Transaction Capabilities Application Part

SS7 protocol suite

SS7 protocols by OSI layer
Application	INAP, MAP, IS-41... TCAP, CAP, ISUP, ...
Network	MTP Level 3 + SCCP
Data link	MTP Level 2
Physical	MTP Level 1

Transaction Capabilities Application Part, from ITU-T recommendations Q.771-Q.775 or ANSI T1.114 is a protocol for Signalling System 7 networks. Its primary purpose is to facilitate multiple concurrent dialogs between the same sub-systems on the same machines, using Transaction IDs to differentiate these, similar to the way TCP ports facilitate multiplexing connections between the same IP addresses on the Internet.

TCAP is used to transport INAP in Intelligent Networks and MAP in mobile phone networks.

TCAP messages are sent over the wire between machines. TCAP primitives are sent between the application and the local TCAP stack, all TCAP messages are primitives but there are primitive that are not messages, i.e. some are only transferred inside the local machine. A TCAP primitive is made up of one or more TCAP components.

A TCAP primitive may be one of the following types:

Unidirectional	A single primitive with no subsequent primitives. Sometimes referred to as a Notice.
Begin	Start a dialog, further primitives will follow.
Continue	Send a subsequent primitive on an existing dialog, further primitives will follow.
End	The last primitive on an existing dialog, Close an existing dialog.
Abort	An error has causing the dialog to close.
Cancel	The invoke timer has expired without a response being received (this is a primitive but not a message)

A Begin primitive has an, up to 32 bit, Originating Transaction ID, Continues also have a Destination Transaction ID, Ends and Aborts only have a Destination Transaction ID.

Each TCAP component may be one of the following types:

Invoke	A new operation is being requested, this may or may not solicit a response
Return Result Last	A final response to an Invoke
Return Result Not Last	A response to an Invoke, further responses will be sent
Return Error	An error occurred
Reject

Invoke components have a signed 7 bit InvokeID which is present in all the other components to identify which invoke they relate to.

TCAP can be regarded as light weight implementation of the OSI defined ROSE, Remote Operations Services Element protocol

External links

• Example capture featuring Begin/End TCAP transaction
• Linkbit Online TCAP Message Decoder