Modem

modem ≡ modulator + demodulator
standards:
- V.21: 300 bit/s, full-duplex
- V.22: 1200 bit/s, half-duplex
- V.32: 4800, 9600 bit/s
- V.34: 28800 bit/s
- V.90: asymmetric, 56 kbps downstream, 34 kbps upstream
- V.92: asymmetric, 56 kbps downstream, 48.6 kbps upstream
digital line is faster than analog line
exclusive speed:
- provider is directly connected to client’s ATE (automatic telephone exchange)
- no ADC (high noise) on ATE, only DAC → channel is asymmetric
voice or data because spectra overlap

xDSL

reuses telephone cable infrastructure
- higher quality is still required because of higher frequencies
asymmetric by design
always on
FDM
- uses spectrum, not allocated to voice (F_low = 26 kHz)
- does not affect telephone
- frequencies above 8 kHz are not switched by ATE
standards:
- Annex A: iniital
- Annex B:
  - spectrum is shifted to higher frequencies
  - does not interfere with analog alarm systems
- Annex M: ADSL2+
- Annex L: ADSL2+, longer reach
DSLAM – DSL access multiplexer
splitter is required on client, because phones produce feedback in higher frequencies – can be interpreted as command if not filtered