ISUP (ISDN User Part) is the integrated services digital network user part in No.7 signaling system. In the system, ISUP performs level-4 functions and is one of the several parallel user parts. In the OSI (Open System Interconnection) model, ISUP is equivalent to the functions at layers 4-7. On the basis of TUP, ISUP is composed of the control protocols added with non-voice bearer services and those of supplementary services. Therefore, ISUP fully supports the basic bearer services and supplimentary services of ISDN users and can implement the complete functions of TUP (Telephone User Part) and DUP (Data User Part).
- Flexible message structure similar to SCCP. There is a smaller quantity of messages than TUP, but a message carries much more information and can meet future needs.
- Many enhanced functions are stipulated, especially the end-to-end signaling, which enables the messages to be transparently transmitted between ISDN users.
- Brief and clear signaling procedures.
- Powerful functions, capable of supporting various voice, non-voice and supplementary services
ISUP messages are transmitted over signaling links on the MTP layer and are the same in format as the telephone MSU (Message Signal Unit) of TUP. Besides, ISUP is the SIF (Signal Information Field) in a processing message. Nevertheless, the SIF processed in ISUP is an octet stack and is composed of the route flag, circuit identification code, message type code, mandatory fixed part, mandatory variable part and optional part. The general format and sending sequence of an ISUP message are shown in Figure 6-1:
Figure6-1 General format andsending sequence of an ISUPmessage
When a message is sent, first the route flag and finally optional parts will be sent. When a byte is sent, the least significant bit will be first sent.
Each kind of messages of ISUP are composed of some parameters, each of which has a name and is encoded by the byte. A parameter may have a fixed or variable length. Each variable parameter contains a length indicator identifying the nubmer of bytes in the parameter content and the length indicator itself occupies a byte. The contents of various constituents are described as follows
1) Route flag
The route flag in an ISUP message in China has the following format:
Figure6-2 Format of the route flag of an ISUP message
In the above figure, DPC (Destination Point Code) indicates the signaling point code of the destination to where a message will be sent while OPC (Originating Point Code) indicates the code of the signaling point from where a message is sent. SLS (Signaling Link Selection) is used for the encoding of the signaling links under load sharing. It is 8 bits long, but currently only 4 bits are in use, namely, still the lower 4 bits of a CIC.
2) CIC (Circuit Identification Code)
Figure6-3 Format of the CIC of an ISUP message
CIC is the code of the cirucuit between the originating signaling point and the destination signaling point. Currently the lower 12 bits are in use and the remaining 4 bits are spare (being 0000).
3) Message type code
The message code specifies the uniform function and format of each kind of ISUP messages and is mandatory in all messages.
Table6-1 ISUP message type code
Note: those marked with * are not used for the moment and the code Bxxxxxxxx represents a binary number Xxxxxxxx
4) Mandatory fixed part (F)
It means the parameter which is mandatory in a message and is fixed in length. The location, length and order of a parameter are specified by the message type on a unifed basis, therefore a message does not contain the parameter name and length indicator
5) Mandatory variable part (V)
It means the parameter which is mandatory in a message and is variable in length. The start of a parameter is represented with a pointer while the parameter name and the pointer’s sending sequence are hidden in a message type. The quantity of parameters and pointers is speicfied by the message type on a unified basis.
6) Optional part (O)
Composed of some parameters, it may be fixed or variable in length. Each optional parameter should contain the parameter name, length indicator and parameter content. An optional parameter ends with an all-zeros octet.
6.3 ISUP Message Examples
Following describes the two commonly-used ISUP message signaling information codes, namely, the IAM (Initial Address Message) and ACM (Address Complete Message).
6.3.1 IAM
IAM (Initial Address Message) is the message sent in the forward direction to begin occupying outgoing circuits and send numbers and the inforation related to routing and call processing. First, the message structure of an IAM shall be discussed. and then the codes of main parts in the structure.
