Disconnect 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 signaling between the telephone exchange

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

 and a connected party used to indicate that the connected call is being disconnected.

For example 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....

 may indicate to the telephone exchange

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

 that the call is being disconnected by 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....

 by allowing loop current to flow in the line.

Disconnect Supervision Signals

The signal used for Disconnect Supervision varies depending on the type of trunk being used. These signal types include:

1. Digital E&M - Used on T1 carriers with Channel Associated Signalling trunks
2. Analog E&M - Used for analog E&M (earth & magneto, or ear & mouth) signalling trunks
3. Analog Disconnect Tone - Used in many South American countries on analog loop start
   Loop start
   In telecommunications, a loop start is a supervisory signal given by a telephone or PBX in response to the completion of the loop circuit, commonly referred to as 'off-hook'. When idle, or 'on-hook', the loop is at 48V DC...
   
    trunks
4. Analog Open Switching Interval (OSI) - Used in North America on analog loop start
   Loop start
   In telecommunications, a loop start is a supervisory signal given by a telephone or PBX in response to the completion of the loop circuit, commonly referred to as 'off-hook'. When idle, or 'on-hook', the loop is at 48V DC...
   
    trunks
5. Digital ISDN - Used with both PRI and BRI digital trunks

CAS E&M Signalling Basics

For Channel Associated Signalling (CAS) trunks that run (E&M) signaling, there are generally only two states in which a voice channel can be. When there is no call on a channel, the channel is in the Idle, or On-Hook

On-hook

In telephony, the term on-hook has the following meanings:# The condition that exists when a telephone or other user instrument is not in use, i.e., when idle waiting for a call. Note: on-hook originally referred to the storage of an idle telephone receiver, i.e., separate earpiece, on a switchhook...

 state. When there is an active call on a channel, then the channel is in the Seized, or 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...

 state. There is no separate state for Answered.

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

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....

, hence the term Answer and Disconnect Supervision.

Why Answer and Disconnect Supervision is Required

The most obvious reasons for Answer and Disconnect Supervision are for billing—the telephone exchange and the customer's need for an accurate indication of calls through a network. It is standard for telephone companies to not 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. If there was no indication of the call's disconnect, thus no teardown or clearing

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....

 then all channels in the system would eventually be blocked.

How it works

This example shows a 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...

 trunk using E&M wink start signaling only. Other methods can be used, although this is the most common.

Wink start is used to notify the remote side that it can send the Dialed Number Identification Service (DNIS), also referred to as the called number. Automatic Number Identification

Automatic number identification

Automatic number identification is a feature of telephony intelligent network services that permits subscribers to display or capture the billing telephone number of a calling party. In the United States it is part of Inward Wide Area Telephone Service . ANI service was created by AT&T for...

 (ANI) can also be transmitted.

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 DNIS (called number) information. This is done when in-band 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 switch sees the network go on-hook and the switch goes on-hook. ABCD bits = 0000.
3. The audio path is disconnected, and the billing system registers a call stop record.

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