Modems

1. Modems Chapter 17

2. Modems are little devices to use the telephone to talk to other computers. • Modem is an abbreviation for Modulator/ Demodulator • Modems can be internal or external. Basic Knowledge

3. Modem Signals • Modems take analog signals and convert them into digital signals and vice versa. • The data that goes into the modem is called serial communication and is a series of 0’s and 1’s. This data is converted into bytes via the UART(Universal Asynchronous Receiver/Transmitter) so CPU’s can understand information • There are 2 ways of “chopping up” the incoming signals • Asynchronous • Synchronous

4. Asynchronous Organization • The main way to serial ports communicate • The data is “chopped up” into 7 or 8 bit packets. 8-bit is most common • Each Packet starts with a start bit that tells the receiving modem that it is the beginning of a piece of information and it always is zero and ends with a stop bit that tells the receiver the packet is over. • Asynchronous communication packets have an optional parity bit used for error detection. Parity bits are rare • Asynchronous packet settings are described in a particular order: data bits, parity type, then stop bits. • Example: If a system uses 8 data bits, no parity, and 1 stop bit, you’d say 8-N-1.

5. Synchronous Organization • Mainly for uploading and downloading programs to and from your computer. • Protocols are sets of definitions used by communication programs so that both computers are speaking the same language. There are many protocols that basically work the same way. • When a modem is going to send data with synchronous communication it sends a series of standardized bytes called sync bytes, which tell the receiving port that it is about to get information. • After sync bytes the sending modem adds a start-of-text(STX) marker then sends the data. • After data transmission the packet ends with an end-of-text(ETX) marker and error checking characters(ECC) • The receiver responds with an acknowledge if data is good or a no acknowledge if there is an error.

6. FTP’s and Baud Rate • File Transfer Protocol • FTP is used to enable you to exchange files with mainframes, mini’s, or other PC’s. It should be included in the software package that comes with your modem • Refer to pages 1046-1047 for variations of FTP. • Baud Rate • Baud rate is the time that a modem uses as its carrier frequency • Baud rate speed is measured in bits per second • Example: If baud rate was 2400 and modulated at 2 bits per cycle the bps would be 4800

7. Flow control • Flow Control (Handshaking) • Process by which 2 serial devices verify a conversation. • During data transfer there are 2 conversations when flow control is needed, Local(between modem and com port) and end-to-end(between modems). • Flow control can come in hardware and software. Look at pg. 1049 to learn about each.

8. Interface Signals • CD – Carrier signal detected • RD or RX – Receive Data • TD or TDX – Transmit Data • DTR – Data Terminal Ready • DSR – Data Set Ready • RTS – Request To Send • CTS – Clear To Send

9. Modem Commands • Commands of modems are known as the command set. • There is no standard for modem commands, but there is a main set that most companies follow. It is the famous AT command set. • AT commands are entered through the keyboard. • A list of AT commands is on pg. 1052-1053

10. Modem Commands • AT – checks to see that modem is plugged in • ATH – Hangs up • ATM1 – turns the speaker on • ATM0 – turns the speaker off • ATD – dials a number • ATZ – resets the modem

11. Talking to serial ports and telephone lines • Telephone Lines • A phone line is guaranteed to handle speeds only up to 2400 baud • To verify you are getting the best quality is to call the phone line. • If the company thinks the voltage drop is too much, they will condition the line. Conditioning is adding a little voltage to the line to compensate for the voltage drop. • Talking to serial ports • Data communication equipment is the device that sends data. Data Terminal Equipment is the device that receives data. The connection between the 2 is called the handshake. • Chart on page 1054

12. UART’s • Heart of many serial ports. • It takes parallel information from data buses and makes it serial information to be sent out to a modem. • It’s important to know what type of UART is in your COM ports. You can find out the type by checking the diagnostics tab/ More Info button under the Modems applet in the control panel under windows 9x. In Windows 2000 use the Phone and Modem Options applet. • For UART’s check pg. 1056

13. Fax Modems • Fax technology was invented in 1842 by Alexander Bain • Modems send data in single dots which are converted to text. • 4 different classes or groups of fax standards • Groups 1 and 2 were analog standards for fax modems operating at 300 baud. • Group 3 was standardized for digital facsimile devices to communicate over phone lines. Fax transmission had a max speed of 14400 bps. • Within group 3, there are several different classes that explain devices using different speeds and methods of communication. • Any good fax modem software can run G3 • Chart of Fax standards on 1058

14. Modem Negotiations • Standardized series of responses between two modems so they can determine optimal speed, data compression, error detection, etc. • Chart pg. 1059-1060

15. TAPI • TAPI is a set of dynamic link libraries (DLL) that enables Windows 9x to perform telephone functions. • A good TAPI installation will configure the modem type, I/O address and IRQ, local phone number, long distance dialing codes, and etc. to enable it to use the modem. • TAPI is also used for answering machines, handle faxing, call forwarding, caller ID, and paging

16. Installation and Troubleshooting • Modems rarely fail, but problems usually are COM port problems • Set up and verify the com ports • Make sure you install nonconflicting I/O addresses and interrupts. • Check modems in device manager to verify the modem is working properly. • Windows 9x and 2000 PnP technology makes installation very easy. To do it manually use the add new hardware wizard. • Verify the Modem’s BIOS • Modern modems all have on board BIOS • A better term for BIOS would be command set • The BIOS is stored on the flash ROM of a modem. • BIOS can be corrupted easy by simply removing and reinserting the modem

17. Set Up the Correct Command Set • All that Windows communication programs need to handle a modem are the latest IFN files for your modem and version of Windows. • If Windows doesn’t verify the modem try a different brand. • Windows 2000 • For dial configuration use the phone and modem option and the control panel • Windows 2000 will not detect some internal modems so you must do manual installation.

18. When Modems Break • Check COM port and IRQ port • Program called modem doctor is very good for this • Modem doctor verifies COM ports and tests your modem and tells what kind of shape its in. • If your software says there is no modem • Make sure modem is looking at correct port • Check that there is no conflict with other devices • Reinstall all software and reconfigure your modem. • Modem works sporadically • Make sure you have the right modem installed. • Try different type. • Check phone lines in house.