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Ethernet Synchronization Message Channel (ESMC)

Ethernet Synchronization Message Channel (ESMC) is a logical communication channel. It transmits Synchronization Status Message (SSM) information, which is the quality level of the transmitting synchronous Ethernet equipment clock (EEC), by using ESMC protocol data units (PDUs).

Ethernet Synchronization Message Channel Overview

Ethernet Synchronization Message Channel (ESMC) is a logical communication channel. It transmits Synchronization Status Message (SSM) information, which is the quality level of the transmitting synchronous Ethernet equipment clock (EEC), by using ESMC protocol data units (PDUs). ESMC support is based on the ITU G.8264 specification.

Synchronized Ethernet with Ethernet synchronized Message Channel (ESMC) is supported on MX204 and MX10003 routers.

A Synchronous Ethernet interface is configured to operate in the following modes:

  • Nonsynchronous mode—In this mode, the Synchronous Ethernet interface does not process the ESMC message and does not extract the quality level information.

  • Synchronous mode—In this mode, the Synchronous Ethernet interface processes the ESMC message and extracts the quality level information. While operating in synchronous mode, the ESMC messages transmit the quality level.

You can enable ESMC on a Synchronous Ethernet port by adding the port to a list of ESMC interfaces. The ESMC messages are transmitted through the port indicating the quality level of the clock it is capable of driving and the ESMC messages are received (if the other endpoint supports ESMC) with the quality level of the transmitting clock. The MPC receiving the ESMC messages on its configured Synchronous Ethernet ports extracts the quality level and transmits it to the Routing Engine. The clock selection algorithm on the Routing Engine collects the ESMC data from each of the ESMC-enabled ports to select the clock sources.

The clock selection process supports revertive and nonrevertive modes. When the clock selection process has selected two clock sources—a primary and a secondary—and the active primary clock source degrades over a period of time and then improves again, this primary clock source again becomes the active clock source only if revertive mode is enabled. If nonrevertive mode is set and the secondary clock source is currently active (due to a previous degradation of primary clock source), the primary clock source is not reactivated even after its quality improves.

The clock selection is based on the following three operational modes:

  • Forced free-run—In this mode, you can set the clock source either from a free-run local oscillator or from an external qualified clock. For MX80 routers, the free-run clock is provided by the local oscillator. For MX240, MX480, and MX960 routers, the free-run clock is provided by the Switching Control Board (SCB).

  • Forced holdover—This mode is an internal state the synchronous Ethernet Equipment Clock (EEC) goes into, when an upstream clock source that the system locks on to is no longer available. You cannot configure this mode because it is an internal state.

  • Automatic selection—In this mode, the system chooses up to two best upstream clock sources. The system then uses the clock recovered from one of the sources to generate a frequency of 19.44 MHz and clock the transmit side of the Ethernet interfaces. If no upstream clock with acceptable good quality is available or if the system is configured in free-run mode, the system uses the internal clock. Automatic clock selection is based on the quality level, priority, signal fail, and external commands.

For more information about clock selection, see Configure External Clock Synchronization for MX Series Routers.

The synchronous EEC is in free-run mode when the chassis is switched on or restarted. When a synchronous EEC locks on to an upstream reference clock source at least once for a continuous period of 60 seconds, the EEC will have stored sufficient Synchronous Ethernet data in a replay holdover buffer. In case of failure of a reference clock source, the system goes to holdover mode and uses the replay data in the holdover buffer to service the downstream Synchronous Ethernet clients.

When a Modular Port Concentrator (MPC) with an EEC restarts (because of either a system crash or a manual restart), the holdover buffer data gets erased. Therefore, downstream Synchronous Ethernet clients cannot be serviced. This is also applicable when a new MPC containing an EEC is inserted into the system.

In a practical deployment scenario, the status display of holdover mode is invalid only when the chassis is switched on or restarted.

