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A modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from light emitting diodes to radio.

A modem modulates outgoing digital signals from a computer or other digital device to analog signals for a conventional copper twisted pair telephone line and demodulates the incoming analog signal and converts it to a digital signal for the digital device.

In recent years, the 2400 bits per second modem that could carry e-mail has become obsolete. 14.4 Kbps and 28.8 Kbps modems were temporary landing places on the way to the much higher bandwidth devices and carriers of tomorrow. From early 1998, most new personal computers came with 56 Kbps modems. By comparison, using a digital Integrated Services Digital Network adapter instead of a conventional modem, the same telephone wire can now carry up to 128 Kbps. With Digital Subscriber Line (DSL) systems, now being deployed in a number of communities, bandwidth on twisted-pair can be in the megabit range.

Types of Modems

 Landline Modems

 Wireless Modems

 LAN Modems

A. Landline Modems:

Landline modems are modems which connect to the public switched telephone network (PSTN). To connect to PSTN, these modems have a jack known as RJ-11, or regular phone jack. A telephone cable with a RJ-11 plug connects the modem to the nearest phone jack, which also conforms to the RH-11standard. Landline modems can be further classified into the followings types:

1. Internal modems: This device is a circuit board that plugs into one of the expansion slots of the computer. Internal modems usually are cheaper than external modems, but when problems occur, fixing and troubleshooting the modem can sometimes prove to be quite difficult. The telephone line plugs into the modem port in the back of the computer. Most internal modems come installed in the computer you buy. Internal modems are more directly integrated into the computer system and, therefore, do not need any special attention. Internal modems are activated when you run a communications program and are turned off when you exit the program. This convenience is especially useful for novice users.

Internal modems usually cost less than external modems, but the price difference is usually small. The major disadvantage with internal modems is their location: inside the computer. When you want to replace an internal modem you have to go inside the computer case to make the switch.

2. External modems: This device is attached to the back of the computer by way of a cable that plugs into the modem port. It is usually less expensive and very portable. It can be used with other computers very easily by unplugging it and plugging it into another computer. This is the simplest type of modem to install because you don’t have to open the computer. External modems have their own power supply and connect with a cable to a computer’s serial port. The telephone line plugs into a socket on the rear panel of the modem.

Because external modems have their own power supply, you can turn off the modem to break an online connection quickly without powering down the computer. Another advantage over an internal modem is that an external modem’s separate power supply does not drain any power from the computer. You also can monitor your modem’s connection activity by watching the status lights.

3. Voice/data/fax modems: This device can be hooked up to your telephone and used to send information to your computer. Your computer can also send information to a fax machine. Most computer modems are modems with faxing capabilities.

4. PC Card modem: These modems, designed for portable computers, are the size of a credit card and fit into the PC Card slot on notebook and handheld computers. These modems are removed when the modem is not needed. Except for their size, PC Card modems are like a combination of external and internal modems. These devices are plugged directly into an external slot in the portable computer, so no cable is required other than the telephone line connection. The cards are powered by the computer, which is fine unless the computer is battery-operated. Running a PC Card modem while the portable computer is operating on battery power drastically decreases the life of your batteries.

B. Wireless Modems:

Wireless modems are radio transmitters/receivers installed into mobile computing devices (i.e. devices that are used while you are moving such as mobile phones, laptops etc.) Using wireless modems, one can connect to a network while being mobile. Unlike landline modems, wireless modems do not plug into an RJ-11 jack.

C. LAN Modems:

LAN modems allow shared remote access to LAN (Local Area Network) resources. LAN modem comes fully preconfigured for single particular network architecture such as Ethernet or Token Ring and/or particular network software such as IPX, NetBIOS, NetBEUI etc.

Working of Modem

Modems convert analog data transmitted over phone lines into digital data so that computers can read; they also convert digital data into analog data so it can be transmitted. This process involves modulating and demodulating the computer’s digital signals into analog signals that travel over the telephone lines. In other words, the modem translates computer data into the language used by telephones and then reverses the process to translate the responding data back into computer language.

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