PDF of this tutorialPlain Old Telephone Service (POTS)
Prior to the mid-1960s, the service logic, as shown in Figure 1, was hardwired in switching systems. Typically, network operators met with switch vendors, discussed the types of services customers required, negotiated the switching features that provided the services, and finally agreed upon a generic release date for feature availability. After this, the network operator planned for the deployment of the generic feature/service in the switching network fabric.
Figure 1. Plain Old Telephone Service
This process was compounded for the network operator with switching systems from multiple vendors. As a result, services were not offered ubiquitously across an operator's serving area. So, a customer in one end of a city, county, or state may not have had the same service offerings as a person in another part of the area.
Also, once services were implemented, they were not easily modified to meet individual customer's requirements. Often, the network operator negotiated the change with the switch vendor. As a result of this process, it took years to plan and implement services.
This approach to new service deployment required detailed management of calling patterns, providing new trunk groups to handle calling patterns. As customer calling habits changed (longer call lengths, larger calling areas, and multiple lines in businesses and residences) the demand on network operators increased.
Stored Program Control (SPC)In the mid-1960s, stored program control (SPC) switching systems were introduced. SPC was a major step forward because now service logic was programmable where, in the past, the service logic was hardwired. As a result, it was now easier to introduce new services. Nevertheless, this service logic concept was not modular. It became increasingly more complicated to add new services because of the dependency between the service and the service-specific logic. Essentially, service logic that was used for one service could not be used for another service. As a result, if customers were not served by a SPC switching system, new services were not available to them.
Common Channel Signaling Network (CCSN)
Another aspect of the traditional services offerings was the call setup information—that is, the signaling and call supervision that takes place between switching systems and the actual call. When a call was set up, a signal and talk path used the same common trunk from the originating switching system to the terminating switching system. Often there were multiple offices involved in the routing of a call. This process seized the trunks in all of the switching systems involved. Hence, if the terminating end was busy, all of the trunks were set up unnecessarily.
The network took a major leap forward in the mid-1970s with the introduction of the common channel signaling network (CCSN), or SS7 network for short. Signaling system number 7 (SS7) is the protocol that runs over the CCSN. The SS7 network consists of packet data links and packet data switching systems called signaling transfer points (STPs).
The SS7 network (see Figure 2) separates the call setup information and talk path from the common trunk that runs between switching systems. The call setup information travels outside the common trunk path over the SS7 network. The type of information transferred included permission for the call setup and whether or not the called party was busy.
Common Channel SignalingSS7 technology frees up trunk circuits between switching systems for the actual calls. The SS7 network enabled the introduction of new services, such as caller ID. Caller ID provides the calling party's telephone number, which is transmitted over the SS7 network.
Figure 2. Common Channel Signaling
The SS7 network was designed before the IN concept was introduced. However, telephone operators realized that there were many advantages to implementing and using SS7 network capabilities.
