network protocols handbook.pdf

308Protocols GuideSS7 / C7 ProtocolProtocol NameISUP: ISDN User PartProtocol DescriptionThe ISDN User Part (ISUP), a key protocol in the SS7 / C7 signallingsystem, defines the protocol and procedures used toset-up, manage, and release trunk circuits that carry voice anddata calls over the public switched telephone network (PSTN)between different switches. ISUP is used for both ISDN andnon-ISDN calls. A simple call flow using ISUP signaling is asfollows:Call set up: When a call is placed to an out-of-switch number,the originating SSP transmits an ISUP initial address message(IAM) to reserve an idle trunk circuit from the originating switchto the destination switch. The destination switch rings the calledparty line if the line is available and transmits an ISUP addresscomplete message (ACM) to the originating switch to indicatethat the remote end of the trunk circuit has been reserved. TheSTP routes the ACM to the originating switch which rings thecalling party’s line and connects it to the trunk to complete thevoice circuit from the calling party to the called party.Call connection: When the called party picks up the phone, thedestination switch terminates the ringing tone and transmits anISUP answer message (ANM) to the originating switch via itshome STP. The STP routes the ANM to the originating switchwhich verifies that the calling party’s line is connected to thereserved trunk and, if so, initiates billing.Call tear down: If the calling party hangs-up first, the originatingswitch sends an ISUP release message (REL) to release thetrunk circuit between the switches. The STP routes the REL tothe destination switch. If the called party hangs up first, or if theline is busy, the destination switch sends an REL to the originatingswitch indicating the release cause (e.g., normal release orbusy). Upon receiving the REL, the destination switch disconnectsthe trunk from the called party’s line, sets the trunk state toidle, and transmits an ISUP release complete message (RLC) tothe originating switch to acknowledge the release of the remoteend of the trunk circuit. When the originating switch receives (orgenerates) the RLC, it terminates the billing cycle and sets thetrunk state to idle in preparation for the next call.Protocol StructureThe ANSI and ITU-T have slightly different ISUP format. ITU-TISUP message format:Routing label (5bytes)Circuit identification code (2 bytes)Message type code (1 byte)Parameters – varies according to message type values• Routing label - The routing label is used by the relevantuser part to identify particulars to which the messagerefers. It is also used by the Message TransferPart (MTP) to route the message towards its destinationpoint.• Circuit identification code - The allocation of circuitidentification codes to individual circuits is determinedby bilateral agreement and/or in accordancewith applicable predetermined rules.• Message type code - The message type code uniquelydefines the function and format of each ISDN UserPart message. Each message consists of a numberof parameters. Message types may be:Address completeAnswerBlockingBlocking acknowledgementCall progressCircuit group blockingCircuit group blocking acknowledgementCircuit group queryCircuit group query responseCircuit group resetCircuit group reset acknowledgementCircuit group unblockingCircuit group unblocking acknowledgementCharge informationConfusionConnectContinuityContinuity check requestFacilityFacility acceptedFacility rejectForward transferIdentification requestIdentification responseInformationInformation requestInitial addressLoop back acknowledgementNetwork resource managementOverloadPass-alongReleaseRelease completeReset circuitResumeSegmentationSubsequent addressSuspendUnblockingUnblocking acknowledgementUnequipped CIC