Understanding SS7 Call Flow

In the world of telecommunications, the Signaling System 7 (SS7) protocol serves as the backbone for connecting and routing voice, data, and signaling messages between different networks and service providers. SS7 call flow represents the sequence of events and signaling messages that occur during a telephone call or a communication session. This article aims to provide a comprehensive understanding of SS7 call flow, its key components, and the role it plays in enabling seamless communication.

The Significance of SS7 Call Flow

SS7 call flow is crucial for establishing, maintaining, and terminating phone calls, regardless of whether they are traditional circuit-switched calls or modern Voice over IP (VoIP) sessions. It ensures efficient call routing, call setup, and enables various supplementary services such as call forwarding, call waiting, and call transfer.

Components of SS7 Call Flow

• a. Service Switching Point (SSP): The SSP initiates the call and is responsible for the initial signaling message transmission. It collects the dialed digits or the user’s input, queries the database to determine the routing information, and sends the Initial Address Message (IAM) to the Service Control Point (SCP).
• b. Signal Transfer Point (STP): The STP acts as a vital intermediary in the SS7 call flow. It receives signaling messages from the SSP, determines the destination of the message based on the Point Code (PC) routing, and forwards it to the appropriate destination STP or SCP. STPs play a crucial role in routing messages between different networks and service providers.
• c. Service Control Point (SCP): The SCP acts as a central database for service logic and subscriber-related information. It receives the IAM from the SSP via the STP and processes the request by querying the relevant databases. It provides instructions to the SSP regarding call routing, call forwarding, and other supplementary services.
• d. Signaling Transfer Point (STP) (again): Once the SCP processes the request and determines the call routing information, it sends the appropriate message to the STP for further transmission to the SSP. The STP ensures that the signaling messages reach the correct destination and are properly routed within the SS7 network.
• e. Service Switching Point (SSP) (again): Based on the instructions received from the SCP, the SSP establishes the call path, sets up the necessary connections, and generates the appropriate response signaling messages to proceed with call setup. It sends the Address Complete Message (ACM) back to the originating SSP to indicate that the call setup is successful.
• f. Signaling Transfer Point (STP) (again): The STP routes the ACM message back to the originating SSP, acknowledging the successful establishment of the call path. This allows the originating SSP to proceed with the call ringing or alerting phase.
• g. Service Switching Point (SSP) (again): Upon receiving the ACM, the originating SSP generates the Call Progress Message (CPG), indicating the call progress to the calling party, such as the ringback tone. It sends this message to the destination SSP.
• h. Destination Service Switching Point (SSP): The destination SSP receives the CPG from the originating SSP and performs the necessary actions, such as ringing the called party’s phone or presenting a voicemail option. It generates a response signaling message, such as the Answer Message (ANM), and sends it back to the originating SSP.
• i. Signaling Transfer Point (STP) (again): The STP routes the ANM message from the destination SSP back to the originating SSP, indicating that the called party has answered the call. The originating SSP proceeds to connect the voice path between the calling and called parties, enabling them to communicate.

Call Termination and Call Release

• a. Service Switching Point (SSP) (again): When either party decides to end the call, the SSP that initiated the termination sends a Release Message (REL) to inform the other SSP about the call termination. This message includes the appropriate cause code to indicate the reason for call release, such as call completion or call failure.
• b. Signaling Transfer Point (STP) (again): The STP routes the REL message to the destination SSP, ensuring that both parties are informed of the call termination.
• c. Destination Service Switching Point (SSP) (again): Upon receiving the REL message, the destination SSP releases the voice path between the parties, terminates the call setup, and sends a Release Complete Message (RLC) back to the originating SSP.
• d. Signaling Transfer Point (STP) (again): The STP routes the RLC message back to the originating SSP, confirming the completion of the call termination process.

Supplementary Services

SS7 call flow also supports various supplementary services that enhance the calling experience. These services include:

1. Call Forwarding: When a subscriber activates call forwarding, the SSP sends the appropriate messages to the SCP to retrieve the forwarding destination number. The SCP then provides the necessary instructions to the SSP to establish call forwarding. The call flow for call forwarding involves additional signaling messages such as the Forward Call Indicator (FCI) and Call Forwarding Busy (CFB).
2. Call Waiting: If a subscriber has call waiting enabled, the SSP sends a Call Waiting Indicator (CWI) to the SCP. Upon receiving an incoming call while the subscriber is already engaged in a conversation, the SSP generates the Call Waiting Message (CWT) and sends it to the subscriber’s device, indicating the presence of an incoming call.
3. Call Transfer: Call transfer allows a subscriber to transfer an ongoing call to another party. The call transfer process involves signaling messages such as the Call Transfer Message (CTM), Call Transfer Request (CTR), and Call Transfer Complete (CTC). These messages enable the subscriber to initiate and complete the call transfer operation.

Conclusion

SS7 call flow forms the foundation of telecommunication systems, facilitating efficient call setup, call routing, and call termination. Understanding the sequence of signaling messages exchanged between the various components, such as SSPs, SCPs, and STPs, is crucial for ensuring seamless communication and enabling supplementary services.

As telecommunications technology continues to evolve, SS7 call flow remains a vital protocol in the modern communication landscape, enabling reliable and efficient connections between individuals and businesses worldwide.