3com 3C886 User Manual

Note that employing data compression upon pre-compressed files, such as a ZIP 
archive, may actually increase the time required for file transmission. As such, 
V.42bis is preferable in these circumstances, as it is capable of recognizing 
pre-compressed files and withholding additional, unnecessary compression. 

The V.90 56K ITU standard allows modems to receive data at up to 56 Kbps over 
the standard, public switched telephone network (PSTN). V.90 technology 
overcomes the theoretical limitations imposed on previous analog modems by 
exploiting the digital connections that most Internet and on-line service providers 
use at their end to connect to the PSTN, such as a T1 or an ISDN BRI line.

Typically, the only analog portion of the PSTN is the phone line that connects your 
home to the telephone company’s central office (CO). Over the past two decades, 
the telephone companies have been replacing portions of their original analog 
networks with digital circuits. However, the connection from your home to the CO 
will likely remain analog for some years to come.

V.90 technology takes advantage of the typical network configuration found 
when an analog modem (such as your 56K LAN Modem) accesses a digitally 
connected ISP. By bypassing the analog-to-digital conversion in the downstream 
path, 56K technology can use nearly all of the available 64K network bandwidth. 
(Upstream data, typically less sensitive, travels at the standard V.34 rate of 33.6K.) 

Based on “encoding” rather than “modulation”, the result is download speeds 
once thought not possible. Older, V.34 modems treat the PSTN as if it were entirely 
analog; they therefore cannot take advantage of the bandwidth made available 
when one end of the connection--that of your ISP, for instance--is completely 
digital.

Note that several factors may affect your 56K LAN Modem’s performance. Current 
FCC rules limit download speeds to 53 Kbps. Poor line conditions or heavy traffic 
may impact performance, just as with older V.34 modems. In real applications, 
speeds typically range from the 40s to the low 50s (Kbps), with the average in the 
mid-to upper 40s.There are cases where performance is in the 30s, and others 
where true V.90 performance may not be possible. Note that both your phone line 
as well as your service provider must be V.90 capable; this means that your service 
provider must conform to the V.90 standard. Lastly, V.90 requires that there be no 
more than one analog-to-digital conversion in the downstream path; PBX devices, 
such as those found in corporate telephone systems or some hotels, may 
introduce additional conversions. 

For assistance with locating an 56K service provider in your area, or for a more 
detailed explanation of 56K technology, visit 3Com’s 56K Web page at 

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