Before we go any further, let me state that I am a former, not a current employee of Telelogic.
Ever heard of Swedish software? Neither did I, until I joined Telelogic. The Swedes make 1st rate software, but have 3rd rate marketing. It's something of the total opposite of Microsoft. More than anything, I believe it's Telelogic's bad marketing that's been holding back the acceptance of SDL. They've got a gem of tool, but they're squandering their opportunities.
Europeans are typically a bit more subdued in their marketing compared to their American counterparts, but Swedes are even more subdued. I remember the Swedes told me that the product will sell itself. Uh huh. And I suppose that Java would have reached the same level of notoriety without the enormous marketing effort of Sun MicroSystems.
Currently Telelogic has the best selection of tools centered around SDL. They've got an SDL editor, SDL compiler, SDL debugger, SDL simulator, and MSC editor. The software is also reasonably stable. You can work for days on end without seeing a crash. You can't get any better than Telelogic.
Be aware that Telelogic has chronic problems with misnaming everything. What is known as a debugger, Telelogic calls a simulator. What should be known as a simulator, Telelogic calls a validator.
Telelogic also has a UML tool. This can work as a high level modeling language on top of SDL, and Telelogic has a UML-SDL integration. However, I must confess I've not been a great fan of UML in the event driven world. For protocol systems, it seems enough that one can specify block interfaces, and that can easily be done in SDL. Also UML is more centered around objects, whereas many protocol designs are centered around RTOS tasks.
(Testing and Test Control Notation)
Alongside SDL, I think this is Telelogic's other gem. TTCN is a language designed for testing. Forget about writing test scripts in a proprietary language or a language that wasn't designed for testing. Come and see what you've been missing out on.
The biggest advantage of TTCN is conformance test suites. TTCN is a standardized language for which standards committees have written conformance test suites. When a product conforms to a conformance suite, that product should be compatible with all other products that conform to that test suite. This is a boon for all those telecom products. In fact, the 3rd generation mobile phone protocols use TTCN as the standard for the conformance test suite.