CHAPT E R 5
ISUP and TCAP
This chapter reviews the ISUP and TCAP protocols and their functions within the Public Switched
Telephone Network (PSTN).
Basic ISUP Signaling
ISDN User Part (ISUP) defines the protocol and procedures used to set up, manage and release trunk
circuits that carry voice and data calls over the PSTN. ISUP is used for both ISDN and non-ISDN calls.
Calls that terminate within the same switch do not use ISUP signaling. (See Figure 5-1.)
In some parts of the world, such as China, the Telephone User Part (TUP) protocol supports basic call
processing. TUP handles analog circuits only; digital circuits and data transmission capabilities are
supported by the Data User Part protocol.
ISUP Message Format
ISUP information is carried in the service information field (SIF) of an MSU. The SIF contains the
routing label followed by a 14-bit (ANSI) or 12-bit (ITU) circuit identification code (CIC). The CIC
indicates the trunk circuit reserved by the originating switch to carry the call.
The CIC is followed by the message type field  IAM, ACM, ANM, REL, RLC  which defines the
contents of the remainder of the message.
Each ISUP message contains a mandatory part that includes fixed-length parameters. Sometimes the
mandatory fixed part is comprised only of the message type field.
The mandatory fixed part may be followed by a mandatory variable part and/or an optional part. The
optional part contains parameters which are identified by a one-octet parameter code followed by a
length indicator ( octets to follow ) field. (See Figure 5-2.)
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Chapter 5 ISUP and TCAP
Basic ISUP Signaling
Figure 5-1 Basic ISUP Signaling
STP STP
SS7 Links
SSP SSP
Voice Circuit
Figure 5-2 ISUP Message Format
ANSI ISUP Message ITU-TISUP Message
O O
0
Subservice Field Service Indicator Subservice Field Service Indicator
0
f f
1 f
f DPC Member DPC Low-Order Octet
1
s s
OPC Low-Order
2
DPC Cluster
e 2 DPC High-Order 6-bits e
Routing
2 bits
t t
Label
3
3 DPC Network OPC Middle-Order Octet
4
4 OPC Member 4-bit SLS/SLC OPC High-Order 4-bits
5
CIC Low-Order Octet
5 OPC Cluster
6
6 4-bit SLS/SLC CIC High-Order 4-bits
OPC Network
3-bits
7
7
Signaling Link Selection Message Type
spare
8
8
CIC Low-Order Octet
2-bits Interpretation varies
|
9
CIC High-Order 6-bits
spare according to
Message Type variable
n
10
Message Type
11
Interpretation varies
|
according to
Message Type variable
n
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Chapter 5 ISUP and TCAP
Basic ISUP Signaling
ISUP Message Types
IAM
An initial address message (IAM) is sent in the  forward direction by each switch in the circuit between
the calling party and the destination switch of the called party. An IAM contains the called party number
in the mandatory variable part and may contain the calling party name and number in the optional part.
(See Figure 5-3.)
Figure 5-3 ANSI and ITU-T Initial Address Message (IAM) Format
ANSI Initial Address Message (IAM) ITU-T Initial Address Message (IAM)
O 1 1 O
f f
SIO and Routing Label SIO and Routing Label
|
f |
f
s s
8 5
CIC Low-Order Octet CIC Low-Order Octet
e e
t 4-bits t
2-bits
9 CIC High-Order 6-bits CIC High-Order 4-bits 6
spare
spare
10 value = 1 value = 1
Message Type Message Type 7
11 Nature of Connection Indicators 8
Nature of Connection Indicators
Mandatory
12 Forward Call Indicators bits H...A 9
Forward Call Indicators bits H...A Fixed Part
13 Forward Call Indicators bits P...I 10
Forward Call Indicators bits P...I
Mandatory
Fixed Part
14 11
Calling Party Category Calling Party Category
15 12
Offset of 1st Mandatory Var. Parameter value = 3 Transmission Medium
value = 2
16 Offset of 2nd Mandatory Var. Parameter
value = 0 if no Offset of Mandatory Var. Parameter 13
optional part
value = 0 if no
17 14
Offset to Start of Optional Part
Offset to Start of Optional Part
optional part
18 Length Indicator of User Service Info
Length Indicator of Called Party No. 15
Mandatory
Mandatory
Variable
User Service Information
Called Party Number
Variable
Part
No. of Octets = Length Indicator value
No. of Octets = Length Indicator value
Part
Length Indicator of Called Party No.
