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Modem as a Fax
Modems can also be designed to include a fax transmitting and receiving function
similar to a fax card. Since the modem's interface with the computer is the standard
serial RS-232 interface, this interface is used for both modem and fax operations.
Since the data throughput of a serial RS-232 interface is limited, only compressed
fax image data should be carried through this serial interface. Fax image coding and
decoding must be done in the computer. Modem/Fax, also called faxmodem, can be
either an external stand-alone unit or a plug-in card. External stand-alone units can
be connected to any computer with a standard RS-232 serial port.
EIA Class 1 and Class 2/2.0 Fax Commands
The EIA class 1 and class 2 fax commands are a set of AT fax commands defined by
EIA/TIA (Telecommunications Industry Association) for controlling faxmodems from a
computer through the serial RS-232 interface. Faxmodems and fax software supporting
this standard will work together. Class 1 commands control the details of how the mo-
dem does on-line negotiation and control jobs whereas Class 2 commands allow the mo-
dem to do many negotiations automatically. The Class 1 command set is also called the
TIA-578 standard. Several revisions of the class 2 standard exist. Implementations con-
forming to different revisions may not work together. A formally approved version is the
Class 2.0 command set, also called the TIA-592 standard.
Autodetection of Fax or Data Call
Fax and data modems have different handshaking signal sequences at the beginning
of a call as specified by the ITU-T. It is possible to automatically detect whether an
incoming call is from a modem or a fax device by testing and recognizing its initial
handshaking sequence. A modem with this capability can make a computer process
both data and fax calls on the same phone line. An example is that a BBS (Bulletin
Board System) can also receive faxes. ZyXEL modems can automatically detect data
and fax calls. ZyXEL also includes a fax reception and printing program to allow a
BBS to receive faxes and print them out.
Caller ID
Normally called Caller Number Delivery (CND), this is a service available from
phone companies in several parts of the USA and some other countries. With this
service, the phone company sends calling party information along with the ring sig-
nal to the called party. The major part of this information is the telephone number
of the calling party or caller ID. ZyXEL modems can detect and report caller ID.
Note that different countries have different caller ID schemes. Please check whether
the scheme used in your country is supported by the modem.