ZyXEL Communications U-1496 series Manual de usuario descargar pdf (Pagina 9)

signal is sent; character timing is recovered from the data

itself.

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.

WHAT IS UART

UART (Universal Asynchronous Receiver Transmitter) is the device

used in a DTE or DCE for asynchronous data reception and

transmission. The normal UART device used in PCs is of the

NS16450 type. For high-speed serial data receiving (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.

MODEM STANDARDS AND SPEEDS

The CCITT (Comite International Telegraphique et Telephonique, now called

ITU-TSS (International Telecommunications Union - Technical Standardization Sector), is the

international standard-making body for telecommunication. They draft "recommendations". The

recommendations they made for modem applications have a "V" prefix and are called V-series

recommendations. The commonly used CCITT standards for 2-wire dial-up line are:

V.32bis 14400/12000/7200 bps

V.32 9600/4800 bps

V.22bis 2400 bps

V.22 1200 bps

V.21 300 bps

V.23 1200/75 bps

Note: bis is the French word for second.

In the USA, Bell Systems used to create the de facto standard

such as Bell 212A for 1200 bps modems and Bell 103 for 300 bps

modems. Everyone is following the CCITT standard now for newer

and higher speed modems.

ZyXEL U-1496 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. Only

two wires connect 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 signal transmission

in both directions. This is the type of phone service most people

have in their home or office. Since the same two wires are used

for both transmitting and receiving signals, an echo of the

transmitting signal will also be received and it is the modem

receiver's task to remove the echo before demodulation.

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ZyXEL Communications U-1496 series Manual de usuario Pagina 9