Integrated Services Digital Network User Part (ISUP) provides the signaling functions required to control circuit-switched voice/data calls and supplementary services. ISUP is also used extensively in the GSM core network for controlling calls between MSCs and between the GMSCs and the external PSTN.
ISUP call control is achieved by the exchange of ISUP messages. These messages have a fixed structure consisting of a header to indicate message type, mandatory fixed parameters, and optional parameters. Figure 2-7 illustrates the ISUP message format.
Figure 2-8 shows an ISUP initial address message (IAM) message decode. The Mandatory fixed and variable parameters are as follows:
1. Message type
■ 01hex for IAM
2. Nature of connection indicator
■ Satellite indicator: no satellite circuit
■ Continuity check indicator: continuity check not required
■ Echo suppressor indicator: O/G half-echo suppression not included
3. Calling party category
■ Ordinary calling subscriber
4. Transmission medium requirement
■ Transmission medium requirement: 3.1-kHz audio
5. Called party number
Figure 2-7 ISUP message format.
Figure 2-8 ISUP IAM protocol decode.
The rest of the parameters are optional. The IAM message is the longest ISUP message. It may contain up to 29 optional parameters.
Table 2-1 lists the ISUP messages and opcodes. There are 49 defined ISUP messages.
Figure 2-9 shows a basic call setup initiated by a fixed line subscriber to a mobile subscriber. To make the example simple, the signaling message flow within the GSM network is not shown.
1. On receiving a SETUP message from one of its subscribers, indicating origination and dialed digits, the local exchange analyzes the called party number and, on realizing that the call is to be routed to another exchange, uses the built-in SSP functionality to build an IAM message. This message contains all the necessary information that is required to route the call to the destination exchange.
2. An intermediate exchange, on receipt of the IAM, analyzes the destination address and other routing information and sends the IAM message to a succeeding exchange.
3. On receiving an IAM message, the GMSC (destination, in this example) uses the GSM procedures to locate the mobile subscriber and notify it of the incoming call.
4. The GMSC sends the ACM message back to the originating exchange via the intermediate nodes to indicate that the complete address of the called party has been received.
5. On receiving the ACM, the originating exchange passes an ALERTING message to the calling party.
6. On answer from the called mobile subscriber, the GMSC sends an ANM message to the originating exchange via the intermediate nodes.
7. The originating exchange sends a CONNECT message to the calling party to complete the call setup.
8. In the example shown in Figure 2-9, the calling party initiated the call release by sending a DISCONNECT message to the originating exchange.
9. The originating exchange then sends the REL message to the intermediate node and returns a RELEASE message to the calling party.
10. The intermediate node, on receiving the REL, returns an RLC to the originating exchange and forwards the REL to the destination exchange.
11. The GMSC, on receiving the REL, sends a DISCONNECT message to the called party and returns an RLC message back to the intermediate node.
TABLE 2-1 List of ISUP Messages
Mnemonics |
Opcode (hex) |
Message name |
ACM |
06 |
Address complete |
ANM |
09 |
Answer |
BLO |
13 |
Blocking |
BLA |
15 |
Blocking acknowledgment |
CMC |
1D |
Call modification completed |
CMRJ |
1E |
Call modification reject |
CMR |
1C |
Call modification request |
CPG |
2C |
Call progress |
CRG |
31 |
Charge information |
CGB |
18 |
Circuit group blocking |
CGBA |
1A |
Circuit group blocking acknowledgment |
GRS |
17 |
Circuit group reset |
GRA |
29 |
Circuit group reset acknowledgement |
CGU |
19 |
Circuit group unblocking |
CGUA |
1B |
Circuit group unblocking acknowledgment |
CQM |
2A |
Circuit query |
CQR |
2B |
Circuit query response |
CVR |
EB |
Circuit validation response |
CVT |
EC |
Circuit validation test |
CSVR |
25 |
Closed user group selection and validation request |
CSVS |
26 |
Closed user group selection and validation response |
CNF |
2F |
Confusion |
CON |
07 |
Connect |
COT |
05 |
Continuity |
CCR |
11 |
Continuity check request |
DRS |
27 |
Delayed release |
EXM |
ED |
Exit |
FAA |
20 |
Facility accepted |
FAD |
22 |
Facility deactivated |
FAI |
23 |
Facility information |
FRJ |
21 |
Facility reject |
FAR |
1F |
Facility request |
FOT |
08 |
Forward transfer |
INF |
04 |
Information |
INR |
03 |
Information request |
IAM |
01 |
Initial address message |
LPA |
24 |
Loopback acknowledgment |
OLM |
30 |
Overload |
PAN |
28 |
Pass along |
REL |
0C |
Release |
RLC |
10 |
Release complete |
RSC |
12 |
Reset circuit |
RES |
0E |
Resume |
SAM |
02 |
Subsequent address message |
SUS |
0D |
Suspend |
UBL |
14 |
Unblocking |
UBA |
16 |
Unblocking acknowledgment |
UCIC |
2E |
Unequipped circuit identification code |
USR |
2D |
User-to-user information |
Figure 2-9 ISUP call setup.