ISDN User Part (CCS7 in Wireless Networks)

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

ISUP message format.


Figure 2-7 ISUP message format.

ISUP IAM protocol decode.

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

ISUP call setup.

Figure 2-9 ISUP call setup.

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