Modem(Modulation and Demodulation)

By | January 5, 2013

Modems :

The word medem is an acronym for Modulator and DEModulator and is typically used to send digital data overa phone line . The dta is modulaated into signal by the sending modem and this signal is compatible with the phone line. The receiving modem demodulates the signal back into digital signal back into a data bus.

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Thus , a modem is device which performs the following:

  • Modulates an analog carrier signal to encode digital information.
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  • Demodulates a carrier signal to decode the transmitted information.
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The modem operates in the one of the following modes:

Data mode :  data is sent to and from the computer over the telephone lines.

Command mode: the computer sends data to the modem. This data can be called as command sent by the computer to the modem. The modem listen to this command and follows it.

TYPES OF Modems:

There are two types of modems, namely:

Internal – An internal modem is a modem card that you can plug into an expansion slot on the motherboard. Internal modems do  not take up the desk space because they are fixed on the motherboard.

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These modems use the power supply of the computer and thus are switched on whenever the computer is on. An internal modem it is physically remove the telephone connection from the modemespecially during lightening. Lightening causes the internal modem to burn through the connected telephone line. At times, due to this the motherboard may also burn.

External: an external modem is connected to the PC through  cable. Which is plugged into a serial port on the back of the system unit. External modems have their own power cords. Before establishing the connection, you must switch on the modem.

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Apart from this classification, the modems are further classified into 56k modem,V90,ISDN,DSL,ADSL,USB modem and Data card.

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Dial up modem:

Dial up modem uses normal telephone lines (copper lines ) to transmitt analog signals. Dial up modem provides maximum theortical data transfer speed of 40-50 Kbits/s.  Modern dial up modem providing maximum theortical speed of 56Kbits/s.

Phone lines are noisy and quality of modem are the factors which affect the connection speed. In extremely noise environment dial up modem provides data transfer speed of 20 kKbits/sec or lower. Online gaming and video confrence are very difficult with dial up modem because dial up connection uusually have high latency. Out of all modems, dial up modem provides slowest speed for data transfer.

ISDN Modem:
integrated Service Digital Network(ISDN) is a set of international communication standatrds for transmitting digital data over a telephone lines. It enables to send the data , voice and video over digital telephone lines or even an ordinary telephone wire. It trnasmitt data at the speed of of 64 Kbps.

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ISDN needs equipment to perform conversion between the digital signals and analog signals. This ISDN modem can be an internal or external modem . ISDN modem provides high-speed Internet access, therefor, your ISDN also needs an ISDN modem.

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DSL Modem:

You can connect to the internet using the standards modem,LAN modem ,ISDN conncetion or the Digital Subscriber Line(DSL) connection. DSL connection is high speed connection that allows the phone wires to transmitt modulated digital content at high speed. DSL can carry both voice and data at the same time. Due to which you need  not install another phone line.

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A DSL modem provide high-speed access to the information through the telephone lines. DSL modems also known as DSL trnasceiver or ATU-R. you can connect DSL Modem to a PC through USB or Ethernet connection.

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ADSL Modem:

ADSL(Asymmetric Digital Subscriber Line) is a high speed data communication technology. It allows you to transmit digital information at a high bandwidth using exiting copper telephone lines. ADSL is an “always on” service( as  long as your computer is powered on); this allows you to leave the downloading of larger files to continue without you.

The distinguish characterstic of ADSL over DSL is that ADSL offers high speed in downstream direction (from your internet Service Provider(ISP) to you) and much slower speeds in upstream direction(from you to ISP). The data stream speed of ADSL ranges from 512 Kbps to8Mbps.

USB Data Card:

USB data card is a device used with computer or laptops to provide mobile wireless internet access . USB data card always comes with a SIM card and software setup CD. Depending upon the manufacture , this software and SIM cards may be different . User has to install the setup files for the first time to access the internet.

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When the user connects the data card to the port of the PC , immediately modem intilization begins. Once the modem is intiliazed, it checks for the port authentication is completed succesfully,the connection is  established. If the port authentication or modem intialization fails , then the USB Data Card is unable to establish the internet connection. Data Transfer Rate(DTR) is 3.1 Mbps and this type of services is provide mostly by Reliance,Tata, Idea and MTS ISP.

 

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