ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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COST 285
Modelling and Simulation Tools for Research in
Emerging Multi-service Telecommunications
Francesco Potortì
ISTI (Istituto di Scienza e Tecnologie dell’Informazione)
CNR (Consiglio Nazionale delle Ricerche)
Pisa – Italy
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Main scientific interests
• Communication protocols, especially at the MAC level and
transport level
• Satellite multiple access protocols in TDMA mode
• Countermeasures against the atmospheric fading of
satellite signals
• Mobile ad hoc networks with using 802.11 and Bluetooth
protocols, especially at the MAC level
• Quality of service on the Internet
• TCP over satellite and terrestrial wireless
• Simulation in all of the above topics
• Characterisation of wireless channels (802.11, GPRS)
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Affiliation
• ISTI (Istituto di Scienza e Tecnologie dell’Informazione) is
part of the CNR (Consiglio Nazionale delle Ricerche)
• It was born in Pisa, in 2002, as the result of a merger
between CNUCE-CNR and IEI-CNR
• I am a member of the Wireless Networking Group,
composed of about 10 persons
• I am currently involved into two research projects:
– TANGO, an Italian research project aimed at multilayer, multiservice
Internet networks
– IS-MANET, an Italian research project aimed at mobile ad hoc
networks in hostile environments
• I promote the use of free software in research environments
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Simulation expertise
• Implementation of MTG, an Ethernet traffic generator used
to measure the performance of FODA/IBEA
• Implementation and performance measurement of
FODA/IBEA, a multiple-access satellite system
• Implementation of fracas, a discrete-time simulator for
framed access channels, and its use to evaluate the
performance of several satellite access systems
• Implementation of a fractal traffic generator
• Implementation of GaliLEO, a prototypal event-driven
simulator for LEO satellite systems
• Analytical analysis of TCP over satellite channels
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Fracas
the Framed Channel Access Simulator
Francesco Potortì
• A very specialised simulator for communications
protocols written in standard C
• Very small and very fast
• Able to study the behaviour and the performance of
multiple-access protocols, usually at the MAC level
• Used until now for satellite channel access protocols
• Provides unsophisticated but comprehensive statistics
• Wrapper in Python for independent replications
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Scope of the simulation
• Multiple-access methods that use a time-framed
communications channel
• In principle, usable for any such type of channel: the framed
channel of FDDI2, but not FDDI, unless the time is artificially
subdivided in frames, putting a lower boundary on time
resolution
• In practice, always used for geostationary satellite access
• The core is very small, consequently
– the access method can be as general as possible, with only the time
frame constraint
– programming is not easy, and requires knowledge of all the inner
structures of the simulator
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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General architecture
generator
worker
Station 1
worker
requester
allocator
generator
statistics
collector
worker
worker
generator
Station 2
worker
generator
worker
generator
generator
allocation delay
Station 3
generator
Station 4
frame allocations
Station Stat.
Station
1
2
3
Station
4
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Traffic generators
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Each produces a number of Transmission Units per frame
Each generator is attached to a single station
Any number of generators can be attached to a station
Built-in generators include:
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two-state periodic fixed rate — can be used for one-shot
two-state periodic Poisson — can be used for one-shot
two-state Markov-modulated Poisson
two-dimensional (NxM states) Markov-modulated models VBR traffic
fractional Gaussian white noise models generic aggregate traffic
specialised generators model batch interactive traffic
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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The stations and the allocator
• All traffic is expressed as the number of Transmission Units
• TRUs are produced by generators, queued at the stations,
and sent according to the allocations in the current frame
• TRUs are not received; Fracas only simulates sending
• For each frame, each station
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Queues traffic produced by attached generators
Drops traffic exceeding the queue length
Sends queued TRUs filling the allocation in the current frame
Asks the allocator for allocation in a future frame
• For each frame, the allocator sets up the allocation for the
future frame as TRUs available to each station
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Emulator core loop
do {
frame_number += 1;
for_all_stations_do {
input = run_generators (this_station);
queue += input;
sent = min (queue, allocation);
queue -= sent;
request = compute_request (this_station);
}
compute_allocations (frame_number + allocation_delay);
gather_statistics (frame_number);
} while (! Stop_condition ());
run_workers_and_print_results ();
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Allocator policies
• An allocator defines an allocation policy, which is an
algorithm for computing the allocations for each station at a
future frame based on their requests in the current frame
• Built-in policies are those that we really used in simulation
work; others may be added as necessary
• Currently implemented policies:
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fixed TDMA: a fixed assignment to each station
FODA/IBEA: developed at CNUCE, experimented with a prototype
