Potential new case studies for behavioural types: Switching software systems

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Aubrey Chambers
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1 Potential new case studies for behavioural types: Switching software systems Faculty of Engineering University of Rijeka Croatia

2 Summary Introduction Switching software System requirements Related work on formalisms Concrete protocol examples Cases study: system description Call flows Problems: previous experiences Switching software perspective

What is switching software? Switching software in telecommunication network Picture taken from: http://commons.wikimedia.

3 What is switching software? Switching software in telecommunication network Picture taken from: le:wac_telephone_operators_operate _the_victory_switchboard_during_the _Potsdam_Conference_in_their_head quarters_in..._-_nara_-_ jpg

4 Switching software Telecommunication exchanges are developed more then 50 years Main concepts are space division and time division Numerous formal approaches were developed Still we face with number of failures

5 System requirements System has to provide: Parallel execution of multiple different requirements, for number of users e.g. Systems implementing MSC logic has to cope with more than milion requests in parallel, provide number of different standard protocol interactions high availability for its users If certain malfunction happen the peers has to be timely informed, and all related resources properly released, avoid congestion situations properly dimensioned aviod load response by the required time Real time system, a system with a real-time constraints Interoperable with other vendors equipment Inside logic has to provide external protocol compliance

6 Easy to maintain System requirements System has to provide: system structured into number of logical functions Well defined and separated logical functions Easy to trace system dynamics Easy transformed from objact code back to original code

7 Switching software systems: Programming languages examples PLEX: used in Ericsson AXE telephone exchange Concept of program segments: data encapsulated by set of procedures (modules) that could access data, other modules can not Software is living modular system and basic requirements are flexibility, manageability, ease of modification, ease of handling, etc. CHILL: Used in 1240 ITT telephone exchange Has to support more than telephone calls simultaneously, fully distributed control Concept of finite message machines Predefined set of input messages and each produces a finite set of replies No FMM has direct access to memory of another FMM all communication through messaging FMM may reside on the same or different machines message handler implements routing functionality among the machines for each messages Erlang Reliable system but in presence of errors

8 Switching software systems 1: Application of formal methods to the verification of communication protocols the most commonly used methods for ensuring the correctness of a system are simulation and testing exhaustive for any reasonably complex system is imposible Errors can sometimes occur only for specific execution sequences which are difficult if not impossible to reproduce or debug, making an exhaustive analysis necessary Knowledge that the design logic of a system is correct help us to avoid critical system faults the errors during implementation are less likely to be fundamentall logical errors that are very difficult to understand and solve

9 Switching software systems 2: ITU-T Standardisation, Z series Unified Model ling Languague UML diagrams, Message Sequence Chart MSC Specification and escription Language SL (ITU-T) for the description of communication protocols including concurrent and real-time aspects System is represented as set of blocks and processes, Processes interact asynchronously via signals that are placed into and consumed from queues. The communication structure is given by signal routes that connect individual system components. Used as automatic verification techniques like model checking high-level languages and formalisms particularly made for realtime and communication systems, CHILL

10 Concrete protocol examples Signalling network and protocols in celular telecomunication network

11 Network communication architecture Networks are defined and modeled at different abstraction levels Application Plane Control Plane ata /Resource Plane Network architecture Open Systems Interconnection Model

12 Case study Signalling network in mobile communication network We can consider as high priority data traffic network eveloped according 3GPP standardisation body regulations

13 Evolution of telecommunication network 3rd Generation Partnership Project Standards for cellular telecommunications network technologies

14 Signalling netowrk evolution Phase 1: Introduction of ATM transport and new BICC protocol BSS RANAP MTP3b SAAL ATM BSSAP MTP3 MTP2 ATM MSC Server MGW ATM BICC ISUP MTP3 MTP2 GMSC Server MGW ISUP MTP3 MTP2 ISN, PSTN, PLMN UTRAN

15 Signalling netowrk evolution Phase 2: Sppliting of network architecture- new GCP protocol BSS RANAP MTP3b SAAL ATM BSSAP MTP3 MTP2 MSC Server Poslužitelj ATM BICC MTP3b SAAL ATM GCP MTP3b SAAL ATM ATM GMSC Poslužitelj Server ISUP MTP3 MTP2 ISN, PSTN, PLMN UTRAN MGW MGW

16 BSSAP MTP3 MTP2 BSS UTRAN Signalling netowrk evolution Phase 3: Migration to IP network RANAP BSSAP M3UA SCTP IP LL SGW ATM RANAP MTP3b SAAL ATM MSC Server Poslužitelj MGW BICC ISUP M3UA MTP3 SCTP MTP2 IP LL GCP M3UA SCTP IP LL IP GMSC Poslužitelj Server MGW ISUP MTP3 MTP2 ISUP LL M3UA SCTP IP SGW ISN, PSTN, PLMN

17 Protocol example Interaction between horizontal and vertical protocol RANAP/ATM MSC Poslužitelj ISUP/ GCP/ATM ATM MGW

18 System AXE based MSC node PLEX signals several million lines of code ivided into more then 1000 modules More then 1 million of simultaneous requests Priority levels Resources Real time with time constraints

19 Call set up Server node Internal protocol 1.BICC_IAM 9. BICC_APM 13. BICC_APM BICC 2.Internal msg 3.Internal msg 6.Internal msg 20. Internal msg Transit 2.Internal msg 3.Internal msg 6.Internal msg 20. Internal msg BICC 12. BICC_IAM 15. BICC_APM 21. BICC_COT 22.. BICC_APM 25. BICC_ACM 24.Internal msg 24.Internal msg 23. BICC_ACM 28. BICC_ANM 27.Internal msg 27.Internal msg 26. BICC_ANM Eksternal protokol Internal interface Resource API Eksternal potokol 4. A Re qu est 5. A Re ply 8. N O T I F Y 10. N O TI FY Re ply 14. M O I F Y 17. M O I F Y Re ply 18. N O T I F Y 19. N O T I F Y Re ply Eksternal protokol 7. A Re qu est 11. A Re ply 16. M O I F Y 22. M O I F Y Re ply 23. N O T I F Y 24. N O T I F Y Re ply Gateway node

20 Call flows Example: Session Initiation Protocol _ip_comm/sip/proxies/2-2/administration/guide/ver2_2/eflows.pdf

21 Problems (1) Several levels of abstractions How to define right structure - and secure type safety between internal and extenal intefaces The most critical issues in the maintainance from the aspect How hard is to locate the fault Interactions between system functions and network functions

22 Problems (2) Protocol interactions E.g. Node recovery, congestion control, etc. System internal optimisations aiming to increase the system performances (e.g. ual seizure shortcut in internal communication ) Partially executed system procedures may affect inconsistencies Order of release procedure is important (Control process) System configuration change that may take longer time because of lower priority Shared resources and correct release procedures, relinking of blocks, traffic interference Message multiplication, congestion and time constraints Timers in the system and their interaction (are some timers long enough) Priorities of error cases

23 Switching software perspective Connectivity software, implemented in software switch include call agents, call servers and media gateway controllers Becomes to be increasingly important Internet of Things anything can communicate with anything and anybody M2M interaction Software efined Network software can reconfigure itself at runtime Open Source community - Everybody could contribute to distributed application development Consequences of failure are higher then ever Problems of dynamic restructuring of system design becomes the most important mechanism

3GPP TS V4.3.0 ( )

3GPP TS V4.3.0 ( ) TS 29.205 V4.3.0 (2006-06) Technical Specification 3rd Generation Partnership Project; Technical Specification Group Core Network; Application of Q.1900 series to bearer-independent Circuit Switched (CS)