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- In ISDN networks everything is carried as a stream of bits.
- Converting digital telephone signals into voice, and vice versa, is done
by CODECs.
- Converting data into analogue telephone signals, and vice
versa, is done by modems.
- A CODEC is the device that allows telephony between an ISDN and the
analogue network.
- This same CODEC can be used to allow modem and fax calls to cross the same
boundary.
- Modems and CODECs always work in pairs - they can be nested together but
each must have a partner.
- A Terminal Adapter is an interface between the ISDN and any non-ISDN
device, such as a computer or an analogue phone
- If analogue devices need to be connected to the ISDN, then the Terminal
Adapter will need to perform the function of a CODEC, and have analogue
(POTS) ports available for modems and fax machines to plug into.
- Internal ISDN adapters can use dedicated chips on the card, or software
running on the PC, to implement the digital and analogue conversions,
thereby removing the requirement for any physical analogue devices like fax
machines and modems.
- There are three kinds of device used in data communications with ISDN:
- Conventional Modems - these require Terminal Adapter to connect to the
ISDN
- ISDN Modems - which combine the functionality of CODEC and modem
- Terminal Adapter - allow analogue devices to connect to the ISDN and
contain a CODEC for this purpose
- ISDN Adapters - merely pass a stream of bits between a protocol driver
and the ISDN
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