System Management
256
Basic System Configuration Guide
3HE 11010 AAAC TQZZA Edition: 01
6.4.7 Network Timing Reference (NTR)
On the 7705 SAR-M, the 6-port DSL Combination module and 8-port xDSL module
support network timing reference (NTR) clock recovery. Using NTR, a synchronized
clock can be derived from the xDSL physical layer or the SHDSL interface. NTR
delivers a highly accurate synchronized clock while eliminating the need for
advanced synchronization hardware in the DSL modem, thereby reducing the overall
cost of the network.
NTR is equivalent to physical layer synchronization and, at cell sites, is the
preference for delivering frequency synchronization over a DSL network. Alternative,
packet-based methods of synchronization, such as ACR and IEEE 1588v2 PTP,
cannot offer the same level of accuracy as physical layer synchronization due to the
inherent PDV characteristics of DSL.
On the 8-port xDSL module, a single NTR timing reference is available to signal back
to the 7705 SAR-M. On the 6-port DSL Combination module, there are two DSL
interfaces and therefore two separate NTR timing references available to the
7705 SAR-M: one for SHDSL and one for xDSL. On SHDSL interfaces, NTR locks
the DSL symbol clock directly to the reference clock. On xDSL interfaces, NTR maps
DSL frame phase difference bits information between the reference clock and the
DSL free-running clock.
6.4.7.1 NTR on xDSL Interfaces
On xDSL interfaces, all CPE lines must be connected to the same LT because the
clock source for all lines must be identical. While operating in VDSL2 mode, all pairs
on an 8-port xDSL module must have their VDSL2 DMT signals aligned.
When NTR on xDSL is in use, a message is sent to the 7705 SAR-M indicating which
pair is currently being used to derive NTR. However, once all lines are in show-time
mode, NTR is carried on all lines. If there is an NTR status change from one pair to
another, a status change is indicated in the CLI for the 7705 SAR-M. The status
change is also visible through the NSP NFM-P.
The chipset automatically selects the line with the best signal-to-noise ratio on the
pilot tone. If there is a line drop, or if the signal-to-noise ratio degrades, the system
automatically switches NTR to another line in show-time mode to recover clock
synchronization. When NTR is locked to a particular line, the status is updated and
indicated in the CLI and on the NSP NFM-P.
If the line carrying NTR is taken out of show-time mode, there may be phase drift
during the switchover if a phase delta difference has been missed.
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