Optional Parameter Code
Sequence
Optional Parameter Length Indicator
may repeat
Called Party Number
No. of Octets = Length Indicator value
Optional Parameter
No. of Octets = Length Indicator value
Optional Parameter Code Optional
Sequence
Part
may repeat
End of Optional Parameters Indicator
Optional Parameter Length Indicator
Optional Parameter
Optional
No. of Octets = Length Indicator value
Part
End of Optional Parameters Indicator
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Basic ISUP Signaling
ACM
An address complete message (ACM) is sent in the  backward direction to indicate that the remote end
of a trunk circuit has been reserved. The originating switch responds to an ACM message by connecting
the calling party s line to the trunk to complete the voice circuit from the calling party to the called party.
The calling party hears ringing on the voice trunk. (See Figure 5-4.)
Figure 5-4 ANSI and ITU-T Address Complete Message (ACM) Format
ANSI Address Complete Message (ACM) ITU-T Address Complete Message (ACM)
O O
1 1
f f
SIO and Routing Label SIO and Routing Label
f f
| |
s s
e e
8 5
CIC Low-Order Octet CIC Low-Order Octet
t t
2-bits
9 CIC High-Order 6-bits 4-bits spare CIC High-Order 4-bits
6
spare
10 value = 6 value = 6 7
Message Type Message Type
Mandatory Mandatory
11 8
Backward Call Indicators bits H...A Backward Call Indicators bits H...A
Fixed Part Fixed Part
12 9
Backward Call Indicators bits P...I Backward Call Indicators bits P...I
Mandatory Mandatory
13 10
Offset toStart of Optional Part* Offset toStart of Optional Part*
Variable Part Variable Part
14 Optional Parameter Code Sequence Sequence 11
Optional Parameter Code
may repeat may repeat
15 12
Optional Parameter Length Indicator Optional Parameter Length Indicator
Optional Parameter Optional Parameter
Optional Optional
No. of Octets = Length Indicator value No. of Octets = Length Indicator value
Part Part
End of Optional Parameters Indicator End of Optional Parameters Indicator
value = 0 value = 0
*value = 0 if no optional part *value = 0 if no optional part
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Basic ISUP Signaling
REL
A release message (REL) is sent in either direction indicating that the circuit is being released due to a
specified cause indicator. An REL is sent when either calling or called party hangs up the call
(cause = 16). An REL is also sent back to the calling party if the called party is busy (cause = 17).
(See Figure 5-5.)
Figure 5-5 ANSI and ITU-T Release (REL) Message Format
ANSI Release Message (REL) ITU-T Release Message (REL)
O 1 1 O
f f
SIO and Routing Label SIO and Routing Label
|
f |
f
s s
8 5
CIC Low-Order Octet CIC Low-Order Octet
e e
t t
2-bits
9 CIC High-Order 6-bits 4-bits spare CIC High-Order 4-bits 6
spare
Mandatory Mandatory
10 7
Message Type = 12 Message Type = 12
Fixed Part Fixed Part
value = 2 value = 2
11 Offset of 1st Mandatory Var. Parameter Offset of 1st Mandatory Var. Parameter 8
value = 0 if no
12 Offset to Start of Optional Part Offset to Start of Optional Part 9
optional part
Length Indicator of Cause Indicators Length Indicator of Cause Indicators
13 10
Mandatory Mandatory
Release Cause Indicator Parameter Variable Variable Release Cause Indicator Parameter
No. of Octets = Length Indicator value Part Part No. of Octets = Length Indicator value
Optional Parameter Code Optional Parameter Code
Sequence Sequence
may repeat may repeat
Optional Parameter Length Indicator Optional Parameter Length Indicator
Optional Parameter Optional Parameter
Optional Optional
No. of Octets = Length Indicator value No. of Octets = Length Indicator value
Part Part
End of Optional Parameters Indicator End of Optional Parameters Indicator
value = 0 value = 0
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Basic ISUP Signaling
RLC
A release complete message (RLC) is sent in the opposite direction of an REL to acknowledge the
release of the remote end of a trunk circuit and to end the billing cycle, if appropriate. (See Figure 5-6.)
