Answer supervision is a term in telephony

Telephony

In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....

, describing a situation whereby the called party

Called party

The called party is a person who answers a telephone call. The person who initiates a telephone call is the calling party....

 indicates to the central office that the call is being answered.

CAS E&M Signaling Basics

For digital channel associated signaling (CAS)

Channel Associated Signaling

Channel Associated Signaling , also known as per-trunk signaling , is a form of digital communication signaling. As with most telecommunication signaling methods, it uses routing information to direct the payload of voice or data to its destination. With CAS signaling, this routing information is...

 Digital Signal 1 (T1)

Digital Signal 1

Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...

 trunks that run E and M signaling (earth & magneto, or ear & mouth signaling)

E and M signaling

E&M is a type of supervisory line signaling that uses DC signals on separate leads, called the "E" lead and "M" lead, traditionally used in the telecommunications industry between telephone switches....

, there are only two voice channel states. A channel is idle/on hook when there is no call on it or seized/off-hook

Off-hook

In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...

/energized by an active call. There is no separate state for answered. It mimics an analog loopstart or groundstart line.

After a channel is initially seized, each device must indicate the progress of a call. The progress indicators include whether a call is answered or remains unanswered, and when a call is answered, which party disconnects first. These call progress states are important as telephony systems need to know when the call was attempted, answered, and cleared, hence the term answer and disconnect supervision

Disconnect supervision

Disconnect supervision is a term in telephony describing signaling between the telephone exchange and a connected party used to indicate that the connected call is being disconnected....

.

Why Answer and Disconnect Supervision Is Required

The most obvious reason for answer and disconnect supervision is for billing—the telephone exchange and the customer need an accurate indication of calls through a network. It is standard for telephone companies not to charge for unanswered or unsuccessful calls. All call detail record

Call detail record

A call detail record , also known as call data record, is a data record produced by a telephone exchange or other telecommunications equipment documenting the details of a phone call that passed through the facility or device...

s (CDRs) produced should indicate a call was unanswered or unsuccessful, and therefore, incur no charge from the billing system.

Secondly, some systems may not cut through the audio path until there is a positive indication that the called party answered the call—there may not be an audio connection until the answer signal is sent.

Lastly, the channel should become free to take new calls when the previous call clears

Clearing (telecommunications)

Clearing, in telecommunications means:* A sequence of events used to disconnect a call and return to the ready state. It is sometimes, particularly in the context of common channel signaling, called teardown....

. Without an indication of the old call's disconnect, the trunk would not become available.

How it Works

This example shows E&M wink start signaling only. Other methods can be used, although this was the most common in 20th century private circuits.

Wink start

Wink pulsing

Wink is used both in connection with DC signaling on a trunk, and with indicator lamps on a key telephone.In telephone switching systems, wink pulsing is recurring pulsing in which the off-condition is relatively short compared to the on-condition. In Wink start trunks, the exchange at the...

 is used to notify the remote side or PBX that it can send the Dialed Number Identification Service (DNIS), also referred to as the Called Number.

For an incoming call, this occurs:

1. Calling switch goes off-hook. It sends ABCD bits = 1111.
2. The called switch sends wink. The ABCD bits transition from 0000 to 1111 for 200 ms, then back to 0000.
3. The calling switch sees the wink, and then proceeds to send the "DNIS" (called number) information. This is done when inband multifrequency/dual tone multifrequency (MF
   Multi-frequency
   In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...
   
   /DTMF) tones are sent.
4. The called switch goes off-hook when the call is answered. It sends ABCD bits = 1111.
5. The audio path is connected, parties can talk, and the billing system registers a call start record.

In an outgoing call the same procedure occurs, but the calling switch and called switch exchange roles.
These occur when a disconnect from the calling party happens:

1. Calling switch goes on-hook by sending ABCD bits = 0000.
2. The called router sees the network go on-hook and the router goes on-hook. ABCD bits = 0000.
3. The audio path is closed, and the billing system registers a call stop record.

For a disconnect from the called party to the calling party, these steps are reversed.