The easiest way to connect computers over long distances is over telephone lines; you can reach almost anywhere by telephone; You need a modem. Modems convert communications signals from a form the computer can understand to a form the phone system can convey and vice versa. Modulation is the process of converting a digital signal from a computer into an analog signal the telephone system will accept. At the other end of the connection, another modem interprets those analog signals the telephone system has conveyed and converts them back into digital form so the receiving computer can understand them.
- Transfer Speed: Modem speed is often discussed in baud rates or BPS. Baud rate refers to the oscillation of a sound wave on which a single bit of data is carried. BPS means the amount of data transferred in a second. By applying special techniques that manipulate analog signals so that more than 1 bit of data can be transferred in a single cycle of the signal, a modem can encode data and achieve higher throughput.
- Compression: Compression can greatly speed up the transfer of some types of information, such as text and graphics. When establishing connection with another modem, compress the data flowing between the two modems. This compression happens without the intervention of the two computers involved. Compression makes modem communication faster.
- Error Detection and Correction: The simplest form of error detection, which all asynchronous modems can perform, is parity checking. In parity checking, there must be an odd number of 1 bits in a data word transmitted, in which case the modem is using odd parity, or there must be an even number of 1 bits in data word transmitted, in which case the modem is using even parity. Parity checking does not correct data errors. It simply signals that an error has occurred. It is up to the receiving computer to request that the transmitting computer resend the data. Another commonly used method to make sure data is sent intact is counting the number of data word sent and received, making sure they are the same and calculating a checksum, on both the sent and received data to make sure the checksums match.
Types of Modem:
The types of modems can be divided into two broad categories, depending on how they coordinate their data transmission:
Asynchronous modems were developed specially for use with telephone lines and they are the most common type of modem. When your computer sends data to another computer using an asynchronous modem connection, the data is first divided into bytes. The bytes are sent 1 bit at a time and the byte is preceded by a start bit and followed by a stop bit.
The computers do not coordinate in terms of when data will be sent. The receiving computer uses the start and stop bits to recognize when it has an entire byte of data. The start and stop bits and other coordination mechanisms in asynchronous communications can take up as much as 25% of data traffic.
Synchronous modems can transmit more data than asynchronous modems because they use careful timing and coordination between modems to send large block of data called frames, having multiple bytes.
Synchronous modems also do some things that asynchronous modems do not always do, including:
- Arrange data into block
- Add control information
- Check information to provide error control