Re-Timing - Alcatel-Lucent 1643 AMS User's Operation Manual

Timing provisioning

Re-timing

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Introduction
When, for instance, a 2 Mbit/s PDH signal enters an SDH network, it is mapped into a
VC-12. This VC-12 itself is aligned to a TU-12 in larger containers (for instance
VC-4) and then transported through the network. A pointer is carried along to indicate
the position of the VC-12 in the container. During transport through the SDH network,
the VC-12 may shift in the TU-12. As a consequence, the pointer will also change,
since it dynamically indicates the position of the VC-12.
In other words, the payload can float in the container. Due to this floating, the phase
information of the original 2 Mbit/s signal inside the VC-12 can no longer be used.
This problem can be overcome using (2 Mbit/s) retiming. The main idea of retiming is
the enforcement of timing integrity at the cost of data integrity.
Re-timing
A phase difference might occur between the tributary signal entering the SDH Network
and the network element clock of the node where this signal leaves the SDH network.
Even if the whole SDH network is synchronized with one reference clock, this clock
can be out of phase with the original clock of the tributary signal. To clock the signal
out with the SDH-PRC frequency, the signal is unpacked (from its virtual container)
and stored in an elastic buffer. From there it is sent out, and then the retimed signal
will carry the SDH-PRC frequency. Now it can be used as a timing reference outside
the SDH network.
Retiming is possible on the following tributary interfaces:
• 1.5 Mbit/s interfaces
• 2 Mbit/s interfaces
• X.21 interfaces.
Important! Due to the nature of the X.21 protocol, enforcement of timing integrity
does not go at the cost of data integrity on this interface.
Self-timed
In the self-timed mode, the timing signal is derived from the bitrate of the, in case of 2
Mbit/s re-timing, VC-12. The bitrate may fluctuate due to pointer adjustments within
the SDH network. Therefore the self-timed mode is not suitable for carrying a timing
reference through the SDH network.
Retiming diagram
In the following figure, the dotted lines indicate the timing sources and outputs that are
used in the network. The solid lines indicate the data transport. A 2 Mbit/s signal
entering an SDH network and leaving it with 2 Mbit/s re-timing.
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7-14 Alcatel-Lucent - Proprietary
See notice on first page
365-312-807R7.2
Issue 4, May 2007