Figure 5-6 ANSI and ITU-T Release Complete (RLC) Message Format
ANSI Release Complete (RLC) Message ITU-T Release Complete (RLC) Message
O 1 1 O
f f
SIO and Routing Label SIO and Routing Label
|
f |
f
s s
8 5
CIC Low-Order Octet CIC Low-Order Octet
e e
t t
2-bits
9 CIC High-Order 6-bits 4-bits spare CIC High-Order 4-bits 6
spare
Mandatory Mandatory
10 7
Message Type = 16 Message Type = 16
Fixed Part Fixed Part
ISUP Call Sequence
Call Initiated
See Figure 5-7 and Figure 5-8 as you review the following messaging sequence:
1. Calling party goes  off hook on an originating switch (SSP) and dials the directory number of
the called party.
1a. Originating SSP transmits ISUP IAM to reserve an idle trunk circuit. The IAM includes OPC,
DPC, CIC, dialed digits, CPID, and calling party name (Caller ID option).
1b. IAM is routed via home STP of originating SSP.
2. Destination switch (SSP) checks the dialed number against its routing table and confirms that
the called party s line is available for ringing.
2.a Destination SSP transmits ACM to the originating SSP via its home STP to confirm that the
remote end of the trunk circuit has been reserved.
2b. The STP routes the ACM to the originating SSP which connects the calling party s line to the
trunk to complete the voice circuit. The calling party hears ringback tone.
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Chapter 5 ISUP and TCAP
Basic ISUP Signaling
Figure 5-7 ISUP Call Initiation (1)
STP STP
2a. ACM
SS7 Links
1a. IAM
2b. ACM
1b. IAM
SSP SSP
Voice Circuit Identification Code (CIC) = 5
2.
1.
Called Party
Calling Party
Figure 5-8 ISUP Call Initiation (2)
Exchange 1 Exchange 2
Subscriber A Subscriber B
SSP SSP
STP
Off-Hook
Dial Tone
Digits IAM
IAM
Setup
Alerting
ACM
Ringback ACM
Off-Hook
ANM
Connect ANM
Conn Ack
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Basic ISUP Signaling
ISUP Call Answered
See Figure 5-9 as you review the following messaging sequence:
3a. Called party goes off-hook. Destination switch terminates ringing tone and transmits an ISUP
answer message (ANM) to the originating switch via its home STP.
3b. STP routes ANM to originating switch which verifies that the calling party is connected to the
reserved trunk. Billing is initiated.
Figure 5-9 ISUP Call Answered
STP STP
3a. ANM
SS7 Links
3b. ANM
SSP SSP
Voice Circuit Identification Code (CIC) = 5
1. 2.
Calling Party Called Party
ISUP Call Released
See Figure 5-10 and Figure 5-11 as you review the following messaging sequence:
4a./b.If the calling party hangs up first, the originating switch sends an ISUP release message (REL)
to release the trunk between the two switches. If the called party releases first, the destination
switch sends an REL message to the originating switch to release the circuit.
5a. When the destination switch receives the REL, it disconnects and idles the trunk, and transmits
an ISUP release complete message (RLC) to the originating switch to acknowledge the release
of the remote end of the circuit.
5b. When the originating switch receives or sends an RLC, the billing cycle ends and the trunk state
is returned to idle.
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Basic ISUP Signaling
Figure 5-10 ISUP Call Release (1)
STP STP
5a. RLC
4b. REL
SS7 Links
4a. REL
5b. RLC
SSP SSP
Voice Circuit Identification Code (CIC) = 5
1. 2.
