Hardware Reference
In-Depth Information
Today, dial-up modems are used primarily as secondary communications devices and
many computers users never use a modem. To learn more about modem standards and
protocolspriorto56Kbps,bitsandbaudrates,modulationstandards,error-correctionpro-
tocols, and data-compression standards, see the relevant sections in
Chapter 16
,
“
Internet
aged with this topic.
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 manu-
facturers 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
the33.6Kbpsbarrierhavechangedseveraltimes.Tounderstandhowthisadditionalspeed
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 be-
cause 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
communicationsrateoveranall-analogPSTNisapproximately35Kbps,dependingonthe
noise present.
However, because many parts of the United States urban telephone system are digit-
al—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
). There-
fore,onlyonedigital-to-analogconversionisnecessary,insteadoftwoormore.Theresult
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 sup-
ported 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.
Figure 16.6
V.90-based 56Kbps connections enable you to send data at standard analog modem rates
(33.6Kbps maximum) but enable you to receive data nearly twice as fast, depending on line conditions.
