ocr: switchblock speech circuit group switcholock signalling chonnel - data link data channel voice - trequency ichonnel mod dem dem Ivoice mod processor signol lling trequency signailing - processor L terminal - ichonneu terminal voice Trequency link dato channel signalling chonnel signalling link common chonnel signalling system note:- VF channel and modems may be reploced by digital arcurt Fig- 11.1 Basie schernarie common channel signalling