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Asynchronous (or Asymmetrical) Digital Subscriber Line, better known as ADSL or DSL for short, is a type of high-speed Internet service that runs over standard copper telephone lines. ADSL Internet uses a different frequency band than telco voice traffic, making it possible to be online while using the landline telephone at the same time. The traditional competitor to ADSL has been cable Internet, and more recently fiber optic service (FiOS). Where available, basic ADSL packages remain the most affordable choice for high-speed Internet.
Asynchronous or Asymmetrical service refers to DSL packages that have faster downstream speeds relative to the upstream speed. In other words, data being passed from the Internet to your computer (downstream data) travels faster than data passed from your computer to the Internet (upstream data). The reason for this asynchronous arrangement is that normally the data going from your computer to the Internet consists of very small request packets, such as calls for Web pages or email. The amount of data traveling back to your computer, however, is substantial, requiring greater bandwidth to avoid slow-downs and an unsatisfying experience.
As a matter of course, businesses might require uploading large files across a private network that tunnels through the Internet, known as a Virtual Private Network (VPN). In this case a Synchronous DSL (SDSL) package can be had at a higher cost. SDSL Internet features upstream and downstream rates that are matched, making uploading extremely fast.
ADSL Internet is not available in all regions. Where offered, the subscriber must be within the service parameters of a local distribution point, or DSL Access Multiplier. (DSLAM). The further one is physically located from the DSLAM, the more the signal degrades, resulting in slower speeds. As a result, ADSL Internet packages are guaranteed to provide speeds that fall within a range for each plan type. People who live closer to the DSLAM will enjoy speeds at the top of the plan’s designated range, while people who live further away will achieve speeds in the lower range. Plans are also tiered, so one can pay more money to get a faster plan.
An advantage of ADSL Internet is that it is the only high-speed service that offers slower speeds for people on a budget or for those just coming up from dial-up modems, who will no doubt be impressed by the improvement without having to break the bank. In newer housing and commercial developments where fiber optic lines are being installed in lieu of copper lines, residents must choose between cable or FiOS, which can provide superior speed but at a higher price point.
It is quite easy to find out if ADSL Internet is offered in your area. One can shop for DSL providers online, then enter the address of the premises where DSL is required to see what packages are available. Once subscribed to the service, one need only setup a DSL modem that can be leased from the provider or purchased outright from a third party. The modem will have a connecting port for the phone line coming from the wall jack, a port to plug in your landline, and two or more ports to connect additional computers, depending on the model.
DSL modems that support multiple computers, known as a local area network (LAN), include a built-in router that will allow all machines to share the same ADSL Internet connection. The modem/router can be wireless so that connected machines do not need to be physically tethered to the DSL modem. This will require a wireless network card installed in each computer, or wireless network adapters can be used. The ADSL modem/router and network cards must also support a common wireless standard or protocol. Purchasing wireless network components that support all current wireless standards is another option.
ADSL Internet is an “always on” service. The modem and Internet connection can be left on 24/7, though for added security it is wise to disconnect computers from the Internet when not in use.
It's worth mentioning that DSL is getting phased out in a lot of areas on favor of all fiber optic networks. AT&T is the one leading the charge on that shift and has spent a lot of cash replacing its copper network with fiber optics. That investment wouldn't have been made unless AT&T believed that fiber optic networks are "the future" in terms of Internet connectivity.