Telecommunication systems
- 33/33.040/33.040.01 Telecommunication systems in general
- 33/33.040/33.040.20 Transmission systems
- 33/33.040/33.040.30 Switching and signalling systems
- 33/33.040/33.040.35 Telephone networks
- 33/33.040/33.040.40 Data communication networks
- 33/33.040/33.040.50 Lines, connections and circuits
- 33/33.040/33.040.60 Powerline telecommunications
- 33/33.040/33.040.99 Other equipment for telecommunication systems