1. IAM message structure
Its first octet is the message type code and next is the mandatory fixed part. Its type is indicated by F. Then, following the F type is the mandatory variable part, whose type can be indicated by V. Following the mandatory variable part is the optional part, whose type can be indicated by O.
The message structure of IAM is shown iin Table 6-2.
Table6-2 IAM message structure
Note 1: this parameter can be repeated.
Note 2: for further study.
2. IAM message code
The code of each part in the message structure can be found in the specifications. Following will list the codes of several main parts to see what each code means.
According to the bit sending sequence (from right to left), the meanings of the connection nature indicator code are as follows.
1 Connection nature indicator
Bit BA: satellite indicator
00: no satellite circuit in the connection
01: one segment of satellite circuit in the connection
10 two segments of satellite circuit in the connection
11: spare
Bit DC: continuity check indicator
00: continuity check not required
01: continuity check required in this circuit
10: continuity check in progress in the previous circuit
11: spare
Bit E: Echo control device indicator
0: outgoing semi-echo control device not included
1: outgoing semi-echo control device included
Bits F~H: standby
2 Forward call indicator
Bit A: national/international call indicator
0: calls processed according to national calls
1: calls processed according to international calls
In an originating country, this bit can be set as any value. In an international network, this bit will not be checked. In a destination country, this bit in a call from an international network should be set as 1.
Bit CB: end-to-end mode indicator (note)
00: no end-to-end mode available (the mode of hop-by-hop forwarding available)
01: Transfer mode available
10: SCCP mode available
11: Transfer mode and SCCP mode available
Bit D: Interworking indicator (note)
0: no interworking encountered (NO.7 signals in all directions)
1: Interworking encountered
Bit E: End-to-end information indicator (note)
0: No end-to-end information available
1: end-to-end information available
Bit F: ISDN user part indicator (note)
0: ISDN user part not used in all directions
1: ISDN user part used in all directions
Bit HG: ISDN user part preferred indicator
00: ISDN user part preferred in all directions
01: ISDN user part not needed in all directions
10: ISDN user part needed in all directions
11: spare
Bit I: ISDN access indicator
0: originating access to non--ISDN
1: originating access to ISDN
Bit KJ: SCCP mode indicator
00: no indication
01:connectionless mode available
10:connection-oriented mode available
11: connectionless and connection-oriented modes available
Bit L: spare
Bit P--M: reserved for national use
(Note: bits B~F and J~K constitute a protocol control indicator).
Caller category
Caller category: mandatory fixed, slightly different from the ISUP in a fixed network, with the length of an octet (H-A). Its codes are as follows:
Bit HGFEDCBA
00000000 caller category unknown (receive only)
00000001
to spare
00001000
00001001 operator (no intrusion function)
00001010 common user, used between a mobile exchange and a local exchange/tandem exchange
00001011 preferred user, used between mobile exchanges
00001100 data call
00001101 test call
00001110
to spare
11101111
11110000 common, no charge, used between a mobile exchange and a toll exchange (including the international exchange)
11110001 common, periodic, used between a mobile exchange and a toll exchange (including the international exchange)
11110010 common, user table, immediate (receive only from a local exchange/tandem exchange)
11110011 common, printer, immediate (receive only from a local exchange/tandem exchange)
11110100 preferred, no charge, used between a mobile exchange and a toll exchange (including the international exchange)
11110101 preferred, periodic, used between mobile exchange and a toll exchange (including the international exchange)
11110110
to spare
11111111
4 Transmission medium request
00000000 voice
00000001 spare
00000010 64kb/s unrestricted
00000011 3.1KHZ audio
00000100 voice (service 2)/64kb/s unrestricted (service 1) alternative (reserved)
00000101 64kb/s unrestricted (service 1)/voice (service 2) alternative (reserved)
00000110 64kb/s preferred
00000111 2×64kb/s unrestricted
00001000 384kb/s unrestricted
00001001 spare
00001010 1920kb/s unrestricted
00001011 spare
to
11111111 spare
Called number
The parameter field of the called number is in the format as shown in Figure 6-3.
