Message Sequence Charts

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Message Sequence Charts

• Clive Jervis
• Rapporteur Z.120

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Message Sequence Charts ITU-T Standard Z.120

• 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

• Topics
• Overview
• Uses
• Language
• Future
• Summary

• MSC is
• used widely in industry
• has good tool support
• strong ties to other ITU languages

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Overview
msc call_set_up
mobile_2
base_1
mobile_1
network
base_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

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MSCs in the Lifecycle
MSCs Used Formally Throughout Lifecycle
System/Integration Testing
ITU, ETSI Standards
Test Generation
System Requirements
TTCN
Automation Requires Formal Languages
Test Generation
Box Testing
Box Requirements
TTCN
Code Generation
ASN.1 Everywhere!
Code Generation
SDL Design
Code
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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)

Requirements VV
SDL Tracing
SDL Verification
TTCN Generation
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Basic Features
msc call_expiration
base
manager
network
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
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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
network
base
identify
initialise
receive initialise
send initialise
receive identify
Three traces!
send identify
receive identify
receive initialise
send initialise
receive initialise
receive identify
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High-Level MSCs
Structuring MSCs

• 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 gt100 basic
MSCs structured through 3 levels of high-level
MSCs
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In-Line Expressions
Structuring event behaviour within an MSC

• In-line expressions
• alternative
• parallel
• optional
• loop
• exceptional
• Operands
• non-deterministic choice
• may be guarded
• events interleaved with those outside
  expression

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Time Constraints/Data

• Real-Time Constraints Message Contents
• Data Dependent Behaviour Parameterisation

• Time Constraints
• absolute (tracing)
• relative (specification)
• single point, intervals
• constrain regions

• Data
• static variables
• - parameterises MSC
• - global to MSC
• - also instances, etc.
• dynamic variables
• - local to instance
• - assigned in actions
• - declared MSC Document
• underspecification
• - dont care values

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Instance Decomposition
Splitting an Instance Into Constituent Processes
msc call
network decomposed as network_call
base
manager
initiate
ready
set_up
msc network_call

• Hierarchical view of processes
• Instance structure defined in enclosing MSC
  Document
• Internal messages hidden in upper view

manager_handler
base_handler
initiate
ready
initiate_set_up
set_up
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Other Features
MSC a Comprehensive Specification Language

• MSC Document
• module encapsulation
• all declarations
• definition of data interface
• inheritance/decomposition

msc call

• Control Flow
• alignment with UML
• method calls
• suspension regions
• Gates
• connecting messages across boundaries

mobile
base_2
base_1
network
call initiate
set_up
call_set_up
g
h
done
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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!
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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

• Improved productivity comes through
• automating manual processes
• Automation requires formal languages