Cisco 400-051 Exam

Which two analog telephony signaling methods are most vulnerable to glare conditions? (Choose two.)

  • A. FXS Loop-start
  • B. FXO Ground-start
  • C. E&M Wink-start
  • D. E&M Delay-dial
  • E. E&M Immediate-start
  • F. E&M Feature Group D
Answer: Option A. E.

The loop start signaling method is more common and is typically used by residential phone lines. When a voice port is configured with loop start signaling, the device (telephone) closes the circuit loop that signals the CO voice port to provide dial tone; an incoming call is signaled on the CO by supplying a predefined voltage on the line. The loop start signaling method has one main disadvantage in that it has no method of preventing both sides of the connection from attempting to seize the line at the same time; this condition is referred to as glare. Because of this, loop start signaling is typically not used on high demand circuits.
With immediate-start, the calling side of the connection seizes the line by going off hook on the E-lead and address information is sent using dual-tone multifrequency (DTMF) digits. Immediate start signaling is vulnerable to glare just like loop-start signaling.

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