ADSL operation, modem connection and topology

ADSL (Asymmetric Digital Subscriber Line) is a DSL technology that gives high-speed internet over the existing copper telephone pair while the same pair still carries ordinary telephone (POTS) calls. It is asymmetric: downstream (to the user) is much faster than upstream, which suits web browsing and video. Standard: ITU-T G.992.1 (up to about 8 Mbps down, 1 Mbps up); ADSL2+ (G.992.5) gives up to 24 Mbps down.

Working principle: frequency division of the copper line

The copper pair can carry frequencies up to about 1.1 MHz over short loops. ADSL divides this band:

 POTS  guard   upstream        downstream
 |---|------|----------|------------------------|
 0   4k    25k       138k                    1104k Hz

Modulation: DMT (Discrete Multitone). The band is divided into 256 sub-channels of 4.3125 kHz each. Each sub-channel uses QAM, and the modem measures the signal-to-noise ratio of each sub-channel and loads more bits on good ones and fewer (or none) on noisy ones. This adapts to line length and noise. (Some early systems used CAP modulation.)

Modem connection and topology

 Customer premises            Telephone exchange
 [PC]--Eth--[ATU-R modem]       +----------+
              |                 | Splitter |--> PSTN
 [Phone]--[Splitter]==copper====|  (POTS)  |    switch
                     pair       |   DSLAM  |--> ISP /
                   (< ~5 km)    | (ATU-C)  |   internet
                                +----------+

• ATU-R (ADSL Transceiver Unit, Remote): the ADSL modem at the user's home; connects to PC or router by Ethernet/Wi-Fi/USB.
• Splitter / microfilter: a low-pass/high-pass filter that separates voice (below 4 kHz) from data, so telephone and internet work at the same time.
• Local loop: the existing twisted pair; speed falls with distance (useful up to about 5 km).
• DSLAM (DSL Access Multiplexer): at the exchange; contains many ATU-C modems, one per line, and aggregates their traffic onto a high-speed link (ATM or Ethernet) to the ISP network.
• POTS splitter at the exchange sends voice to the telephone switch.

The topology is a star: each subscriber has a dedicated line to the DSLAM, so bandwidth on the loop is not shared, unlike cable modem networks.

Operation

1. On power-up the ATU-R and ATU-C perform training: they test each DMT sub-channel and agree the bit loading.
2. User data from the PC is framed, error-coded (Reed–Solomon, interleaving) and modulated onto the upstream sub-channels.
3. The DSLAM demodulates and forwards the data to the ISP; return traffic is sent on the downstream sub-channels.
4. Voice continues in the 0–4 kHz band through the splitters, unaffected.

Advantages: uses existing copper, always-on, simultaneous voice and data, dedicated loop. Limitations: speed depends on distance and line quality; upstream is limited.