TUCoPS :: Phreaking Technical System Info :: loopst.htm

Basics of Telephony Loop Start Signaling
Loop Start Signaling

Loop Start Signaling

The following diagrams depict Loop Start signaling states:

     Advantages of Loop Start:


     No need for accurate ground references between the Central Office/FXS and

     the PBX/Telephone.

     Generally, TIP/RING wires may be reversed at the PBX/Telephone with no

     adverse impact on operations - (some older DTMF keypads may have problems

     with a T/R reversal, causing no output of tones).

     Disadvantages of Loop Start:


     There is no FAR-END Disconnect Supervision.  When the remote

     handset/line hangs-up, there is no provision for the local CO/FXS to

     notify the PBX of the disconnection (Not a problem with a telephone).

     There is poor "glare" resolution.  "Glare" occurs when both the local end

     (PBX/Telephone) and the remote end (CO/FXS) attempt to access the circuit

     at the same time. Unfortunately, with Loop Start, the PBX/Telephone is

     not informed of an inbound seizure until ringing is detected; The ring

     "cadence" is normally 2-seconds ON, 4-seconds OFF (U.S.).

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