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Windows was not designed for the Internet (WAN) thus, the serial port setting was not optimized for modem to have a higher data byte flow. Users can now can buy cheap serial port cards to overcome the problem.

Since the introduction of the PCs the serial port act as an interface between the PCs and a modem, which are commonly restricted to serial rates of 115,200 bps and with an ISDN line running at 128,000 bps, the serial port becomes a bottleneck.

As modem performance improves, transfer of data at speeds of 28.8Kbps, 33.6Kbps, 56.6Kbps and 128Kbps and above, they can easily reach the serial port theoretical maximum

Notes: 
The latest V92 compliance 56K modem (if supported by your ISP) will now allow you to up-load (send emails) at the maximum theoretical speed of 50 Kbps. Whereas the older V90 compliance modem up-load speed are limited to 33.6

Data flow between the serial port and the modem is governed by a Universal Asynchronous Receiver Transmitter UART chip, which can contain no data buffers, single-byte data buffers or 16-byte data buffers.

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Setting serial port to the maximum can optimize a PCs process data to the theoretical maximum data flow between serial port and the modem is governed by UART

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All data flows between the serial port
and the modem via the UART

Most of today's desktop PCs and notebook are equipped with either 8250, 16450, 16550A which contain single-byte data buffers.

Since most serial port cannot transmit or receive data at the maximum rate of 115Kbps, today's modem far exceed serial port capacity.

In system with non-buffered UART each byte of data must be processed before the next arrives, to ensure that data bytes are not lost.

If the data byte is not processed before the next one arrives, a data overrun occurs and the data byte is lost. If this occurs the entire data packet containing the lost byte must be retransmitted, resulting in reduced throughput.

Manufacturers sought to overcome serial port limitations by implementing bus-based modems or intelligent serial boards.

Whatever being done,
the communications board continues to process information one character at a time. While some parallel-port architecture transfer data in 8 bit segments, eliminating the need for a UART type interface.

More info available on why modem rarely
connects at the maximum speed? See bits.bytes

First check your PCs to find out if it is using an older UART chip that is build-into the motherboard or a PCI UART serial card.

Plugging in a newer 16650 UART card into the PCI slot will replace the existing ones or can easily be configured to over-ride the existing build-in mother board UART chip.

www.byterunner.com for 16650 UART

www.lavalink.com 16650 UART PCI or ISA

www.turbocom.com for UART 16750

Chip Buffer Space MAX DTE Rate
----- -------------- ------------
16550 - 16byte buffers - 115200 b/s

16650 - 32byte buffers - 460800 b/s

16750 - 64byte buffers - 921600 b/s

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