Modems

Chapter 17

Basic Knowledge

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.

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

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.

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.

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

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.

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

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

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

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

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

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

Modem Negotiations Standardized series of responses

between two modems so they can determine optimal speed, data compression, error detection, etc.

Chart pg. 1059-1060

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

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

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.

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.