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Sorin Manolache — UML Modelling Case Study: A GSM BTS
UML Modelling Case Study: A GSM BTS
Sorin Manolache, [email protected]
Linköping University, IDA/ESLAB
1
September 9, 2005
Motivation
• Investigate the versatility of UML as an implementation
independent specification language for complex hardware/
software real-time systems
• Possibly perform architecture exploration on the model
Therefore...
September 9, 2005
2
Specification Requirements
• executable specification
• (explicit) specification of time constraints and execution time
• specification of concurrency
• modelling of resources (hw: processors, comm. links, memory,
etc.; sw: shared data, message queues, locks, etc.)
• specifying the binding
• scheduling policies and scheduling units
• etc. [UML Profile for Scheduling, Performance and Time —
http://www.omg.org/cgi-bin/doc?ad/99-03-13]
3
September 9, 2005
Application Characteristics
Application implies:
• data oriented processing
• control oriented processing
• protocol processing
• analog part
• concurrency
September 9, 2005
4
The GSM BTS (1)
Base
Station
Controller
(BSC)
Radio
interface
Abis
interface
Network
Switching
Subsystem
(NSS)
Images with courtesy of Mike Pratt
http://www.prattfamily.demon.co.uk/mikep/
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September 9, 2005
The GSM BTS (2)
BTS
Measurements Processing Subassembly
Radio
Subassembly
Baseband Subassembly
Abis
Interfacing Subassembly
Basestation Control Functions
Radio
interface
September 9, 2005
Abis
interface
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Model Architecture
LAPDm PI
Radio
BuG
...
TRX
BC
BC
FU
FU
Radio
interface
LAPD PI
FU
Abis
BlG
TRX
...
BC
...
FU
PI – protocol interpreter
FU – functional unit
BC – baseband controller BuG – burst generator
TRX – transmitter-receiver BlG – block generator
Abis
interface
Protocol processing
Control oriented
Data oriented
Specified at a high level of abstraction
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September 9, 2005
Main Packages of the Model
• FunctionalUnits
• BasebandCtrl
• TRX
• PIs
• Abis
• Radio
• Globals
• Shell
September 9, 2005
8
The FunctionalUnits Package
• operates on the data path
• the DSP algorithms are specified in C++
• no control functions (very primitive statechart)
• simple interface (in command, out notification) ⇒ event-driven
• instrumented for timed simulation
• examples of units:
– CRC encoders, CRC decoders
– burst interleavers, burst deinterleavers
– convolutional encoders, convolutional decoders
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September 9, 2005
FunctionalUnits Class Diagram
ProcessingUnit
int procDelay
Interleaver
SpeechInterleaver
September 9, 2005
DataInterleaver
ControlInterleaver
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FunctionalUnits Behaviour
evCmd(burst)
active
idle
tm(procDelay) /
itsController->GEN(evCompl(outBuf));
for (k = 0; k < NORMAL_BLOCK; k++) {
val = block->getBitAt(k);
b = ((4 * currBlock + (k % 8)) % 8);
j = 2 * ((49 * k) % 57) + (k % 8) / 4;
outBuf[b]->setBitAt(j, val);
}
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September 9, 2005
The BasebandCtrl Package
• control oriented
• instrumented for timed simulation (execution deadlines of
managed functional units)
• example of baseband controllers:
– SpeechDownLkCtrl
– DataUpLkCtrl
– CtrlDownLkCtrl
September 9, 2005
12
BasebandCtrl Class Diagram
ConvEnc 0,1 DownLk
ConvEnc 0,1 DownLk
0,1 CRCEnc
0,1
0,1 CRCEnc
Interl
0,1
Interl
Data
ConvEnc
Data
DownLk
Data
Interl
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September 9, 2005
BasebandCtrl Statechart
active
deinterl
evBursts/deI->GEN(evDeICmd(params->bursts));
idle
evDeICompl/report(result);
convDec
evConvCompl
[else] errConv
[params->status==0]
[params->status==0]
watchdog tm(deadline)
September 9, 2005
CRCDec
evCRCDecCompl
[else]
errCRC
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The TRX Package
• control oriented
• controls signalling, uplink and downlink transmission
• allocated and deallocated channels
• “database” (keeps track of allocated channels and their types)
• time-triggered
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September 9, 2005
The TRX Statechart
main
uplink
signalling
access
[TSN()==3]
inactive
idle
