Message Sequence Charts Clive Jervis Rapporteur Z.120

Message Sequence Charts ITU-T Standard Z.120 Topics: Overview Uses Language Future Summary Current Version: MSC 2000 Previous Version: MSC ’96 Next Version: MSC 2004 Standard consists of: graphical grammar ASCII grammar static & dynamic semantics formal semantics, Annex B examples Z.120 UK USA RMTR air_in taxi_in taxi_out air_out MSC is: used widely in industry has good tool support strong ties to other ITU languages ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019

ITU-T - Bangalore Workshop, 30-31st August 2001 Overview msc call_set_up mobile_1 base_1 network base_2 mobile_2 up_call_req up_call_req down_req down_req call_ack call_ack up_call_resp up_call_resp down_resp down_resp Intuitive, widespread informal use Focuses on external interactions rather than internal behaviour Only notation that shows complete system interactions Good for partial specifications - i.e. scenarios Can be used independently of other languages Universal data language interface ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019

ITU-T - Bangalore Workshop, 30-31st August 2001 MSCs in the Lifecycle UK USA RMTR air_in taxi_in taxi_out air_out MSCs Used Formally Throughout Lifecycle ITU, ETSI Standards System/Integration Testing UK USA RMTR air_in taxi_in taxi_out air_out UK USA RMTR air_in taxi_in taxi_out air_out Test Generation System Requirements TTCN Automation Requires Formal Languages UK USA RMTR air_in taxi_in taxi_out air_out UK USA RMTR air_in taxi_in taxi_out air_out Test Generation Box Testing Box Requirements TTCN Code Generation ASN.1 Everywhere! Code Generation SDL Design Code ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019

ITU-T - Bangalore Workshop, 30-31st August 2001 MSC Uses Formality Enables Useful Tool Support Verification & Validation: feature interactions race conditions tracing Tracing: model validation application code validation test validation Design Verification: model checking SDL upholds MSCs Test Generation: conformance testing test purposes (one-2- many) test specification (one-2-one) UK USA RMTR air_in taxi_in taxi_out air_out Requirements V&V UK USA RMTR air_in taxi_in taxi_out air_out SDL Tracing UK USA RMTR air_in taxi_in taxi_out air_out SDL Verification UK USA RMTR air_in taxi_in taxi_out air_out TTCN Generation ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019

ITU-T - Bangalore Workshop, 30-31st August 2001 Basic Features msc call_expiration base network manager Time Progresses Downwards initiate handler wait(5) ready ‘handle call’ call_set_up No linearity No synchronicity Interleaving expired terminate instance message timer co-region create reference local action stop msc call_set_up network ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019

ITU-T - Bangalore Workshop, 30-31st August 2001 Semantics Single MSC defines a set of event traces Events interleaved Timing constraints can reduce possible traces msc call_expiration A message consists of: a send event a receive event Semantics prescribes no buffering mechanism mobile base network identify initialise send initialise receive initialise Three traces! receive identify send identify receive identify receive initialise send initialise receive initialise receive identify ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019

ITU-T - Bangalore Workshop, 30-31st August 2001 High-Level MSCs Structuring MSCs msc call_blocking initiate_call call_refused call_set_up terminate_call call_proceeding References can be to basic MSCs or high-level MSCs start point end point reference branching looping parallel (not shown) guards (not shown) Single telecom. feature may have >100 basic MSCs structured through 3 levels of high-level MSCs ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019

ITU-T - Bangalore Workshop, 30-31st August 2001 In-Line Expressions Structuring event behaviour within an MSC msc MyLife mobile_1 mobile_2 infra req_refused alt call_req t(5) t when idle otherwise call_ack call_accept In-line expressions: alternative parallel optional loop exceptional Operands: non-deterministic choice may be guarded events interleaved with those outside expression ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019

Time Constraints/Data Real-Time Constraints • Message Contents Data Dependent Behaviour • Parameterisation Time Constraints: absolute (tracing) relative (specification) single point, intervals constrain regions base manager network set_up(time_out + 3) terminate expired (“z1”, _, x) initiate ready msc call_expiration(time_out: time) [5, 10] @10:00 x := f(_, 5) Data: static variables - parameterises MSC - global to MSC - also instances, etc. dynamic variables - local to instance - assigned in actions - declared MSC Document underspecification - “don’t care” values ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019

Instance Decomposition Splitting an Instance Into Constituent Processes msc call network decomposed as network_call base manager initiate ready set_up Hierarchical view of processes Instance structure defined in enclosing MSC Document Internal messages hidden in upper view msc network_call base_handler manager_handler initiate ready initiate_set_up set_up ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019

ITU-T - Bangalore Workshop, 30-31st August 2001 Other Features MSC: a Comprehensive Specification Language mscdocument network_manager inst base_handler(x : integer); inst call_controller; msg initiate(integer, integer, bool); call_set_up call_proceeding terminate_call MSC Document module encapsulation all declarations definition of data interface inheritance/decomposition Control Flow alignment with UML method calls suspension regions Gates connecting messages across boundaries msc call mobile base_1 network base_2 call initiate set_up call_set_up g h done ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019

ITU-T - Bangalore Workshop, 30-31st August 2001 Future Features Z.120 MSC 2004 Message Channels - superseding gates - organising messages through channels - buffering semantics Presentation Formats - TTCN Graphical presentation format, Z.142 - semantic restrictions, e.g. tracing Formal Semantics, Z.120 Annex B - Currently compliant only with MSC ’96 Instantiations of Universal Data Interface - SDL interface scheduled 2002, Z.121 - ASN.1 interface? - TTCN-3 interface? UML alignment - OMG UML v2.0 sequence diagrams Plenty to do Please Contribute! ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019

ITU-T - Bangalore Workshop, 30-31st August 2001 Summary MSC is a rich language suitable for requirements specification many uses across lifecycle ‘engineer friendly’ language used in telecom standards Good tool support getting more sophisticated industrial use integrated with SDL, TTCN tools Standardisation Continues interesting problems to work on good forum for insight, new ideas learn latest methods/technology Topics: Overview Uses Language Future Summary UK USA RMTR air_in taxi_in taxi_out air_out Improved productivity comes through automating manual processes Automation requires formal languages ITU-T - Bangalore Workshop, 30-31st August 2001 05.02.2019