Table6-3 Format of the parameter of the called number
a) Odd/even indicator
0 even address signal
1 odd address signal
b} Address nature indicator
0000000 spare
0000001 = subscriber number
0000010 spare, reserved for international uses
0000011 national (valid) number
0000100 = international number
0000101 spare
to
1101111 spare
1110000 reserved for national use
to
1111110 reserved for national use
1111111 spare
c) INN (Internal Network Number) indicator
0 routing to an internal network number enabled
1 routing to an internal network number disabled
d) Numbering plan indicator
000 spare
001 ISDN (telephone) numbering plan
010 spare
011 data numbering plan
100 user telegraphy numbering plan
101 reserved for national use
110 reserved for national use
111 spare
e) Address signal
0000 digit 0
0001 digit 1
0010 digit 2
0011 digit 3
0100 digit 4
0101 digit 5
0110 digit 6
0111 digit 7
1000 digit 8
1001 digit 9
1010 spare
1011 code 11
1100 code 12
1101 spare
1110 spare
f) Fill code
If an address signal is an even nubmer, then a fill code (0000) will be inserted after the last address signal.
6.3.2 ACM
ACM (Address Complete Message) is a message sent in the backward direction indicating that all address signals needed during the routing of this call to the called have been received. Likewise, following will first have a look at the message structure of an ACM, and then the codes of main parts in the structure.
1. ACM message structure
The message structure of ACM is shown in Table 6-4.
Table6-4 ACM message structure
Note: this parameter can be repeated
2. ACM message code
The code of each part in the message structure can be found in the specifications. Following will list the codes of several main parts to see what each code means.
According to the bit sending sequence (from right to left), the meanings of the connection nature indicator codes are as follows.
1 Backward call indicator
A backward call indicator has two octets, whose bits are encoded as A~P according to the sending sequence.
Bit BA: charging indicator
00 no indication
01 no charging
10 charging
11 spare
Bit DC: called subscriber status indicator
00 No indication
01 unallocated number
10 space time connection
11 spare
Bit FE: called category indicator
00 no indication
Common subscriber
Payphone
11 spare
Bit HG: end-to-end mode indicator (note)
00 end-to-end mode unavailable (only the mode of hop-by-hop forwarding available)
01 Transfer mode available
10 SCCP mode available
11 Transfer mode and SCCP mode available
Bit I: interworking indicator (note)
0 interworking not encountered
1 interworking encountered
Bit J: end-to-end information indicator (note)
0 no end-to-end information available
1 end-to-end information available
Bit K: ISDN user part indicator (note)
0 ISDN user part not used in all directions
1 ISDN user part used in all directions
Bit L: Hold indicator
0 hold not requested
1 hold requested
Bit M: ISDN access indicator
0 terminal accessed to non-ISDN
1 terminal accessed to ISDN
Bit N: echo control device indicator
0 incoming semi-echo control device not included
1 incoming semi-echo control device included
Bit PO: SCCP mode indicator
00 no indication
01 Connectionless mode available
10 connection-oriented mode available
11 connectionless and connection-oriented modes available
Note: bits G~K and O~P constitute a protocol control indicator.
2 Optional backward call indicator
Bit A: Inband information indicator
0 no indication
1 Inband information or proper code pattern currently available
Bit B: call change possible indicator
0 no indication
1 call change possible
Bit C: simple segmentation indicator
0 no additional information sent
1 additional information sent with the segmentation message
Bit D: MLPP user indicator
0 no indication
1 MLPP user
Bits E~H: reserved for national use
3 Call reference
Call reference includes the call identification and signaling point code.
a) Call identification
The binary code represents the identification number allocated to a call.
b) Signaling point code
Signaling point code related to the call identification.
