notes05 - System Modeling and Simulation TELCOM 2120...

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1 System Modeling and Simulation TELCOM 2120 Network Performance Joseph Kabara Telecommunications Program University of Pittsburgh Spring 2008, notes5 TELCOM 2120: Network Performance 2 Simulation Simulation: the process of designing a mathematical or logical model of a system then conducting experiments with the model to describe, explain and predict the behavior of the system Strengths – Allows details to be included. – Can compare alternate system designs. – Can control time scale. – Existing system not required Weaknesses – Difficult to generalize results. – Difficult to consider all cases/parameter values. – Hard to determine sensitivity. – Time to develop and execute simulation. – Effort to validate model and analyze output data. Spring 2008, notes5 TELCOM 2120: Network Performance 3 System Definitions System Definitions 1. Parameters – quantities that are fixed or can be controlled 2. Variables – quantities that are determined by functional relationships 3. State Variables – the minimum set of variables to completely describe a system at a point in time Systems properties 1. Static – state variables are independent of time 2. Dynamic - state variables are a function of time 1. Continuous time – states are continuous function of time 2. Discrete time - states only defined at certain time points 3. Combined – system contains both continuous and discrete variables 3. Continuous State – state variables can take on values from a continuous range 4. Discrete state – state variables can only take on values from a discrete range 5. Deterministic - state variable can be predicted with certainty 6. Stochastic – state variables include some source of randomness
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2 Spring 2008, notes5 TELCOM 2120: Network Performance 4 System Definitions Computer system and communication networks are dynamic continuous time, discrete state, stochastic systems. Note that states can only take values from a discrete range and are constants between changes. Point in time where the state changes – an “event time’’ What causes the state change is called an ``event’’ – For example - arrival of a packet to a router queue Such systems are called Discrete Event Systems event times Spring 2008, notes5 TELCOM 2120: Network Performance 5 Simulating Discrete Event Systems 1. States Collection of variables necessary to characterize the system at any time point 2. Entities objects processed in simulation – for example packets or phone calls 3. Attributes characteristics of entities (e.g., packet length, type and destination) 4. Resources Substances/items that the entities engage in or use (e.g., buffer space at router, tokens in a FDDI network, bandwidth on a link) 5. Activities Duration of time of whose length is known when it begins For example, transmission time of a packet on a link 6. Delay Duration of time of unspecified length which is not know until it ends For example – time for a packet to travel from node A to node B in a network Spring 2008, notes5
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notes05 - System Modeling and Simulation TELCOM 2120...

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