QSIG

Chapter 4 covers QSIG, a messaging framework that fosters feature transparency between Private Branch Exchanges (PBX). For CallManager, QSIG is almost a protocol within a protocol within a protocolthe QSIG feature messages wrapped in ISDN signaling communicated to MGCP gateways.

QSIG messages are called application protocol data units (APDU). They tend to contain a handful of fields at most. CallManager uses QSIG APDUs to implement the following features:

• Call completion
• Call diversion
• Call transfer
• Name services (calling name presentation and restriction, alerting name presentation and restriction, connected name presentation and restriction)
• Message waiting indicator
• Path replacement

The sections that follow describe the APDUs required for each of these features and the encoding rules that CallManager uses for the fields within the messages.

Note

You'll notice frequent use of the term PBX. In the QSIG section, PBX is used interchangeably for CallManager clusters as well as legacy PBXs.

 

Call Completion

The QSIG call completion feature allows a user who calls a destination that is busy or that does not answer to set a watch over the called party. When the called party becomes available, The IP phone prompts the calling user to redial the destination.

Call completion relies on the following APDUs:

• ccnrRequest and ccbsRequest Communicate from one PBX to another that a user wants to start monitoring the called party.

  The ccnrRequest and ccbsRequest APDUs contain the following fields:

  numberAIndicates the number of the monitoring party.

  numberBIndicates the number of the party to be monitored.

• ccExecPossible Communicates the availability of the called user to the PBX that set the watch.

  The ccExecPossible APDU contains the following fields:

  numberAIndicates the number of the monitoring party.

  numberBIndicates the number of the party to be monitored.

• ccCancel Indicates the caller's or the PBX's desire to cancel the callback.

  The ccExecPossible APDU contains the following fields:

  numberAIndicates the number of the monitoring party.

  numberBIndicates the number of the party to be monitored.

• ccRingout Indicates that an incoming call is the result of a callback attempt; contains no APDU fields.
• ccSuspend Pauses the watch when the calling user is busy at the time of the called party's availability; contains no APDU fields.
• ccResume Resumes the watch when the caller is idle; contains no APDU fields.

Call Diversion

The QSIG call diversion feature causes the displays of the calling and called parties to update when a call is forwarded from one user to another.

Call diversion relies on the following APDUs:

• divertingLegInformation1 Communicates diversion information from the forwarded phone to the caller while the call diversion is being processed.

  The divertingLegInformation1 APDU contains the following fields:

  diversionReasonIndicates why the diversion occurred (for example, the destination was busy or failed to answer).

  subscriptionOptionDetermines whether to deliver number or name to the user.

  nominatedNrIndicates the call forwarding target to the caller.

• divertingLegInformation2 Communicates diversion information from the forwarded phone to the forward target while the call diversion is being processed.

  The divertingLegInformation2 APDU contains the following fields:

  diversionCounterIndicates the number of times that the call has been previously diverted, to minimize the effect of call forwarding loops.

  diversionReasonIndicates why the diversion occurred.

  originalDiversionReasonIn the case of multiple diversions, indicates why the original called party diverted the call.

  divertingNrIndicates the number of the device that diverted the call.

  originalDivertingNrIn the case of multiple diversions, indicates the number associated with the original target of the call.

  redirectingNameIndicates the display name associated with the diverting user.

  redirectingNameIn the case of multiple diversions, indicates the display name associated with the original target of the call.

• divertingLegInformation3 Communicates diversion information from the forward target to the caller when the call forward has completed.

  The divertingLegInformation3 APDU contains the following fields:

  presentationAllowedIndicatorIndicates whether the caller is permitted to see the name of the forward target.

  redirectionNameIndicates the display name associated with the user to whom the call was diverted.

Call Transfer

The QSIG call transfer feature causes the displays of the transferred and transfer destination to update when a user transfers a call from one party to the other.

Call transfer relies on the following APDUs:

• ctComplete Communicates information about the transferred party to the transfer destination upon completion of the transfer.

  The ctComplete APDU contains the following fields:

  redirectionNumberIndicates the transferred-to number if sent to the calling party. If sent to the transferred-to user, it represents the calling party number.

  redirectionNameIndicates the transferred-to name if sent to the calling party. If sent to the transferred-to user, it represents the calling party name.

• ctActive Communicates when a ringing transfer destination answers the call.

  The ctActive APDU contains the following fields:

  connectedAddressIs sent to the calling party and indicates the transferred-to number.

  redirectionNameIs sent to the calling party and indicates the transferred-to name.

• ctUpdate Communicates display information about the transferred party to the transfer destination and vice versa, once the transfer is complete.

  The ctUpdate APDU contains the following fields:

  redirectionNumberIndicates the transferred-to number if sent to the calling party. If sent to the transferred-to user, it represents the calling party number.

  redirectionNameIndicates the transferred-to name if sent to the calling party. If sent to the transferred-to user, it represents the calling party name.

Message Waiting Indicator

The QSIG message waiting feature allows a voice mail system attached to one PBX to deliver message waiting indications to users connected to another PBX.

QSIG message waiting relies on the following APDUs:

• mwiActivate Communicates the number of the phone whose message waiting indicator must be lit from the system hosting voice mail to the system hosting the phone. This APDU contains the servedUserNr field, which indicates the number of the party whose lamp state needs to be altered.
• mwiDeactivate Communicates the number of the phone whose message waiting indicator must be extinguished from the system hosting voice mail to the system hosting the phone. This APDU contains the servedUserNr field, which indicates the number of the party whose lamp state needs to be altered.

Name Services

The QSIG name services features ensure that a caller can see the name of the called party (when permissible) and that a called party can see the name of the calling party (when permissible).

QSIG name services features include the following:

• Calling name presentation and restriction
• Alerting name presentation and restriction
• Connected name presentation and restriction

QSIG name services APDUs communicate two main pieces of information:

• The name of the calling, alerting, or called party
• The presentation indicator associated with that party, which determines whether the recipient of the message can see the name of the sender

Path Replacement

The QSIG path replacement feature permits PBXs that have been involved in a call diversion or call transfer to optimize the signaling path after the diversion or transfer completes. This process can free up trunk circuits when a call has hairpinned through the network.

QSIG path replacement relies on the following APDUs:

• prPropose Allows one PBX to propose to another PBX that the path be optimized.

  The prPropose APDU contains the following fields:

  callIdentityProvides a numeric key that the receiving PBX should encode in the replacement call so that the sending PBX can identify the new incoming call.

  reroutingNumberIndicates the number that the receiving PBX should use when placing the replacement call. This number is not the number of the actual party involved in the call but is instead a number associated with the CallManager node that is hosting the party.

• prSetup A PBX embeds this APDU in a call setup attempt to the target PBX to indicate that the incoming call is not a new call but is an optimized call attempt that should replace the incoming leg of the original call. This APDU contains the callIdentity field, which indicates the numeric key that will be provided in the prPropose sent by the initiating PBX of the path replacement operation.