As noted in OSI model - 4, a communication entity is likely to have two logical planes, signaling and transmission. It depends on standards/specification to decide as how it would be taken care - whether use same communication link for signaling and transmission or use different ones or may be some combinations of it. During earlier analog world, the signaling and transmission use to be over same links. Even today, the (traditional public telephone network) landline phones that we use fall in the same category.
Typically this phone will be set up in such a way that when we pick up the receiver, a simple electrical circuit will be completed, signaling that I am going to dial now. Dial tone will be sent over this circuit/line. And when you dial a digit, your phone will send certain frequency (or combination of certain frequencies) over same circuit/line. The device at the other end will detect this waveform signal and know the dialled digit. Once called phone start ringing, the same line will be used for ringing sound. And of course, when called party answers the call, voice will be transmitted over same line. So here we use same line for both signaling and transmission.
Before CCS6/SS7, on network side too, signaling and transmission used to be on the same link which would be reserved during setup. Refer the diagram below.
Even though the other phone is ringing, the communication links are reserved in network. This is inefficient - 1) If called party do not answer till timeout, reserved links are not used, 2) Links reserved in network will not carry any transmission information (i.e. voice for voice call) during setup and so underused. Note that these links are bidirectional. 3)Depending on number of telephone switches connecting in-between (STD call will go thru more switches than local call, and ISD call more than that for STD call), there will be more wastage of communication links.
Concept of translation or routing: When you make a call, the number gets "translated" at switch. Let us take an example.
When person A makes a call to B, Switch1 from its translation/routing table decides that B is on the same switch and it don't have to route the call further. But when A calls C, Switch1 routes to Switch2 as per routing table.
Now let's say person A activates call forward and configures that call to be forwarded to D. Switch1 updates routing table accordingly. This call forward is known only to Switch1. If C calls A, Switch2 routes the call to Switch1. As call forward active on A, Switch1 routes the call to Switch3. So instead of only one trunk between Switch2 and Switch3 being used, two trunks are being used (between 1 & 2 and 1 & 3).
All switches maintain their own routing tables. As number of users grow, technologies improve, new switches will be added into the network, old ones will be replaced, this all would require updation of routing tables in all of the switches.
It is apparent that there is a need for switches to interact with each other for purposes other than only call setup. As an enhancement, we can as well have global routing table for not found or special numbers. This table can be made accessible to all switches over simple signaling links.
So it seems we need to separate signaling from transmission.
Common Channel Signaling 6(CCS6), which evolved into Signaling System 7(SS7), recommended to have separate signaling links and transmission links. That in a way mean, we can have signaling network separate from transmission network with transmission network obeying orders from signaling network.
Let us see how we can do it, refer the diagram below.
Now we do not have inefficiency related to transmission link. Transmission links will be required at later stage in call setup, making it more efficient. Both directions of link need not be reserved. Also as signaling information is smaller amount of data compared to transmission data, signaling link can be time shared between number of connections. That adds up to saving.
Call forward scenario can be handled in organised and better way.
Consider a popular example of pizza number. We would like to keep the pizza number same throughout city, state and may be even country, when user dial a pizza number, the call should go to nearest pizza delivery center. Signaling network - independent of transmission network - can intelligently convert the dialled number to nearest pizza center number using global routing table accessible in signaling network and instruct transmission network to complete the call accordingly.
CCS6/SS7 allows us to think of and implement these enhanced features. Today's GSM network also use the same concept. In fact GSM network use SS7 as signaling and transmission backbone.
Ever wondered when you are going around the city, how your GSM mobile network finds your mobile ? Try to catch the similarity with above pizza example. Your mobile talks (i.e. signals) to network that "I am here" as and when location changes. Network keep updating that in its "location register". Most of these location update signaling (and number of other GSM signaling) uses underlying SS7 network's signaling capabilities.
References: SS7 book by Russell, SS7 tutorial from Performance Technologies.
Let us study SS7 network and protocols in later articles.
© Copyright Samir Amberkar 2010| . | SS7 Index | » Network |