Sunday, December 26, 2010

The simplest SS7 network

As I wrote in the first post, I am currently working at the ITU-T Signalling Sistem no. 7 (SS7) protocol suite. 
I've already implemented the MTP2 and MTP3 (the latter is fairly complex indeed!) so I can use them to open this blog with an example of a simple SS7 network. The simplest one.

As stated in ITU-T Q.705 recommendation:

"The most elementary signalling network consists of originating and destination signalling points connected by a single signalling link."

... so here it is: 

The example network is made up of the two signalling points A and B, addressed with point code 101 and 202 respectively, and directly connected with one linkset composed of a single link, which I assigned the signalling link code 5.

Here is a capture file showing what happen when the network is put into operation: 
  • first, there is the link alignment at the MTP2 level (SIOS,SIO,SIN messages); 
  • then, the signalling link test at the MTP3 level (SLTM and SLTA messages);
  • as the link is available (reception of the SLTA) there is the SP restart phase, which I shortened to 10 seconds (it should last around 1 minute, really) ended with the delivery of the TRA("traffic restart allowed") message;
  • and at last, the exchange of some user data.

The capture can be viewed using Wireshark and configuring it to decode the DLT=148 with the MTP2 dissector (to do so, select Edit->Preferences->Protocols->DLT_USER->Edit Encapsulations Table, and add an entry for DLT=148 with Payload Protocol=MTP2).

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