 The signaling standard 7 as a signaling network can be thought of as a 7 layer protocol service model. In this module, we should look at it one by one. Specifically, we'd see how an analogy can be seen between the ISO OSI reference model and the SS7 protocols, and we'd also quickly recap their operations. SS7 is known to provide a suite of services. So these can be layered in the form of five ISO OSI reference model layers, which are seven of these five are of relevance to SS7. Let's look at it in detail. On the left hand side, we have the seven layers of the ISO OSI reference model. Just to make you quickly understand a mnemonic for understanding the ISO OSI reference model, you can think about a famous mnemonic known as all people seem to need data processing from top to bottom, and I'll go now from bottom to up. That is, please do not throw sausage pizza away. Using and recalling these mnemonics would help you understand the order in which these layers are organized. Now let's look at how the SS7 protocols are mapped onto the OSI model. We see going from bottom to upwards, we have MTP layer one, layer two, layer three, which correspond to the physical, the data link, and the network layer. Then we have starting from the transport layer to the application layer, we have these particular protocols such as the telephone user part, the transaction capabilities part, the ISDN user part, etc. Like similarly, we also have the mobile user part. If you look at all of these, these are the application layers which cover the scope of more than a single layer. In fact, these cover broadly four layers. We also have another particular part or another particular layer called the signaling connection control part. We look at each of these one by one. Let's look at the message transfer part layer one. It is akin to the physical layer. It is defined in IT recommendations Q.702. What it does is exactly in the same way as the ISO OSI reference model. It defines the physical and the electrical characteristics of the signaling link, including the hardware interfaces that are going to carry these signals. The MTP layer two belongs to the data link layer, and it is based and defined in the IT recommendations Q.703. Similar to the data link layer, it concerns with only the link level. And it defines reliable transfer of signaling from the source to destination, which are the two endpoints on a single link. The MTP layer three is similar to the network layer. It is defined in again Q.704. It performs routing operations, which as we saw were determined by the STP in the last lecture. And what we see here is that this routing is done for control messaging only. It is to be understood very clearly that we are looking at the signaling network, not the voice network as such. Regarding the upper layers, as we saw, these upper layers cover mostly top three or four layers. And these are defined for special technologies for which these are used. For instance, for voice telephony, for mobile voice, we have the mobile user part. For ISDN integrated services, we have ISUP. Now these upper layers actually can use the transaction capabilities part. The transaction capabilities part, we shall shortly see, determines the negotiation of capabilities in the form of messages between end parties. And it can use the signaling connection control part. The signaling connection control part, as you can see, is defined by Q.711 to 719. It provides two of the services, either a connection less service or a connection oriented service between the two end points, that is SSP nodes in the overlay network. The capabilities are actually exchanged between the two parties using the ITUT standard, the Q.770 to 779. What it comprises is the message format, which exchange the capabilities between the two end points, of course using STPs for routing, and also for the database access. Now this TCAP is used for database access as in SCP. The important consideration here is that since it is using SCP for certain supplementary services as we saw earlier, it means that this TCAP is to be used using SCCP. That is, in order to exchange the capabilities, we must use the underlying services of SCCP either in a connection oriented manner or in a connection less manner depending on what the user application necessitates.