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3.1.1. Data Terminal Connected to an ALC
For a DTE connected to an ALC via a modem, the situation is straightforward. Because the
output of the modem is an analogue signal it is possible to connect a modem directly to an ALC
as indicated in figure
information of the terminal is converted into an analogue signal by the modem and offered to
the ALC. The ALC converts the analogue signal (carrier with the digital user information) into
a digital 64 kbps signal which is switched through the FIN. The ATU converts the digital signal
back into an analogue signal. The analogue signal on the trunk line now is the same analogue
signal (modulated carrier) as at the modem side. The connection is formed between the
internal modem and the modem of the external DTE.
MODEM
V.24
Modulated carrier (or speech)
*
Figure 3-1 Analogue Connection to an Analogue Trunk.
Two types of modems can be used:
-
Automatic dialling modem.
If the modem supports V.25 bis or Hayes commands, the user can type the trunk access
code and the external number directly from the DTE for an outgoing call. The modem
transfers this information as impulses or keytones to the ALC.
-
Manual dialling modem.
The user must dial the destination DNR from a telephone set connected to the modem.
The telephone set transfers the dial information to the ALC as impulses or keytones. Once
the `speech' path to the called party is established, the telephone line is switched through
to the digital part of the modem. With a incoming data call the extension starts ringing and
the extension user must answer the data call and switch the telephone line to the digital
part of the modem. In this configuration the telephone can also be used for normal speech,
although not at the same time a data connection is present.
Most modems offer an automatic answering option.
When the path is established the modems at both sides try to synchronise and if successful the
26
Figure 3-1 Analogue Connection to an Analogue
IMP
DNR
*
64 kbps
CV
ALC
SN
IMP
64 kbps
ATU
Trunk.. The digital
Trunk
*
CV
line
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