VnL-DA: VBR allocation developed at CNUCE
FEEDERS: distributed allocation scheme developed at CNUCE
DRIFS : distributed allocation scheme developed at CNUCE
CFRA: developed at ENST - Toulouse (FR)
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Output statistics
• At each frame, several observables are collected
• Each worker computes a different statistics on one or more
observables, including:
– TRUs input, queued, dropped, allocated, requested, sent by stations
– allocation unused by stations
– transmission delay, either per frame or per TRU
• Some workers produce their results at emulation time
• Others produce a result at the end of the emulation
• A Python wrapper around Fracas is used to obtain
estimates of a statistics inside a given confidence interval
using independent replications of the same emulator run
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Classes of traffic
• Fracas distinguishes among four different classes of traffic,
hierarchically ordered as follows: stream, vbr, interactive,
bulk, whose names reflect their supposed usage
• When emptying the station queues, the simulation core
starts from stream, and gives any unused allocation to
lower-grade traffic classes
• Usage of the classes is optional; possibilities include:
– gathering statistics for traffic generated in different classes
– defining different allocation policies for each class
• Use of classes enables the definition of complex allocation
strategies
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Fracas summary
Francesco Potortì
• A lightweight, portable simulator specialised for the study
of multiple-access allocation schemes
• Traffic generators, allocation policies and statistics
computations can each be added as separate modules
• Many built-in modules are implemented and have been used
in research work
• Fracas has been validated by checking against a prototypal
implementation of the FODA/IBEA allocation algorithm
• A paper on the architecture of Fracas has been published
on Telecommunications Systems in 1999
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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GaliLEO progress report
Laurent Franck and Francesco Potortì
• A comprehensive simulator for satellite constellations,
targeted towards LEO/MEO communication systems
• Experience from previous projects (SimToc, LeoSim,
Fracas), involves different academic institutions
• Free software entirely implemented in Java
• Possible studies include:
– algorithms for cell frequency reuse
– QoS routing for both UDL and ISL
– QoS aware channel access techniques
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Capacity of global coverage
• Emulation of an entire
constellation
• Definition of individual earth
stations and traffic
generators
• Complete map of traffic
patterns
• Ability to individually follow
any traffic connection
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Study of a limited region
• Emulation over a limited
geographical region
• Detailed and realistic traffic
generation
– complex access techniques
– complex frequency reuse
strategies
• Nearby network simulated
by mathematical description
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Basic terminology
• A UDL (up-down link)
covers a satellite’s
footprint
• A UDL is made of beams,
each covering partially
overlapping cells
• Satellites are connected
by ISLs
• A station is fed by one or
more traffic generators
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Architecture of GaliLEO
• An event scheduler is at
the core of the simulation
engine
• The core modules come
with the simulator
• Custom modules can be
added at will
• Custom modules include
traffic generators,
constellation layouts,
access protocols etc.
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
The two inner layers
• The simulation engine provides
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the event scheduler
the framework for building modules
the communications facility between modules
the input file structure, statistics gathering and display facility
• The standard modules provide some basic features:
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a Leonet and a polar constellation
a deterministic periodical traffic generator
scalar resources for allocation in stations and satellites
simple routing and load-dependent adaptive routing
basic allocation strategies for stations and satellites
basic station and satellite structures
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ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
The simulation engine
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ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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Demo setup
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12 stations evenly distributed between 45°N and 45°S
Leonet constellation (15 satellites on 3 orbits)
ISL capacity is 20 connections, UDL’s is 80 connections
Measuring the connection blocking probability
For each connection, the first and last satellites are those
with max elevation
• The demo is made of three steps, global traffic is the same
for all steps:
– 6 Erlangs per station, static routing
– 4 adjacent stations at 10 Erlangs, others at 4 Erlangs, static routing
– same traffic as previous case, adaptive routing
ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
Demo results
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ISTI - Consiglio Nazionale delle Ricerche – Pisa – Italy
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GaliLEO summary
Laurent Franck and Francesco Potortì
• A big simulator for LEO/MEO communication systems
• Both overall earth view and geographically limited studies
are considered
• Very modular; standard modules available, custom modules
written in Java can be added
• Programming environment is Java
• Work in progress: development is open to contributions
• Some features usable today for research: demo available
• Home page at http://galileo.tesa.prd.fr/