Connection States

Connection states are among the most important concepts in telephony. Each and every connection has an individual state that determines the condition of the connection and what can and cannot be done with that connection. The life cycle of a device's participation in a call (or call progress) is represented by the sequence of states through which the corresponding connection transitions. A state transition indicates that some service was performed on the connection by the switching implementation. The switching services that may be applied to a connection at any time are determined by its connection state.

Figure 3-16 Connection state diagram

3.4.1 The Seven Connection State Model

There are seven connection states (described below), and they are permitted to transition as shown in the state diagram in Figure 3-16.

Initiated

While a device is requesting service or dialing a digit sequence to initiate a call, the

corresponding connection is in the initiated state. The initiated state typically is entered when the device goes off-hook^3-10 and, if supported, when the user is prompted^3-11 to take the device off-hook. The media stream received by the device, if any, is initially dial tone followed by silence after the first digit or command is issued.

Connected

After a call has been created and the telephone system is establishing connections with other devices, the connections where media stream channels are allocated and the associated media stream(s) are flowing, are in the connected state.

Null

A connection is said to be in the null state if it no longer exists. A nonexistent connection is synonymous with a connection in the null state. Occasionally a connection is said to

''transition through the null state." This is a simple way of saying that the original association between a device and call was replaced with a new one.

3-10 Off-hook — In general usage, the term off-hook refers to an action to be taken on a device, typically lifting a telephone handset, signifying that the device is requesting service. The term comes from the days when a telephone's microphone hung from a hook. The use of the term off-hook as it applies to the operation of analog telephone sets is explained in Chapter 8.

3-11 Prompting — The term prompting refers to an indication, typically audible, a telephone set makes to indicate that it should be taken off-hook in order to progress out of the initiated state. Prompting is distinct from ringing, which is an indication made by a device that a connection is in the ringing mode of the alerting state. Prompting is used when some service is initiated for a device but is unable to go off-hook without manual intervention.

Alerting

While an attempt is being made to connect a call to a device, the connection representing the association between the call and device is in the alerting state. There are actually three modes of the alerting state that determine what type of action the device can take:

• Entering distribution mode

• Offered mode

• Ringing mode

These modes will be discussed in Chapter 5.

The fact that a connection is in the alerting state is independent of whether the corresponding device is indicating an incoming call in some way (by ringing, for example).

Typically for voice calls, all of the other active (e.g., connected) connections to the call will hear ringback,^3-12 while a connection in the alerting state is associated with the call.

Fail

The fail state indicates that call progress was stalled for some reason and an attempt to

associate a device and a call (or keep them associated) failed. The most common example of this state is attempting to connect to a device that is busy.

In most cases, all of the other active (e.g., connected) connections to the call will hear a busy tone, or another appropriate failure tone, while a connection in the fail state is associated with the call.^3-13

3-12 Ringback — Ringback is the "ringing" sound heard after you have placed a call and are waiting for it to be answered. It is not the actual sound of a phone ringing, but rather a sound the switching implementation generates to supply the caller with feedback on call progress.

3-13 Blocked — One case where the fail state may not be associated with an audible tone is the case where a bridged connection is blocked from a call. This is described in section 4.4.3.

Hold

When a connection is in the hold state, it continues to associate a particular device with a call (signaling information continues), but the transmission of associated media streams is

suspended. Depending on the implementation, the media stream channel(s) associated with the connection's media stream(s) may or may not be deallocated while the connection is in the hold state. A channel that is not deallocated is said to be reserved.

Hold should not be confused with mute. Mute is a telephone set feature which will be

described later in this chapter. Mute deals with turning off a speaker or microphone and is not related to the transmission of media streams or the allocation of media stream channels.

Queued

A connection is in the queued state when call progress is suspended pending subsequent application of certain switching services. Like the hold state, connections in the queued state do not have active media streams and the associated channel may or may not be deallocated.