Modems

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The current A+ Core Hardware exam is likely to target your knowledge of analog dial-up modem commands and troubleshooting customer-related modem issues.

There are many types of modems on the market today. Popular modem types include cable, wireless, Digital Subscriber Line (DSL), and analog dial-up modems. For the purpose of the A+ Core Hardware Service Technician exam, we will focus on analog dial-up modems.

Modem is an acronym for Modulate Demodulate. A computer sends data from the CPU to a modem in digital format. The modem (modulator) converts digital data to analog format, which can be sent over a Plain Old Telephone Service (POTS) line. When the analog signal reaches the receiving modem, it is converted back to digital format, which can be understood by the receiving computer.

Typical 56Kbps analog modems are connected to a system internally or externally. An internal modem is inserted into an expansion slot on the motherboard. An external modem is connected to an RS-232c standard serial port with a 9-pin or 25-pin modem cable, or a USB port with a USB cable connector. After an analog modem has been connected to the computer system, either internally or externally, it can then be connected to a traditional phone jack with an RJ11 connector.

As discussed earlier in this chapter, a computer works with data in units of 8 bits, called a byte. A serial port transmits data only one bit at a time. A Universal Asynchronous Receiver Transmitter (UART) chip is used to break down incoming bytes into bits that can be transmitted serially (one bit at a time) out of the serial port. There are three types of UART chips:

• 8250. The first UART chip. Used in XT and AT computers. It has a 1-byte buffer.

• 16450. Introduced in the AT with a 2-byte buffer.

• 16550A. Introduced with the 486 computers. Common in Pentiums. Uses 16-byte, First-In First-Out (FIFO) buffering.

A modem, like any other device attached to a computer, requires a software driver in order to be recognized by the operating system. If a modem software driver is outdated, the user may experience intermittent problems with the connection to the Internet or another modem. Another consideration is that if your dial-up connection to the Internet is unusually slow or unstable, you should first contact your local phone company and have them clear the noise on the phone line.

You can test the ability of an analog modem to send and receive signals properly with an analog loopback adapter or a loopback plug.

AT Modem Commands

The Hayes Microcomputer Products Company developed some of the earliest modems. Many modems are compatible with the modem standards set forth by Hayes (Hayes compatible). Hayes developed a set of modem commands that can be used to control modems manually. Today, the control and configuration of modems is generally handled by the operating system. Table 5.6 identifies some of the basic Hayes-compatible AT modem commands that you should be familiar with for the A+ Core Hardware exam.

Table 5.6: AT Modem Commands

AT Command	Command Action
ATA or A0	Answer call
ATD or ATDT	Dial the number specified
ATE	Echo (show) command on screen
ATH and ATH0	Hang up or disconnect modem
ATZ	Reset the modem
XON/XOFF	Modem flow control

Modem Flow Control (Handshaking)

When using dial-up communications, it is typical for a sending device to deliver data faster than a receiving device can accept it. Flow control, or handshaking, is a verification process that two communication devices use to verify that proper communication is taking place. Several flow-control protocols are available to assist with the smooth transmission of data from a Data Terminal Equipment (DTE) device, such as a serial port, to Data Communication Equipment (DCE) device, such as a modem, and vice versa. These protocols are as follows.

• XON/XOFF. This software flow control mixes control characters in with the data to perform handshaking between devices. When the receiving device's buffer is full, it sends a request, or XOFF message, to the sending device. When the buffer on the receiving device is ready to accept more data, an XON message is sent to the sending device, and transmission resumes. This is a commonly used type of flow control, although it does have a high margin of error.

• RTS/CTS (Request To Send/Clear To Send). A dependable, commonly used form of hardware flow control between a computer and a modem, the RTS signal represents a computer, and the CTS signal represents a modem. If a computer is not ready to receive data, it drops its RTS signal. Its attached modem in turn drops its CTS signal and refuses to accept incoming data. When the computer is ready to receive data, it raises its RTS signal, which in turn raises its attached modem's CTS signal, and data acknowledgment can resume.

• XMODEM. An error-checking method used to verify that data is not corrupted or lost, XMODEM sends data in 128-byte blocks.

• YMODEM. Data is sent in 1024-byte blocks over a dial-up connection. YMODEM error-checking protocol is faster than XMODEM and is less reliable than ZMODEM.

• ZMODEM. This method also sends data in 1024-byte blocks. ZMODEM protocol is faster than XMODEM. ZMODEM can resume or restart a file transfer at the point it previously failed.

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