---
title: "xDSL access technologies and types"
subject: "Internet and Intranet"
chapter: 7
key: "semester_8/internet_and_intranet:xdsl-technologies"
source: https://www.bctengineer.com/q/semester_8/internet_and_intranet/xdsl-technologies
site: BCT Engineering
---

# xDSL access technologies and types

Internet and Intranet · Chapter 7: Internet and Intranet Applications

- Asked in: 2072 Magh, 10 marks; 2073 Magh, 4 marks; 2070 Bhadra, 4 marks (asked 3 times)
- Video: [Broad Band Networks   01 Lecture 1   Fixed BB   xDSL](https://www.youtube.com/watch?v=wHI8E8HT2m4) — Dhanuka Navodya
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/internet_and_intranet/xdsl-technologies
- Whole chapter: https://www.bctengineer.com/md/semester_8/internet_and_intranet/chapter-7

## Question

Discuss the different xDSL technologies. (List the DSL access technologies with their types.)

## Short answer

xDSL technologies provide high-speed digital access over existing copper lines. Main types include ADSL (asymmetric, up to 24 Mbps), SDSL (symmetric), and VDSL (very high, up to 100 Mbps). A splitter separates voice and data; DSLAM at the exchange connects lines to the ISP backbone.

## Answer

**DSL (Digital Subscriber Line)** technologies give high-speed digital access over the ordinary copper telephone line by using higher frequencies than voice. The "x" stands for the different types. A splitter/filter separates voice (0-4 kHz) from data, so the phone and internet work at the same time. Speed falls as the distance from the exchange increases.

#### Main types

| Type | Full name | Downstream / upstream | Distance | Notes |
|---|---|---|---|---|
| ADSL | Asymmetric DSL | up to 8 Mbps / 1 Mbps | up to 5.5 km | Home internet; uses 1 pair; shares line with voice |
| ADSL2 | ADSL second gen | 12 Mbps / 1 Mbps | 5.5 km | Better modulation and diagnostics |
| ADSL2+ | | 24 Mbps / 1 Mbps | 3 km | Doubles bandwidth to 2.2 MHz |
| G.Lite (ADSL Lite) | Splitterless ADSL | 1.5 Mbps / 512 kbps | 5.5 km | No splitter needed; easy home install |
| RADSL | Rate-adaptive DSL | 0.6-7 Mbps / 128 kbps-1 Mbps | 5.5 km | Rate set by line quality |
| HDSL | High-bit-rate DSL | 1.544 / 2.048 Mbps both ways | 3.6 km | Symmetric, uses 2-3 pairs; replaces T1/E1; no voice |
| SDSL | Symmetric DSL | 1.544 / 2.048 Mbps both ways | 3 km | 1 pair; businesses |
| IDSL | ISDN DSL | 144 kbps both ways | 5.5 km | Digital data only |
| VDSL | Very-high-bit-rate DSL | 13-52 Mbps / 1.5-2.3 Mbps | 300 m-1.5 km | Fibre-to-the-curb |
| VDSL2 | | up to 100 Mbps both ways | under 500 m | IPTV, triple play |

#### Features common to xDSL

- Uses the existing copper pair, so no new cable is needed.
- Always on, no dial-up, with a separate voice channel (in ADSL).
- Uses DMT or CAP modulation; a modem at the user end and a **DSLAM** at the exchange connect many lines to the ISP backbone.
- **Asymmetric** types (ADSL, VDSL) suit internet browsing and downloads; **symmetric** types (HDSL, SDSL) suit business servers and leased-line replacement.

```text
 PC--[DSL modem]--splitter--copper--[DSLAM]--ISP network
 Phone-------------+   (voice goes to PSTN switch)
```

Limits: speed depends on distance and line quality, and there is interference (crosstalk). In Nepal, ADSL/VDSL service is provided by Nepal Telecom and ISPs.

## Check yourself

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

1. How can the phone and internet work together on an ADSL line?
   A. Data uses 0-4 kHz and voice uses higher frequencies
   B. A splitter/filter separates voice (0-4 kHz) from data at higher frequencies
   C. Voice and data alternate in time slots
   D. Voice is converted to digital and sent as data
   Answer: B. DSL uses higher frequencies than voice, and a splitter/filter separates the voice band from the data. (Key point: Voice and data separation)

2. Which DSL types are symmetric and suit business servers and leased-line replacement?
   A. ADSL and VDSL
   B. HDSL and SDSL
   C. ADSL2 and ADSL2+
   D. G.Lite and RADSL
   Answer: B. HDSL and SDSL give equal rates both ways; ADSL and VDSL are asymmetric and suit browsing and downloads. (Key point: Symmetric vs asymmetric)

3. How does the achievable speed of DSL vary with the distance from the exchange?
   A. Speed increases with distance
   B. Speed falls as distance increases
   C. Speed is independent of distance
   D. Speed doubles every km
   Answer: B. Higher speed types like VDSL work only over short distances (300 m-1.5 km), and speed falls as the loop gets longer. (Key point: Distance and speed)

4. Which statement is correct?
   A. The DSLAM is at the user and the modem is at the exchange
   B. The DSLAM at the exchange connects many lines to the ISP backbone, and G.Lite needs no splitter
   C. G.Lite needs a splitter at every home
   D. DSLAM separates voice only
   Answer: B. The modem is at the user and the DSLAM at the exchange; G.Lite (splitterless ADSL) eases home installation. (Key point: DSLAM and G.Lite)