Transmission and DTE Types
Modems communicate over the telephone line by converting data from analog
to digital form and vice versa. The technique is fundamentally the same
whether the data transmission mode is synchronous or asynchronous. For more
information about the transmission mode, click:
Synchronization
In most cases, communications over the telephone lines is synchronous, regardless
of the mode selected. The interface between the modem and the DTE depends
on the mode selected. 300 bits/s communications (e.g., when online with
a Smartmodem 300) are always asynchronous between modems. At speeds of 1200
bits/s and higher, the modem-to-modem links are Asynchronous between the
DTE and the modem, synchronous and asynchronous communication differ significantly.
The DTE hardware and software are usually different as well. A modem operating
with a synchronous DTE could connect to another modem operating with an
asynchronous DTE, as the carrier signals are no different. But the data
passed over the link might not be intelligible to their respective computers.
The interface between the modem and the DTE is generally asynchronous for
all personal computer-to-modem communications. When the modem-to-DTE link
is synchronous, the DTE must be a synchronous device such as a synchronous
adapter card installed within the computer. The two types of transmission
differ in the techniques used to separate the characters that are transmitted.
Asynchronous transmissions use bits to indicate the start and stop of the
character. Synchronous transmissions use clocking signals.
Hayes modems provide several communication modes to adapt to a variety of
environments and operating demands. When a communications mode is selected
that supports an asynchronous DTE at 1200 bits/s or greater, the modem converts
the asynchronous data into synchronous data that is compatible with the
modem-to-modem carrier signals. In some of the communications modes that
support asynchronous DTEs, the start and stop bits are also converted to
synchronous data over the modem-to-modem link. Naturally, the modems on
both ends of the link must be in a compatible communications mode if the
data is to be restored at the receiving DTE. For this reason, communications
modes that support asynchronous DTEs and which actually send the start and
stop bits over the modem-to-modem link are often referred to as asynchronous
connections.
The communications mode is controlled by the &Q command, except for Smartmodem
300 and those Smartmodem 1200s that support only asynchronous mode.
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