How multiple telephones(Landline) transmit signals simultaneously? How the bandwidth is divided individually to make transmission happen individually?
Merging multiple individual signals/services/data paths together is called multiplexing. Depending how you actually do that there are several technologies:
- time division multiplexing (TDM) - there are distinctive time slots that iterate over the services (e.g. most ISDN variants)
- frequency division multiplexing (FDM) - each service uses a dedicated frequency band over copper (e.g. xDSL over POTS/ISDN)
- wavelength division multiplexing (WDM) - each service uses one or more dedicated wavelengths/colors over fiber (e.g. for SONET or Ethernet WAN/MAN)
When it comes to packet switching, multiplexing a large number of data streams comes naturally (essentially a kind of dynamic TDM).
For POTS landlines, both TDM and FDM are (were) used. Both usually exclude the use of additional FDM for DSL.