Probe Generation Model

• Constant Bit Rate (CBR): In this case, the traffic is generated as a deterministic model and the interval between packet generation is constant. Depending on the probe generation format and the measures of interest, the size of packets may be varied in this model.

• Markov Modulated: The traffic is generated according to a continuous-time Markov reward model with finite states space. In this case a reward rate is associated to each state in the model. Let $r_{j}$ be associated to state $j$. Then the transmission rate when the model is at state $j$ is $r_j$ bps. The model can be specified using the Tangram-II modeling environment [#!Tangram98!#,#!Tangram00!#], that generates the state transition rate matrix for the model and associated reward rates. (Any other tool can be used to generate the transition matrix, provided that the file format is that required by the generator.)

  The traffic generator tool first obtains the state transition probability between any two states as follows. Let $\Delta_{j}$ be the sum of the output rates out of state $j$, that is, $\Delta_{j} = \sum_{k} \lambda_{j,k}$, where $\lambda_{j,k}$ is the transition rate from state $j$ to $k$. Then the transition probability from state $j$ to $k$ is $Prob_{j,k} = \frac{\lambda_{j,k}}{\Delta_{j}}$.

  Assume that the system is at the state $j$. Since the amount of time in $j$ is a random variable exponentially distributed with rate $\Delta_j$, a sample of this random variable is first generated. Clearly, the number of bytes that must be sent till be next transition is $r_{j} \Delta_{j}$. The traffic generator then generates a random sample to determine the next state from the state transition probabilities (the $P_{j,k}$) and the process continues till the total generation time is reached.

• Trace: The tool can generate the traffic based on a trace file. This trace must be in a specific format, where each line contains in the first collum the amount of time since the last packet sent and in the second collum the packet size. It is important to note that the trace file can be obtained from a simulator or from any real multimedia data such as from a video coded in MPEG. Especific tools can generate these trace files from a MPEG video file. The Tangram-II simulator is completely integrated with the traffic generator and, if this is the simulator of choice, the modeler can generate traffic with different characteristics including long range dependent traffic models such as FARIMA and FBM. Therefore, a wide range of options is available to the user.

• Packet Pair: Similar to the CBR generation, this module can generate traffic in a constant interval. But, in spite of only one packet, a pair of them is sent. Also as in the CBR model, depending on the probe generation format, the size of packets may be varied in this model.