Modems

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 driven diodes to radio.

 

INTRODUCTION

The need to communicate between distant computers led to the use of the existing phone network for data transmission. Most phone lines were designed to transmit analog information - voices, while the computers and their devices work in digital form - pulses. So, in order to use an analog medium, a converter between the two systems is needed. This converter is the MODEM which performs MODulation and DEModulation of transmitted data. It accepts serial binary pulses from a device, modulates some property (amplitude, frequency, or phase) of an analog signal in order to send the signal in an analog medium, and performs the opposite process, enabling the analog information to arrive as digital pulses at the computer or device on the other side of connection.

Modems, in the beginning, were used mainly to communicate between DATA TERMINALS and a HOST COMPUTER. Later, the use of modems was extended to communicate between END COMPUTERS. This required more speed and the data rates increased from 300 bps in early days to 28.8bps today. Today, transmission involves data compression techniques which increase the rates, error detection and error correction for more reliability.

In order to enable modems of various types and different manufacture to communicate, interface standards were developed by some standard organizations

Today's modems are used for different functions. They act as textual and voice mail systems, facsimiles, and are connected or integrated into cellular phones and in notebook computers enabling sending data from anywhere. The future might lead to new applications. Modem speeds are not expected to be increased much over today's 28.8 kbps. Further dramatic speed increases will require digital phone technology such as ISDN and fiber optic lines.

New applications might be implemented such as simultaneous voice and data. Videophones are an example of this.

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CLASSIFICATION OF MODEMS

The modems can be classified according to their characteristics:

• Range
  • Short Haul
  • Voice Grade (VG)
  • Wideband
• Line Type
  • Dial-up
  • Leased
  • Private
• Operation Mode
  • Half Duplex
  • Full Duplex
  • Simplex
• Synchronization
  • Asynchronous
  • Synchronous
• Modulation
  • AM
  • FM/FSK
  • PM
  • TCM
• Data Rate

 

Example:

The most familiar example is a voiceband modem that turns the digital data of a personal computer into analog audio signals that can be transmitted over a telephone line, and once received on the other side, a modem converts the analog data back into digital.

Types Of Modems:

1) External modems: It is the second term we have to consider from different types of computer modem. An External modem can be used to the same purpose and in the same conditions as internal computer modem. However external modem is a small box that uses other kind of interfaces to be connected to the computer.

2) USB modem: It could be a serial modem, named thus because it uses the serial port to connect to the computer. Usually installed on the back of the computer, the serial port is an easy-to-install option for the external modem. The same small box, on the other hand, can be an USB modem which normally uses USB port usually placed on the back or in front of the computer.

3) Dial-Up modem: First of all external modem can be a Dial-Up modem but more expensive than the internal.

As well as another type of external modem you may consider two important types of modem: cable modem and DSL modem if you need high-speed internet services. All ISPs usually supply a specialized modem named digital modem in the broadband package.

It’s important to notice that cablemodem has to connect to an Ethernet card, placed on the computer PCI slot to provide a broadband internet connection to the user. This is true if you opt to use Ethernet connection. However you won't need it if your option goes to use an USB connection.

4) Cable modem. The cable modem uses a coaxial cable television lines to provide a greater bandwidth than the dial-up comuter modem. An extremely fast access to the Web is providing by the cable modem with downstream transmission up to 38 Mbits/s and an upstream transmission up to 1 Mbits/s.

Unfortunately this transmission rate fluctuates with the number of users because of the shared bandwidth on which the cable technology is based.

5) DSL modem. DSL (Digital Subscriber Line) modem is exclusively used for connections from a telephone switching office to the user. This technology, available and frequently usable, split up into two main categories:

6) ADSLor Asymetric Digital Subcriber Line is used in North America and supports from 1.5 Mbits/s up to 9 Mbits/s of downstream transmission rate and up to 3 Mbits/s of upstream transmission rate.

7) SDSL. SDSL or Symetric Digital Subcriber Line is used in Europe and has the same data rates for downstream and upstream transmission which is 128 Kbits/s.

Radio modems

Direct broadcast satellite, WiFi, and mobile phones all use modems to communicate, as do most other wireless services today. Modern telecommunications and data networks also make extensive use of radio modems where long distance data links are required. Such systems are an important part of the PSTN, and are also in common use for high-speed computer network links to outlying areas where fibre is not economical.

Even where a cable is installed, it is often possible to get better performance or make other parts of the system simpler by using radio frequencies and modulation techniques through a cable. Coaxial cable has a very large bandwidth, however signal attenuation becomes a major problem at high data rates if a digital signal is used. By using a modem, a much larger amount of digital data can be transmitted through a single piece of wire. Digital cable television and cable Internet services use radio frequency modems to provide the increasing bandwidth needs of modern households. Using a modem also allows for frequency-division multiple access to be used, making full-duplex digital communication with many users possible using a single wire.

Wireless modems come in a variety of types, bandwidths, and speeds. Wireless modems are often referred to as transparent or smart. They transmit information that is modulated onto a carrier frequency to allow many simultaneous wireless communication links to work simultaneously on different frequencies.

Broadband

ADSL modems, a more recent development, are not limited to the telephone's voiceband audio frequencies. Some ADSL modems use coded orthogonal frequency division modulation (DMT).

Cable modems use a range of frequencies originally intended to carry RF television channels. Multiple cable modems attached to a single cable can use the same frequency band, using a low-level media access protocol to allow them to work together within the same channel. Typically, 'up' and 'down' signals are kept separate using frequency division multiple access.

New types of broadband modems are beginning to appear, such as doubleway satellite and power line modems.

Broadband modems should still be classed as modems, since they use complex waveforms to carry digital data. They are more advanced devices than traditional dial-up modems as they are capable of modulating/demodulating hundreds of channels simultaneously.

Many broadband modems include the functions of a router (with Ethernet and WiFi ports) and other features such as DHCP, NAT and firewall features.

Mobile modems and routers

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Modems which use mobile phone lines (GPRS, UMTS, HSPA, EVDO, WiMax, etc.), are known as cellular modems. Cellular modems can be embedded inside a laptop or appliance, or they can be external to it. External cellular modems are datacards and cellular routers. The datacard is a PC card or ExpressCard which slides into a PCMCIA/PC card/ExpressCard slot on a computer.

The best known brand of cellular modem datacards is the AirCard made by Sierra Wireless. (Many people just refer to all makes and models as AirCards, when in fact this is a trademarked brand name.)Nowadays, there are USB cellular modems as well that use a USB port on the laptop instead of a PC card or ExpressCard slot. A cellular router may or may not have an external datacard (AirCard) that slides into it. Most cellular routers do allow such datacards or USB modems. Cellular Routers may not be modems per se, but they contain modems or allow modems to be slid into them. The difference between a cellular router and a cellular modem is that a cellular router normally allows multiple people to connect to it (since it can route, or support multipoint to multipoint connections), while the modem is made for one connection.