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Home > Resources > Technical Resources > Black Box Explains > WAN > What is a DSL modem?

What is a DSL modem?

What is a DSL modem?

A DSL "modem" is a baseband modem (MOdulator/DEModulator) that delivers high-speed communication for short-distance interconnection or last-mile access. A DSL modem uses special coding techniques to get maximum throughput from the low bandwidth of voice-grade copper wire. At either end of the copper phone line (also known as "twisted copper pairs"), a DSL modem is installed. By adding ordinary telephone splitter boxes, the line can handle simultaneous voice connections.

Benefits of DSL modems
DSL can provide virtually instantaneous transmission of data, voice and video over ordinary copper phone lines. It provides a cost-effective, uninterrupted LAN interconnection as well as fast access to mission-critical information on corporate intranet servers and the Internet.

  • Cost reduction
    When compared to traditional leased lines or equivalent services, DSL is a less expensive solution without any recurring monthly equipment fee.
  • Increased speed
    DSL's bandwidth enables customers to increase performance up to 100 times greater than conventional modems.

What are the DSL standards?

Digital Subscriber Line (DSL)
DSL is the foundation of all XDSL services. In fact, DSL is the root physical layer for ISDN BRI service, which is two 64-kbps bearer (B) channels and one 16-kbps data (D) channel (2B+D) all bundled together to provide a functional 128-kbps pipeline that can support simultaneous transmission of voice and data, fax, or video signals.

Asymmetric DSL (ADSL)
Asymmetric DSL (ADSL) is used to provide cost-effective, high-speed, local loop access for Internet and other applications where data flows downstream to end users faster that it does upstream from them. ADSL provides asymmetric transmission over one pair of copper telephone wires, and it allows the telephone line to be used simultaneously for voice and data transmission.

Integrated DSL (IDSL)
An integrated DSL is a line that uses ISDN technology to deliver transmission speeds from 64 kbps up to 144 kbps. It allows for extended distances between the user location and the central office. In contrast to ISDN, it is a dedicated service for data communications only.

Symmetric DSL (SDSL)
Unlike most other forms of DSL service, SDSL is symmetric, which means you get the same amount of bandwidth to and from the Internet.

Multi-rate symmetric DSL (MSDSL)
As its name indicates, MSDSL operates at a variety of rates, which is an important improvement over HDSL. MSDSL was developed to enable service providers to offer increased flexibility to their customers.

High-bit-rate DSL (HDSL)
High-bit-rate DSL is similar to SDSL and has symmetrical transmission capabilities. Most E1 lines installed today use this technology.

High-bit-rate DSL 2 (HDSL2)
HDSL2 is a second generation of HDSL. It has the same features as regular HDSL but uses only a single twisted copper pair. This is a distinct advantage in certain areas where unused copper pairs are becoming rare. HDSL2 is currently being developed as an industry standard.

Single-pair High-bit-rate DSL (G.SHDSL)
G.SHDSL (also known as SHDSL) is a standard developed by the International Telecommunications Union (ITU) that addresses the worldwide SDSL market. This technology encompasses all functions that are currently provided by the European SDSL standard and HDSL2, including overlapped phase trellis-coded interlocking spectrum (OPTIS) spectral shaping.

Rate-Adaptive Asymmetric DSL (RADSL)
Rate-adaptive DSL uses the CAP modulation technique to offer asymmetric data transmission rates. The line rate is automatically adjusted to optimise the signal quality under the line condition.

Very high-bit-rate DSL (VDSL)
VDSL, which stands for very-high-bit-rate DSL, delivers high bandwidth over an existing single pair of copper wires. VDSL is the highest-speed technology in the XDSL family.

XDSL Technologies
Name Standard Wire Data transfer rate
ADSL none 1 Pair 1.5 to 9 Mbps downstream; 16 to 640 kbps upstream
IDSL none 1 Pair 64 to 144 kbps upstream and downstream
SDSL none 1 Pair 160 kbps to 1.544 Mbps upstream and downstream
MSDSL none 1 or 2 Pairs 64 kbps to 2.304 Mbps upstream and downstream
HDSL none 1, 2 or 3 Pairs 64 kbps to 1.544 or 2.048 Mbps upstream and downstream
HDSL2 ANSI 1 Pair 1.5 Mbps upstream and downstream
G.SHDSL ITU 1 or 2 Pairs 192 kbps to 2.34 Mbps or 384 kbps to 4.624 Mbps upstream and downstream
RADSL none 2 Pairs 1.5 to 9 Mbps downstream; 16 kbps to 1.08 Mbps upstream
VDSL none 1 Pair 13 to 52 Mbps downstream; 1.5 to 2.3 Mbps upstream
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