Telephone signaling interfaces

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There are many interface standards between analog telephone central office equipment and the subscriber's premise. Single voice paths generally include analog audio connections, either a two-wire circuit or four-wire circuit plus signaling paths to indicate call progress and status information, such as ringing, answer supervision, etc. Some of the known interface types are

• FXO foreign exchange office
• FXS foreign exchange station
• DPO dial pulse originate
• DPT dial pulse terminate
• E and M signaling
• KS Kewlstart
• Loop start
• Ground start