Computers use binary codes of “ON” bit which has a value of 1 and “OFF” bit which has a value of 0 for sending information across short distances to serial peripherals such as serial printers and modems.
Computers voltages from 5 to 15 volts for an “ON” bit and from -5 to -15 volts for an “Off” bit. But to send information over the telephone lines, computers need to modulate this digital information into analog format the telephones use; in other words convert it into variable pitch sound instead of beeps, then demodulate it at the other end so that the receiving computer can understand it. Hence, the device that does this takes its name from MOdulate DEModulate => MODEM.
Modems communicate one bit at a time using serial transfer. The standard serial transfer uses 9 wires of which only two wires actually transfer data, the rest manage the transfer process and provide signal ground, to prevent statics and interference.
Your communication software’s dial command transmits a signal ATDT (ATtention: Dial Tone) followed by a phone number to your modem which goes off-hook – picks-up the phone so to speak – and dials out using DTMF (tone) as opposed to pulse dialing in this case, When the modem on the other end receives a ring, it sends a signal RI (Ring Indicator) to the software telling it to issue a command to pick up the phone. When the remote modem picks up the phone, your modem sends out a hailing tone, and the remote modem responds with a higher pitch tone. The modems then exchange information (cackle) about how to transfer data in a handshake, rate (BAUD – bits per second), type of transfer (ASCII, x-modem etc.) and settings (8N1) to determine the size of data packets the systems will exchange. Hence the word protocol is used to describe this exchange.
Once the communications are established the modems send a DCD (Data Carrier Detect) signal, which continues until the modems either hang up or get cut-off.
A data packet is a group of bits that constitutes a single character. The 8 in the setting 8N1 refers to the number of data bits in the packet. Each packet is enclosed by a Start bit and a Stop bit that tells the receiving modem where the packet begins and ends. The 1 refers to the number of stop bits in the packet
Data packets sometimes contain a special parity bit that the modem use for basic error checking. Parity can be set to Odd, Even or None. When parity is Even the transmitting system sums up the 0s and 1s and gives the parity bit a value of 1 or 0, whichever makes the total an even number. If the receiving modem gets a different result, it assumes that the transmitted data is incorrect and requests a re-transmit from the other modem.
When a communications session is over, each of the computer’s software sends an ATH (ATtention: Hang) command to its modem instructing it to go on-hook, hang-up. Or when the other modem loses the data signal carrier (DCD) it also hangs up.
DTR <-> Data Terminal Ready DCD <-> Data Carrier Detect DSR <-> Data Set Ready ATA <-> ATtention: Answer ATO <-> ATtention: (go) On-line