Introduction to SDL

Introduction to SDL

Bibliography:

[Doldi2001] L. Doldi, (2001), SDL Illustrated.[Edwards2001] S. Edwards, (2001), SDL, http://www.cs.columbia.edu/~sedwards/classes/2001/w4995-02/presentations/sdl.ppt.[ABS2002] A. Alkhodre, J.-P. Babau, and J.-J. Schwarz, "Modelling of real-time constraints using SDL for embedded systems design," Computing & Control Engineering Journal, vol. 13, pp. 189-196, 2002.

Dept. of Software and IT Engineering

Introduction to SDL 2

SDL: Specification and Description Language

• ITU-T (International Telecommunication Union) Recommendation Z.100

• Created for the specification of communication protocols

• Two forms: Textual (SDL/PR) et Graphic (SDL/GR)



SDL

• Two aspects:1. Structure of a specification in SDL2. Behavior of a specification in SDL



Structure: Basic components

• Three basic components in SDL System Block Process



System: Highest level



Block: middle level(s)



Process: lowest level



Communication in SDL

• Signal Models an event that

is communicated between processes, blocks, or systems

• Channel Models the means of

transferring a signal between two blocks, or a block and the external environment

• Signal route Models the means of

transferring a signal between two processes



SDL Signals

• General format: Signal_name(parameters)

• Simplest format start

• Format with values A(true) (message with a Boolean argument) dial(number) get_request(file, username)

• Parameters in SDL can be rather complex if necessary (supports ASN.1)



Signal address

• Source address (process that sent it) Useful when there are more than one

instance of the same process• e.g., in the case of a server that handles

many requests from different clients, each request can be run in a separate concurrent process, which are different instances of the same process



SDL Channels

• Signal channel, between blocks communication delays are uncertain

• Signal route, in the same block (computer), between processes Almost instantaneous



SDL Channels (2)

• A signal can travel across several channels on its way from the source to destination



SDL: Behavior

• A process in SDL is modeled as a finite-state machine (actually extended, communicating FSM). System behavior

• A process can be created or terminated dynamcially



Specification in SDL

• An SDL specification is a group of communicating modules

• An SDL specification is a group of ECFSM (extended communication finite-state machines)



Finite State Machines

• A finite state machine has: States Inputs (events) Transition function Outputs (events)

• Examples…



FSM: Example



(Some) properties of FSM

• Deterministic• Non-deterministic• Complete• Partial• Connected• …



Extended Finite State Machine

• It’s an FSM that has state variables• For complex problems, the state-space

can explode Can be reduced by using state variables

• 3 additional features Input/Output events can have parameters Use of predicates for firing transitions Variables can be modified during a

transition

• Examples…



Example of EFSM



Extended Communicating Finite State Machine

(ECFSM)• It’s an EFSM that can communicate

with other EFSM• Each EFSM has it’s own (input)

queue• If M1 sends a message to M2, the

message is put in M2’s queue



ECFSM: Example- Means a message is sent

+ means a message is received

1

2

-B+A-C

1

2

+B -A

M2M1



SDL Process

• A process in SDL is an ECFSM• Execution rule: take the next signal

(message) from the queue and... Update variables Process branch decisions Send a new signal … Determine the next state End



SDL Process (2)

• Text and graphic forms

state Wait; input Money(a); task x := a; nextstate Pay; input Cancel; nextstate Wait;endstate;

Wait

Money

Pay

Cancel

Waitx := a



SDL Process (3)

• Transition rules At any given state, remove a signal from the

input queue If a transition is defined for this signal, then

• Fire the transition Output signal(s) Modify internal variables, etc.

Else (no transition defined for this signal)• Ignore the signal and stay in the same state



State symbol

• Indicates either the current state or the next state, depending on its use

inactive

ready



Start State

• Indicates the start of a process• Nameless

ready

...



Receive symbol

• Follows a State symbol• Indicates the Signal activating a

transition [Edwards2001]

Idle

Coin Choice Clear

... ... ...



Receiving a signal

• Set of valid signals Contains all signals a process can receive Any signal not in this set will produce an

error

• At a given state, only certain signals can activate a transition If a (valid) signal is received but it has no

defined transition, the signal is destroyed without causing a transition

This is called an implicit transition



Save symbol

• Allows a signal to be accepted and stored again into the input queue (without losing it) Useful when signals can get re-ordered

[Edwards2001]Idle

Coin Clear Choice

A “Choice” signal that arrives in this state will be deferred to the next



Output symbol

• Sends a signal to another process Explicit forms « VIA channel » et

« TO process »

• The channel by which the signal is sent is determined by the signal’s type

A(s)



Explicit output

• VIA channel or TO process e.g., a process that

needs to resend the same signal it has received

To avoid receiving again the same signal, a channel can be specified

There are other cases…

“VIA” the channel named MB



Local variables

• A process has local variables

• Shared variables Modified only by the

process itself Read by other

processes (in certain cases)

• Declared textually



Important!

There are no global variables

in SDL!!!



Variable types

• integer, Boolean, real, character, string Operators +, -, *, etc.

• Arrays, enumerations et sets



Task symbol

• Assignment statements, expressions

• Informal text Incomplete

specification, to be detailed later

Useful during simulation



Decision symbol

• Branch according to a condition Expression form

a=e

Informal‘message’



Process Creation symbol

• Creates (instantiates) a new process after a transition Channels aren’t

duplicated Make sure that a

message is sent to the right instance

• Specification of the number of instances

CallHandler

CallHandler(0,63)

[Edwards2001]



Process creation

• Convention: independent handling of a request Like a daemon in Unix (lpd, httpd, etc.) e.g., create a new server process to

handle a file-transfer request• The process “dies” when the file has been sent

• Set the maximum number of process instances because resources are limited



Process creation

• Process is running at all times (one and only one instance)

• Process starts out as inactive. No more than one instance.

• No processes running at first, but up to 64 instances.

CallHandler(1,1)

CallHandler(0,1)

CallHandler(0,64)



Terminating a process

• Only a process can terminate

‘last task’



Timers

att_ack

TB(r)

reset(T)

Timer T;

A(s)

... ...

set (now+10, T)

A(s)

Once a timer has expired, the process receives a message.

A timer is declare like a variable

Duration of timer is specified here

Timer is deactivated



SDL: Inside information…

• SDL has two “close cousins” ASN.1 MSC

• They are always together on projects that are serious.


Introduction to SDL

Examples

43

Documenting Software Architectures: Views and Beyond

by Paul Clements; Felix Bachmann; Len Bass; David Garlan; James Ivers; Reed Little; Robert Nord; Judith StaffordPublisher: Addison-Wesley Professional

Pub Date: September 26, 2002


Introduction to SDL

Examples

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Introduction to SDL

Example: (MSC)

45





Introduction to SDL

Exercise

• http://www.comnets.uni-bremen.de/typo3site/uploads/media/Exercise6.pdf

46

Introduction to SDL

Documents

Transcript of Introduction to SDL