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A PC's COM1 and COM2 are asynchronous serial ports. Most PCs' and Unix sys-
tems' serial data communications are asynchronous. The serial data communication
on an IBM mainframe or mini is synchronous.
UART
A UART (Universal Asynchronous Receiver Transmitter) is the device used in a DTE
or DCE for asynchronous data reception and transmission. The standard UART de-
vice used in PCs is of the NS16450 type. For high-speed serial data transfers (38400
bps and up), the PC may not serve the UART fast enough and data may get lost. In
this case, a UART with data buffer is needed, such as the NS16550A type device.
Modem Standards and Speeds
The ITU-T or ITU-TSS (International Telecommunications Union - Telecommu-
nications Standardization Sector), is the international standard-making body for tel-
ecommunications. They draft recommendations. The recommendations they make
for modem applications have a "V" prefix and are called V-series recommendations.
The commonly used ITU-T modem standards for 2-wire dial-up line are:
In the USA, Bell Systems used to create de facto standards such as Bell 212A for
1200 bps modems and Bell 103 for 300 bps modems. Everyone follows the ITU-T
standards now for newer and higher-speed modems.
ZyXEL 2864 series modems support all the above mentioned modem standards and
are compatible with existing modems.
Type of Telephone Line
The commonly used phone service is a 2-wire dial-up line. There are only two wires
connecting the modem to the phone company's central office. The same two wires
are used for DC current feeding, ringing, dialing, on/off-hook monitoring, and sig-
*. bis is the old French word for second.
Standard
Speed (bps)
V.34 28 800 - 2 400
V.32bis 14 400 / 12 000 / 7 200
V.32 9 600 / 4 800
V.22bis
*
2 400 / 1 200
V.22 1 200
V.21 300
V.23 1 200 / 75