Signaling Connection Control Part
In addition to the basic MTP message transfer, the Signaling Connection Control Part (SCCP) as described in Q.711 offers connectionless and connection-oriented services. It provides logical signaling connections within the SS7 network. These services complete requirements found in the OSI Reference Model at Level 3. Therefore, the combination of SCCP and MTP are referred to as the Network Service Part (NSP).
The SCCP is used for transferring circuit- and call-related signaling information of the ISDN User Part (ISUP), with or without setting up a logical connection. For information transfers, the SCCP might be responsible for mapping network addresses in the messages (i.e., global titles) to a signaling relationship within the network. An example would be mapping the first six digits of a calling card number to a destination point code for the database to verify the card. The global title has a variable coding to allow telephone number, addresses, or the use of point codes for identification. The bit pattern of the title informs the receiving SP of the type of information contained in the title field.
Although the ITU-T standard allows SCCP to support connection-oriented service, ANSI networks support connectionless service only. Also, while the international and ANSI standards indicate that ISUP can obtain MTP services either directly or through SCCP, access through SCCP has not been implemented. Only direct ISUP access to MTP is supported.
Connectionless and Connection-Oriented SCCP Services
The four protocol classes of SCCP services are listed below. For the connectionless services, the SCCP maps network addresses to the proper signaling points. It can then rely solely on MTP services, or establish an SCCP connection. The SCCP message set consists of connection establishment and release messages called network protocol control information; the data transfer set is called the network service data units.
- Class 0: Basic connectionless
- Class 1: Sequenced (MTP) connectionless
- Class 2: Basic connection-oriented
- Class 3: Flow control connection-oriented
Within the connection-oriented classes, two connections can be established: a temporary signaling connection or a permanent signaling connection. Only basic connectionless service has been actually implemented.
Global Title Translation
When a service switching point (SSP) receives a request to validate a calling card number, it gets the card number and the associated identification number, but might not know the point code address and subsystem number (SSN) of the database to query. Similarly, in establishing a cellular call, the SSP might have the mobile identification number (MIN) and electronic serial number (ESN) of the telephone, but not know the destination point code (DPC) and SSN of the home location register (HLR) to interrogate. Mapping what is called a global title address to the DPC and SSN requires the services of a specially equipped STP. There could be only a couple of these in an SS7 network.
To perform global title translation, the SSP gives the global title to the SCCP, which constructs a message directed to the STP that can provide the service. The DPC in the message will be that of the target STP. The SCCP process in the STP maps the global address to the DPC and SSN of the destination database. The SCCP then passes the message to the MTP, which can properly route it to the destination.