SS7 protocol was defined by ITU-T in Q.700 series (in 1980), national variants defined by ANSI (USA), ETSI (Europe), RRC (China), and TTC(Japan).
Below is network proposed by SS7.
SSPs (Service Switchin Points) are basically telecom switches, in charge of call setup and tear down.
STPs (Signal Transfer Points) provide network layer functions - major function being routing messages to make sure message reach intended entity.
SCP (Service Control Points) provide database functions.
1st diagram shows two networks with above SS7 points and their interfaces. 2nd diagram is simplified to show that STP form a backbone for network. It suggest that SSPs can talk to each other transparently with the help of STPs. These STPs are organised in such a way that there are sufficient number of paths from one SSP/SCP to another SSP/SCP.
All above SS7 points are recognised by (unique) physical address called "Point code".
Let us take an example of call setup.
A makes a call to B. SSP1 from its routing table finds out that B is located on SSP2. So it sends a setup message over a signaling link to STP1, mentioning Point Code of SSP2 as destination and its own as origination. STP1 does not have direct path to SSP2, so it routes the message to STP2. STP2 finding point code of SSP2, sends the message to SSP2. SSP2 notes that the called number is of B and it is present with it. So SSP2 triggers ringing to B (If SSP2 do not have B, it will route the call further to next level of SSP (through STP network) and will wait for reply from it.). At the same time, SSP2 sends (sort of) acknowldgement message back, mentioning SSP1's point code. Now this message can go through the same path or different path than the message sent by SSP1. SSP1, upon receiving message from SSP2, sends ringing tone to A. At this or later time, one way or two way transmission link from SSP1 and SSP2 may be reserved. Once B answers or times out, STP network will be used to next set of signaling exchange (call answered or tear down respectively).
Now consider a call to 800 number. This being a special number, SSP1 may not have information of where it is to be routed. In this case, it needs to query SCP for the routing information (basically a routable number that can used to locate the destination SSP). SCP contain multiple number of databases. SSP1 may or may not know the point code of SCP and the database that is to be queried. If SSP1 knows this information, it will send query message to SCP (via STPs). If SSP1 do not know point code and/or database of SCP, it will still send query by mentioning the 800 number (and leaving database/point code information not specified). STP network will insert this information and will route the message to appropriate SCP. SCP will send back the routing information to SSP1. SSP1 will then go ahead with the call as usual.
We will see SS7 Layers in next article.
References: Q.700 standard from ITU-T, SS7 book by Russell, SS7 tutorial from Performance Technologies, Wikipage on SS7.
© Copyright Samir Amberkar 2010
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