Hardware Reference
In-Depth Information
speed modem communications, but inaudible noise easily can degrade a 56Kbps connection
to the point at which there is only a marginal increase over a 33.6Kbps modem, or even no
increaseatall.Ifyoudohaveaproblemwithlinenoise,gettingasurgesuppressorwithnoise
filtration might help.
Hotel connections through telephones with data jacks typically provide slow connections
with any type of modem. Even if you have a V.90- or V.92-compliant 56Kbps modem, you
will be lucky to achieve even a 24Kbps transmission rate. The analog-to-digital conversions
that occur between your room's telephone and the hotel's digital PBX system eliminate the
possibility of using any of the 56Kbps standards the modem supports because they depend
on a direct digital connection to the CS.
56Kbps Standards
To achieve a high-speed connection, both modems and your ISP (or other hosting service
to which you connect) must support the same 56Kbps technology. The first 56Kbps chip-
sets were introduced in late 1996:
• U.S. Robotics' x2 used Texas Instruments (TI) chipsets.
• Rockwell's K56flex was supported by Zoom and other modem makers.
These rival methods for achieving performance up to 56Kbps were incompatible with
each other and were replaced in 1998 by the ITU's V.90 standard.
Unfortunately, the 56Kbps name is rather misleading, in regard to actual transmission
speeds. Although all 56Kbps modems theoretically are capable of this performance on
top-quality telephone lines, the power requirements for telephone lines specified in the
FCC's Part 68 regulation limit the top speed of these modems to 53.3Kbps.
V.90
V.90 was introduced on February 5, 1998, and was ratified by the ITU-T on September
15, 1998. Its ratification ended the K56flex/x2 standards “war.” Shortly thereafter, most
modemmanufacturersannouncedupgradeoptionsforusersofx2andK56flexmodemsto
enable these products to become V.90 compliant. Most modems manufactured since 1998
support the V.90 protocol.
V.92
56Kbps protocols, such as the early proprietary x2 and K56flex and the ITU V.90 stand-
ard, increased the download speed from its previous maximum of 33.6Kbps to 56Kbps.
However,uploadspeedswithanyofthese56Kbpstechnologiesarelimitedtoamaximum
of 33.6Kbps. Other shortcomings included the amount of time it takes the user's modem
to negotiate its connection with the remote modem and the lack of uniform support for
call-waiting features.
