ADSL / DSL overview

ADSL (Asymmetric Digital Subscriber Line) provides broadband internet over existing telephone lines. Key characteristics:

  • Asymmetric — faster download than upload (typical: 24/1 Mbps)
  • Distance-sensitive — performance degrades with distance from exchange
  • Shared medium — multiple users in area share backhaul capacity

Line quality factors

Your ADSL performance depends on:

  1. Distance from exchange — copper attenuation increases with length
  2. Cable quality — old, damaged, or improperly terminated lines lose signal
  3. Interference — electrical noise from appliances, poor grounding
  4. Contention — other users in area during peak times

Key metrics

Attenuation

Signal loss measured in dB. Lower is better:

  • 0-20 dB: Excellent (close to exchange)
  • 20-40 dB: Good (typical residential)
  • 40-60 dB: Marginal (may experience sync issues)
  • 60+ dB: Poor (unstable or won't sync)

Check your router's DSL statistics page for downstream and upstream attenuation.

SNR Margin

Signal-to-Noise Ratio margin. Higher is better:

  • 15+ dB: Excellent stability
  • 6-15 dB: Good (recommended minimum 6 dB)
  • 3-6 dB: Marginal (may drop during interference)
  • < 3 dB: Poor (frequent disconnections expected)

Sync Rate vs Throughput

  • Sync rate — maximum line speed (shown in router)
  • Throughput — actual usable speed (always lower due to overhead)

PPPoE adds ~8 bytes per packet overhead. IP/TCP/Ethernet overhead further reduces effective throughput by ~10-15%.

Common issues

Frequent disconnections

Causes:

  • Low SNR margin (< 6 dB)
  • Line interference (check for faulty phone equipment, extension wiring)
  • DSLAM issues at exchange
  • Power supply problems (router losing power)

Diagnostics:

  1. Check router logs for disconnect patterns
  2. Note attenuation and SNR values during stable period
  3. Disconnect all phone extensions, test with router at main socket
  4. Try different router/modem

Slow speeds

Causes:

  • High attenuation (distance from exchange)
  • Congestion (peak time contention)
  • Interleaving enabled (adds latency but improves stability)
  • Internal wiring issues

Diagnostics:

  1. Compare sync rate to throughput — should be within 10-15%
  2. Test at different times of day (peak vs off-peak)
  3. Bypass all internal wiring, connect at NTE5 test socket
  4. Check for local interference sources

High latency

Causes:

  • Interleaving (error correction adds 10-50ms)
  • Congestion at ISP or backhaul
  • Poor quality line forcing lower sync rates
  • Bufferbloat (excessive router buffering)

Fixes:

  • Request interleaving disabled (only if line is stable)
  • Use QoS on router to prioritize traffic
  • Upgrade to better quality service if available

PPPoE fundamentals

PPPoE (Point-to-Point Protocol over Ethernet) authenticates DSL connections:

  • Username/password — provided by ISP
  • MTU typically 1492 — standard 1500 minus 8 bytes PPPoE overhead
  • Session-based — maintains authenticated connection

Common PPPoE errors

  • Authentication failed — wrong username/password (check for typos)
  • Timeout — ISP BRAS not responding (check physical link)
  • Session limit — ISP allows only one connection (disconnect other devices)

Router configuration basics

Essential settings:

  1. PPPoE credentials — enter ISP-provided username/password
  2. MTU — set to 1492 for PPPoE (test with ping: ping -f -l 1464 8.8.8.8)
  3. DNS — use ISP DNS or public resolvers (1.1.1.1, 8.8.8.8)
  4. Firewall — enable, but don't block necessary services
  5. Wi-Fi security — WPA2 or WPA3, strong password

See router security basics for hardening checklist.