ModemsThe main reason for using a modem is that the commercial grade telephone circuit is limited to a bandwidth of 300 to 3400 Hz, which gives you a bandwidth of 3. 1 KHz which is not wide enough to transmit digital signals without the introduction of distortion. Therefore, the digital signal must be converted into an analog signal before being transmitted over the telephone network. This also applies at the far end, so the received analog signal must be converted back to its original digital form. This process is carried out by a piece of equipment called a MODEM and for this to be achieved there must be a modem at each end of the circuit. The ITU-T calls a modem data communications equipment (DCE), and the EIA calls it circuit terminating equipment (CTE). A modem may also be required to establish, maintain, and terminate each connection made through the phone. network either using a leased circuit or a dial-up connection via the PSTN. It is common to find modems that offer automatic dialing and answering and can also restore a communications failure by finding an alternate channel. Before two modems can send data to each other, they must first configure their respective modems with each other. This sequence of events is called the handshake. An interface is the means by which a terminal is connected to the modem and ensures that electrical and mechanical compatibility is achieved. Using a home phone via PSTN. Note 1. The use of a modem in a data circuit. A modem works by transmitting digital data from the terminal or computer to an encoder (except FSK modems) where the bit stream is encoded in dibits, tribits or quabits. This coded signal...... middle of paper ...... distance between the two modems, if it is about 25 km and it has a continuous DC path, then a short distance modem , an A-line driver or a modem eliminator that could be used for such devices would be considerably cheaper. Another important consideration would be whether the modem was on a dedicated leased circuit or a dial-up connection. For a large international company with the amount of data transfer per day that would be expected, a leased circuit would be used because they cannot afford to waste time as they need to complete these data transfers as quickly and efficiently as possible. possible. However, someone who only needs to be 'online' with the Internet and would only use the Internet for short periods would not find it cost effective to have a leased circuit and therefore a normal connection via the PSTN would be more favorable..