Synchronous Ethernet
Traditionally, Ethernet-based networks use the physical layer transmitter clock derived from an inexpensive +/-100 ppm crystal oscillator and the receiver locks onto it. There is no need for long-term frequency stability because the data is packetized and can be buffered. For the same reason, there is no need for consistency between the frequencies of different links.
However, you can derive the physical layer transmitter clock from a high-quality frequency reference by replacing the crystal with a frequency source traceable to a primary reference clock. This does not affect the operation of any of the Ethernet layers, for which this change is transparent. The receiver at the far end of the link locks onto the physical layer clock of the received signal and gains access to a highly accurate and stable frequency reference. Then, in a manner similar to conventional hierarchical network synchronization, this receiver can lock the transmission clock of its other ports to this frequency reference, and a fully time-synchronous network can be established.
The advantage of using synchronous Ethernet (SyncE), compared with methods that rely on sending timing information in packets over an unclocked physical layer, is that it is not influenced by impairments introduced by the higher levels of the networking technology (packet loss, packet delay variation). Therefore, the frequency accuracy and stability may exceed those of networks with unsynchronized physical layers.
SyncE allows operators to gracefully integrate existing systems and future deployments into a conventional industry-standard synchronization hierarchy. The concept behind SyncE is analogous to SONET/SDH system timing capabilities. It allows you to select any (optical) Ethernet port as a candidate timing reference. The recovered timing from this port can then be used to time the system. You can then ensure that any system output is locked to a stable, traceable frequency source.
Supported reference interfaces
Any Ethernet interface can be configured as the underlying reference interface for an instance, with the following exceptions:
- management port
- SFP+ ports 33 and 34 (on 7220 IXR-D5)
SyncE interface limitations
On the 7220 IXR-D5, SFP+ ports 33 and 34 do not support SyncE (frequency or SSM).
Clock source quality level definitions
Clock source quality levels have been identified for tracking network timing flow. These levels make up all the defined network deployment options described in Recommendation G.803 and G.781. The Option I network is a network developed on the original European SDH model. The Option II network is a network developed on the North American SONET model.
In addition to the QL values received over SSM of an interface, the standards also define additional codes for internal use. These include the following:
-
QL INVx is generated internally by the system if and when an unallocated SSM value is received, where x represents the binary value of this SSM. With SR Linux, all these independent values are assigned as the single value of QL-INVALID.
-
QL FAILED is generated internally by the system if and when the terminated network synchronization distribution trail is in the signal fail state.
There is also an internal quality level of QL-UNKNOWN. This is used to differentiate from a received QL-STU code, but is equivalent for the purposes of QL selection.
The following table lists the synchronization message coding and source priorities for SSM received.
| SSM value received on port | Internal relative quality level | |
|---|---|---|
| SyncE interface in SDH mode | SyncE interface in SONET mode | |
|
0010 (prc) |
0001 (prs) |
1. Best quality |
|
0000 (stu) |
2 |
|
|
0111 (st2) |
3 |
|
|
0100 (ssua) |
0100 (tnc) |
4 |
|
1101 (st3e) |
5 |
|
|
1000 (ssub) |
6 |
|
|
1010 (st3/eec2) |
7 |
|
|
1011 (sec/eec1) |
8. Lowest quality qualified in QL-enabled mode |
|
|
1100 (smc) |
9 |
|
|
10 |
||
|
1110 (pno) |
11 |
|
|
1111 (dnu) |
1111 (dus) |
12 |
|
Any other |
Any other |
13. QL_INVALID |
|
14. QL-FAILED |
||
|
15. QL-UNC |
||
The following table lists the synchronization message coding and source priorities for SSM transmitted.
| Internal relative quality level | SSM values to be transmitted by interface of type | |
|---|---|---|
| SyncE interface in SDH mode | SyncE interface in SONET mode | |
|
1. Best quality |
0010 (prc) |
0001 (PRS) |
|
2 |
0100 (ssua) |
0000 (stu) |
|
3 |
0100 (ssua) |
0111 (st2) |
|
4 |
0100 (ssua) |
0100 (tnc) |
|
5 |
1000 (ssub) |
1101 (st3e) |
|
6 |
1000 (ssub) |
1010 (st3/eec2) |
|
7 |
1011 (sec/eec1) |
1010 (st3/eec2) |
|
8. Lowest quality qualified in QL-enabled mode |
1011 (sec/ eec1) |
1100 (smc) |
|
9 |
1111 (dnu) |
1100 (smc) |
|
10 |
1111 (dnu) |
1111 (dus) |
|
11 |
1111 (dnu) |
1110 (pno) |
|
12 |
1111 (dnu) |
1111 (dus) |
|
13. QL_INVALID |
1111 (dnu) |
1111 (dus) |
|
14. QL-FAILED |
1111 (dnu) |
1111 (dus) |
|
15. QL-UNC |
1011 (sec/eec1) |
1010 (st3/eec2) |
Timing reference selection based on quality level
For a synchronous Ethernet interface that supports Ethernet Synchronization Message Channel (ESMC) or for PTP, the clock class provides a quality level value to indicate the timing source of the far-end transmitter. These values provide input to the selection process on the nodal timing subsystem. This selection process determines which input to use to generate the signal on the SSM egress ports and the reference to use to synchronize the nodal clock, as follows:
-
For the reference inputs, if the interface configuration supports the reception of a QL over SSM or ESMC, the quality level value is associated with the timing derived from that input.
-
For the reference inputs, if they are synchronous Ethernet ports and the ESMC is disabled, the quality level value associated with that input is QL-UNKNOWN.
-
For the reference inputs, if the interface configuration supports the reception of a QL over SSM or ESMC, but the quality level value received over the interface is not valid for the type of interface, the quality level value associated with that input is QL-INVALID.
-
For the reference inputs, if they are external synchronization ports, the quality level value associated with the input is QL-UNKNOWN.
-
For the reference inputs, if they are synchronous Ethernet ports and the ESMC is enabled, but no valid ESMC Information PDU has been received within the previous 5 seconds, the quality level value associated with that input is QL-FAILED.
-
If you have configured an override for the quality level associated with an input, the node displays both the received and override quality level value for the input. If no value has been received, the associated value displays instead.
After the quality level values have been associated with the system timing inputs, the reference inputs and the external input timing ports are processed by the system timing module to select a source for the SSU. This selection process is as follows:
-
Before an input can be used as a potential timing source, it must be enabled using the ql-selection command. If ql-selection is disabled, the priority order of the inputs for the Synchronous Equipment Timing Generator (SETG) is defined using the priority parameter configured under each instance.
-
If ql-selection is enabled, the priority of the inputs is calculated using the associated quality level value of the input and using the priority parameter configured under each instance. The inputs are ordered by the internal relative quality level based on their associated quality level values. If two or more inputs have the same quality level value, they are placed in order based on where they appear in the priority. The priority order for the SETG is based on both the reference inputs and the external synchronization input ports.
-
After a prioritized list of inputs is calculated, the SETG and the external synchronization output ports are configured to use the inputs in their respective orders.
-
After the SETG and external synchronization output ports priority lists are programmed, the highest-qualified priority input is used. To be qualified, the signal is monitored to ensure that it has the expected format and a frequency that is within the pull-in range of the SETG.
Advanced G.781 features
The central clock of the node supports several advanced features of the ITU-T G.781 standard. These include the specification of minimum acceptable QL values for the input references and the specification of a Wait To Restore timer for input references. These features allow for more options in the management of the synchronization topology.