Transmission technologies
The old analogue transmission systems have mostly been
substituted by digital transmission systems where the information
is presented and transferred in digital form.
Digital systems use electrical transmission, radio transmission or
optical transmission with modulated light. Since all digital
branching and switching technologies are electrical, there is a
need of conversions between the optical and radio forms.
The major transmission systems used today are:
- PDH (Plesiochronous Digital Hierarchy),
- SDH (Synchronous Digital Hierarchy),
- SONET (Synchronous Optical Network).