Why SS7 Was Developed

The introduction of the SSX infrastructure has allowed for the telco to provide a better QoS for its customers. It is intentionally stated as SSX because there are several common channel-signaling systems. From a historical point of view, there was SS4, SS5, SS6, and now SS7. SS4, SS5, and SS6 are legacy systems that are not discussed in detail because they are no longer used and are outside the scope of this book.

Prior to the creation of the SSX infrastructure, call control was inefficient. In the past, if you wanted to place a call from City A to City B, you had to reserve a trunk through all interconnecting switches between the locations to see if the dialed number was available for connection. This seems like normal operation, but if the circuit was busy or unavailable, the trunks reserved through the network were used for no reason. Furthermore, they could have been used for other calls being placed through the network. Why would this be such a big issue? If a call can't be completed, then it can't be billed. Those trunks could have been used for billable calls in the same time period.

In the SS7 infrastructure, the circuits are reserved but not connected through the network until the call is answered. In the event that the call cannot complete, the circuits that have been reserved for the call are released immediately. Although it sounds the same, the circuits in an SS7 network are typically reserved for a shorter period of time.

SS7 helps alleviate this problem because the signaling takes a separate path to get to the remote switch. If the remote circuit is busy, it can return that information out of band and release the circuit at the same time. The SS7 network is also able to reroute signaling traffic based on network congestion or failure. This is referred to as network resiliency.

Another reason that SS7 was developed was due to the slow call setup time. In-band signaling is slower to set up and tear down calls, and although it had been improved, call setup time was still too slow for the fast growing infrastructure. SS7 improved upon the call setup speeds by adding a separate link to the network and by providing a message-based signaling network. The bit states associated with DTMF tones take longer to set up and tear down calls. The purpose of this signaling link was for nothing more than transporting signaling information from node to node in the network.

The ideology behind the creation of SS7 was to create a scalable and flexible network infrastructure that allowed for common signaling messages for call control and also to allow for the integration of newly developed protocols. The various upper-layer protocols, also referred to as user or application parts, allow for everyone to use the same base structure while providing specific services to the end user.

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