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compatible (regarding the communication protocol) a connection between the devices can be
established. The connection is circuit switched, a continuously established connection
between two data parties.
Without the modem higher communication speeds between two internal data parties can be
obtained as the modem is often the limiting factor in data communication.
In larger systems, different ISPBX units are connected using 2 Mbps digital inter-unit links and
the network acts as if it is one large PBX. The users are not aware of the separate units in the
system. This applies both for voice and for data communication.
The terminals in the fully integrated network of these units can be located within one physical
location. In this case all units are located on the same premises (interconnected via short 2
Mbps inter-unit lines). Such a network of terminals can be called a Local Area Network (LAN).
It is also possible that the units are located further apart (interconnected via leased 2 Mbps
lines through the PSTN for instance) and that the terminals are spread over the entire ISPBX
network. In such a case the network of terminals can be called a Wide Area Network (WAN).
1.1.2. Data Communication over an Analogue Route
A modem connection can be established over an analogue trunk or tie line to a data party
located in another PBX or in the PSTN. The analogue trunk or tie line is connected to an
Analogue Trunk Unit (ATU).
If the data party is connected via a modem to an ALC, then a data call to an ATU can be
established as if it was a normal voice call. An outgoing call is made by dialling the trunk access
code to the required destination and if necessary the external number of the data party. The
dialled digits are transferred over the line in the form indicated by the bundle properties. An
incoming call can be directly connected to the ALC after analysing the incoming number.
Operator assistance can be useful for setting up the call, since this type of data call is
established using a speech connection. After the connection between the two parties is
established the modems are inserted at both sides and the data transfer can take place.
If a data party is connected to a DLC, then conversion is required to enable data to be
transferred over the analogue trunk line. A modem must therefore be inserted between the
DLC and the analogue trunk line. This is accomplished using a Modem Line Unit (MLU) or a
SOPHO-LAM P375.
If a trunk or tie line is dedicated for data calls then a modem can be permanently connected
to the line.
3
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