DSL Distance Limitations


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• The tradeoff between different DSL variants is reach vs. speed.

• Maximum Reach numbers are best-case assuming "clean" copper.

© 2004 Cisco Systems, Inc. All rights reserved. BCRAN v2.1—4-4

The trade-off among various DSL types is reach versus speed. The longer the local loop, the lower the maximum speed the DSL connection can support.

For example, VDSL supports the highest speed but it has the shortest distance limitation.

For ADSL, the maximum distance is typically about 18,000 feet (5,460 meters). To support the maximum ADSL download speed of 8 Mbps, the CPE must be very close to the CO, within several thousand feet.

The maximum speed listed in the figure assumes that there are minimal local loop impairments. Here are some of the many local loop impairments that will influence the maximum speed of the DSL connections and the ability to obtain DSL service in an area:

■ Loading coils in the local loop: Loading coils will cut off (block) the DSL frequency. Loading coils are used to improve POTS quality on long local loops. They are effectively low-frequency band pass filters. Loading coils must be removed from the local loop to support DSL.

Distance from CO to the DSL CPE: The longer the distance, the lower the speed.

Gauge of wire used in the local loop: Thicker wire supports higher speeds.

Wire gauge change: Changes in wire gauge cause an impedance mismatch that can reduce speed.

Bridge taps: Bridge taps in the local loop cause reflections that can reduce speed.

Crosstalk: Crosstalk between different wires in the same bundle can cause interference that can reduce speed.

AM radio: AM radio interference can also reduce speed.


This topic describes ADSL fundamental concepts.



• ADSL is designed to coexist with POTS, unlike most other DSL types.

• ADSL provides slow upstream speed for uploading (sending) low-data-rate requests.

• ADSL provides fast downstream speed for downloading bursts of rich graphics and multimedia content.

• ADSL features three basic modulation techniques:

- Carrierless Amplitude and Phase (CAP) modulation

- Discrete MultiTone (DMT)

- Consumer/Mass-Market DMT (G.lite)

NOTE: The type of modulation must match the provider.

© 2004 Cisco Systems, Inc. All rights reserved. BCRAN v2.1—4-5

ADSL features three basic modulation techniques:

■ Carrierless Amplitude and Phase (CAP) modulation

■ Discrete Multitone (DMT) modulation

■ Consumer/mass-market DMT (G.lite). This technique is the most popular.

DMT is a line code that is implemented in ITU 992.1 (G.dmt), ITU 992.2 (G.lite), and ANSI T1.413 Issue 2. DMT divides the 1-MHz spectrum offered by a telephone line into multiple 4-kHz subchannels. Each subchannel is optimized based on the local loop characteristics.

In contrast, CAP relies on a single channel for upstream and another single channel for downstream.

An installer must check with the service provider to determine which modulation technique is being used. The modulation method used must correspond with the ADSL CPE and the ADSL modems on the DSLAM.

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