Understanding Asynchronous Modems

Understanding Asynchronous Modems
Technically,
modems
are modulator/demodulators, but most people define them by their highlevel
function: modems connect devices to the telephone network. The modem connects the computer
or router to the phone network and might incorporate a pass-through for an analog phone
set. Although the phone cannot be used while the computer is connected to a remote location, this
does afford a non-concurrent role for the installation—only the phone or the data connection can
be used at any given time.
Modems are considered
data communications equipment (DCE)
, whereas computers and
routers are
data terminal equipment (DTE)
. The connection between the modems, or DCEs, is
analog
in nature, meaning that bits are defined by an analog waveform that is continuous and
variable. DTE connections are
digital
in nature; this means that each bit has a clear 0 or 1 value
defined by voltage to denote the bit. It is important to remember that asynchronous refers to
clocking and not a digital or analog transmission.
Clocking
is provided in asynchronous connections
with start and stop bits, which typically results in 10 bits per byte of data—8 for the
byte of data and 1 each for the start and stop markers.
Unlike asynchronous connections, synchronous connections have precise clocking to denote
the data bits; in these connections, bytes can begin only on the downbeat of the synchronous
drum, for example. There really isn’t a drum in synchronous signaling. Rather, bits are sent in
sync with the clocking pulse; it’s similar to taking a dance step for every drumbeat, with the
dance step being the data. For an asynchronous connection there are actually three distinct connections
(DTE to DCE, DCE to DCE, and DCE to DTE), which are illustrated in Figure 23.1.
As you can see in Figure 23.1, the DTE-to-DCE bandwidth is uncompressed and is four times
that of the modem connection, assuming optimum compression. Remember that this figure represents
an optimal situation rather than a realistic one. Therefore, it is unlikely that either the
DTE-to-DCE or DCE-to-DCE connections will normally see this level of performance. Some of
this is attributable to the DCE-to-DCE limitations; however, limitations also exist in the serial
interface from the PC to the modem.
The
Universal Asynchronous Receiver/Transmitter (UART)
is a chip that controls asynchronous
communications to and from a device. It can buffer inbound and outbound data. Most
UARTs are limited to the speed of 115.2Kbps, which is insufficient for 56Kbps connections, and
the most capable UART provides for only a 56-byte receive buffer and a 64-byte transmit buffer.
Even with these relatively large buffers this may be insufficient for maximum throughput.
In current computer designs, the UART is virtually disregarded as a component in the communications
system. This is because most systems today provide sufficient buffering systems to
address the volume of packets that come with 56Kbps asynchronous transmissions—specifically,
the 16550 UART (16550 is a part number). In the early days of PCs, the most common chipset
was the 8250 UART from National Semiconductor. It contained a single buffer of sorts—it could
hold a single byte of data. Any transmission speed greater than 19,200Kbps was too fast for
the UART to forward properly. This was a substantial cause of performance problems with the
original deployments of 28.8Kbps modems.
Again, this issue is not of much consequence in modern communications systems. Unless
you are installing a 386 or older computer (which by 2005 is very unlikely), you should find
that 16550 UARTs (or better) were used for the serial ports. Please note that most internal
modem cards include either the 16550 UART or a proprietary buffering system to alleviate
these problems.

An asynchronous end-to-end connection
Computer or router
DTE
Computer or router
DTE
Modem
DCE
Modem
DCE
224Kbps 56Kbps
DCE to DCE data
compressed 4:1
224Kbps