8264, and
G.781. It is an evolution of the conventional Ethernet and Ethernet + SDH and
SONET-based synchronization. SyncE is used to synchronize and send clock
information to remote sites on the network. For SyncE to work, each network
element along the synchronization path must support SyncE. SyncE provides only
frequency synchronization, not related to time or space.
SyncE provides the
Ethernet physical layer network (PHY) level frequency distribution of known
common precision frequency references. Clocks for use in SyncE are compatible
with the clocks used in the SONET/SDH synchronization network. To achieve
network synchronization, synchronization information is transmitted through the
network via synchronous network connections with performance of egress clock.
In SONET/SDH the communication channel for conveying clock information is SSM,
and in SyncE it is the ESMC.
SyncE is a standard for
distribution of frequency over Ethernet links. Other standards (IEEE Std. 1588
Precision Time Protocol [PTP], IETF Network Time Protocol [NTP], and so on)
have been and are being developed or enhanced for high-quality time
distribution and Adaptive Clock Recovery (ACR) requirements.
To maintain the timing
chain in SONET/SDH, operators often use SSM. Information provided by SSM
Quality Levels (SSM-QL) helps a node derive timing from the most reliable
source and prevent timing loops. The SONET/SDH header has a QL information
present in the S1 bytes of its header. Hence, the SONET/SDH does not require
any specific channel for QL information exchange. As the Ethernet does not have
the QL information in its header, it requires ESMC for QL information. Because
Ethernet networks are not required to be synchronous on all links or in all
locations, a specific channel, the ESMC channel defined in G.8264, provides
this service. ESMC is composed of the standard Ethernet header for an
organization-specific slow protocol, the ITU-T OUI; a specific ITU-T subtype;
an ESMC-specific header; a flag field; and a type, length, value (TLV)
structure: the use of flags and TLVs aimed at improving the management of
Synchronous Ethernet links and the associated timing change.
For more information,
- ITU-T G.8261: Timing and
synchronization aspects in packet network
- ITU-T G.8262: Timing
characteristics of Synchronous Ethernet equipment slave clock
- ITU-T G.8264: Distribution of
timing through packet networks
- ITU-T G.781: Synchronization
Network clocking uses
the Synchronization Status Messages (SSM) mechanism to exchange the Quality
Level (QL) of the clock between the network elements. In Ethernet, Ethernet
Synchronization Message Channel (ESMC) is used for SSM exchange.
The two important
protocols used for SyncE are:
- Synchronization Status Messages
- Ethernet Synchronization
Messaging Channel (ESMC)
Synchronization Status Messages (SSM)
Network elements use
Synchronization Status Messages (SSM) to inform the neighboring elements about
the Quality Level (QL) of the clock. The non-ethernet interfaces such as
optical interfaces and SONET/T1/E1 SPA framers uses SSM. The key benefits of
the SSM functionality:
- Prevents timing loops.
- Provides fast recovery when a
part of the network fails.
- Ensures that a node derives
timing from the most reliable clock source.
Ethernet Synchronization Messaging Channel (ESMC)
To maintain a logical
communication channel in synchronous network connections, ethernet relies on a
channel called Ethernet synchronization Messaging Channel (ESMC). This is based
on IEEE 802.3 Organization Specific Slow Protocol standards. ESMC relays the
SSM code that represents the Quality Level (QL) of the Ethernet Equipment Clock
(EEC) in a physical layer.
The ESMC packets are
received only for those ports configured as clock sources and transmitted on
all the SyncE interfaces in the system. These packets are then processed by the
Clock selection algorithm and are used to select the best clock. The Tx frame
is generated based on the QL value of the selected clock source and sent to all
the enabled SyncE ports.
Clock Selection Algorithm
The clock selection
algorithm selects the best available synchronization source from the nominated
sources. This algorithm exhibits nonrevertive behavior among the clock sources
with the same QL value, and always selects the signal with the best QL value.
For clock option SDH, the default is revertive, and for clock option SONET, the
default is nonrevertive.
The following parameters
contribute to the selection process:
- Quality level (QL)
- Signal fail through QL-FAILED
- External commands (Manual,
Auto-revertive and so on)
Clock Selection Modes
A clock selection is
said to be the best, when the clock source is configured with the highest QL
and with the highest priority (for the ones with equal QL).
The following are
different clock selection modes:
- Manual—the clock selector
is manually set to the chosen clock source. If the manually selected clock
source fails, then, the clock selector goes to the holdover state.
- Selected—the clock selector
selects the clock manually, however, the highest priority selected clock source
becomes the Source.
- NonRevertive—the clock
selector selects the best clock source only done when the selected clock fails.
- Revertive—the selection of
the best clock source is constantly searched for.
- Holdover—the clock selector
is forced to the holdover state.
- Freerun—the clock selector
is forced to the free run state.
Manual mode is used to
force selection of a specific source. It is also used to switch back to the
primary source if auto-nonrevertive mode is selected and the failure is
cleared. Selected mode is used to freeze the current clock source, in case of a
failure on switchover.