DIGITAL SUBSCRIBER LINE

After traditional modems reached their peak data rate, telephone companies developed another technology, DSL, to provide higher-speed access to the Internet. Digital subscriber line (DSL) technology is one of the most promising for supporting high-speed digital communication over the existing local loops.

Topics discussed in this section:

• ADSL
• ADSL Lite
• HDSL
• SDSL
• VDSL

ADSL

The first technology in the set is asymmetric DSL (ADSL). ADSL provides higher speed (bit rate) in the downstream direction (from the Internet to the resident) than in the upstream direction (from the resident to the Internet). That is the reason it is called asymmetric. Unlike the asymmetry in 56K modems, the designers of ADSL specifically divided the available bandwidth of the local loop unevenly for the residential customer. The service is not suitable for business customers who need a large bandwidth in both directions.

Note

The existing local loops (twisted-pair

lines) can handle bandwidths up to

1.1 MHz.

Traditional phone has a low-pass filter in front of it, which limits its bandwidth to 4KHz.

Note

ADSL is an adaptive technology.

The system uses a data rate

based on the condition of

the local loop line.

Distance between residence to switching office

Size of the cable

Signaling used

Discrete Multitone Technique

The modulation technique that has become standard for ADSL is called the discrete multitone technique (DMT) which combines QAM and FDM.

There is no set way that the bandwidth of a system is divided.

Each system can decide on its bandwidth division.

Typically, an available bandwidth of 1.104 MHz is divided into 256 channels.Each channel uses a bandwidth of 4.312 kHz. bandwidth can be divided into the following:

1. Voice. Channel 0 is reserved for voice communication.
2. Idle. Channels 1 to 5 are not used and provide a gap between voice and data communication.
3. Upstream data and control. Channels 6 to 30 (25 channels) are used for upstream data transfer and control.
4. Downstream data and control. Channels 31 to 255 (225 channels) are used for downstream data transfer and control.

Discrete multitone technique (QAM + FDM)

Bandwidth division in ADSL

Customer site: ADSL modem

Splitter and data line need installation (maybe expensive)

ADSL Lite (universal ADSL or spliterless ADSL: does not need additional installation from telephone company

Telephone company site

HDSL

The high-bit-rate digital subscriber line (HDSL) was designed as an alternative to the T-1 line (1.544 Mbps).

• The T-1 line uses alternate mark inversion (AMI) encoding, which is very susceptible to attenuation at high frequencies. This limits the length of a T-l line to 3200 ft (1 km). For longer distances, a repeater is necessary, which means increased costs.
• HDSL uses 2B1Q encoding, which is less susceptible to attenuation.
• A data rate of 1.544 Mbps (sometimes up to 2 Mbps) can be  achieved without repeaters up to a distance of 12,000 ft (3.86 km).
• HDSL uses two twisted pairs (one pair for each direction) to achieve full-duplex transmission.

SDSL

• The symmetric digital subscriber line (SDSL) is a one twistedpair version of HDSL.
• It provides full-duplex symmetric communication supporting up to 768 kbps in each direction.
• SDSL, which provides symmetric communication, can be considered an alternative to ADSL.

VDSL

• The very high-bit-rate digital subscriber line (VDSL), an alternative approach that is similar to ADSL, uses coaxial, fiberoptic, or twisted-pair cable for short distances.
• The modulating technique is DMT. It provides a range of bit rates (25 to 55 Mbps) for upstream communication at distances of 3000 to 10,000 ft.
• The downstream rate is normally 3.2 Mbps.

Summary of DSL technologies

ADSL Lite: does not need additional installation

from telephone company