DSL Modems

January 24, 2012

A DSL (Digital Subscriber Line) Modem allows you to connect to the Internet Broadband services typically offered by your local telephone provider. Although most customers will refer to this device as a “DSL Modem”, technicians at your local telephone company or ISP provider will most likely call it a DSL transceiver. Regardless, the basic job of the DSL modem is to convert the signal from your telephone company into the Ethernet signal required by your computer or home network.

How Does A DSL Modem Work?

Unlike cable modems which require a coaxial cable for their connection to the Internet Broadband, DSL modems make use of the existing copper telephone wires which most people already have in their homes. This works because standard telephone over wires uses only a small portion of the bandwidth available. Using electronic technologies, DSL is able to make use of the remaining bandwidth to carry its signal.

The beauty of DSL is that the signal can exist on the same wires used by your voice phone server. It does this by using a special form of DSL called ADSL, otherwise known as asymmetric DSL, by dividing up the frequencies in your signal and using more of them for the download of Internet data than upload. The assumption is that most typical customer download more data than they upload.

While this kind of signal works well for most customers, ADSL is limited in the distance it can travel with service deteriorating as the signal approaches these limits. The maximum distance a DSL signal can travel is 18,000 feet (5,460 meters).

The ADSL technology has improved over the years resulting in ASDL2 and ASDL2+. ASDL2 increases download speeds to 12 Mbps while ASDL2+ increases it to as much as 24 Mbps.

DSL begins its trip to your home from equipment at your local telephone provider called DSL Access Multiplexer or more simply DSLAM. The DSLAM is one of the main differences from cable broadband data where the home office uses a Cable Modem Termination System (CMTS). The key difference is that cable users share the network signal coming from the CMTS whereas the DSL customers have a dedicated signal from their DSLAM.

The DSL signal travels through the telephone companies local wire network where it arrives at your home through the standard wiring. The telephone cable connects to your DSL modem using a standard RJ11 connection. The DSL modem separates the DSL frequencies and converts them into the 1s and 0s requirement for computer networks and provides a RJ45 so that you can connect a standard Ethernet cable between the modem and your computer or network router.

Types of DSL (ADSL Alternatives)

While ADSL is probably the most common, it’s not the only kind of DSL signal.

  • VDSL – Very high bit-rate DSL. This is a much faster connection (52 Mbps downstream) but can work over only small distances – about 4,000 feet (1,200 m). It works in many places because telephone companies are replacing the main feeds in many neighborhoods with fiber optic cables. The fiber will run along main streets, with junction boxes near the end of your neighborhood street, and then be converted to copper using a VDSL gateway in the phone company’s junction box for your neighborhood. A VDSL transceiver in your home translates the signal into your required Ethernet connection. AT&T U-verse makes use of this technology for their high-speed internet broadband services.
  • SDSL – Symmetric DSL. SDSL provides the same speed for downloads and uploads. It comes with a cost though as you cannot use the same signal for your standard analog phone.
  • RADSL – Rate-adaptive DSL. This is similar to ADSL but with the modem intelligence allowing it to vary the connection speed based on the distance and quality of the signal.
  • Uni-DSL – Universal DSL. This is a newer technology developed by Texas Instruments. It’s based on the ADSL2+ and VDSL2 technologies and becomes more cost effective as the installation of fiber is deployed further from the telephone provider office and closer to the customers. It can provide download data rates up to 200 Mbps. Uni-DSL has been viewed as a trade off from running fiber optical cable to every home, which becomes cost-prohibitive.

How to Buy a DSL Modem

Buying a DSL modem is not as standardized as the DOCSIS specification used by cable companies. It’s important to verify what your local DSL provider requires. Frequently, they will offer a free modem as part of signing up for a 1 or 2 year contract for DSL Internet broadband. Just don’t get yourself in the position of buying the wrong modem and finding this out when your local telephone technician arrives at your home to hookup the DSL connection. If your equipment is wrong, they may charge you for another service call or your broadband connection may not work properly.