How is DSL different from today's analog modems?

In order to achieve data transmission rates of up to 300 times faster than analog modems, DSL technologies use a wider band of frequencies. Also, because DSL uses a digital signal, unlike today's analog modems, DSL transmissions do not pass through the regular analog voice telephone network. This element of DSL can clear the "congestion" that a lot of dial-up Internet traffic causes (the cause of delayed dial tone in certain high-Internet use areas).

What are the different varieties of DSL?

Several different types of DSL have been developed, and each provides different benefits. The table below summarizes the information in the narrative that follows it.

What is HDSL and why was it developed?

Following the completion of ISDN technology design (which provides 128 kbps connections), attention was turned to developing a higher speed DSL for the delivery of high speed data lines, leased by telephone companies to businesses. These traditional leased data line services operate at T1 (1.54 Mbps) rate in the USA and E1 (2 Mbps) in Europe. In order to achieve 4 km range with the technology at the time, two copper pairs had to be used. The data stream is divided into two streams each of half rate (i.e. 784 kbps), and these are independently transmitted over two cables and then the data is recombined at the receiving end. Today HDSL is the most well proven and heavily deployed DSL. It is used as an alternative for providing T1 or E1 type leased lines and is usually cheaper and faster than other traditional methods, which previously involved the deployment of special cables with repeaters at 1 km intervals or optical fiber cables.

What is SDSL?

Symmetric Digital Subscriber Line (SDSL) is a derivative of HDSL that uses only one of the two cable pairs. It transmits only on a single-pair, usually at the 784 kbps (half-T1) data rate. Although not originally intended to operate simultaneously with POTS, Orckit has managed to achieve POTS sharing so that residential or small office users can share the same telephone for data transmission and voice or fax telephony.

What is HDSL2?

HDSL2 is an advanced form of HDSL that has the same features as regular HDSL, but uses only a single twisted copper pair. (See a more in-depth description in our HDSL2 White Paper)

What is ADSL?

Asymmetric Digital Subscriber Line (ADSL) was conceived originally by researchers at telephone companies for video-on-demand type applications, but has since become focused on providing higher speed Internet services, such as the World Wide Web. ADSL is faster in the downstream (towards the customer) direction and slower in the upstream direction. Some applications, such as downloading from databases using control commands (browsing on the Internet), do not always demand symmetric data rates and can take advantage of an asymmetric system.

As a minimum, ADSL provides T1 rate or higher in the downstream direction and 64 kbps or higher in the upstream. The "enhanced performance" ADSL Standard provides for multiple channels with total downstream rates as high as 8 Mbps, plus bi-directional channels up to 768 kbps. Since ADSL was designed for residential or small-office, home-office (SOHO) type services, is was designed from the outset to operate with POTS simultaneously on the same line, so that an additional copper line would not need to be installed at each location.

Services which deliver their data in packets (e.g. Internet) require that packets delivered downstream are acknowledged before more will be sent and therefore the upstream data rate (maximum acknowledgment rate) will often dictate the maximum useful downstream throughput. A maximum ratio of downstream-to-upstream of 10-1 is a generally accepted rule of thumb.

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