Hardware Reference
In-Depth Information
used, and the value for this parameter is nearly always set to none. Other possible parity values
you might see in a communications software package are odd, even, mark, and space.
•
Data bits
—This parameter indicates how many bits are actually carried in the data portion of
the packet (exclusive of the start and stop bits). PCs typically use 8 data bits, but some types of
computers use a 7-bit byte, and others might call for other data lengths. Communications
programs provide this option to prevent a system from confusing a stop bit with a data bit.
•
Stop bits
—This parameter specifies how many stop bits are appended to each byte. PCs
typically use 1 stop bit, but other types of protocols might call for the use of 1.5 or 2 stop bits.
In most situations, you will never have to modify these parameters manually, but the controls are
almost always provided. In Windows, for example, if you open the Modems control panel and look at
the Connection page of your modem's Properties dialog box, you will see Data Bits, Parity, and Stop
Bits selectors.
56Kbps Modems
At one time, the V.34 annex speed of 33,600bps (33.6Kbps) was regarded as the absolute speed limit
for asynchronous modem usage. However, starting in 1996, modem manufacturers began to produce
modems that supported speeds of up to 56,000bps. These so-called “56K” or “56Kbps” modems are
now universal, although the methods for breaking the 33.6Kbps barrier have changed several times.
To understand how this additional speed was achieved, you must consider the basic principle of
modem technology—that is, the digital-to-analog conversion.
As you've learned, a traditional modem converts data from digital to analog form so it can travel over
the public switched telephone network (PSTN). At the destination system, another modem converts
the analog data back to its digital form. This conversion from digital to analog and back again causes
some speed loss. Even though the phone line is physically capable of carrying data at 56Kbps or
more, the effective maximum speed because of the conversions is about 33.6Kbps. An AT&T
engineer named Claude Shannon came up with a law (Shannon's Law) stating that the maximum
possible error-free data communications rate over an all-analog PSTN is approximately 35Kbps,
depending on the noise present.
However, because many parts of the United States urban telephone system are digital—being
converted to analog only when signals reach the telephone company's central office (or central
switch)—it's possible to “break” Shannon's Law and achieve faster download rates. You can, in
some cases, omit the initial digital-to-analog conversion and send a purely digital signal over the
PSTN to the recipient's CO (see
Figure 16.6
)
. Therefore, only one digital-to-analog conversion is
necessary, instead of two or more. The result is that you theoretically can increase the speed of the
data transmission, in one direction only, beyond the 35Kbps specified by Shannon's Law—to nearly
the 56Kbps speed supported by the telephone network. Prior to the new ITU V.92 standard, the
transmission in the other direction was still limited to the V.34 annex maximum of 33.6Kbps.
However, both the modem and the ISP must have support for the ITU V.92 standard to overcome this
limitation for uploading speeds.
