Here's how, where and why the 56K Faxmodem hits top speed| GCN

Pros
and cons:
+        Impressive speed under right conditions
+        Easy to install and upgrade
–        Tricky implementation for V.90
standard


Real-life requirements:
Pentium PC, Windows 3.x or Win9x, 4M RAM, 2M free on hard drive, CD-ROM drive, serial
cable, 56K-compatible phone line, V.90 digital modem at other end


V.90 56-Kbps modems are a bit like skittish racehorses—capable of blazing speed
but affected by their surroundings.


The GCN Lab recently tested the U.S. Robotics 56K Faxmodem from 3Com Corp. to find out
what kind of performance boost it could bring.


The international V.90 standard has ended confusion over modem makers’ competing
x2 and K56flex standards. But V.90 also has changed the underlying technology.


Previous modem protocols treated the public switched telephone network as an analog
environment to ensure consistent performance over lowest-common-denominator connections.


But people use V.90 modems primarily to reach Internet service providers, and most of
their networks are digital.


Conversion from digital to analog causes no increase in signal noise. But noise does
increase when calls follow the opposite route, from analog to digital. For V.90 to work at
its peak, there can be no analog-to-digital conversions between the digital V.90 modem and
the public network.


If the right kinds of systems are present along the entire route of the call, and a
digital V.90 modem takes the call at the other end, you can connect at the legal maximum
53.3-Kbps rate.


The Federal Communications Commission set that rate in response to concerns raised by
telephone companies.


A V.90 modem can reach 56 Kbps only if dialing into another V.90 modem over a digital
circuit.


This might be trunk-side channelized T1 or an Integrated Services Digital Network
primary- or basic-rate connection.


Dialing out from a private branch exchange line drops the maximum speed to the V.34
rate of 28.8 Kbps.


Another limitation is that the 56-Kbps rate applies only to data coming downstream from
a digital modem. Data traveling upstream is limited to a maximum 33.6 Kbps in theory and
31.2 Kbps in practice, once again because of digital and analog conversions.


If you have seen references to 115.2-Kbps throughput in relation to 56-Kbps modems,
they pertain to the maximum throughput under V.42bis data compression. It can double
throughput, but only in bursts on sustained downloads.


Overall, the U.S. Robotics 56K Faxmodem performed well. Dialing into digital V.90
modems of bulletin boards and Internet providers, it routinely connected at 50 Kbps to 52
Kbps. Dialing out from an office PBX line, it usually put through about 26.4 Kbps—a
condition of the V.90 standard, not the fault of the modem.


When two of the U.S. Robotics 56K modems called each other, they connected at 47.8 Kbps
under the proprietary x2 protocol. Putting a PBX between the modems again reduced the rate
to 26.4 Kbps.


The modem was easy to set up, and the U.S. Robotics Connections CD-ROM included had not
only the needed drivers but a number of software packages, trial software and utilities.


To streamline future upgrades, the U.S. Robotics 56K Faxmodem comes with its own
internal flash ROM. There is nothing to upgrade this modem to at present, but we did try a
V.90 upgrade to a U.S. Robotics Courier V.Everything x2 modem as part of this review. It
took all of 10 minutes to get the modem ready to go.


The U.S. Robotics 56K Faxmodem is an excellent choice for telecommuters and Internet
users who need extra speed with little installation hassle at reasonable prices.  
 

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