4 User to user indicator
Bit A: Type
0 request
1 response
If bit A equals to 0 (request):
Bit CB: service 1
00 no information
01 spare
10 request, not mandatory
11 request, mandatory
Bit ED: service 2
00 no information
01 spare
10 request, not mandatory
11 request, mandatory
Bit GE: service 3
00 no information
01 spare
10 request, not mandatory
11 request, mandatory
Bit H: spare
If bit A equals 1 (response):
Bit CB: service 1
00 no information
01 not provided
10 user to user information provided
11 spare
Bit ED: service2
00 no information
01 not provided
10 provided
11 spare
Bit GF: service 3
00 no information
01 not provided
10 provided
11 spare
Bit H: spare
6.4 ISUP Basic Signaling Flow
6.4.1 Features of ISUP in an MSC (M900/1800)
Besides the above-mentioned difference in the caller category, there are some other major differences:
1: MSC adopts the group code sending mode. That is, MSC does not send any SAM, but sends the called data at one time. However, the receiving is in the group and overlay modes.
2: First party release mode is adopted in a mobile system and also applied to special number calls.
3: There must be the optional parameter “Caller number” in the IAM message of ISUP transmitted between MSCs.
4: If a mobile user is the called, the called number in the IAM message sent by GMSC/MSC is MSRN (roaming number).
5: ”Call Forwarding on mobile subscriber Not Reachable (CFNRc)” is added into the forwarding type.
6: Such mobile-related parameter values as “mobile subscirber absent” are added to the cause indicator parameters.
6.4.2 Signaling Procedures of ISUP
1: Basic signaling procedure of successful call connection
Figure6-4 Basic signaling procedure of successful call connection
As shown in Figure 6-5, IAM message is the first message sent by the outgoing MSC and carries complete information, such as the called number, caller category, transmission medium request and caller number. When the called is idle, an ACM message will be sent back and ringing starts. The called answers and sends back an ANM message, and the converation begins. If the called hooks on, REL should be sent directly to clear inter-office circuits and RLC message will be used for acknowledge. If the caller hooks on, the processing is the same except that the directions of messages are different (either party in ISUP can initiate the action of releasing inter-office trunk circuits).
CLP_TAGGING_NO_SCR. Basic signaling procedure of unsuccessful call connection
Figure6-5 Basic signaling procedure of unsuccessful call connection
As shown in Figure 6-6, the outgoing MSC sends IAM messages and is ready to establish a call. However, the incoming MSC finds that the subscriber cannot be paged due to called subscriber busy or base station subsystem fault or that the called subscriber is not reachable due to other reasons. Then, it sends an RELmessage to notify about the connection failure. The “cause value” parameter in the REL will inform the outgoing MSC of the cause for this unsuccessful connection.
The REL sent by the outgoing MSC to clear inter-office trunk circuits will be confirmed by RLC.
3: Signaling coordination between ISUP and No.1
Figure6-6 Signaling coordination between ISUP and No.1
In a mobile network, No.7 signaling mode is used between MScs and the signaling coordination between ISUP and No.1 will occur only in a mobile-to-fixed call. As shown in Figure 6-7, the tandem exchange converts the information in an IAM message into a series of No.1 signals. If the called is idle (KB=1), an ACM message will be sent back and ringing starts. The called answers and sends back an ANM message. Then, the converation begins. When the called fixed subscribers hooks on, he cannot release inter-office circuits, and must send an SUS (suspend) message to the outgoing MSC to request for disconnection. Meanwhile, the outgoing MSC shall send an REL message to start clearing the inter-office trunk circuits. If a call fails, the tandem exchange will, based on different KB values sent back by PSTN, add the corresponding cause value to the REL message sent back by the outgoing MSC.
