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voice or a modulated carrier with digital information.
The diagram below shows the connection of two DTEs using modems. Here a single unit
configuration is illustrated (one switching network). A connection made between units uses 2
Mbps inter-unit links between the switching networks.
MODEM
SN
MODEM
V.24
V.24
5404
5404
CV0
CV0
DTE
DTE
Speech
Speech
A
D
64 kbps
64 kbps
D
A
ALC
DCE
Speech or modulated carrier
Speech or modulated carrier
Compatible modems
CV0 CV0 : allowed
Figure 2-1 Analogue Connection between Terminals within the ISPBX.
The end user terminal is a DTE Equipment (DTE). The modem is a Data Circuit Terminating
Equipment (DCE). A normal flat cable can be used for connecting both devices. By using two
compatible modems two devices with any type of interface can be connected. The type of
interface is of no interest as long as the ISPBX receives the call set-up information. For detailed
information on the functioning of an interface, such as V.24, see the data communication
literature.
The calling modem modulates the digital information from the calling data party on a carrier
and this modulated carrier is transferred through the `analogue' network in the speech band.
The receiving modem demodulates the incoming signal and the digital information is
transferred to the destination data party. The two modems must be compatible with each
other (use the same transmit and receive frequencies). In the case of full duplex
communication Frequency Division Multiplexing is used. The calling data party uses the low
frequency band (within the speech band) to send information while the called party uses the
high frequency band as backward channel.
Depending on the type of modem, call set-up may be done using a normal telephone set
(rotary dial or keytone), using keyboard dialling from the DTE or automatically under control
of the DTE software. For example, V.25bis commands or AT-commands can be used for call
set-up. The modem transfers the digits to the network as pulses or keytones. If a telephone
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