computer network: modeling and simulation -abhaykumar kumbhar computer science department
TRANSCRIPT
Contents:• Introduction.• Modeling.• Simulation.• How to develop a Model.• Classification .• NS simulator.• OPNET.• An Ethernet Case Study.• Requirements of a good Network Simulating Tool• Evolution.• Conclusion and Future Work
Introduction:• Performance analysis of computer networks :
increase in size and geographical extent.• The size of the networks and the inherent
complexity of network protocols complicate this analysis.
• Analysis techniques such as queuing models have difficulty modeling dynamic behavior retransmission timeouts and congestion.
• Simulation offers a better method of studying computer networks, since one can simulate the details of actual protocols.
• Simulation is an analysis and evaluation tool.• Every simulation is tight to a model that
represents the real world.• Inconsistent modeling
• Modeling itself is a crucial step towards meaningful results.
• Aim :systematic modeling techniques -increase the quality and validity of results
Simulation:
system under study(has deterministic rules governing its behavior)
observer
“real” life
system boundary
exogenous inputsto system
(the environment)
computer programsimulates deterministic rules governing behavior
pseudo random inputsto system
(models environment)
program boundary
observer
“simulated” life
Why Simulation?
• study system performance, operation• real-system not available, is complex/costly or
dangerous (eg: space simulations, flight simulations)
• quickly evaluate design alternatives (eg: different system configurations)
• evaluate complex functions for which closed form formulas or numerical techniques not available
When to Use Simulation?• Whenever Mathematical Analysis Is Difficult
or Impossible.
• For Validating Analytic Models.
• For Experimentation Without Disturbing an Operational System
How to develop a model:
• Determine the goals and objectives• Build a conceptual model• Convert into a specification model• Convert into a computational model• Verify• Validate
Classification of Simulation Tools:• GPPL: General Purpose Programming Language• PSL: “Plain” Simulation Language• SP: Simulation Package
NS Simulator:
• Began as REAL in 1989• Developed by UC Berkeley• Public domain SP• Object-oriented• Written in C++ and object-oriented tcl (Otcl)• Network components are represented by
classes
OPNET:
• Developed by OPNET Technologies Inc.• Commercial SP• Object-oriented• Totally menu-driven package• Built-in model libraries contain most popular
protocols and applications• Simulation task made easy
An Ethernet Case Study:• Bridging the Gap Between Reality and
Simulations• We set up our test-bed using two nodes
connected by a 3 meter cross-over Ethernet cable.
• a simple test-bed experiment and attempt to replicate the obtained results by simulation in ns-2,QualNet and OPNET.
• Network details• set packet size of the application to 1472
bytes. This was to ensure that the maximum Ethernet frame of 1500 bytes would be transmitted
Delay experienced by packets in saturated case with non-blocking sockets
particularly interested in the saturation performance of the system and we chose multiple rates near the maximum link bandwidth
• The maximum delay of the system is experienced when the source bufferis full.
• This delay is given as max delay = tx delay ×buffer size in packets.
• The transmission delay for one packet is the minimum delay.
• The buffer size is then calculated as max delay/min delay. Hence a buffer size of 78 packets was arrived at 94/1.2 ≈ 78packets ≤ 128KB.
• Setting up simulations in QualNet:--the link propagation delay being negligible, set it to 25 μs to model test-bed network characteristics.--queue size from its default value of 50 KB to 117 KB, which corresponds to 78 packets of 1500 bytes each.
• Setting up simulations in OPNET– Ethernet Station Advanced (ESA).– The ESA has a traffic generator built over the
Ethernet MAC.– the queue length to infinity.– OPNET models queues as a set of sub-queues,
allowing the possibility of different application traffic to map on to different queues. By default only one sub-queue is created in the Ethernet MAC process model and this is a hidden parameter. This sub queue is accessed as a process interface of the Ethernet MAC process model. The sub-queue packet capacity was set to 78 packets.
• Setting up simulations in NS-2– configurable parameters: node objects, link
bandwidth, maximum length and type of the interface queue and delay.
– queue length to 78. – maximum propagation delay is 25 μs for 10 Mbps.
Requirements for Network Simulation tools :
• Model development simplicity• Modeling flexibility• Fast modeling• Animation• Different kinds of implemented components• Component adaptability• Creating new components• Static capabilities of a simulator• Graphs
Evolution of Network Simulation Tools:
• “Zeroth ” Generation — General Purpose Languages- Fortran, C/C++, Pascal, Basic
• “First” Generation — General Purpose Queueing System Simulations
-GPSS, SLAM, SIMSCRIPT
• “Second” Generation — Application Specific: Computer Systems and Wide-Area Communication Networks
• “Third” Generation — Integration of Second Generation Languages-With a Graphics-Oriented Analysis and Modeling Environment
-SES/Workbench-OPNET
References:
1:Advanced Modelling and Simulation Methodsfor Communication Networks Jože Mohorko, Fras Matjaž, Klampfer Saša.
2:www.sciencedirect.com3:www.ieeexplore.org