3Com Dual 56K User Manual page 144

144 A A: N P
PPENDIX ETWORKING RIMER
throughput. On the LAN side, computers on a typical network can communicate
with each other at up to 10 Mbps. When dialing up to a long distance location
using 56K, you can establish a network connection speed of up to 56 Kbps
without compression or up to 115.2 Kbps with compression. If you are dialing
into the Internet, the speed of the router providing access must also be
considered. In addition, the Internet itself may have speed limitations.
What is a modem? The word "modem" is derived from the combination of two words:
modulator/demodulator. The main function of a modem is to convert the analog
signal of the telephone line into the digital signal required by your computer.
What is the difference Digital information is processed according to two finite states, expressed as either
between analog and ON or OFF. Digital signals are therefore often referred to as binary. Most telephone
digital signals? lines, however, are analog, meaning that their signals are continuously varied
along fluctuating rates and values. Before the advent of online services, the
primary use for the telephone network was for transferring voice traffic,
accomplished by converting the spoken voice into an electric signal of varying
frequencies. Therefore, in order for your computer to understand the incoming
analog data, a translation must take place; this is essentially the task of a modem.
What is error correction? Error correction is a method by which modems verify the integrity of the data they
are receiving. If an error is found, the corrupted or damaged packet of
information, referred to as a frame, is resent. While this may impose a delay on the
transmission speed, the data across the connection will be nearly 100% error-free.
The current error correction standard is known as V.42. It is the most common
error correction protocol in use today.
What is Data Data compression is a method of reducing the amount of bandwidth required
Compression? when transmitting a file over a network. This is accomplished by condensing any
duplicate characters into a more compact form. For example, the series of
numbers 0101 0101 0101 0101 0101 0101 0101 may be more simply expressed
as "8 times 0101". Two common compression protocols are MNP-5 and V. 42bis.
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.
How does 56K The V.90 56K ITU standard allows modems to receive data at up to 56 Kbps over
technology work? 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 Dual 56K LAN Modem) accesses a digitally