Hardware Reference
In-Depth Information
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.
uploading.
56Kbps Limitations
Although 56Kbps modems can increase data transfer speeds beyond the limits of V.34 modems
(which topped out at 33.6Kbps), they are subject to certain limitations. For example, you can't buy
two 56Kbps modems, install them on two computers, and achieve 56Kbps speeds. One side of the
connection (the side opposite the PC) must use a special digital modem that connects directly to the
PSTN without a digital-to-analog conversion.
Therefore, 56Kbps modems can be used at maximum speeds only to connect to ISPs or other hosting
services that have invested in the necessary infrastructure to support the connection. Because the ISP
has the digital connection to the PSTN, its downstream transmissions to your computer are
accelerated. If both sides of the connection support standards predating V.92, your communications
back to the ISP are not accelerated.
Also, only one digital-to-analog conversion can be in the downstream connection from the ISP to your
computer. This is dictated by the nature of the physical connection to your local telephone carrier. If
additional conversions are involved in your connection, 56Kbps technology will not work for you;
33.6Kbps will be your maximum possible speed.
With the way the telephone system has had to grow to accommodate new exchanges and devices, even
neighbors down the street from each other might have different results when using a 56Kbps modem.
Caution
56Kbps modem communications are highly susceptible to slowdowns caused by line noise.
Your telephone line might be perfectly adequate for voice communications and even lower-
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
increase at all. If you do have a problem with line noise, getting a surge suppressor with noise
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
