A DSL (Digital Subscriber Line) modem is necessary to connect to the Internet once you have subscribed to an ISP (Internet Service Provider) for DSL service. The modem is used to communicate directly with the ISP.
The role of the modem is to communicate with the ISP and translate the data it receives to a protocol that the computer can understand. It must be translated because the methods that are used by telephone lines are not used by computer networks.
The majority of DSL modems are external boxes which are connected to the computer through the Ethernet port. Occasionally modems connect through the USB (Universal Serial Bus) port.
The DSL modem separates the available frequencies of the phone line into sections. This allows the user to be on the Internet and make phone calls simultaneously.
As stated in the article "ADSL Filters Explained" on ePanorama.net, frequencies in the range of 25 kilohertz to 1.1 megahertz will be used to transmit digital data, while frequencies in the range of 300 to 3400 hertz will be used by the telephone.
Modems allow all data to pass through and do not offer a level of security for the user, such as routers can via firewalls.
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