Synchronization is an important part of all SONET products. Alcatel-Lucent 1665 DMX is designed for high performance and reliable synchronization and can be used in a number of synchronization environments. Each Alcatel-Lucent 1665 DMX can be provisioned to free run from an internal oscillator, line time from an incoming high-speed interface, or get external timing from the digital synchronization network via DS1/E1 references. Alcatel-Lucent 1665 DMX also supports distribution of timing references using DS1/E1 timing outputs.
Alcatel-Lucent 1665 DMX supports three synchronization reference configurations:
External timing from a Stratum 3 or better office clock (typical CO installations should be synchronized with DS1 or E1 timing references from a Stratum 3).
Line timing from incoming high-speed OC-3/OC-12/OC-48/OC-192 signal (for small COs or remote sites).
Line timing from tributaries derived from incoming low-speed OC-3/12/48 signal.
Free running from the multiplexer's internal Stratum 3 Timing Generator (no synchronization inputs).
These timing modes are supported by the embedded Stratum 3 Timing Generator in the OC-12, OC-48 or OC-192 circuit pack. The four basic timing modes can be combined into various network configurations. Line timing from high-speed lines is not available when using the LNW80 in the MAIN slots; line timing from tributaries is available when using the LNW80.
Internal timing functions such as reference interfaces, the on-board clock elements, and timing distribution, are provided by the Stratum 3 Timing Generator. The timing generator distributes clock and frame signals, derived from the selected reference source, to the transmission packs.
In external timing mode, the timing generator accepts two DS1 or E1 references from an external Stratum 3 or better clock. This Stratum 3 (or better) clock would typically be traceable to a primary reference source (PRS). The DS1/E1 references from the clock synchronize the local Alcatel-Lucent 1665 DMX with other network equipment operating under the Alcatel-Lucent 1665 DMX primary clock source. A high-stability digital phase-locked loop (DPLL) removes transient impairments on the DS1/E1 references for improved jitter performance.
The PRS is equipment that provides a timing signal whose long-term accuracy is maintained at 10-11 or better with verification to universal coordinated time, and whose timing signal is used as the basis of reference for the control of other clocks in the network. Universal coordinated time is a time and frequency standard maintained by the US National Institute of Standards and Technology.
Beginning in Release 9.1, the initial default state of the DS1 external timing source is in-service (IS). If the external timing source is invalid or missing, the system declares two alarms, inc. DS1 sync. ref. LOS and holdover mode active . Prior to 9.1, the default state of the timing source was AUTO and the external timing source was not alarmed.
In line timing mode, the TG3 derives local shelf timing from the incoming service OC-3, OC-12, OC-48, or OC-192 signal in the Main, Function Unit, or Growth slot. When provisioned for automatic protection switching and one of the OC-n references is corrupted or becomes unavailable, the TG3 makes a revertive or nonrevertive protection switch to the other reference without causing timing degradations. Synchronization reference mode switching can be either revertive or nonrevertive. You can also provision a primary reference. If all OC-n timing signals are lost, the TG3 switches to holdover mode. The TG3 normally switches back to line timing mode when a reference is no longer corrupted and clock mode switching is set to revertive. It can also be provisioned to require a manual switch. Switching between two OC-n reference inputs can also be done using a manual command. All optical circuit packs can derive a 6.48 MHz timing signal from an incoming optical line. DS1/E1 Sync Out is always derived from the provisioned line timing source.
In free running mode, no mode switching is performed. The timing generator derives timing from a high stability temperature-compensated, voltage-controlled crystal oscillator that has an end of life performance of ± 4.6 ppm. This oscillator is capable of Stratum 3 accuracy. Only one Alcatel-Lucent 1665 DMX in a subnetwork can be provisioned in the free running mode. All other Alcatel-Lucent 1665 DMX NEs in the subnetwork must be line timed to this free running system to avoid performance degradation.
Alcatel-Lucent 1665 DMX also supports a DS1 or an E1 timing output feature that facilitates network timing distribution. E1 timing outputs are available with the LNW59, LNW82, or LNW504 VLF Main packs. The DS1/E1 timing output is derived from the OC-n line rate, so it is not subjected to multiplexing or pointer processing effects. The result is a DS1/E1 traceable to the far-end source with extremely low jitter and wander. The timing output can be locked to an OC-n line or the OC-n source can be automatically selected using synchronization messages. In either case Threshold AIS may be enabled to insert AIS if the synchronization message of the OC-n source matches a provisioned threshold.
The frame format on the DS1 output is provisionable as superframe format (SF) or extended SF (ESF). The E1 frame format can be provisioned as FAS or CRC4. The DS1/E1 is a framed all-ones signal under normal conditions or an AIS signal under failure conditions.
With the use of the LNW59, LNW82, or LNW504 VLF Main pack, Alcatel-Lucent 1665 DMX supports E1 timing references. SDH synchronization conforms to industry standards for frequency accuracy, pull-in, hold-in and pull-out ranges, wander, jitter, noise tolerance, noise transfer, transient response, holdover performance (per ITU-T G.813), and for SDN Sync Status Messaging (per ITU-T G.704).
With the use of VLF Mains, both external timing references are monitored simultaneously, even when the Timing Generator (TG) is in holdover mode. So, if an external reference fails, the Out of Lock (OOL) status of the standby reference is already known and the decision to switch to the standby reference can be made with certainty that the standby reference is not in an OOL condition. If the TG is in holdover mode, or if it has switched references with revertive reference switching enabled, it can monitor the OOL reference(s) for a non-OOL condition and switch to the good reference with certainty that the reference is in fact not in an OOL condition. It will also autonomously clear the OOL alarm.
With non-VLF MAINs both external timing references are monitored for LOS, LOF, AIS, BER & EOOF failures, but only the active reference can be monitored for frequency offset OOL failures, and only when the TG is locked to the reference. Frequency offset of either external reference cannot be monitored when the TG is in holdover or free-running modes.
When a failure of the active reference occurs, if reference switching is enabled, and if the standby reference doesn't have an LOS, LOF, AIS, BER & EOOF failure, the NE switches to the standby reference and locks to it, and then monitors its frequency offset for an OOL condition. If the standby reference does have an OOL failure, the TG goes into holdover mode and issues a major sync alarm for both references failed (subject to ASAP provisioning).
When line or external timing is used, in case of unprotected synchronization reference failure, the TG3 will switch to "holdover mode" and continue to provide system timing using the internal oscillator to maintain the last known good reference frequency. If the DS1/E1 timing output is enabled for network synchronization, DS1 or E1 AIS is inserted on detection of unprotected optical reference failure.
| November 2011 | Copyright © 2011 Alcatel-Lucent. All rights reserved. |