Serial modems originated in the 1960s with point-to-point serial data transmission, which,
at the time, was the most economical way to transmit data over long distances. In the
terms of network architecture, a typical 1960s' computer network block diagram was a
"star" type-the same architecture which is used with direct digital control (DDC) in instrumentation
systems. In the star architecture the main computer is the hub. All things are
either transmitted to or from the hub; no lateral transmissions are permitted. Most loops
are local, though several could operate through modems. Note that if you replace the word
hub or main computer with Internet you will have the world as envisioned by those who
are net-centric.

Over the years, some things in the modem world became more or less standardized. The
modem's parameters were expressed in terms of the (predivestiture) AT&T (Bell System)
specifications. These specifications are both historical and contemporary. Long-distance data
communication, although more rapid than two decades ago, still uses some of the same
specifications as well as all the old technical jargon.

Definitions

Let's start our discussion of modems by defining key terms and the technical rationale for them.

Asynchronous

Generally, asynchronous means that it may occur at any time and is not tied to a clock. The
old "start-stop" teletypewriter signal with its one start bit and one (1.45 or 2) stop bit(s) is
a good example. This signal started out using motor speed as the main synchronizing
element, and the start-stop bits synchronized each character. In today's vernacular, any
start-stop signal is assumed to be asynchronous.

Synchronous

Synchronous generally means tied to a common clock, the clock signal being transmitted
along with the data. Originally meaning a signal that had no need for start-stop synchronization,
synchronous used bit timing so each bit of data was accounted for.

Baud Rate and Bits per Second

Baud rate and bits per second are often used interchangeably, but this usage is incorrect. A
baud is a line modulation rate, that is, what the transmission media needs in order to pass
the data. Line rate in bits per second is the transmission speed of the device that is transmitting
or that the device is capable of receiving. The bit rate (throughput) of a modem
depends on its line rate, the condition of the line, the packetizing method used, and the
amount of data compression. Baud per second is a term for describing a change in transmission
requirement, not a line speed. An ad for a V.90 56 Kbps modem (the line data rate)
is for a modem that requires a 1200-baud line, that is, a line that has a bandwidth great
enough for 0.0008333-second rectangular pulses.

Preamble

A preamble is a physical layer signal that is used to synchronize connected devices. In many
systems, a burst of carrier (or clock for baseband) is sent to synchronize receivers. This is followed
by bit patterns to synchronize the bit timing, then control patterns (characters or a
unique bit arrangement) to synchronize messages.

Synchrony

Synchrony in the context of modems relates to the way two modems will get their bit-rate
clocks in phase-or how they will know when they are in time with each other. Many
modems will supply a "receive" or "recovered" clock. This may be used to synchronize (or
even drive) the data terminal equipment (DTE). In most modern communications devices,
the transmit signal's transitions will be performed at the transmitter's clock time so the
transmitter's clock timing is inherent in the data signal's transitions. These may be recovered
on the receive end and the receive DTE's clock adjusted until it is in phase with the transmitter's
clock.

As one works through the various modems and networks available, a pattern emerges: first,
frequency synchrony is achieved, then bit synchrony. Though methods differ, this will be the
pattern in all synchronous systems. Generally, an asynchronous modem will not supply a
recovered clock, and a synchronous must.