An introduction to UML 2 for modelling communications

1
An introduction to UML 2 for modelling
communications

• Assumptions some familiarity with well-known
  UML concepts such as class diagrams.
• Scope This is not a full UML 2 tutorial.

2
Classes Active and Passive

• In UML 1, classes had
• Attributes instance variables, etc.
• Operations methods that can be called.
• In UML 2, there can be
• Passive classes as above
• Active classes classes with defined behaviour,
  and that can react to arriving signals.
• Denoted with double vertical borders.

ClassName
ActiveClass
Attributes
Attributes
Operations
Operations
A passive class
An active class
3
Active Classes

• Active classes can have
• Ports defined ports through which signals can
  be sent or received.
• Composite structure
• Sub-components
• Communications paths among sub-components

4
Signals and Channels

• Signals can be sent between active classes.
• A signal is defined by a class with the
  stereotype ltltsignalgtgt.
• Signals can carry parameters.
• Sending and receiving a signal is somewhat
  analogous to calling a method in another object.
• The path that a signal takes from sender to
  receiver is specified by channels between
  objects.

5
Interfaces and Signals

• The concept of interface was already in UML 1,
  where it specifies a set of operations that must
  be implemented by one or more classes.
• In diagrams, an interface appears as a class
  symbol with the stereotype ltltinterfacegtgt.
• Because UML 2 considers sending a signal to be
  similar to a method call, signals are allowed to
  be specified in interfaces as well.
• Using an interface is the preferred way to define
  a group of signals.
• Regular methods can be included as well.

6
Interfaces and Signals
7
Ports

• A port is attached to an active class.
• The port has
• A name.
• An interface specifying the signals that can be
  received.
• An interface specifying the signals that can be
  sent.
• Two types of ports
• Connected to internal communication channels (by
  default).
• Connected to the state machine for the class
  instance (a behaviour port).

In interface
Out interface
A behaviour port
8
Composite Structure

• A composite structure diagram shows the
  relationship among internal components of a
  class, in terms of communication paths.
• The class may have one or more communications
  ports through which signals can be sent or
  received.
• The ports are connected either to
• Internal components
• Channels connect the ports of the class to the
  ports of the internal components.
• Channels can be unidirectional (one direction
  only) or bidirectional (both directions).
• The state machine behaviour of the class (a
  behaviour port).

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Object instance references
instance name
class name
10
Composite Structure
11
Behaviour

• In a passive class, the behaviour is specified by
  the method implementations.
• In an active class, the behaviour is specified by
  an extended finite state machine.
• The finite state machine is initialized by the
  objects constructor, and the state machine will
  then react to signals that are received by the
  object instance.

12
Extended Finite State Machines

• An extended finite machine consists of
• States
• Events
• Transitions
• Variables
• The general operations is that the EFSM is in a
  current state. When an event occurs, the
  associated transition for the current state and
  event is executed. The EFSM then moves into
  another state.
• Variables may be set or used by events and
  transitions.

13
States

• In a state, no activity is occuring. The EFSM is
  waiting for an event.
• While in a state, other EFSMs have a chance to
  execute.
• State symbol in diagrams
• Special states
• The initial state
• The final state
• The history state

Idle
H
14
Special states

• The initial state is the state in which the EFSM
  is in when the state machine initializes.
• Once the EFSM starts, it cannot return to the
  initial state.
• Entering the final state terminates the state
  machine execution.
• The history state is used to indicate that after
  executing a transition, the EFSM should remain in
  the same state it was in when the transition
  started.

15
Multiple and Generic States

• In a state symbol at the head of a transition,
  more than one state name can be specified.
• Use a list of names, separated by commas.
• In any of the states in the list, the transition
  can be taken for the specified event.
• The state name is used to refer to all
  states, if there is a transition that can be
  performed in any state.
• Specifying (list of state names) means all
  states except those in the list.
• The history state is often useful in these
  situations to return to the same state.

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Events

• In order to exit a state, an event must occur.
• There are three types of events
• A signal arrives (an input signal).
• No signal has arrived, but some boolean condition
  on the EFSM variables is true (a guard condition)
• A signal has arrived AND a guard condition is
  true.
• The transition is not taken if only one of the
  above is true.
• Special case no event is needed to exit the
  initial state.

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Events

• Input signal
• Receiving this signal will set the variable var
  to the value of the data parameter carried with
  the signal.
• The type of var must match the type of the
  signal parameter.
• Guard Condition
• The value of var must be gt 6 for the transition
  to be taken.
• Input signals are checked before guard
  conditions.
• If two guard conditions become true for the same
  state, it is not defined as to which of the
  transitions will be taken.
• Combined
• The guard condition is specified as text within
  the input symbol.

18
Transitions

• A transition is a sequence of tasks to perform
  whenever an event occurs in a specified state.
• Tasks can include
• Actions doing computations, calling passive
  methods, setting or cancelling timers.
• Decisions altering control flow based on
  conditions.
• Sending output signals.
• A transition always terminates with a state
  symbol.
• The EFSM will then move into this state and wait
  for a suitable event to resume execution (unless
  the final state is reached).

