Digital Subscriber Line (DSL) technology is available in both asymmetrical and symmetricalconfigurations.

Asymmetrical DSL (aka ADSL) - The incoming speed from the Internet on an ADSL connection is faster than the outgoing speed to the Internet.  ADSL is best used for light web surfing and downloading email.  It is not designed for sending large files, large Email attachments or uploading files to the Internet.  It is also not designed for downloading very large files because each packet of information in a large downloaded file must be acknowledged as it is received.  This can use over half of the available upload bandwidth of an ADSL circuit.   When this happens, a high speed ADSL circuit can choke on the outbound traffic and become very slow.

Symmetrical DSL (aka SDSL) - While the download speed of ADSL is usually faster, SDSL service is usually faster overall than ADSL because the incoming speed from the Internet and the outgoing speed to the Internet are exactly the same.   SDSL is preferred when you want remote users to have access to a shared database, file or email server on your network.  It is also preferred when you are downloading extremely large files.  These transactions take advantage of the maximum speed available in both directions, which is why a symmetrical line is needed.  SDSL is also much better for remote screen sharing applications, such as Timbuktu.

Symmetrical IDSL is another symmetrical variation of DSL that is popular for locations over 2 miles from the telephone equipment office with DSL equipment or locations that are served by fiber optic cable instead of copper telephone wire.  IDSL is the least popular of the DSL services because the maximum speed is limited to 128K.

Your Decision - Several factors will influence your decision in selecting which DSL configuration to use.

• Intended Use - If you intend to do simple web surfing and Email, ADSL is a good choice.  If you plan to upload and download large files, SDSL is the preferred choice.
• Number of Users - The more users you have, the greater the demand on your outbound bandwidth will be, and the more likely you will benefit from using SDSL.
• Service You Qualify For - The service(s) your location qualifies for limits your options.   If your location is served by copper telephone wire between the telephone equipment facility with DSL equipment and your location, you may qualify for ADSL service if the distance is less than 2 miles, SDSL service if the distance is less than 2.5 miles and IDSL service if the distance is less than 3 miles.  If your location is served by fiber optic wire between the telephone equipment facility with IDSL equipment and your location, you may qualify for IDSL service if the distance is less than 3 miles.  Please check with the CrossLink DSL Department to determine which DSL services and speeds your location qualifies for.
• VOIP - If you intend to use your DSL connection for Internet telephone service then SDSL is a better option because each Voice Over Internet Phone number reduces the incoming and outgoing bandwidth that is available for Internet access on a DSL circuit by up to 90K.  Since ADSL has significantly less outgoing than incoming bandwidth, an SDSL or T1 Data Circuit is usually the preferred choice when using Internet phone service.
• Budget - How much do you have to spend on a quality Internet connection?  ADSL usually cost less than SDSL or IDSL services.

Consultation - If you are not sure what type of DSL service you require please contact the CrossLink DSL Office at 888-686-3863 (703-642-1120 in the Washington, DC area).  Our staff will be happy to assist you in defining your requirements and implementing a solution that will work best for you.