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BPS/CPS Rates
When calculating a BPS (bits per second) or CPS (characters per second) data transfer rate, there are several considerations. One consideration is the compression ratio of the file. Text files may have up to a 2:1 compression ratio, while graphics files containing large single-colored areas may have up to a 4:1 compression ratio. The more the MNP or V.42 compression algorithms can compress a file, the greater the transfer rate.
Archive files are compressed as the files are added to the archive. The modem compresses these files very little (if at all). While your software may show a relatively low transfer rate, when you extract files from the archive you'll extract two or more bits for every one bit that your modem received. Your effective transfer rate is much higher than your software might reflect.
Another consideration is whether your software displays the modem to modem rate (the highest is 14.4 kilobytes per second) or modem to system rate. If the sending modem compresses the data at a 4:1 ration, your modem will send data to your computer at up to 57.6 kilobytes per second. Additionally, all packet communications add start bits, stop bits and error correction information to each packet. So, for every 128 bits your modem receives, your modem strips out 8 bits of overhead before passing the remaining 120 bits on to the computer.
The condition of your phone line also affects the transfer rate. A good, clean line will allow higher transfer rates than a noisy line.
You must also consider the UART (Universal Asynchronous Receiver Transmitter) used on your serial port. A buffered UART, such as the 16550, is essential for reliable high-speed data transfer. In order to achieve 57,600 bps on a 14.4EX modem (or 38,400 on a 9600EX modem) the data must be compressible at a 4:1 ratio, the line must be clean, and the UART must buffer the data flow.
If the host system supports multitasking and multiple lines, it could limit your effective data transfer rate because the computer has to alternate sending bits to everyone else who's on-line when you are. The serial data rate between the host computer and host modem can also limit the data transfer rate. If the host's serial port has a top speed of 38,400 bps, that's the fastest your modem will receive data from the host.
This applies to:
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