Difference between DSL and ADSL

Many of us may already know that there is not much difference between DSL and ADSL; the latter is a type of the former. To understand better, one should foremost construe the terms, DSL and ADSL, individually. Let us first see what DSL is all about, before we start to discuss on ADSL.

What is DSL?

DSL (Digital Subscriber Line), also referred as xDSL (x stands for different DSL techniques), is a group of technologies that provide high-speed Internet access, by using the ordinary telephone lines. It converts the ordinary telephone line into a high-speed passage for digital audio, video and data. DSL technology is widely used for business and personal purpose. The technology is simple as the setup required for network already exists. It does not keep the telephone line busy as is the case in the Internet access used in the Dial-up connection. However, the speed depends on the distance between the Internet service provider and the user. More the distance, less will be the speed of the Internet access.

How does DSL work?

POTS (Plain Old Telephone Service) uses copper wired (twisted cable) network, for the exchange of voice information. It uses analog signal transmission and requires a very low bandwidth (0-3400Hz), thus a wide bandwidth remains unused. DSL uses this unused bandwidth to provide Internet service. In DSL technique, the unused high range of frequency is used for broadband Internet access and the low range is used for voice transmission. In this technique, there is no analog to digital and digital to analog conversion. Digital data is directly fed into the computer using the DSL modem, thus allowing a wider range of bandwidth to be used. Splitters are used to split the low and high frequency signals into two bands. Filters are installed in phones to avoid interference between the range of frequencies used for DSL and telephonic conversion.

Difference between DSL and other Internet Services

Internet Service	Upstream Speed	Downstream Speed
DSL	128 Kbps to 384 Kbps	3 Mbps to 6 Mbps
Dial Up	56 Kbps	56 Kbps
Cable	768 Kbps to 1.5 Mbps	8 Mbps to 16 Mbps
Satellite	128 Kbps to 256 Kbps	512 Kbps to 1.5 Mbps
Wireless	128 Kbps to 768 Kbps	384 Kbps to 2.0 Mbps

Types of DSL Techniques

There are basically two types of DSL techniques -
Asymmetric DSL: ADSL, RADSL, VDSL are types of Asymmetric DSL
Symmetric DSL: SDSL, HDSL, SHDSL are types of Symmetric DSL

In Asymmetric DSL the bandwidth allotted for upstream and downstream is unequal, whereas in Symmetric DSL, it's equal. Let us look into the concept of upstream and downstream to understand the broad uses of the technologies.
Downstream: Data transfer from the server to the user is called downstream. For e.g. downloading a song from any website.
Upstream: Data transfer from the user to the server is called upstream. For e.g. uploading an image to a website.

Difference between DSL Techniques

As we already know, ADSL is one type of DSL technique. To get a brief overview of the various DSL techniques, refer to the box given below.

Type of DSL	Upstream Speed	Downstream Speed	Distance Limit
ADSL - Asymmetric DSL	16 Kbps to 640 Kbps	1.5 Mbps to 6.1 Mbps	9,000 feet to 18,000 feet
VDSL - Very High Speed DSL	1.5 Mbps to 2.3 Mbps	1.6 Mbps to 52.8 Mbps	1000 feet to 4500 feet
HDSL - High Data Rate DSL	1.544 Mbps	2.048 Mbps	12,000 feet
SDSL - Symmetric DSL	1.544 Mbps to 2.048 Mbps	1.544 Mbps to 2.048 Mbps	12,000 feet
RADSL - Rate Adaptive DSL	272 Kbps to 1.088 Mbps	640 Kbps to 202 Mbps	9,000 feet to 18,000 feet

What is ADSL?

ADSL is a type of asymmetric DSL which allots more bandwidth for downstream than upstream. The downstream speed (1.5-9 Mbps) is higher than the upstream speed (1.5 Mbps). The downstream speed depends on the distance of the user from the service provider. The speed increases with decrease in the distance. 1.5 Mbps downstream speed can be achieved for 18,000 feet distance; while 9 Mbps speed is possible for distance of 9,000 feet. Most of the Internet users aim at high downloading, as compared to uploading on network. ADSL is widely used for Internet connections used at home and small businesses. With the help of ADSL, large unused bandwidth of the copper wired network can be utilized for high speed data transfer. With help of ADSL, one can talk on the telephone and simultaneously access the Internet. Following are other types of Asymmetric DSL:
RADSL: The speed of data transfer is adjusted automatically depending on the quality of telephone line and distance from the service provider. The downstream speed is adjusted higher than the upstream speed. RADSL provides a faster speed for premises close to the service provider.
VDSL: It provides a high speed of data transfer for short distance, by connecting to ONU (Optical Network Unit), which is a combination of fiber optic and copper wire networks.

The most important advantage of DSL technology, is that it uses existing telephonic network for high speed Internet access, without keeping the phone line busy.