Schmid Watson Installation Guide Page 23

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LTU/NTU SZ-DOC-W5-1.pd
f
Operating Manual
V
ersion: 1.5
Revision: 2002-09-10 14
3.2.4.2 External Clock Mode
NTU/
LTU
LTU
DSL
Tx
Rx
Rx
Tx
2 Mbit/s2 Mbit/s
INP 2048 kHz
Figure 3-3: External Clock Mode
In “External Clock” mode, the 2048kHz input clock is fed directly in the LTU in case of the
minirack or via the ACU clock input in case of the plug-in LTU. The external clock is used as the
E1 reference clock.
If the “External Clock” option is enabled, the primary E1 clock source is the external clock. If no
external clock is present at the 2048kHz clock input, the E1 transmit clock is used as the clock
source. If no signal is received at the E1 Port, then the internal clock is used as the clock source.
If the “External Clock” option is disabled, the primary E1 clock source is the 2Mbit/s transmit
clock. If no signal is received at the E1 Port, then the internal clock is used as the clock source.
The external clock is never used to drive the E1 Rx direction.
Note:
There is neither an external clock input nor a clock output on the NTU side. The E1 Tx clock rate
is defined by the incoming E1 Tx data rate. The Rx clock rate is the recovered Tx clock rate of
the remote side or the local internal clock. The primary E1 Rx clock source is the recovered
2048kHz clock.
3.3 ISDN PRA Interface
In PRA mode, the DSL modem offers the functions of an ISDN PRA NT1, a LT or a combination of
the functions of NT1 and LT. This makes it possible to use two set-ups:
The slave modem at the customer premises is configured as NT1; the master modem at the
central office is configured as LT.
The slave modem at the customer premises is configured as NT1 & LT. Direct access to the
exchange is established by a bit-transparent DSL-Link. All data, including time slot 0, from
the exchange must be transmitted transparently (also time slot 0) to the PRA-NTU, therefore
the DSL equipment providing the line termination is E1 working in transparent mode (see
Figure 3-4).
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