CCNA Voice Certification/Analog Versus Digital

Analog Phones
Analog phones use electrical signals to send voice and signaling over a line. Each analog phone's circuits consist of two wires: a positive ground (also called the tip) and a negative battery (also called the ring). When the phone's receiver is on the hook, the tip and ring wires are separated and the circuit is broken. When the receiver is lifted off the hook, the wires connect and allow an electrical current to flow between the telephone company and your phone. This method of signaling is known as loop start signaling and is typically used in a home environment.

While loop start signaling may be okay in a home environment, it is not without its flaws. For instance, have you ever picked up a phone and started dialing only to realize that someone else was already on the line? This is known as glare, a problem with loop start signaling that occurs when you pick up a phone with an incoming call before the phone has time to ring. This doesn't cause a very large problem at your house, but in a business environment that runs a key system incoming calls could accidentally be redirected to employees trying to make outgoing calls.