6 — Network timing reference support in ISAM
6.2 ISAM clock system and NTR extraction
High level description of the external port selection for NTR
Figure 6-3 gives a high level description on how the external port is selected that will be used for NTR extraction. This is valid for BITS and SyncE which both are linked to physical ports.
An ISAM hardware configuration has a number of external ports RJ45-a, RJ45-b, SFP-1,…, SFP-n, XFP-1,…, XFP-m available on NT-A, and possibly also on NT-B, and NTIO, in case the latter are also present. Not every port can be used for synchronization input. Hardware design of the specific ISAM boards determine which ports can be used for SyncE input (some Ethernet ports) or BITS input (some RJ45 ports), and this will be then a subset of the total number of external ports (see Figure 6-3).
Figure 6-3 Port selection for external NTR (SyncE and BITS)
The operator needs to configure which of these ports are valid inputs for NTR in his network deployment. Maximum 2 ports can be configured for this (T and U in Figure 6-3).
The ISAM clock subsystem will then dynamically select one of these 2 ports as NTR reference, according to the actual quality of the NTR signals on these ports, configured priority of these ports, and so on, according to the ITU Rec G.871 section 5.6 criteria and selection algorithm.
External
6 — Network timing reference support in ISAM
Possible External NTR sources
The ISAM supports the following external NTR clock sources:
• One BITS / SSU interface per NT faceplate:
This interface supports a 2.048 MHz plain clock signal, an E1 framed signal, or a DS1 framed signal. For ETSI markets, the default expected input is an E1 framed signal.
SSM is not supported on this interface.
BITS has been a very common way of clock distribution in PDH/SDH networks for already a long time, and is therefore available in many COs. Even after migration from PDH/SDH networks to Metro Ethernet, it is still available in many cases for clock distribution. And because Synchronous Ethernet requires new specific hardware not yet available on first generations of Metro Ethernet networks, BITS is still an important option for providing NTR to ISAMs in COs.
• One or more Synchronous Ethernet interfaces on the NT or NTIO faceplates:
This can be only supported on optical 1 GE, 2.5 GE and 10 GE interfaces, and not at other speeds (for example, 100 Mbps), nor on any electrical interface.
SSM reception and processing can be enabled on these interfaces.
Further network rationalization is the driver to move all functions to the Metro Ethernet, so the PDH/SDH network becomes completely obsolete. Consequently, over time, SyncE will become the more important solution for NTR. Since SyncE-support requires specific hardware, upgrades of some nodes in the Metro Ethernet network may be required.
Figure 6-1 and Figure 6-2 give a high-level view of the possible interfaces to external NTR sources for both the 24Gbps and the 100Gbps/320Gbps NT family,
respectively. More detailed information on the actual capabilities of specific boards is available in the Product Information document and/or the UDS. Also there one can find which ports on these boards can be used as external NTR sources (and which ones not).
Single NT clock operation
Figure 6-4 shows the NTR configuration with a single NT board, and with an NTIO board added as a possible option. The internal system NTR clock can be
synchronized to any of the external NTR sources described in the previous subsection: BITS, SyncE.
6 — Network timing reference support in ISAM
Figure 6-4 ISAM configuration for NTR provisioning with single NT.
The 8 kHz NTR signal generated by the internal system NTR clock is distributed to the subscriber interface logic on the LT boards.
Up to two ports can be configured as valid external NTR input ports (see “High level description of the external port selection for NTR”). One will be the reference, and the other one is for protection (see “Clock protection: Overview”).
If all available external NTR clock sources fail, then this clock will switch to Hold-over mode, if locking to the external NTR clock source was completed at the time of failure.
In case no valid external NTR clock source is connected during system start-up, the internal NTR clock will remain in free-running mode, that is, it will adapt to the output frequency of its local oscillator.
Clock protection: Overview
When applications are running on equipment connected to ISAM which require NTR, it is important that this NTR signal is provided uninterrupted, and that protection is available against degradation or failure of selected external NTR sources. This is supported in the following ways:
• Switching to another redundant external NTR clock source, if available (see
“Clock protection: External NTR source protection”).
• An internal NTR clock hold-over function (see Figure 6-5), which continues to apply the last known clock correction data to the internal NTR clock, in order to keep the NTR clock to dependent equipment as stable as possible during absence of external references.
• Switching to a second NT with identical NTR clock system when the active NT fails (see “Clock protection: NT redundancy”)
Active NT
NTR clock source selection NTR clock generation PHY
6 — Network timing reference support in ISAM Figure 6-5 States and state transitions for the internal NTR clock
Clock protection: External NTR source protection
Up to two ports can be configured as valid external NTR input ports (see “High level description of the external port selection for NTR”). One port will be the reference, the other port is for protection. If the reference fails, then the other selected NTR input port will be used for clock synchronization.
NTR clock source failure is detected from:
• Loss of Signal
• A signal frequency that falls outside the capture range of the internal system NTR clock
• Failure to receive SSM messages on an SSM enabled Synchronous Ethernet link during more than 5 seconds
• Reception of SSM messages with a QL value below the configured threshold value.
