Basic System Configuration Guide System Management
Edition: 01 3HE 11010 AAAC TQZZA 257
6.4.7.2 NTR on SHDSL Interfaces
NTR for SHDSL is carried equally across all lines because all lines must connect
back to the same LT on the same DSLAM. NTR for SHDSL operates in auto-detect
mode. The 6-port DSL Combination module automatically selects an SHDSL pair
that will be used to extract NTR and transmit to the SSU. The SHDSL pair is selected
based on clock activity monitoring, coarse frequency monitoring, and chipset level
indications on the active status of individual lines.
The auto-detect algorithm on the 6-port DSL Combination module selects an SHDSL
pair used for NTR by first checking SHDSL pair 1. If pair 1 is not considered an
acceptable source, the algorithm checks each pair in sequence until it finds an
acceptable source or reaches SHDSL pair 4. The ID of the in-use line is displayed in
CLI; however, it is not user-configurable.
When NTR on SHDSL interfaces is in use, the status is indicated to the 7705 SAR-M.
The pair currently being used to derive NTR is shown in the CLI and is updated to
the NSP NFM-P. 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 6-port DSL Combination module automatically selects the SHDSL pair for NTR
to use based on the selection algorithm. 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 will be phase drift
during the switchover and clock recovery may enter the holdover state if this was the
only external timing reference available. If this happens, the 6-port DSL Combination
module selects a new SHDSL line if one is available.
6.4.8 Synchronous Ethernet
Synchronous Ethernet is a variant of line timing that derives the physical layer
transmitter clock from a high-quality timing reference, traceable to a primary
reference clock. Synchronous Ethernet uses the physical layer of the Ethernet link to
distribute a common clock signal to all nodes in the network. Each node has a local
or system clock that determines the outgoing clock rate of each interface. The
system clock of each node in the network is derived from the incoming clock at an
input interface or from a dedicated timing interface; for example, a BITS port.
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