4: Signaling coordination between ISUP and TUP
Figure6-7 Signaling coordination between ISUP and TUP
Because both ISUP and TUP belong to the SS7 system, their messages and signaling coordination are very similar. The difference between them is that the disconnection of inter-office trunk circuits in TUP is initiated by the outgoing MSC while either party in ISUP can initiate the disonnection. In addition, if a call is unsuccessful, the same REL message in ISUP is used to carry different cause value parameters while different messages should be used in TUP to end different calls.
6.4.3 Implementation of ISUP in the M900/1800 System
The M900/1800 MSC system software adopts modular design and the communciation between various software modules is achieved by means of messages. ISUP-related modules in this system are as follows: call-related CC (Call Control) module, MT (Maintenance Test) module related to circuit management and MTP (Message Transfer Part). Their relationships are shown in Figure 6-8:
Figure6-8 Relationships between ISUP and CC/MT/MTP
The message between CC and ISUP is called the call control primitive, which is adapted from the protocol Q.931. The message between MT and ISUP is called the circuit management primitive, including the blocking, unblocking, reset, query and continuity check of a circuit/circuit group, and is adapted from the protocol Q.764. The message between MTP and ISUP adopts the MTP primitive as stipulated in the protocol Q.701.
ISUP in M900/1800 is divided into two function modules: CPC and CSC. CC analyzes the called roaming number of the subscriber or the incoming trunk, and selects the outgoing trunk circuit. Then, it encapsulates related information into a call control primitive and sends it to the CPC of ISUP. ISUP converts it into an inter-office message to send to MTP, which transmits it to the peer exchange. MTP receives the inter-office message and sends it to ISUP. Then, CPC converts it into a call control primitive and sends it upward to CC for corresponding processing.
Circuit management process and call control process are almost the same except that CSC is the module for processing circuit management messages in ISUP. Besides, the circuit management message about the peer end received from MTP is processed by CSC alone, which only reports the corresponding information to MT.
6.5 Summary
This section describes the ISUP, one of the user parts of level 4 functions in the No.7 signaling mode. ISUP specifies the contents of signaling messages transmitted between telephone switching exchanges and provides the functions of call control and circuit management. The foucus is on the format of an ISUP message and the signaling procedures of a call (including the coordination with other signaling modes). In addition, the difference between the ISUP in M900/1800 systems and a fixed network, along with their modes of implementation, are briefly described.
6.6 Exercises
1: Draw up the format of an ISUP message and the composition of an IAM message.
2: Draw up the ISUP signaling procedure of a successful call.
6.7 Key to the Exercises
2-1 The basic features of a common channel signaling system are as follows: signaling channel and traffic channel are fully separated, and the signaling information of all trunk lines and all communication services are transmitted over common data links in the form of messages.
2-2 Signaling point, signaling transfer point and signaling link
2-3 Direct connection mode and quasi-direct connection mode
3-1 message transfer user MTP SCCP ISUP TUP TCAP MAP
3-2 MSU (Message Signal Unit) LSSU (Link Status Signal Unit) FISU (Fill-in Signal Unit)
MSU SIF SIO unit length
4-1 signaling data link bidirectional 64kbps analog
4-2 signal unit delineation signal unit localization error detection and correction initial localization signaling link error monitoring
Flow control Processor error control
4-3 signaling message processing function signaling network management function message routing message discrimination message allocation signaling service management signaling link management signaling route management
5-1 connection-oriented and connectionless services Class 0 basic connectionless services
class 1 ordered connectionless services
Class 2 basic connection-oriented services class 3 connection-oriented services of flow control
5-2 SPC (Signaling Point Code) SSN (SubSystem Number) GT (Global Title)
5-3 05H 06H 08H 07H
6-1 transaction sublayer component sublayer
6-2 tag length value
7-1 location registration/erasure reset after location register fault user management authentication & encrytion route management function of IMEI access processing and paging processing of supplementary services handover short message services operation and maintenance
8-1 IAM
8-2 group sending mode not sen