Per external NTR source type, the following protection is supported:
• BITS input redundancy always requires 2 NT boards, since maximum one BITS input interface is available on NT boards. If the reference BITS input fails, then the BITS input on the other NT will be used as NTR, even if this other NT board is in standby mode. The ISAM is in general HW-ready to support this type of BITS input redundancy, but up to this release, SW support for this has been implemented on NANT-A only. BITS input redundancy is not supported on other NTs, but this will be planned in a future release.
• SyncE source redundancy is supported with all input ports either on one NT board, or on one NT board and NTIO board.
Furthermore, also any mix is supported when both inputs are on the same NT, or on one NT and NTIO. Example, BITS as the
- update holdover memory - lock clock to selected reference
Locked mode Holdover mode
- freeze holdover memory - lock clock to holdover memory
Free-run mode
6 — Network timing reference support in ISAM
reference for NTR, while SyncE as NTR source protection.
However, such combinations are expected to be less common in the field, since either the long-existing BITS on the PDH/SDH network is used, or else this network has been completely outphased and the network has moved fully to metro Ethernet aggregation and uses SyncE.
Clock protection: NT redundancy
Also in ISAM configurations with NT redundancy, the NTR function should restore and this to the same quality, when an NT fails and the redundant NT takes over. This is supported when the next restrictions are taken into account:
• In case SSU / BITS is applied, a valid signal has to be provided to both NT board front plates. This will guarantee that the system NTR clock on the stand-by NT board can be synchronized to the network in case the active NT board HW fails or is removed.
The BITS signal on the stand-by NT board cannot be configured independently, it will take the same configuration as the former active NT board BITS signal in case of NT board switch-over. This BITS signal cannot be monitored while the NT is in stand-by mode (Note: Although some NT's support active/active operation, this only refers to the data plane, since the control plane is still active/standby.)
Note that this configuration does not support redundancy of BITS input (see previous subsection on external NTR sources), except for NANT-A.
• In case NT redundancy needs to be provided with SyncE for NTR, the SyncE input(s) should be connected to the NTIO board which has connections to both NTs. Note that in this way, also SyncE input redundancy can be supported.
Once the redundant NT has taken over from the failing NT and has arrived in a stable state, the NTR function will be compliant to the typical related standards. These standards also define the maximum allowed phase jump during a transient effect.
Switch-over from a failing NT to a redundant NT is one of these transient effects, and ISAM does exceed in that case the maximum allowed phase jump. Since such NT switch-overs are exceptional, and since phase jumps may be filtered to some extent by end-user equipment, the impact on services is expected to be limited.
Future SW releases will improve the NTR functions of this subsection and relax the restrictions.
Detailed behavior of internal system NTR
The operator can configure the following elements regarding NTR:
• The external NTR source(s) to be used:
• BITS/SSU
• Synchronous Ethernet interfaces
• Enabling and disabling of the reception of SSMs that carry a QL, on the one or two external NTR clock sources that have been configured as nominated for network synchronization purposes by the operator.
The default setting is “DISABLE”. For the BITS/SSU interface, this setting cannot be changed
6 — Network timing reference support in ISAM
• The QL value applied for an external NTR clock source, in the algorithm that performs the selection of one external NTR clock source from up to two configured as nominated, and in case reception of SSM for that NTR clock source is disabled.
The default setting for the value is equal to “QL-PRC” (code 0010b) for ETSI, and “QL-PRS” (code point 0000b) for ANSI.
• The target QL value that is applied as minimum threshold for eligibility of an external NTR clock source, in the algorithm that performs the selection of one external NTR clock source from up to two configured as nominated, and in case reception of SSM for that NTR clock source is enabled.
The default setting for the value is equal to “QL- DNU” (code 1111b).
• The static relative priority to be applied for an external NTR clock source, in the algorithm that performs the selection of one external NTR clock source from up to two configured as nominated, in case the respective Quality Levels (QL) of the two sources are identical. The QL for each of both NTR clock sources can be either communicated via the Synchronization status Messages, or is fixed to a default value.
• Revertive or non-revertive operation of the external NTR clock signal selection.
The default setting is “Revertive mode”
• Override of synchronization to any external NTR clock source, and forcing of free-running or hold-over mode for the internal NTR clock function.
• The target QL to be applied as minimum threshold for the internal system NTR clock, for generating an SSU / BITS out signal.
The default setting for this target QL value is equal to “QL- DNU” (code 1111b).
The system performs the following autonomous NTR clock management functions
• Monitoring of the signal status (signal present, frequency within the capture range) and the QL of up to two external NTR clock sources that are configured by the operator as nominated.
• Selection of the external NTR clock source that fits best the selection criteria, from up to two sources configured as nominated. Selection happens as specified further.
• Disabling of the SSU / BITS output signal(s) in case the QL, which can be attributed to the internal system NTR clock, drops below the configured threshold.
The operator can retrieve the following information
• The status of BITS / SSU and / or Synchronous Ethernet interfaces nominated as external NTR source(s): “not available”, “available but not used”, “used”.
• The number of switch-over actions between nominated external NTR clock sources. In revertive mode, switch-over between nominated external NTR clock sources may happen without further alarm generation.
