The ISDN (Integrated Services Digital Network) User Part or ISUP is part of Signaling System No. 7 (SS7), which is used to set up telephone calls in the public switched telephone network (PSTN). It is specified by the ITU-T as part of the Q.76x series..
When a telephone call is set up from one subscriber to another, several telephone exchanges could be involved, possibly across international boundaries. To allow a call to be set up correctly, where ISUP is supported, a switch will signal call-related information like called party number to the next switch in the network using ISUP messages.
The telephone exchanges may be connected via T1 or E1 trunks which transport the speech from the calls. These trunks are divided into 64 kbit/s timeslots, and one timeslot can carry exactly one call. Regardless of what facilities are used to interconnect switches, each circuit between two switches is uniquely identified by a circuit identification code (CIC) that is included in the ISUP messages. The exchange uses this information along with the received signaling information (especially the called party number) to determine which inbound and outbound circuits should be connected together to provide an end to end speech path.
In addition to call related information, ISUP is also used to exchange status information for, and permit management of, the available circuits. In the case of no outbound circuit being available on a particular exchange, a release message is sent back to the preceding switches in the chain.
ISUP variants Different ISUP variants exist. ITU-T specifies the variant used in the international network. In Europe ETSI releases its own ISUP specification which is close that of the ITU-T.. ITU-T ISUP is used for international connections and is the base for some national ISUP variants. Most countries have their own variation of ISUP to cover national requirements. ANSI specifies variations of ISUP utilized under the North American Numbering Plan; however, some countries under the NANP differ in their support of some procedures (for example, LATA is meaningless within Canada. Also, RBOCs support Telcordia procedures not fully specified by ANSI.) Some countries outside the NANP support ANSI-based variants (e.g. Mexico).
While these variations of ISUP differ in subtle ways, the vast majority of ISUP message type, parameter type, and parameter field code-points, and related fundamental call processing procedures, agree across all variants.
ITU-T specification versions 1980 – ISUP Yellow Book 1984 – ISUP Red Book 1988 – ISUP Blue Book 1991 – ISUP Q.767. 1992 – ISUP'92 White Book (segmentation, compatibility, new supplementary services) 1997 – ISUP'97 (new procedures, IN CS1, new supplementary services)
According to ITU-T Q.761 section 2.4.1 ISUP interworking ISUP'92 is backwards compatible with ISUP Blue Book and Q.767 for basic call procedures and supplementary services except for some procedures (e.g. number portability).. Additionally the compatibility features introduced in this version ensure forward compatibility with newer versions.
Message types An ISUP message contains a fixed header containing the circuit identification code and the ISUP message type, followed by a mandatory fixed-length parameter part, a mandatory variable-length parameter part, and an optional parameter part that are dependent on the type of message being sent. ISUP messages can be sent using the services of the Message Transfer Part, or, less often, the Signalling Connection Control Part. These messages are transmitted in various stages of call setup and release. The most common messages are: Initial address message (IAM) — First message sent to inform the partner switch that a call has to be established on the CIC contained in the message. Contains the called number, type of service (speech or data) and optional parameters. Subsequent address message (SAM) — For networks that support overlap dialing procedures, and then in the case that the IAM did not contain the full called number, one or more SAMs follow containing additional digits. This message is not supported by networks that only support en bloc dialing procedures. Address complete message (ACM) — Message returned from the terminating switch when the subscriber is reached and the phone starts ringing, or when the call traverses an interworking point and the intermediate trunk is seized. Call progress (CPG) — Contains additional information about the progress of a call. Normally sent after the ACM when the status of the call changes from that reported in the ACM. Answer message (ANM) — Sent when the subscriber picks up the phone, a resource is connected or answer supervision is returned by an interworking point. Normally charging starts at this moment. It is required that the call be cut through in both directions by this point. Connect (CON) — Sent when the call is answered by an automatic terminal. This message replaces the ACM, CPG and ANM for calls that are answered by automatic terminals. Release (REL) — Sent to clear the call when a subscriber goes on hook. This is also sent (in direct response to an IAM) if the terminating switch determines that the call cannot be completed. In either case, the terminating switch provides a cause value in the message to explain the reason for the release, e.g., "User busy". Release complete (RLC) — Acknowledgment of the release – the circuit is idle afterward and can be used again.
Sample call flow This is a very basic call flow involving only two telecom switches which exchange the ISUP messages. The subscriber interfaces are not covered here and are only listed for a better understanding.
A subscriber telco switch A telco switch B B subscriber Off hook Dial digits ---> -- IAM --> -Ringing -> <-- ACM -- Off hook <-- ANM -- ----------------------- Conversation ----------------------- On hook -- REL --> On hook <-- RLC --
Detailed call flows are provided in ITU-T Recommendation Q.784.1.
Below is a detailed exchange of ISUP messages involving two ISDN telecom switches. The report was from an Alcatel S12 digital switch.
