A modem converts the digital data signals into analog data signals. Modem stands for modulation demodulation. They can be installed inside the computer in an expansion slot available for it. External modems are also available and they can be connected to a computer through a serial or USB port. Two general types of modems are:-
Standard modem
Windows modem
The standard modem use generic device drivers and they can be internal as will as external ones. One the other hand a window modem is an internal plug and play device. It needs a special device driver provided by the window operating system to function properly. The internal modems do not require much physical configuration. They can be installed into a compatible expansion slot. The external modem is connected through one of the COM port to the computer through a cable called a null modem cable. Most of the home computers use a DSL modem which bridges the data from the phone line to a format usable by the network interface card and computer. Here DSL stands for Digital Subscriber Line a symmetrical DSL (SDSL) modem can send and receive at the same speed. The DSL bandwidth can be shared by several computers and a router allows this. A cable modem provides a higher download speeds and lower upload speed and it does not need phone line rather it uses satellite transmission. A splitter is used to separate two signals in case of a cable modem. If we consider the modem from computer attachment point of view than there are two types of modems:-
Internal modem
External modem
Internal modem: - An internal modem is plugged directly into ISA or PCI expansion bus. It contains its own UART (Universal Asynchronous Receiver/ Transmitter) which is responsible for manipulation data into and out of serial form. A UART converts parallel data into serial data or at least forms the foundation of serial port. Then the modulator circuit converts the serial data into audio signals which is coupled to telephone lines through telephone interface and RJ-11 type connector. At the receiving end the telephone interface separates receives signals and passes it to demodulator that converts it into data signals or serial data. The UART then converts the serial bits into parallel data and is placed on system bus. The telephone interface performs many jobs like combining and separating audio data, generation dual tone multi frequency dialing signals (DTMF), detection of incoming ring, alerting UART to begin negotiation of a connection, driving the speaker that is used to hear dial tone, dialing signals etc. The controller circuit controls the overall operation of the modem and switches the modem between control mode and data operation mode. It accepts commands from the modulator and stores the operating system parameter into NVRAM which in which in case of power loss or reset conditions can be reloaded.
External Modem: - The functions of an external modem are very much similar to that of an internal modem. The main difference between internal and external modems is that the external modem does not have an inbuilt UART; rather it relies on an existing serial port that is already configured in the computer. External modems are generally faster and easier to operate than internal modems because of it. Also an external modem needs an AC adapter for its power supply while internal modems are powered directly from expansion bus. Reset the working of external modem is exactly same as that of internal modem
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