1 - Your communications program sends a voltage across the 20 pin serial port, in which the modem is connected. The signal voltage is known as Data Terminal Ready (Data Terminal Ready) or simply DTR. This signal tells the modem that the PC was turned on and ready to transmit data. At the same time, the PC detects a voltage of the modem on pin 6 - a sign of Receiving Terminal Ready (Data Set Ready) or DST - which lets him know that the modem is ready to receive data or instructions. In normal connection, both signals must be present before something else can happen.
2 - Using a standard query language, called Hayes Hayes modems because it was in she became popular, the program communication sends a command to the modem via the line 2, line Data Transmission.
The command prompts your modem skirt hook - ie, connect to the phone line. The program follows another command Hayes asks the modem tones or dial pulses needed to complete the call from a particular phone number. The modem recognizes the command, sending a signal to the PC on line 3, line Receive Data Receive Data (Line).
3 - When the modem on the other end of the phone connection - remote modem - answered the call their local modem sends a tone
communication so that the remote modem know you are being called by another modem. The remote modem responds with a tone
higher. (I can hear both tones if your modem has a speaker.)
4 - When communication is established, your modem sends a signal to the PC Line Detection (CD, or Carrier Detect)
the line 8. The sign informs the communication program the modem is receiving a CD signal, which is a continuous-tone
certain frequency, which will later be modulated to transmit data.
5 - The two modems exchange information on how they will manage the sending of data, a process known as handshake (Handshake). Both modems must be in agreement on the transfer speed, the number of bits that compose the data packet - for example, a single character - how many bits will signal the beginning and end of a packet, if modems will use a parity bit for error checking and will operate in half-duplex or full-duplex. If the local system and the remote does not use the same configuration, sending characters that will not make any sense or not will communicate in any way.
The Transmission Velocity - Although transmission speeds are typically expressed in baud - the number of
frequency changes that occur for a second - the term is outdated, being more appropriate bits per second
the current situation. The baud modems in the first 300 bits per second was achieved by sending a frequency
bit 0 to indicate different and one to indicate the bit 1. The analog signal from the telephone line is limited by how fast it can change frequencies, generating the need for different projects to increase the rate of sending data.
The code group allows different frequencies to transmit more than one bit at a time. Broadcasts 1,200 bits per the second, for example, signals are sent, in fact, 600 baud, but three different frequencies are used for
represent the three possible different pairs of binary bits, 0 and 0, 0 and 1, 1 and 1. A similar project combines more more frequencies with binary combinations to reach 2400 bits per second. For transmission rates even faster, the two modems must use the same compression method of data via pattern recognition, often repeated 0s and 1s, using even smaller codes to represent these patterns.
Data Bits - Communication systems may use both seven and eight bits to represent a packet of data (date
packet). In this example, we used eight bits.
Bits Start / Stop - Each data packet uses a single bit to signal the beginning (start) of a character and one or two
bits to signal the end of it. The example given here uses one stop bit (stop bit).
Parity Bit - As a form of error correction, the two systems must agree to use odd parity, even or none.
If they choose odd or even both systems add up the bits contained in the character and add another bit known
as a parity bit. This bit can be either 0 to 1, it is necessary that the total is an even number or odd,
depending on the parity that systems combined. Parity bits are used for error checking.
Half-Duplex/Full Duplex - Both systems need to agree on who will be responsible for showing the text on the computer
site. A system must be configured for full-duplex and half-duplex to another. The system using full-duplex
will be responsible for displaying the text on both systems and will echo any text sent to it by the system
half-duplex. If the two systems do not use complementary duplex settings, not any text appear on the system
Local or each character appears twice.
6 - When the communication program wants to send data, it first sends a voltage to the serial line 4.
In fact, this signal Request Submission (Request to Send, RTS) asks the modem if it is free to receive
PC data. If the remote modem is receiving data that you want to move to the computer and the computer is busy performing another
activity, as the recording disk of previously submitted data, the PC suspend the RTS signal, which means the
modem to stop sending data until it finishes its task, and then restart the RTS signal.
7 - Unless the modem is too busy managing other data to receive new data system, it returns a
Clear to Send signal (CTS) to the PC via line 5 and the PC serial port responds by sending the data to be
transmitted on line 2. The modem into the PC sends the data received from the remote system via line 3. If the modem does not
is able to transmit data at the same speed that is sent, it will drop the signal CTS, asking the PC wait
until he can capture new data and then relay them.
8 - Across the line, the remote modem listens to the data that are coming as a series of tones at different
frequencies. It demodulates these tones into digital signals by sending them to the receiving computer. In fact, both computers
can send and receive signals at the same time, because the use of a standard system of tones enables both modems
sides differentiate the input signals of the output signals.
9 - When you enter your program to terminate a communication session, the program sends another command to Hayes
modem so that it breaks the phone connection. If the connection is broken by the remote system, your modem will send a signal
Line Detection (CD) to your PC, telling the program that the communication ended.
Source: Evolution of Computers
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