CCNA Voice 2

The telephone component responsible for detecting an off-hook condition is the switch hook. The switch hook is closed when the receiver is picked up, resulting in an off-hook condition. In the off-hook condition, the -48Vdc 20ma circuit from the CO switch is completed, which notifies the switch to send the dial tone.
Traditional analogue signaling types are FXO (Foreign Exchange Office), FXS (Foreign Exchange Station) and E&M (Earth & Magneto or Ear and Mouth). The FXO port is connected to the CO switch, while the FXS is connected to the telephone. E&M is a trunk link typically used to connect from a router interface card to the PBX.

When a user picks up a phone before it rings and realizes that he is already connected to the caller, a "glare" has occurred. Loop start circuits are the most susceptible to glares. Ground start circuits are designed to prevent glares, but are typically only used in business lines. The names of the wires used in the loop start circuit is the ring and the tip.

Ground start circuits uses two mechanisms to prevent glares:
1) Answer Supervision
2) Disconnect Supervision

These two supervision mechanisms removes glare completely.

There are currently 5 E&M signaling types. Type 1 is currently the most popular in the United States. The two sides of the E&M are known as the signaling-unit and the trunk side. The trunk side signals an off-hook condition through the E-lead (typically a PBX), while the M-lead is used by the signaling side (typically a router).

In E&M, there are three mechanisms for line seizure (aka Start Dial Supervision):
1) Wink Start - Provides Answer supervision to reduce glare.
2) Delay Start - Solves the problem of Wink Start dialing too soon with some equipment.
3) Immediate Start - Most basic Start Dial Supervision. Does not provide Answer supervision. The originating end goes off-hook after 150ms and begins transmitting.