When an MPC containing an EEC is restarted or a new MPC containing an EEC is inserted into a system that is (already) in holdover mode, the EEC on this MPC cannot be considered to be in holdover mode because it does not have any Synchronous Ethernet replay information in its holdover data buffer. Therefore, you must first fix the system holdover issue before attempting to service the downstream Synchronous Ethernet clients on this MPC. To accomplish this, you must find a suitable upstream reference clock source and let the synchronous EEC lock on to this upstream reference clock source, and then service the downstream Synchronous Ethernet clients on this MPC.

Ethernet Synchronization Message Channel Quality Level Mapping

Ethernet Synchronization Message Channel (ESMC) is a logical communication channel. It transmits Synchronization Status Message (SSM) information, which is the quality level of the transmitting synchronous Ethernet equipment clock (EEC), by using ESMC protocol data units (PDUs). ESMC support is based on the ITU G.8264 specification. In order for an interface to receive or transmit ESMC messages, at least one logical interface must be configured on that interface. If the interface is currently not configured with a logical interface, you must configure a logical interface by using the [set interfaces interface-name unit 0] statement at the [edit] hierarchy level.

The following factors affect the ESMC quality level value that is transmitted out on the interfaces configured at the [edit chassis synchronization esmc-transmit interfaces] hierarchy level:

  • Quality mode

  • Selection mode

  • Conversion of PTP clock class flag

Other than the aforementioned factors, the software phase lock loop (spll) state or the hybrid state impacts the transmitted ESMC quality level when the router is in PTP mode or hybrid mode, respectively.

The following sections explain how the ESMC quality level is handled in various situations:

Synchronous Ethernet Mode

In Synchronous Ethernet mode, the ESMC quality level is handled in the following way:

  • In quality mode:

    • If the quality-mode-enable option at the [show chassis synchronization] hierarchy level is not set, then the configured quality and the priority set for the clock sources are used for the clock selection. The ESMC quality level is based on the configured quality level corresponding to the active clock source.

    • If the quality-mode-enable option at the [show chassis synchronization] hierarchy level is set, then only those clock sources that receive ESMC quality level is higher than or equal to the configured quality are considered for selection. The ESMC quality level value transmitted also depends on the selection mode option as discussed next.

  • In selection mode:

    • If the selection-mode option at the [show chassis synchronization] hierarchy level is set to configured-quality, then the configured quality for the selected, active source is used as the system ESMC quality level value that is transmitted out.

    • If the selection-mode option at the [show chassis synchronization] hierarchy level is set to received-quality, then the received ESMC quality level value from the selected clock source is transmitted out.

  • When no clock sources are locked:

    1. Do Not Use (DNU)/Don’t Use for Synchronization (DUS) quality level is transmitted.

    2. The ESMC quality level value sent out on the selected, active clock source interface is always DNU/DUS.

Precision Time Protocol Mode

In Precision Time Protocol (PTP) mode, you can transmit ESMC quality level values with the following parameters set:

  • The network-option option must be configured at the [edit chassis synchronization] hierarchy level.

  • Synchronous Ethernet sources must not be configured at the [edit chassis synchronization] hierarchy level.

  • The convert-clock-class-to-quality-level option at the [edit protocols ptp slave] hierarchy level must be enabled so that the PTP clock class received from the selected primary is converted to the appropriate ESMC quality level.

Clock class is a value that ranges from 80 through 109 and is used to map the clock class to the set ESMC quality level. The ESMC quality level value is mapped to the clock class value by one of the following methods:

  • Mapping of PTP clock class to ESMC quality level—By default, the standard mappings suggested by ITU-T G.781 specification are used as shown in Table 1 and irrespective of the clock being configured in hybrid mode or pure PTP mode, the outgoing quality level is always based on the PTP clock class mapping. To map the PTP clock class to the ESMC quality level, you must set the convert-clock-class-to-quality-level option at the [edit protocols ptp slave] hierarchy level. For default mapping values, see Table 1.

