The SSP is the local telephone exchange, as noted the SSP's have SS7 links that connect to STP's. The STP's are signaling transfer points "tandem office" acting as a tandeming point to route calls to and from various SSP's and carriers. The STP's are also connected to SCP's, signaling control points. Each SSP will have routes sets configured that point to SCP's, they use these when information is needed on how to route a call. The SCP has databases where they store 800 service information, cname information, local number portability info and and other advanced intelligent network information.
I'll give a basic description of a few call scenarios.
When an SS7 call is placed to number, the originating SSP1 transmits an ISUP Initial Address Message (IAM) to reserve a trunk circuit from the originating SSP1 to the destination SSP2. The IAM includes the originating and destination points codes, the circuit identification code and the global title digits. The IAM is routed via the local STP1 of the originating SSP1 to the destination SSP2.
The destination SSP2 determines if it serves the called party. If so it generates a ringing tone at the called party's line and transmits an ISUP Address Complete Message (ACM) to the originating SSP1 via its local STP2. The ACM indicates that the remote end trunk circuit has been reserved. STP2 routes the ACM to SSP1 which generates a ringing tone to the calling party's line and connects it to the trunk circuit.
When the called party picks up the phone, SSP2 terminates the ringing tone and transmits an ISUP ANswer Message (ANM) to SSP1 via STP2. STP2 routes the ANM to SSP1 which verifies that the calling party line is connected to the reserved trunk and if so starts billing.
If the caller hangs up first SSP1 sends an ISUP RELease message (REL) to release the trunk circuit between the 2 switches. STP1 routes the REL to SSP2. Upon receiving the REL SSP2 disconnects the circuit from the called party's line and transmits an ISUP ReLease Complete message (RLC) to SSP1 to ack the release of the trunk circuit. When SSP1 receives the RLC it terminates billing.
If the called person hangs up first, SSP2 sends a REL to SSP1 indicating the release cause. When REL is received, SSP1 disconnects the circuit from the caller's line and transmits an ISUP ReLease Complete message (RLC) to SSP2. When SSP2 receives the RLC it terminates and stops billing.
I'll give a basic description of a few call scenarios.
When an SS7 call is placed to number, the originating SSP1 transmits an ISUP Initial Address Message (IAM) to reserve a trunk circuit from the originating SSP1 to the destination SSP2. The IAM includes the originating and destination points codes, the circuit identification code and the global title digits. The IAM is routed via the local STP1 of the originating SSP1 to the destination SSP2.
The destination SSP2 determines if it serves the called party. If so it generates a ringing tone at the called party's line and transmits an ISUP Address Complete Message (ACM) to the originating SSP1 via its local STP2. The ACM indicates that the remote end trunk circuit has been reserved. STP2 routes the ACM to SSP1 which generates a ringing tone to the calling party's line and connects it to the trunk circuit.
When the called party picks up the phone, SSP2 terminates the ringing tone and transmits an ISUP ANswer Message (ANM) to SSP1 via STP2. STP2 routes the ANM to SSP1 which verifies that the calling party line is connected to the reserved trunk and if so starts billing.
If the caller hangs up first SSP1 sends an ISUP RELease message (REL) to release the trunk circuit between the 2 switches. STP1 routes the REL to SSP2. Upon receiving the REL SSP2 disconnects the circuit from the called party's line and transmits an ISUP ReLease Complete message (RLC) to SSP1 to ack the release of the trunk circuit. When SSP1 receives the RLC it terminates billing.
If the called person hangs up first, SSP2 sends a REL to SSP1 indicating the release cause. When REL is received, SSP1 disconnects the circuit from the caller's line and transmits an ISUP ReLease Complete message (RLC) to SSP2. When SSP2 receives the RLC it terminates and stops billing.