27-09-2012, 12:48 PM
ISUP and TCAP
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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.)
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.