ZyXEL Elite 2864 User Guide - Page 29

A PC's COM1 and COM2 are asynchronous serial ports. Most PCs' and Unix systems' 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 device 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 - Telecommunications Standardization Sector), is the international standard-making body for telecommunications. 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 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 *. 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- 2-3