6 — Network timing reference support in ISAM
The operator can receive the following alarms
• Unavailability of any nominated external NTR clock source for reasons that include:
• Frequency out of range
• Loss of Signal
• Time-out for SSM reception, if enabled
• Received SSM-QL below the target QL, default or configured
• Unavailability as described above of all nominated external NTR clock sources, with defaulting to hold-over mode for the internal NTR clock
• BITS output signal disabled
• Internal system NTR clock QL drops below the output threshold QL, default or configured.
The ISAM selects the most appropriate NTR clock source for synchronizing its output NTR signals to, and for protecting against failure of external NTR clock sources, as follows:
• In case two external NTR clock sources have been configured by the operator as nominated, and both are active, then selection of the external NTR clock source, to which the internal system NTR clock will synchronize, is subject to the following rules:
• The external NTR clock with highest Quality Level (QL), is selected as actual reference for the internal NTR clock. The QL of an external NTR clock source is communicated by means of SSM messages received on the interface related to the source. If SSM reception is not supported, or disabled on that interface, then a QL value configured by the operator, or a default QL value is applied, as described above.
• In case both external NTR clock sources exhibit the same QL, then their relative priority is determined by the external NTR clock source priority list as configured by the operator.
• After restoration or upgrading of an external NTR clock source, the selection depends on revertive or non-revertive mode setting, as configured by the operator.
• In case only one external NTR clock source has been configured by the operator as nominated, or in case only one is active, then the internal system NTR clock will switch to hold-over mode when this external NTR clock source fails, or is removed.
In hold-over mode, the internal system NTR clock maintains application of the last stored correction values which describe the deviation of the own free-running oscillator signal relative to the external NTR clock source signal which was applied last.
NTR management
6 — Network timing reference support in ISAM
Configuration: external NTR clock source priority list
This command allows the operator to configure two NTR clock sources, with an operator assigned priority between them, as nominated references for the internal system NTR clock. Each of these two sources can be independently designated to be:
• The BITS interface on the faceplate of an NT board.
• The 1GE /10GE interface on the faceplate of an NT board.
• One of the two dedicated 1GE interfaces on the faceplate of a 1GE NTIO board.
The system factory default is “none”: no external clocks are selected. In this case the system automatically selects the internal free-run system NTR clock for downstream NTR timing.
Configuration: SSU/BITS input interface(s)
This command allows the operator to configure the BITS mode of the external clock source to E1, DS1, 2048Khz or auto-select. The BITS mode applies for the system, that is, any configured BITS clock source.
The system factory default is “auto-select”. In this case, the system automatically selects E1 for the system with the NT capabilities for clock device type of E1, or DS1 for clock device type of T1. This setting can be viewed in the clock status command.
When the BITS mode is configured to “auto-select”, the actual BITS mode will display “E1” or “DS1” depending on the NT capabilities.
However, the system does not restrict the manual configuration of “DS1” or “E1” to a specific NT capability of the clock device type.
Configuration: Synchronous Ethernet input interface(s)
This command allows the operator to configure the Ethernet interface(s) which can provide(s) their extracted data clock as external NTR clock source. As mentioned above, 1 or 2 external NTR sources can be configured as clocks for synchronizing the internal system NTR clock to. Therefore, between 0 and 2 synchronous Ethernet links can be designated as external NTR clock sources.
The selected Ethernet interface(s) is (are) identified by means of:
• The board slot: NT-A, NT-B, NTIO slot, or none
• The port type: SFP, XFP or none
• The port number on the board: depends on SyncE port supported, or none The system factory default is “none”.
Configuration: NTR Switching Mode
This command allows the operator to configure the external NTR selection mode to be either:
• Revertive:
the system NTR clock always selects as reference the external NTR clock source with highest QL, or the one configured as preferred by the operator if the QLs of both nominated external NTR clock sources are equal, whenever this clock source is available.
6 — Network timing reference support in ISAM
• Non-revertive:
the system NTR clock keeps the currently selected external NTR clock source as a reference, until it is no longer available for selection, for reasons listed above, or until it is disabled by the operator. This is the case even if another external NTR clock source, with better QL or higher preference as configured by the operator, has become available since the selection of the currently selected external NTR clock source.
The system factory default is “revertive”
Configuration: enabling of Synchronization Status Messaging (SSM)
This command allows the operator to enable or disable the support of Synchronous Status Message (SSM) for the configured NTR clock source(s).
At this time, this configuration is subject to the following restrictions in ISAM:
• SSM support for Synchronous Ethernet interfaces applies only to the reception of SSM frames on Synchronous Ethernet links at the network side.
• SSM frame transmission on Synchronous Ethernet links is not supported.
• In particular, sending of SSM frames with “Do Not Use” (DNU) indication on the transmission side of a Synchronous Ethernet link of which the incoming data clock is applied as external NTR source, is not yet supported. This limitation implies that the ISAM cannot be deployed in ring networks that rely on Synchronous Ethernet for NTR distribution.
• SSM support for BITS-A and BITS-B cannot be enabled yet.
• SSM support for BITS-A and BITS-B cannot be enabled yet.