Uart; Modem Standards And Speeds; Type Of Telephone Line - ZyXEL Communications 2864 Series User Manual

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:
Standard
V.34
V.32bis
V.32
*
V.22bis
V.22
V.21
V.23
*. bis is the old French word for second.
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-
Speed (bps)
28 800 - 2 400
14 400 / 12 000 / 7 200
9 600 / 4 800
2 400 / 1 200
1 200
300
1 200 / 75
2–3