Signaling connection control part

Signaling Connection Control Part (SCCP)

SCCP is the SS7 layer above MTP Level 3. MTP-3 can only route to a node by point code and deliver to a user part. SCCP adds addressing of subsystems inside a node (by Subsystem Number, SSN) and Global Title Translation (GTT), so a message addressed by a dialled number or IMSI can be routed. MTP + SCCP form the Network Service Part, equal to OSI layer 3. SCCP is used by TCAP, MAP and INAP.

It offers four protocol classes:

SCCP message structure

An SCCP message is carried in the SIF of an MTP MSU (Service Indicator = 3).

+---------------------------------------+
| Routing label (DPC, OPC, SLS)         |
+---------------------------------------+
| Message type code (1 octet)           |
+---------------------------------------+
| Mandatory fixed part                  |
+---------------------------------------+
| Mandatory variable part               |
|  (pointers + length + value)          |
+---------------------------------------+
| Optional part (name, length, value)   |
+---------------------------------------+

1. Routing label: DPC (14 bits), OPC (14 bits), SLS (4 bits) in ITU format; used by MTP-3.
2. Message type code: one octet giving the message, e.g. CR (connection request), CC (connection confirm), CREF (refused), DT1/DT2 (data form 1/2), RLSD (released), RLC (release complete), UDT (unitdata), UDTS (unitdata service), XUDT (extended unitdata).
3. Mandatory fixed part: parameters of fixed length and order for that message type, so no name or length is needed, e.g. protocol class, source/destination local reference.
4. Mandatory variable part: variable-length parameters such as called party address, calling party address and data. Pointers at the start give the offset of each parameter; each parameter starts with its length.
5. Optional part: parameters that may or may not appear; each coded as name + length + value, ending with an end-of-optional-parameters octet.

SCCP address (called/calling party)

| Address indicator | SPC | SSN | Global title |

• Address indicator: shows whether PC, SSN and GT are present and whether routing is on GT or on PC+SSN.
• SPC: signaling point code; SSN: subsystem (e.g. HLR = 6, VLR = 7, MSC = 8).
• Global title: digits (e.g. E.164 or E.212 number) with translation type and numbering plan.

Example

A UDT carrying a MAP "send routing info" query from an MSC to an HLR uses connectionless class 0; the called party address holds the subscriber's MSISDN as global title, which the STP translates to the HLR's point code and SSN 6.