Source: https://www.academia.edu/31281572/Chapitre_1_La_T%C3%A9l%C3%A9phonie_Analogique_%C3%A0_Commutation_D%C3%A9partement_Electronique_LMD_Licence_T%C3%A9l%C3%A9communications_TEL_56_version_00_2015.pdf