downlink
dispatch
[else]
[isAlloc()]
idle
evBurstTick [else]
evAlloc/
alloc(params);
September 9, 2005
idle
evAbisBlock/
extract(params);
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The PIs Package
• protocol processing on the datalink layer
• aspects:
– sliding window and transmit–wait–acknowledge
concepts
– segmenting and reassembling ⇒ packets may arrive
out of order
– channel loss
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September 9, 2005
PIs Class Diagram
Link
Channel
Sender
Receiver
LinkLayer
LAPDmSnd
LAPDSnd
LAPDmLk
September 9, 2005
LAPDmRec
LAPDRec
LAPDLk
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Sender Statechart
toTrans
[ready]
idle
[canTrans]
dummy
evStart
trans
[else]
idle
evAck/
reset()
evSendMsg
[else]
tm(trans)
tm(t)/error()
[timerRuns]/
rst()
[else]
evReset
evError
[windowFull]
[windowFull]
running
cannotTrans
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September 9, 2005
Layer 2 Message Transmission
Sender
evSendMsg(msg)
L2Channel
L2ChHelper
Receiver
segment()
tm(trans)
sendMsg(pkg)
evConvey(pkg)
tm(line)
evGetPkg(pkg)
buffer(pkg)
enqueue(ack)
sendAck(ack)
tm(trans)
evConvey(ack)
tm(line)
evAck(ack)
September 9, 2005
20
The Globals Package
• contains various utility classes and functions
• examples:
– classes: BitAddressableBuffer, Queue, BTSConfig
(singleton), GlobalClock (singleton)
– functions: getTSN(), getFrame()
• GlobalClock implements the subscriber pattern
21
September 9, 2005
The Shell Package
• for interfacing the designer with the model
• provides a means to feed “coarse-grained” events from the
outside (MS or BSC)
September 9, 2005
22
Channel Allocation
Shell
ToBSCLink
FromBSCLink
TRX
evAccess(reson)
RACHAccess(reson)
MS
evSendMsg(msg)
evGrant(trx, tsn, mode)
evSendMsg(msg)
BSC
evAlloc(tsn, mode)
23
September 9, 2005
Rhapsody’s Simulation Engine
• the objects of the model are clustered in one or several OS
threads (under the user’s control)
• an event queue and a timeout pool are associated to every
thread
• the time is advanced to the value of next due timeout when all
threads are idle, that is, when
– the objects wait for timeouts to be due, or
– the event queue is empty
• we observed that the result of the timed simulation depends on
how the objects are grouped in threads ⇒ probably because of
the OS the simulator relies upon
September 9, 2005
24
Simulation
• implied a small modification of the tool simulation engine in
order to have access to its clock
• a list of events to be fed to the model can be specified in a file ⇒
possibility of specifying test scenarios
• for telecom applications automatic generation of large set of
events with various probability distributions would be helpful ⇒
an idea would be to define an test scenario specification
language
• impossibility at the moment to feed events to the system at
explicit moment in time ⇒ we adopted two approaches
25
September 9, 2005
Feeding Test Vectors to the Model (1)
• “wrap” the model in an “eventGenerator”
e: doIt();
idle
tm(nextStep)
gen
September 9, 2005
switch (time) {
...
case 48000:
sprintf(msgBuf, “Car passes on
track 2 at %d”, t);
cout << msgBuf << endl;
insertMessage(msgBuf);
itsSensor[5]->GEN(evCar());
nextStep = 1000;
break;
case 49000:
sprintf(msgBuf, “Car passes on
track 2 at %d”, t);
...
}
26
Feeding Test Vectors to the Model (2)
• GUI that allows feeding of events ⇒ dynamic scenario
generation
• applicable when
– coarse-grained events (e.g. evAllocate, evGrant)
– not necessarily interested when an input event arrives
but rather in the response time
27
September 9, 2005
Conclusions
• an industrial-scale application has been modelled in UML
• it comprised heterogeneous aspects: protocol processing, DSP,
control oriented, real-time requirements
• UML leverages the quality of the specification and the ease of
understanding by the visualization of the various relationships it
provides
• the timeout transition is currently the only means for dealing with
real-time aspects
• manual instrumentation is required for correctness checking
• software or hardware generation from UML is equivalent to
synthesis from statecharts or from the language of the
imported/generated code
• limited possibility for architecture exploration
September 9, 2005
28

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