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Transition symbols
insert code here

• Action
• Decision
• Send output signal
• Flow connectors
• To connect flow between diagram pages, or
  different locations on the same diagram.

condition
aSignal( var )
label
label
20
Complete transition example
21
UML code in behaviour descriptions

• Variables must be declared in a text box.
• UML code style is closest in style to C syntax.
• For basic code, this is also fairly similar to
  Java.
• Statements in actions must be terminated by a
  semi-colon
• Multiple statements can occur in one action box.

Integer a Boolean b String c Character d
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Timers

• Timers are uses frequently in communications
  software.
• Life cycle
• A timer is started. It will expire after a
  specified interval, or at a certain time.
• While running, a timer can be cancelled.
• At the expiry time, a timer event occurs.
• Appears as an input signal.
• Multiple timers can be used concurrently.
• Good practice be sure to cancel timers when
  they are no longer needed.
• Otherwise, the timer expiry event will still
  occur, and possibly cause the system to react
  incorrectly.

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Timer Operations (1)

• Set
• Start a timer.
• Appears as a statement in an action box on a
  transition.
• Example set ( timerName, now 5 )
• Reset
• Cancel a timer (whether or not it was running).
• Appears as a statement in an action box on a
  transition.
• Example reset( timerName )

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Timer Operations (2)

• Expiry
• Results in the arrival of a signal with the timer
  name, and is an event.
• Use the input signal symbol.
• Active
• Returns true if a specified timer is active, and
  false otherwise.
• Appears as an expression within a statement in an
  action box on a transition.
• Example booleanVar active( timerName )

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Timer Setup

• Timers must be declared (like variables).
• Example Timer timerName
• Timers can be set to expire
• After an specified duration (relative).
• At a certain time (absolute, this is the default)
• The now pre-defined variable is used to indicate
  the current time, and now x indicates a
  relative duration of x (default units are
  seconds)
• Timers can have parameters
• Allows for multiple instances of timers with the
  same name, running concurrently.
• The parameter value identifies a specific timer
  instance.

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Data in UML models

• Pre-defined types
• Integer
• Boolean
• String
• Array (acts like a Java Map)
• Real (floating point values)
• Additional types for real-time systems
• Bit
• BitString
• Octet
• OctetString
• Time (an absolute time)
• Duration (a time interval)

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Pre-defined types

• Typical operators are available
• Arithmetic , -, , /, , --,
• Logical , , ! also not, and, or, xor
• Comparison , !, gt, lt, lt, gt
• String concatenation
• Predefined Character constants for the ASCII
  non-visible communications characters
• NUL, SOH, STX, ETX, EOT, ENQ, ACK, BEL, BS, HT,
  LF, VT, FF, CR, SO, SI, DLE, DC1, DC2, DC3, DC4,
  NAK, SYN, ETB, CAN, EM, SUB, ESC, IS4, IS3, IS2,
  IS1, DEL

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Real-Time Types

• Time, Duration
• Usual arithmetic and comparison operators
  available
• Bit represents a bit
• Values 0 or 1
• Operators not, and, or, xor, , !
• Conversion mkstring( aBit ) convert a Bit to
  a BitString of length 1.

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Real-Time Types

• Octet represents an 8 bit numeric value, in
  hexadecimal
• Values 00 to ff (not directly assignable ?)
• Operators , !, , -, , /, gt, lt, lt, gt,
  not, and, or, xor, mod
• Shifts
• shiftl( anOctet, numBits ) bit shift left by
  numBits bits
• shiftr( anOctet, numBits ) bit shift right by
  numBits bits
• Conversions
• I2O( anInteger ) convert Integer to Octet
• O2I( anOctet ) convert Octet to Integer
• String2Octet( aString ) convert String (see
  values, above) to Octet.

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Real-Time Types

• BitString represents an arbitrary-length string
  of Bit values
• Operators , !, not, and, or, xor,
  (index) (concatenation)
• Functions
• length( aBitString ) returns the length of the
  BitString
• last( aBitString ) returns the last Bit in the
  BitString
• first( aBitString ) returns the first Bit
  equivalent to aBitString0
• substring( aBitString, firstIndex, numBits )
  the substring from positions firstIndex to
  firstIndexnumBits-1, inclusive.
• Conversions
• String2BitString( aString ) convert String
  representing a binary value to a BitString.
  String characters must be 0 or 1, representing
  bits. Example String2BitString( 01110 )
• Hextring2BitString( aString ) convert String
  representing a hexadecimal value to a BitString.
  String characters must be 0 to 9, or a to f,
  representing four bit values. Example
  String2BitString( 03a9c2f )

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Real-Time Types

• OctetString represents an arbitrary-length
  string of Octet values
• Operators , !, not, and, or, xor,
  (index) (concatenation)
• Functions
• length( anOctetString ) returns the length of
  the OctetString
• last(anOctetString ) returns the last Bit in
  the BitString
• first(anOctetString ) returns the first Bit
  equivalent to aBitString0
• substring(anOctetString , firstIndex, numBits )
  the substring from positions firstIndex to
  firstIndexnumBits-1, inclusive.
• Conversions
• String2BitString( aString ) convert String
  representing a binary value to a BitString.
  String characters must be 0 or 1, representing
  bits. Example String2BitString( 01110 )
• Hextring2BitString( aString ) convert String
  representing a hexadecimal value to a BitString.
  String characters must be 0 to 9, or a to f,
  representing four bit values. Example
  String2BitString( 03a9c2f )