 The PSTNs have seen success for a very long time and one of the key factors that has contributed in entirety to the success of these PSTNs is the presence of signaling network. In this module we shall actually see what signaling is that is effectively used by the signaling network, what is the scope of signaling as in signaling network and what are the variants of signaling used in these networks. So signaling is essentially an exchange of control information. Now this control information is shared between the interested parties which realize or in other words through the call. We can think of these control signals as special alphabets which are understood in exactly the same manner between the two parties. These signals could be either analog or digital. These analog signals are mostly the voltages that we see in the form of electrical signals. The digital signals are usually exchanged between the network elements we shall shortly see. The digital signals are in the form of bits and bytes, the packets and the messages. Now in order to understand the signaling we also should understand how far does the signaling work and what are the network elements which exchange signaling. We call this the scoping of signaling. Scoping of signaling is usually understood in the form of the network element and the customer premises equipment that is a telephone and its local exchange and from the local exchange to the other exchanges. If the scope of the signaling is between the telephone and its local exchange it is called line signaling. This line signaling is mostly done in the form of analog signals. If you recall in old PTCL telephones which were provided by a company called TIP Telephone Industries of Pakistan it was a telephone which would have a dial interface on it. Using that dial interface we would simply dial numbers. Using these dialing numbers through a rotational dialing mechanism we would generate different pulses. So that pulse would actually be sent from the telephone to the local exchange. Similarly for certain tone type telephone industries of Pakistan telephones do you remember when you would dial a 1, a 2 or a 0 you would hear different tones. Now these different tones were meant to signal some instructions or some control information from the telephone to the local exchange. Now the exchange of signals in the form of digital is usually done between the exchanges. This is called exchange signaling. Exchange signaling is only understood by the network elements, the exchanges. To fully comprehend signaling let's move on to another definition of signaling. The first type of signaling from another perspective is called channel associated signaling. Channel associated signaling is also known as per trunk signaling. As the name suggests this is the signaling that is associated to a certain user calling another user that is between a caller party and a call party. This signaling usually carries control information to realize a call between two endpoints. What is the purpose? The purpose is to make a voice call through. These channel associated signals are sent on the same telephone line or on the same frequency or within the same time slot as would the voice be carried into. It means that these channel associated signals are part of the same medium or the same traffic that would also be of the same medium that would also be carrying voice as the traffic. The other type of signaling is the common channel signaling. The name suggests that this common channel is common to maybe a group or all of the users within a certain exchange. This signaling information is usually multiplexed over a common channel and it carries important signaling information on behalf of certain number of users in a certain exchange to other users in another exchange. The very purpose of having a common channel signaling is that number one it is faster because it is doing the same activity that was done by the previous channel for a certain caller and call party pair. Here it is being done for more than one users. That's the first advantage. The second possible advantage is that in the previous specific channel that we saw in the channel associated signaling the signaling was sometimes known as the in band signaling. Here in the letter case in common channel signaling it is the out of band signaling. So out of band signaling has the advantage that this signaling can be initiated even after the call is established. As a quick recall if you just try to remember when you try calling somebody from your mobile phone or from your PTCL phone you actually wait for some time and you let the signaling and all that call processing activity to take place once the call has been established as soon as the call party simply takes the receiver off the hook the call is established. After the call is established you are no more in a position to affect the call because it has been established but using this common channel signaling while a call is on the signaling can still be performed. So if we have these variants of signaling schemes it means that we are now in a good position to understand a signaling network. A signaling network is a combination of network elements that work together to actually receive process and distribute the messages which are associated with signaling with an eventual goal and the goal is to control the behavior of the overall system. What is the system? The system is a telephone network that is meant to realize telephone calls between the parties.