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.[2] 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.
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[3] for basic call procedures and supplementary services except for some procedures (e.g. number portability).[4] 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:[5]
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.[6]
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.[citation needed]
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
8
7
6
5
4
3
2
1
Routing Label
...
CIC Least Significant 8 Bits
Padding
CIC Most Sig. 4 Bits
Message type
Mandatory fixed part
...
Mandatory variable part
...
Optional part
...
The Signalling Information Field (SIF) for all ISUP Message Signal Units (MSU) contain the following components:[7]
Routing Label
Circuit Identification Code
Message Type
Mandatory Fixed Part
Mandatory Variable Part
Optional Part
The Routing Label indicates the Point Codes of the originating and destination nodes in the network; it also includes the Signalling Link Selection field that is used to select between the multiple routes an MSU could take between two nodes.
The Circuit Identification Code is used to specify which trunk between two switches is used to carry a particular call. Note that some versions of ANSI ISUP permit a CIC with 14 significant bits instead of the 12 that are shown.[8]
The Message Type indicates the ISUP message type. The presence and form of the remaining 3 components are determined by this message type.
The Mandatory fixed part, when present, contains the mandatory, fixed-length parameters associated with the message type.
The Mandatory variable part, when present, contains the mandatory, variable-length parameters associated with the message type.
The Optional part, when present, contains the optional parameters permitted to be included in the message type.
When sent using the services of the Signalling Connection Control Part, ISUP messages passed to SCCP in the User Data parameter (NSDU) consist of only the last 4 components (Message Type, Mandatory fixed part, Mandatory variable part, Optional part). The routing label and circuit identification code are not included in the user data passed to SCCP.[9]
