Short note: ADSL

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

 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

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.