"The bottom line is that, in the modems we use today, the throughput in bps
is significantly higher than the 2,400 baud maximum signaling rate. Readily
available modems in the $200 price range can achieve 28.8 Kbps. When
sending plain text, compression schemes such as V.42bis may increase the
effective throughput by up to four times, e.g., to 115.2 Kbps. (The actual
compression ratio is highly dependent on the nature of the data being sent.)
However, unless you're using a 16550 UART in your serial port you will not
be able to handle this throughput. Note that compression does not increase
the line rate. The modem 'decompresses' the data received over the phone
line and presents the higher data rate to your PC."
Pete Holsberg asked "Can you add a word to the group about the relationship of
the throughout and the port speed? I can see that many are confused by
that." Here's my try at an answer:
When using a modem and a PC to communicate over a phone line there are two
"links" involved. The first link is the phone line connection between the
modems at each end of the line. (Data com folks call modems Data Circuit
terminating Equipment, or DCE's.) The theoretical maximum transmission rate
between the modems is about 34,000 bits per second, a limit created by the
properties of dial-up phone lines.
The second link is between the modem and the PC. (Data com folks call the PC
in this application the Data Terminal Equipment, or DTE.) For most of us,
the modem communicates with the PC via the PC's serial data port. In any
case, when we configure the modem and PC we define the data rate which can
be anything up to the maximum allowed by the special purpose IC's called
Universal Asynchronous Receiver Transmitter (UART's) at each end of the
cable connecting the modem and PC. Not so long ago, e.g., in the time of
80386-based PC's, the UART's designed into serial ports were limited to 57.6
Kbps (16450 UART). Now days, serial ports can handle 115.2 Kbps (16550
UART). Newer UART's will go faster.
So, the data rate between the modems is not necessarily the same as the data
rate between modem and PC. This is nice because modern modems can "compress"
some types of data for transmission. For example, a V.34 modem might be
transmitting a text file across the phone line at 28.8 Kbps but, using its
built in V.42bis compression scheme, the 28.8 Kbps data stream could be
decompressed and presented to the PC's serial port at up to 115.2 Kbps. But
this can only happen if the PC's serial port (read UART) can handle that speed.
The bottom line is that the actual throughput to the application in the PC
is limited by both the phone line link and the PC's serial port. It is nice
when the serial port is capable of, and set to use, the highest speed
expected to be input/output to the modem.
However, it should be pointed out that a lot of the data we move around via
modems can not be compressed very much, if at all. And a lot of folks are
not using the latest V.34 28.8 Kbps modems or even the earlier V.32bis 14.4
Kbps modems. So, there probably are not a lot of folks that are really
throughput limited by their serial port speed. But it is something to think
about when buying modems and PC's for communications and when setting up
your communications programs.
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| Richard A. (Rick) Whiting Phone: + 1 612 550 1213 |
| 5780 Rosewood Ln. N. E-mail: rwhiting@winternet.com |
| Plymouth, MN 55442-1411 Packet: W0TN @ WB0GDB.MN.USA.NOAM |
| U.S.A. Fax: Number on request |
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