Chapter 1 General Information
MX-2100/2104
Installation and Operation Manual
1-40 Application Considerations
In the leased line configuration (A) of
, two MX-2100 units
communicate through a leased line using two high-speed modems, or other
transmission equipment (e.g., higher-speed digital multiplexers) capable of
supporting the required main link data rate. The modems provide the transmit and
receive clock signals to the MX-2100 units. In this configuration, the modem
connected to one of the MX-2100 units is selected to provide the timing reference
for the whole link, by selecting its INTERNAL timing mode. The other modem is
set to operate in the LOOPBACK timing mode, thereby ensuring that both
MX-2100 units derive their timing reference from the same source.
In the digital configuration (B) of
, two MX-2100 units communicate
through a digital transmission network, e.g., a national or private carrier network,
or an ISDN network (in the latter case, the connection is usually made by means of
KML.10 main link modules).
Digital data and ISDN networks usually incorporate very accurate timing sources,
which are always preferable. In many cases these networks will not accept data at
a rate that differs from their own rate. In the digital configuration, the modems that
connect the MX-2100 units to the digital data network are set to operate in the
LOOPBACK timing mode. This ensures that the two MX-2100 units derive their
timing reference from the accurate master clock provided by the network.
When operating in the DTE timing mode, buffers are used in the receive and
transmit data paths to allow retiming and to absorb short-term timing instabilities.
Buffers size is
±
8 bits; when operating over satellite links, the receive buffer size
can be increased to
±
256 bits.
External DCE Timing (KML.1, KML.2, KML.3 and KML.4 Modules)
With external DCE timing, MX-2100 provides the main link transmit clock signal to
the modem, and accepts a receive clock signal from the modem. In this timing
mode, it is necessary to specify the transmit timing reference source. Two
reference sources can be used:
•
External
. The MX-2100 main link transmit timing is synchronized to the
transmit clock of a data module channel. The desired channel is selected by
the operator, and must operate in the DTE2 clock mode.
The external mode is very useful when one or more of the MX-2100 channels
are connected to a AT&T DDS link, or to a similar data network. A typical
configuration with the MX-2100 units connected as a tail-end link for channels
using the DDS network is shown in
The DDS network can provide highly accurate timing and will accept only data
with similar accuracy, therefore it is necessary to lock the timing of the
MX-2100 link to the channel timing. This is accomplished by operating the
modem connected to the left MX-2100 in the EXTERNAL timing mode. The
other modem is set to operate in the LOOPBACK timing mode, thereby
ensuring that the right MX-2100 unit derives its timing reference from the
same source.