BENIN 2005-07-15 16:49:16 FR 00121 00000000 G159CA01 L6X8AA47 SWA-ORJ-OBSERVATION SYSTEM REPORT
OBSERVATION/CALL SAMPLING SUCCESSFUL
TYPE OF OBSERVATION: ORIGINATING DATE = 2005-07-15 TIME = 16:48:23:09 CALLING DN = 52250000 CALLING CAT = PRIORITY SUBSC INC EQ NBR = H'31 & 1 RCVD DIGITS = 012625729 CALLED DN = 012625729 OTG TRNKGRP = LAGOS_SC OTG EQ NBR = H'1111 & 16 BEARER = AUDIO31 XFER MODE = CIRCUIT SWITCHED CAUSE = NORMALUN TERM SEIZED = 0: 0: 2: 3 THROUGH SWITCH = 0: 0: 4: 3 ANSWER = 0: 0:14: 6 RELEASE = 0: 0:46: 3
TAXATION INFO
CHARGED DN = 52250000 CALL DURATION = 0: 0:31: 6 COUNTS = 2 TARIFF GRP = 4 TARIFF ID = 7 RATE = 20 RANDOM CNT = 0 SURCH UNITS = 0 CAT UNITS = 0 TARIFF REG = 1
RECEIVER SZD = 0: 0: 0: 0 RECEIVER RLSD = 0: 0: 4: 2 REC EQ NBR = H'13 & 6 REC RLS INFO = FORCED RLSE
REC SIGNAL DIRECTION TIME
REG-I-10 RECEIVED 0: 0: 1: 3 REG-I-1 RECEIVED 0: 0: 1: 6 REG-I-2 RECEIVED 0: 0: 1: 8 REG-I-6 RECEIVED 0: 0: 2: 1 REG-I-2 RECEIVED 0: 0: 2: 3 REG-I-5 RECEIVED 0: 0: 2: 5 REG-I-7 RECEIVED 0: 0: 3: 0 REG-I-2 RECEIVED 0: 0: 3: 3 REG-I-9 RECEIVED 0: 0: 3: 7
INC SIG TYPE = ANALOG-SUBSCR
SIGNAL DIRECTION TIME
SEIZURE RECEIVED 0: 0: 0: 0 DIAL-TONE SENT 0: 0: 0: 0 CLEAR-FW-FW RECEIVED 0: 0:46: 1
OTG SIG TYPE = ISUP-NAT
MESSAGE DIRECTION TIME LENGTH
IAM SENT 0: 0: 2: 3 29
85014040007000010020000B03020604019010620A0603132552000000
SAM SENT 0: 0: 2: 4 13
85014040007000020200028002
SAM SENT 0: 0: 2: 6 13
85014040007000020200028005
SAM SENT 0: 0: 3: 1 13
85014040007000020200028007
SAM SENT 0: 0: 3: 4 13
85014040007000020200028002
SAM SENT 0: 0: 3: 8 13
85014040007000020200028009
ACM RECEIVED 0: 0: 4: 1 15
850141000070000616040129010100
ANM RECEIVED 0: 0:14: 6 18
8501410000700009012D02000039022DC000
SUS RECEIVED 0: 0:45: 4 10
850141000070000D0100
REL SENT 0: 0:46: 1 13
850140400070000C020002849F
RLC RECEIVED 0: 0:46: 3 9
850141000070001000
UNSOLICITED REPORT NO = 00121
Cause codes Release cause codes are used to identify and debug any events occurring in ISDN User Part signaling. Every event in ISUP signaling generates a cause code number. Even for a normal ISUP call, a cause code is generated. There are lot of applications developed based on the cause code from ISUP signaling. Similarly Telecom operators trace for Causecodes to debug any call failures.
Following are the list of cause codes used. Cause codes only defined by number are effectively undefined, and may be used for proprietary solutions.
Unallocated (unassigned) number No route to specific transit network No route to destination Send special info tone Misdialed trunk prefix Channel unacceptable Call awarded and being delivered in established channel Preemption Preemption – circuit reserved for reuse 10 11 12 Suspend Resume 15 Normal call clearing User busy No user responding No answer from user (user alerted) Subscriber absent Call rejected Number changed Redirect to new destination (unallocated destination number ANSI) Unknown business group (ANSI) Exchange routing error (ANSI) Non-selected user clearing Destination out of order Invalid number format Facility rejected Response to STATUS ENQUIRY Normal, unspecified 32 33 No circuit/channel available 35 36 37 Network out of order Permanent frame mode connection out of service Permanent frame mode connection operational Temporary failure Switching equipment congestion Access information discarded Requested channel/circuit not available Preemption (ANSI) Precedence call blocked Resources unavailable, unspecified 48 Quality of service unavailable Requested facility not subscribed Call type incompatible with service request (ANSI) 52 Outgoing calls barred within CUG Call blocked due to group restrictions (ANSI) Incoming calls barred within CUG 56 Bearer capability not authorized Bearer capability not presently available 59 60 61 Inconsistency in designed outgoing access information and subscriber class Service or option not available, unspecified 64 Bearer capability not implemented Channel type not implemented 67 68 Requested facility not implemented Only restricted digital bearer capability is available 71 72 73 74 75 76 77 78 Service or option not implemented, unspecified 80 Invalid call reference value Identified channel does not exist A suspended call exists, but this call identity does not Call identity in use No call suspended Call having the requested call identity has been cleared User not member of CUG Incompatible destination 89 Non-existing CUG Invalid transit network selection 92 93 94 Invalid message, unspecified Mandatory information element is missing Message type non-existing or not implemented Message incompatible with call state or message type non-existent or not implemented Information element non-existent or not implemented Invalid information element contents Message not compatible with call state Recovery on timer expiry Parameter non-existent or not implemented - passed on 104 105 106 107 108 109 Message with unrecognized parameter discarded Protocol error, unspecified 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 Interworking, unspecified.
Message format 87654321Routing Label ...CIC Least Significant 8 BitsPaddingCIC Most Sig. 4 BitsMessage typeMandatory fixed part ...Mandatory variable part ...Optional part ...
The Signalling Information Field (SIF) for all ISUP Message Signal Units (MSU) contain the following components: Routing Label Circuit Identification Code Message Type Mandatory Fixed Part Mandatory Variable Part Optional Part