Unit installation

Chapter 1 Introduction Installation and Operation Manual
1-6 Overview FCD-E1
Timing
Multiple clock source selection provides maximum system timing flexibility, and
supports hierarchical dissemination of timing information.
System Timing
Internally, FCD-E1 uses one system timing source (clock). This system clock
determines the transmit timing of all the E1 links and data ports, and the timing of
most other signal processing operations.
To achieve maximum flexibility in system integration and enable hierarchical
distribution of timing in the system, FCD-E1 enables the user to select the source
to which the master clock is locked. The available options are as follows:
• System clock source locked to the recovered receive clock of the main link, or
sublink (when available).
• System clock source locked to an external clock (e.g., the transmit clock
applied to a user-selected data port). The timing mode of the selected port
must be DTE2.
The DTE2 mode is not available on channels with X.21 interfaces, or on the
Ethernet interface.
• System clock source locked to the internal crystal oscillator, which has an
accuracy of ±50 ppm.
In addition to the selection of a main clock source, you can specify a fallback
source, which is automatically selected in case the main source fails. The fail
criteria are loss of the receive signal on the port selected as the main source, or
inactive RTS line on the selected data port. The internal oscillator always serves as
a fallback source, which is automatically selected in case the other selected timing
sources fail.
Main Link Timing
FCD-E1 recovers the main link receive clock signal, and uses it as the timing source
for the receive path. The main link transmit timing source, which is derived from
the main system clock, can be locked to one of the following sources:
• Recovered receive clock
• External clock signal (derived from one of the synchronous data channels or
from the sublink)
• Internal oscillator.
Sublink Timing
The sublink transmit clock is always locked to the main link transmit clock, and the
receive clock is recovered from the incoming sublink signal.
The receive clock can be selected as an external clock source for the main link
transmit clock.
Synchronous Data Port Timing
The FCD-E1 data port has three timing modes:
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