    Table 1: Default Quality Level to PTP Clock-Class Mapping

    SSM QL (Binary)

    Standard Mappings Given in ITU-T G.781 Specification

    PTP Clock Class
     

    Option I

    Option II

    		  

    0001

    -

    QL-PRS

    80

    0000

    -

    QL-STU

    82

    0010

    QL-PRC

    -

    84

    0111

    -

    QL-ST2

    86

    0011

    -

    -

    88

    0100

    QL-SSU-A

    QL-TNC

    90

    0101

    -

    -

    92

    0110

    -

    -

    94

    1000

    QL-SSU-B

    -

    96

    1001

    -

    -

    98

    1101

    -

    QL-ST3E

    100

    1010

    -

    QL-ST3/QL-EEC2

    102

    1011

    QL-SEC/QL-EEC1

    104

    1100

    -

    QL-SMC

    106

    1110

    -

    QL-PROV

    108

    1111

    QL-DNU

    QL-DUS

    110

  • User-defined mapping of PTP clock class to ESMC quality level—You can manually override the clock class to ESMC mapping by setting the clock-class option at the [edit protocols ptp slave clock-class-to-quality-level-mapping quality level ql-value] hierarchy level.

Unlike Synchronous Ethernet, the DNU/DUS quality level value is not transmitted on the interface on which the PTP primary is configured. In PTP mode, an interface is configured as part of the [edit chassis esmc-transmit interfaces] hierarchy level, and an appropriate ESMC quality level value is transmitted through it. Note that when the PTP clock class value received from the primary changes, the ESMC quality level transmitted also changes appropriately. If there is no valid clock class value as input, then the DNU/DUS value is transmitted on the interfaces configured under the esmc-transmit option at the [edit chassis synchronization] hierarchy level.

To view the current mapping between the clock class and the ESMC quality level, run the show ptp quality-level-mapping operational mode command.

To display the ESMC quality level currently transmitted by the interface, run the show ptp global-information operational mode command in PTP or hybrid mode. Note that when the convert-clock-class-to-quality-level option is disabled or when there is no valid clock class as input, the show ptp global information command does not display the ESMC quality level value.

To view the ESMC quality level transmitted in all modes, run the show synchronous-ethernet esmc transmit detail operational mode command.

Hybrid Mode

Hybrid mode is a combination of PTP and Synchronous Ethernet modes.

The configuration required for transmitting the ESMC quality level in hybrid mode differs from that in PTP mode in the following ways:

  • In hybrid mode, synchronous Ethernet source interfaces must be configured at the [edit chassis synchronization] hierarchy level.

  • In hybrid mode, configuring the convert-clock-class-to-quality-level option is optional. When this option is configured, the outgoing ESMC quality level behavior is the same as that in PTP mode. When the convert-clock-class-to-quality-level option is not configured, the outgoing ESMC quality level behavior is the same as that in Synchronous Ethernet mode.

Feature Mode Changes

When the router configuration is changed from one feature mode to another mode—that is from or to Synchronous Ethernet, PTP, or hybrid mode—the following occurs:

  1. The ESMC quality level is reset to DNU.

  2. Based on the new feature mode, the ESMC quality level is decided:

    • When the reference clock qualifies for Synchronous Ethernet mode.

    • When PTP goes into phase-aligned state or hold-over state in PTP mode.

    • When the hybrid state reaches frequency and phase aligned state in hybrid mode.

Sometimes PTP is required to drive Synchronous Ethernet and ESMC. This scenario occurs when:

  • After certain PTP hops, the network branches out, and one branch of the network requires only frequency synchronization while the other branch requires both phase and frequency synchronization.

  • A packet-based distribution network is located between a time-division multiplexing (TDM), a SONET, and a Synchronous Ethernet network.

In such situations, the clock recovered by PTP is sent over the Ethernet physical transceiver for Synchronous Ethernet, and the ESMC quality level value mapping with the PTP clock class is sent over the interfaces.