ISDN SIGNALING MESSAGES

Many of the VoIP Gateway operations entail the processing of ISDN signaling messages. These messages and their functions are defined in the ITU-T Q.931 Recommendation.

The ISDN Q.931 messages are used to manage ISDN connections on the B channels. These messages are also used (with modifications) by Frame Relay and ATM for setting up calls on demand at a UNI, and for provisioning services between networks at a NNI. Table B-1 lists these messages, and a short explanation is provided here about the functions of the more significant messages:

• ALERTING: This message is sent to indicate that the called user party has been "alerted" and the call is being processed. This message is sent in response to an incoming SETUP message, and it is sent in the backward direction (backwards from the called end to the calling end) after the called exchange has placed ringing signals on the line to the called party.

• CALL PROCEEDING: This message is sent to the call initiator to indicate that the call establishment procedures have been initiated. It also indicates that all information necessary to set up the connection has been received and that any other call establishment information will not be accepted. In ISDN-conformant implementations, the CALL PROCEEDING message is exchanged only at the originating end of the connection.

• CONGESTION CONTROL: This message is employed only on USER INFORMATION messages. As the name implies, it is used to govern the flow of USER INFORMATION messages. In most implementations, congestion control is not used or, if it is used, it is rarely invoked.

• CONNECT: When the called party picks up the telephone and goes off-hook, this action precipitates the invocation of this message. The message is sent in the backward direction (from the called party to the calling party) to signal the call acceptance by the called party.

• CONNECT ACKNOWLEDGE: This message is sent in response to the CONNECT message. It's invocation means that the parties have been awarded the call.

• DISCONNECT: This message is sent when either party (calling or called) hangs up the telephone (goes on-hook). It is a trigger to the network that the end-to-end connection is to be cleared and the resources reserved for the connection are to be made available for another call.

• INFORMATION: As the name implies, this message is sent by either the user or the network to provide more information about a connection. For example, the message may be invoked by an exchange if it wishes to provide additional information about a connection to another exchange.

• NOTIFY: This message is not often used, but is available for the user or the network to provide information regarding a connection. The NOTIFY message contains a field called the notification indicator, which is described in the next section of this chapter.

• PROGRESS: The progress message is part of the call establishment procedure although it is not invoked in a typical implementation. However, it is available to indicate the progress of a call and it ms invoked in situations where interworking is required or where the exchanges need to provide information about in-band information. This information is provided through a field in the message called the progress indicator which is described in the next section.

• RELEASE: This message is invoked in response to the reception of a DISCONNECT message. It is sent by the network or the user to notify its recipient that the equipment has disconnected the circuit that had been reserved for the connection. In essence, it tells the receiver that it should also release the circuit. The RELEASE message is designed also to free and make available the call reference numbers (and the associated resources) associated with the call.

• RELEASE COMPLETE: As the name implies, this message is sent in response to the RELEASE message and it indicates by its invocation that the sender has released the circuit, the call reference and, of course, the resources associated with the connection. The combination of the RELEASE and RELEASE COMPLETE messages means that the circuit has been completely cleared and made available for other calls, and that the call reference is no longer valid.

• RESUME: This message is used for a relatively simple operation, which is to request that the network resume a suspended call. The arrangements for resuming a suspended call vary between network providers, but the idea is to allow users to change their minds (within a brief period of time) upon hanging up.

• RESUME ACKNOWLEDGE: This message is sent by the network in response to the RESUME message. It indicates the completion of a request to RESUME a suspended call.

• RESUME REJECT: This message is sent by the network to indicate that it cannot fulfill the request to resume a suspended call.

• SETUP: The setup message contains more information elements than any of the other Q.931 messages. It is used to begin the call setup procedure. The SETUP message is always issued by the calling user to the network at the originating end and by the network to the called user at the terminating end.

• SETUP ACKNOWLEDGE: This message is sent in response to the SETUP message to indicate that the SETUP message has been received correctly. It is used to indicate that call establishment has been initiated. It may also indicate that additional information may be required to complete the call. For the latter case, the recipient of the SETUP ACKNOWLEDGE is required to send the additional information which is coded in an INFORMATION message.

• STATUS: This message is sent in response to a STATUS INQUIRY message. It may also be sent in the event of certain error conditions that occur at a network node.

• STATUS ENQUIRY: This message is sent by either the user or the network to inquire about the status of an ongoing operation, such as a call in progress. Both the STATUS and STAUS ENQUIRY messages are intended to be flexible enough to allow the implementor latitude in their implementation. The only information element in these messages is the display information element described later in this chapter.

ISDN permits calls to be suspended. The reason for the suspensions are not defined in the specifications. Whatever the reasons, Q.931 provides for three messages to support these operations. They are as follows:

• SUSPEND, SUSPEND ACKNOWLEDGE, and SUSPEND REJECT: The SUSPEND message is sent from the user to request that the network suspend the call. The direction of the message is important in that the network is not allowed to send this message; so, call suspension can only be initiated by the user. SUSPEND ACKNOWLEDGE is an acknowledgment by the network of the reception of the SUSPEND message; it also indicates the completion of the call suspension. SUSPEND REJECT is an acknowledgment by the network of the reception of the suspend message, but it indicates that the network did not suspend the call.

• USER INFORMATION: This message is slightly different from the INFORMATION message described earlier, in that it contains different parameters than the INFORMATION message. The major aspect is the existence of the user-user field which does not reside in the INFORMATION message. As the next section will explain, the user-user field is passed transparently by ISDN to ISDSN users.

• FACILITY: This message is used by either the user or the network to provide additional information about a call. Examples are keypad facility and display information, described in the next section.

• RESTART: This message is sent by the user or the next work to request a restart of a connection. It returns the identified channel to an idle state.

• RESTART ACKNOWLEDGE: This message acknowledges the RESTART message.

Q.931 Example

Figure B-2 provides an example of how a call is set up with the Q.931 messages. The two persons involved in this connection are using conventional telephone handsets that are attached to ISDN terminals, shown in this figure as the calling terminal and the called terminal. The exchange termination's (ET) are located at the central offices.

The calling party goes off-hook and dials the telephone number of the called party. This information is used by the calling terminal to create an ISDN SETUP message, which is sent across the ISDN line to the local ET. This ET acknowledges the message with the SETUP ACK message, and initiates actions to set up a circuit to the next ET, which is shown in the figure with the dashed arrow. The SETUP ACK, and INFORMATION messages are optional, and were described in the previous section. The local ET sends a CALL PROCEEDING message to the calling terminal to indicate that the call is being processed.

At the called end, the SETUP message is forwarded to the called terminal by the terminating ET. This terminal examines the contents of the message to determine who is being called and what services are being requested. It checks the called party's line to see if it is idle, and if so, places the ringing signal on the line. When the ringing signal is placed on the line, the called terminal transmits an ALERTING message in the backward direction, which is passed all the way to the calling terminal. This message indicates to the calling terminal that the called party has been signaled, which allows a ring back signal to be placed on the line to the calling party.

When the called party answers the call, the called terminal sends a CONNECT message in the backward direction, which is passed to the calling terminal. Upon receiving this message, ring back is removed from the line, and the connection is cut-through to the calling party. The CONNECT messages are acknowledged with CONNECT ACK messages.

The on-hook action initiates the ISDN connection termination operations. The DISCONNECT messages are used to indicate that the connection is to be terminated. The RELEASE and RELEASE COMPLETE messages follow the DISCONNECT messages.