---
title: "Short note: ADSL"
subject: "Telecommunications"
chapter: 8
key: "semester_8/telecommunications:short-note-adsl"
source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-adsl
site: BCT Engineering
---

# Short note: ADSL

Telecommunications · Chapter 8: Data Communication

- Asked in: 2074 Chaitra, 4 marks; 2072 Chaitra, 4 marks (asked 2 times)
- Video: [How Broadband ADSL Works - Computerphile](https://www.youtube.com/watch?v=uwtGfyna62I) — Computerphile
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-adsl
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

Write a short note on ADSL.

## Short answer

ADSL is an asymmetric DSL technology on the existing telephone copper pair, with downstream rate higher than upstream. It uses DMT modulation, splitting the line into 256 subchannels, and a splitter keeps voice and data separate. Typical rates are up to 8 Mbps down and 640 kbps to 1 Mbps up, with range about 5.5 km.

## Answer

**ADSL (Asymmetric Digital Subscriber Line)** is a DSL technology that sends high-speed data over the existing copper telephone pair. It is called asymmetric because the downstream rate (exchange to user) is much higher than the upstream rate (user to exchange). This suits home users, who download far more than they upload.

**Key features**
- Uses the same twisted pair as the telephone; voice and data work at the same time.
- Typical rates (ITU-T G.992.1): up to about 8 Mbps downstream and 640 kbps–1 Mbps upstream. ADSL2+ (G.992.5) reaches up to 24 Mbps downstream.
- Works up to about 5.5 km from the exchange; the rate falls as the loop gets longer.
- Always-on connection; no dial-up needed.

**Frequency division of the line**
```text
 0-4 kHz     25-138 kHz        138-1104 kHz
+-------+   +----------+   +------------------+
| Voice |   | Upstream |   |    Downstream    |
+-------+   +----------+   +------------------+
```
The line uses **DMT (Discrete Multitone)** modulation: the band up to 1.104 MHz is split into 256 sub-channels of 4.3125 kHz each. Each sub-channel carries QAM, and more bits are loaded on clean sub-channels and fewer on noisy ones.

**ADSL system**
```text
Phone --+                            +--> PSTN switch
        |                            |
     [Splitter]==copper pair==[Splitter]
        |                            |
PC--[ATU-R modem]           [DSLAM (ATU-C)]--> Internet
```
- **Splitter / microfilter:** separates the low-frequency voice band from the high-frequency data band.
- **ATU-R:** ADSL modem at the customer side.
- **DSLAM:** at the exchange, holds many ATU-C modems and joins the data of many users onto the ISP backbone.

**Advantages:** reuses existing copper, no new cabling; dedicated line (not shared like cable); voice and Internet together.
**Limitations:** speed depends on distance and line quality; low upstream rate; crosstalk in bundled cables.

Example: Nepal Telecom's ADSL broadband service gave home users Internet over their existing landline pair.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In the ADSL frequency division, which order is correct from low to high?
   A. Voice (0-4 kHz), upstream (25-138 kHz), downstream (138-1104 kHz)
   B. Downstream, upstream, voice
   C. Upstream, voice, downstream
   D. Voice, downstream, upstream
   Answer: A. Voice sits lowest, then the narrower upstream band, then the wider downstream band. (Key point: Frequency plan)

2. What does the splitter or microfilter do?
   A. Removes crosstalk
   B. Increases line speed
   C. Separates the low-frequency voice band from the high-frequency data band
   D. Converts analog to IP
   Answer: C. It lets phone and ADSL modem share the same pair. (Key point: Splitter)

3. What happens to ADSL speed as the loop gets longer?
   A. It falls (works up to about 5.5 km)
   B. It stays constant
   C. It increases
   D. It becomes symmetric
   Answer: A. Attenuation and noise reduce the rate with distance. (Key point: Distance limit)