Digital Subscriber Line (DSL) is the most promising for the support of high-speed internet connection over the existing local loops. Basically, it is a set of technologies. This set is also referred to as xDSL, where x can be replaced by A, V, H, or S.
Technologies of Digital Subscriber Line:
- ADSL (Asymmetric DSL)
- ADSL Lite or Universal ADSL or Splitterless ADSL
- HDSL (High-bit-rate DSL)
- SDSL (Symmetric DSL)
- VDSL (Very high bit-rate DSL)
ADSL (Asymmetric DSL):
The very first technology in the set of DSL is ADSL. It is like a 56K modem, i.e. it provides higher speed in the downstream direction than the upstream direction. That is the reason why it is referred to as asymmetric. In this, the available bandwidth is divided unevenly for the residential customers only. This service is not suitable for business customers, which are true of the larger bandwidth.
A new version of ADSL called ADSL Lite or Universal ADSL or Splitterless ADSL. An ADSL modem is directly plugged with a telephone jack and connected to the computer, It uses 256 DMT carriers with 8-bit modulation. Some carriers may not available due to reason that errors generated by voice signals might mingle with them. It is capable of providing a maximum of 1.5 Mbps data rate downstream and 512 Kbps data rate upstream.
HDSL (High-bit-rate DSL):
It is used as an alternative to T-1 line. Encoding used by T-1 line is alternate mark inversion which is susceptible to attenuation at high frequencies. Length limit is 1km and it requires a repeater for a longer distance which results in an increase in cost. Thus, HDSL uses 2B1Q encoding which results in less susceptible to attenuation. With the use of this data rate increases to 2 Mbps and length, the limit increases up to 3.86 km. To achieve full-duplex transmission it uses two twisted-pair cables.
SDSL (Symmetric DSL):
As two twisted cables are used in HDSL, it is the one twisted cable version of HDSL. It supports 768 kbps in each direction and it provides full-duplex symmetric communication. As it provides symmetric communication, so it can also be used as an alternative to ADSL.
VDSL (Very high bit-rate DSL):
It is also used as an alternative approach, i.e. similar to ADSL. It uses a coaxial, fiber-optic, and twisted-pair cable for short distances. It is capable of providing bit rate of 25-55 Mbps for upstream and 3.2 Mbps for downstream
Summary: The following is summary table for technologies of DSL. The values in the following table are approximate and can vary from one implementation to another.
|Technology||Downstream Rate||Upstream Rate||Distance in feet||Twisted Pairs||Line Code|
|ADSL||1.5-6.1 Mbps||16-640 kbps||12,000||1||DMT|
|ADSL Lite||1.5 Mbps||500 kbps||18,000||1||DMT|
|HDSL||1.5-2.0 Mbps||1.5-2.0 Mbps||12,000||2||2B1Q|
|SDSL||768 kbps||768 kbps||12,000||1||2B1Q|
|VDSL||25-55 Mbps||3,2 Mbps||3000-10,000||1||DMT|
Features of DSL:
- It helps to meet the need for networks in different scenarios like DSL, Fiber/Cable, and 3G/4G Dongle.
- It supports ISP service providers, so we can that it is compatible with most of the service providers.
- It also supports high-quality telephone calls.
- There is an available app to manage the modem.