Editing ISDN User Part (section)

== Message format ==
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| colspan="8" style="background:#b1d1f4"|Routing Label
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| colspan="8" style="background:#e5cd4b"|CIC Least Significant 8 Bits
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| colspan="4" style="width:25%; background:#c3c3c3"|Padding
| colspan="4" style="background:#e5cd4b"|CIC Most Sig. 4 Bits
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| colspan="8" style="background:#4e99ff"|Message type
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| colspan="8" style="background:#4e99ff"|Mandatory fixed part
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| colspan="8" style="background:#4e99ff"|Mandatory variable part
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| colspan="8" style="background:#4e99ff"|Optional part
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The ''Signalling Information Field (SIF)'' for all ISUP [[Message Signal Unit]]s (MSU) contain the following components:{{Sfn | ITU-T | 1999a | pp = 3–4}}
* Routing Label
* Circuit Identification Code
* Message Type
* Mandatory Fixed Part
* Mandatory Variable Part
* Optional Part

The ''Routing Label'' indicates the [[Point Code]]s of the originating and destination nodes in the network; it also includes the [[Message Transfer Part|Signalling Link Selection]] field that is used to select between the multiple routes an MSU could take between two nodes.

The ''Circuit Identification Code'' is used to specify which trunk between two [[Telephone exchange|switches]] is used to carry a particular call. Note that some versions of [[ANSI]] ISUP permit a CIC with 14 significant bits instead of the 12 that are shown.{{Sfn |Russell|2002|p= 321}}
# The ''Message Type'' indicates the ISUP message type. The presence and form of the remaining 3 components are determined by this message type.
# The ''Mandatory fixed part'', when present, contains the mandatory, fixed-length parameters associated with the message type.
# The ''Mandatory variable part'', when present, contains the mandatory, variable-length parameters associated with the message type.
# The ''Optional part'', when present, contains the optional parameters permitted to be included in the message type.

When sent using the services of the [[Signalling Connection Control Part]], ISUP messages passed to SCCP in the User Data parameter (NSDU) consist of only the last 4 components (''Message Type, Mandatory fixed part, Mandatory variable part, Optional part''). The [[Message Transfer Part|routing label]] and [[circuit identification code]] are not included in the user data passed to SCCP.{{Sfn | ITU-T | 1999a | p = 3}}