DSL Standards

DSL comes in many flavors to meet different needs. For instance, DSL has limits on how long the local loop can be (the length of the local loop is simply the length of the combined cables that stretch from a house to the CO). Some DSL variants allow the local loop to be much longer, while others allow for only a shorter local loop. For the standards with a shorter local loop, the transmission rates tend to be much higher—a simple design trade-off. ADSL and SDSL tend to be the most popular option in the United States today.

Another architectural difference among the different specifications is that some DSL variants use asymmetric transmission rates, while others use symmetric rates.

Table 15-5 lists the major DSL variants, the standard defining that variant, the speeds, the distance limitations, and encoding.

Table 15-5 DSL Technologies Standards Comparison

DSL Type

Standards

Modulation/Encoding Technique

Speed

Distance Limit

Full-rate ADSL/ G.DMT

ANSI T1.413 Issue 2

Discrete multitone (DMT) or carrierless amplitude phase (CAP)

Downstream speed of 384 to 8 Mbps; upstream speed slower, up to 1.024 Mbps

18,000 feet

G.Lite

ITU-T G.992.1, ITU-T G.992.2

DMT

Downstream speed up to 1.544 Mbps to 6 Mbps; upstream speed up to 640 kbps

18,000 feet

Very-high-data-rate DSL (VDSL)

ETSI and ANSI in process

DMT/single-carrier modulation (SCM)

12.96 Mbps to 52.8 Mbps for both upstream and downstream speed

4500 feet

ISDN DSL (IDSL)

ANSI ETR 080

Two binary one quaternary (2B1Q)

144 kbps for both upstream and downstream speed

18,000 feet

Symmetric DSL (SDSL)

None

2B1Q

768 kbps for both upstream and downstream speed

22,000 feet

High-data-rate DSL (HDSL

ITU G.991.1, ANSI TR 28

2B1Q

1.544 or 2.048 Mbps for both upstream and downstream speed

12,000 feet

G.SHDSL

ITU G.991.2

Trellis-coded pulse amplitude modulation (TC PAM)

192 kbps to 2.360 Mbps for both upstream and downstream speed

28,000 feet

A wide variety of standards bodies and vendor consortiums helped develop these standards. As you might imagine, with so many standards, many different organizations pushed for standards that best met their needs. Over time, the number of differing standards will stabilize.

The following organizations are among the key players in the development of DSL:

■ American National Standards Institute (ANSI)

■ Institute of Electrical and Electronics Engineers (IEEE)

■ International Telecommunications Union (ITU)

■ European Telecommunications Standards Institute (ETSI)

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