 After understanding how a signaling network works, let's look at the most widely used standard, the signaling standard 7 architecture. This architecture is the one which is being used in PSTNs, PLMNs and it is also being adopted by SIGTRAN as in next generation networks. So in this module, we should just look at how SS7 works like an overlay network. What are the entities involved in it and specifically how do these work. So SS7 is a wonderful signaling network which was standardized by the International Telecommunication Union. This belongs to the common channel signaling which is the signaling that we discussed in the preceding lecture and it is being used as of today even in PSTNs and PLMNs. As we recall that compared to circuit switched signaling which was based on the electrical signals, it is based on packet signaling that is bits, bytes, packets and messages. So it means what we expect in SS7 is the exchange of specific control information in the form of packets between different entities that we shall study soon. SS7 is known to be widely used and a universally accepted signaling standard for its effective performance. Certain supplementary services such as call waiting, forwarding, conference, CLI, call barring etc. are also provided in addition to the voice and data services. The SS7 architecture looks more like an overlay on top of a physical circuit switched telephone network. The entities in the overlay network specifically the switching point, the service switching point, the signaling transfer point and the signaling control point are of our concern in today's lecture. So let's go through each one of these one by one. If we look at the overlay network we actually see different entities. These entities in the overlay network are the SSP, the STP and the SCP. These form the logical architecture of the SS7 signaling network. We shall go through each one of these one by one. The service switching point is the first entity with which the telephone call interacts. When the user dials in a telephone number, the service switching point understands that number which is also known as the B number or the global title and translates it into certain format or a message that the SS7 network can understand after which the role of the subsequent entities is going to be determined. When this number is converted into the message then the call establishment process is initiated by opening up a connection with the call party or the destination SSP. The destination service switching point is the one that acknowledges the receipt of these messages and the coordination and overall interaction of these two service switching points is used to establish, manage and terminate the voice connection. It is also interesting that these SSPs cannot communicate to each other directly but these need the support of another entity that is STP we shall shortly see to have connectivity with each other. The signal transfer point is another entity you can think of it like a router or a gateway. A router is used to establish a connection or establish a path between two parties. It routes the signals from the calling SSP to the called SSP by finding the most optimal path between the two. This is with regards to establishing a telephone call. Sometimes the signal transfer point is also used as a gateway when the call is routed from one telco to the other. Another important entity that may not necessarily always be used in establishing a call between two telephones is the signal control point. It provides access to a group of applications in SS7. As we know that the applications are usually considered to be placed at the application layer in the ISO OSI reference model. So the signal control point is more like a database application that is used to provide certain common services used in a telephone network. For instance, you might have heard of 0800 series of numbers which are either free or they are charged. For PLMNs as in mobile network, the mobility related information such as where is the call party at the moment or where from the calling party is calling is required. So all these services are provided to the network using this entity, the signal control point. Let's get back to the overlay network original definition. As you can see from the calling telephone on the left hand side, the SSP is consulted which actually connects to the destination SSP on the overlay network in the bottom to reach the destination telephone via a series of STPs which are shown to you in these rectangular figures. The SSP is shown to you on the right hand side which is only consulted when needed as we said for specific supplementary services.