Disconnect supervision

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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 terminated. For example the called party may indicate to the telephone exchange that the call is being disconnected by the called party by allowing loop current to flow in the line.

1 Overview
2 See also
3 References

Overview

Disconnect Supervision Signals

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

Digital E&M - Used on T1 carriers with Channel Associated Signaling trunks
Analog E&M - Used for analog E&M trunks
Analog Disconnect Tone - Used in many South American countries on analog loop start trunks
Analog Open Switching Interval (OSI) - Used in North America on analog loop start trunks^[1]
Digital ISDN - Used with both PRI and BRI digital trunks

CAS E&M Signaling Basics

For Channel Associated Signaling (CAS) trunks that run (E&M) signaling, there are generally only two states in which a a voice channel can be. When there is no call on a channel, the channel is in the Idle, or On-Hook state. When there is an active call on a channel, then the channel is in the Seized, or Off-Hook 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, hence the term Answer and Disconnect Supervision.

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 to not charge for unanswered or unsuccessful calls. All call detail records (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 (telecommunications) then all channels in the system would eventually be blocked.

How it Works

This example shows a T1 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 (ANI) can also be transmitted.

For an incoming call, this occurs:

Calling Switch goes off-hook. It sends ABCD bits = 1111.
The Called Switch sends wink. The ABCD bits transition from 0000 to 1111 for 200 ms, then back to 0000.
The Calling Switch sees the wink, and then proceeds to send DNIS (Called Number) information. This is done when inband multifrequency/dual tone multifrequency (MF/DTMF) tones are sent.
The Called Switch goes off-hook when the call is answered. It sends ABCD bits = 1111.
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:

Calling Switch goes on-hook by sending ABCD bits = 0000.
The Called Router sees the network go on-hook and the router goes on-hook. ABCD bits = 0000.
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.