Calling Party Called Party
Figure 5-11 ISUP Call Release (2)
Exchange 1 Exchange 2
Subscriber A Subscriber B
SSP SSP
STP
Disconnect
REL
REL
Disconnect
Release
RLC
RLC
Release
Release Ack
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Chapter 5 ISUP and TCAP
TCAP Functions
TCAP Functions
Transactional Capabilities Application Part (TCAP) enables deployment of advanced intelligent network
(AIN) services by supporting information exchange between signaling points using SCCP. TCAP
messages are contained within the SCCP portion of a Message Signal Unit (MSU). TCAP messages
consist of a transaction portion and a component portion.
An SSP uses TCAP to query an SCP to find out the routing number for an 800, 888 or 900 number.
Calling cards are validated using TCAP query and response messages.
Mobile subscribers roaming into a new mobile switching center (MSC) area cause the integrated Visitor
Location Register (VLR) to request a service profile from the subscribers Home Location Register
(HLR) using the Mobile Application Part (MAP) information carried in TCAP messages.
TCAP Transaction Portion
The transaction portion contains the package type identifier. There are several package types:
" Unidirectional Transfers component(s) in one direction only (no reply expected).
" Query with Permission Initiates a TCAP transaction. The destination node may not end the
transaction.
" Response Ends the TCAP transaction. A response to a 1-800 query with permission may contain
the routing number(s) associated with the 800 number.
" Conversation with Permission Continues a TCAP transaction. The destination node may not end
the transaction.
" Abort Terminates the transaction due to an abnormal situation.
The transaction portion also contains the Originating Transaction ID and Responding Transaction ID
which associate the TCAP transaction with a specific application at the originating and destination
signaling points.
TCAP Component Portion
The TCAP component portion contains several possible kinds of components:
" Invoke (Last) Invokes an operation. For example, a Query with Permission transaction may
include an Invoke (Last) component to request SCP translation of a dialed 800 number. The
component is the last component in the query.
" Invoke (Not Last) Similar to the Invoke (Last) component except that the component is followed
by one or more components.
" Return Result (Last) Returns the result of an invoked operation. The component is the last
component in the response.
" Return Result (Not Last) Similar to the Return Result (Last) component except that the
component is followed by one or more components.
" Return Error Reports the unsuccessful completion of an invoked operation.
" Reject Indicates that an incorrect package type or component was received.
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TCAP Functions
Note Components include parameters which contain application-specific data carried
unexamined by TCAP.
Sample TCAP Database Query
This sample query describes how a dialed 800 number is processed using TCAP. See Figure 5-12 as you
review the messaging sequence described below.
1. A subscriber goes off-hook and dials an 800 number. The end office switch (SSP) parses the digit
string and sends an 800 query message to either of its STPs over its A-link.
2. The STP recognizes the 800 query and routes it to an appropriate database via an SCP.
3. The SCP receives the query, extracts the passed information and retrieves a real telephone number
to which the call should be routed.
4. The SCP sends a response message with the information necessary to process the call to the
originating SSP via an STP and an A-link.
5. The STP receives the response and routes it to the SSP.
6. The SSP receives the response and uses the information to route the call. It generates an IAM
message and proceeds with ISUP call setup.
Figure 5-12 Sample TCAP  800 Number Query
800 Query
SCP
STP
SCP
A-links
STP
SSP
Calling Party
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Routing Info
800 Query
Routing Info
Chapter 5 ISUP and TCAP
Review: ISUP and TCAP
Review: ISUP and TCAP
1. What is the principal function of ISUP?
2. Which field in an MSU carries ISUP information?
3. Define CIC.
4. What is the function of an IAM?
5. What does the receipt of an ACM indicate?
6. Which ISUP message initiates call billing?
7. What is the function of an REL message?
8. What is the response to an REL message?
9. What information is included in an IAM?
10. What happens when a called party goes off-hook?
11. What is the principal function of TCAP?
12. Where in an MSU are TCAP messages located?
13. Briefly describe the two portions of a TCAP message.
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