Accurate modelling of communication networks often results in models that are intractable to mathematical analysis and computer simulation becomes the option for modelling and analysis of complex systems. This section provides a description of the modelling concepts, simulation techniques and tools that have been used in the modelling and evaluation of the HWN* system. Algorithms proposed in conjunction with the HWN* infrastructure have been evaluated using OMNET++, which is a discrete event simulation environment with GUI support. OMNET++ is an object-oriented simulation tool, which consists of hierarchically nested modules written in C++. Nested modules are used that implement comprehensive and accurate modelling, which can reflect a good estimation of the actual system performance. In order to introduce events such as discontinuous transmission, session arrival & departure and etc, it is necessary to introduce the notion of time into the simulations. Events are triggered at some time instance in the simulation. For example, the event of session arrival at one node usually triggers the event of the arrival of that packet at a downstream node. Indeed the time delays are often random variables but events allow session arrival and departure processes to be modelled.
The HWN* physical layer needs to calculate the Bit Error Rate (BER) based on the SIR variation influenced by path loss, shadowing and fading models. The simulation of HWN* mainly considers packet level and session level modelling, but not bit level. Fast fading such as multipath fading is averaged out or captured in the BER and Packet Error Rate (PER). The propagation characteristics change from place to place when a MT moves. Thus, the transmission path between the transmitter and the receiver can be modelled from simple direct line of sight (LOS) to one that is severely obstructed by buildings, foliage and other terrain. Three mutually independent propagation phenomena considered are path loss, shadowing and multipath fading.
As complicated simulation models are evaluated, each simulation run may consume random numbers from several streams, which should be from several independent Random Number Generator (RNG) instances. Therefore different random streams or say simulation benches have been use for different tasks. For example, a random stream for generating packets and another random stream for simulating packet delivery rates in the transmission are different and not overlap. For all algorithm and system evaluation, we implement the Mersenne Twister RNG with 623-dimensional equal-distribution. The independent simulation processes run independently of one another and continuously send their observations to the central analyser and control process. This process combines the independent data streams and calculates from these observations an overall estimate of the mean value of each parameter. The Akaroa method provided by OMNET++ is also used which halts a simulation. It is decided by the 95% confidence intervals for all simulations to precise whether the result has enough observations.
Probabilistic models are applied to generate voice traffic, multimedia traffic and web traffic in this simulation environment for QoS oriented results evaluation. Voice traffic is modelled down to a packet activity with a different degree of granularity. Packet based voice source traffic models incorporate a voice activity aspect, which allows transmitting speech data when voice frames are detected, and facilitates discontinues data transmission. Hence, the voice source model has to include a higher degree of granularity and is represented as a sequence of consecutive talk spurts and silence periods within each call. The duration of the active and inactive periods reveals a negative exponential distribution for the duration of both active and inactive periods. The values applied for the investigations of this work are, a mean duration for the active periods of 1.35 seconds and a mean duration for the inactive periods of 1.70 seconds. The activity factor of the source is defined as the proportion of the time that the source is active. Several factors impact the nature of multimedia traffic and its transport requirements such as target quality, compression technique, coding time. The use of video traces is to facilitate algorithm evaluation with respect to QoS. As modeling the video sources always requires the original trace to be fully evaluated first, a simpler approach adopted in this research is to incorporate traces directly into OMNET++. Another benefit from incorporating the video traces directly into network simulators is the vast amount of video source models. Direct utilisation of video traces in OMNET++ facilitates the fastest method to incorporate video sources into existing network models. An interface has been provided for video trace is implemented, which is capable of detecting the different video trace file formats and to feed the data into the simulator accordingly. For particular environments and service types only specific traffic has to be instantiated. Whereas the evaluation of routing and inter-system traffic balancing of HWN* has instantiated all voice, multimedia and web services. Web traffic modelling is represented by a generic model with three levels session, activity and packet level. Session level consists of pages visited in a web session where a client starts an application, uses it for a time and then disconnects from the system. The moments when sessions arrive can be described by a Poisson model. The duration of sessions in real time applications depends on applications and in non real time is controlled by Transmission Control Protocol (TCP). Activity Level consists of a set of object applications such as images, sound and applet. The density of information in one application depends on the application itself. To describe this property, the activity level represents the application as a detailed succession of activity and inactivity periods in an ON/OFF model. ON represents page downloading time followed by an OFF period for the reading time. Packet Level decides the transmission of IP packets. If the User Datagram Protocol (UDP) is used, the packet inter arrivals can follow any distribution with packets following a truncated version of distributions to respect transmission limitation. In case of TCP, inter-arrivals of packets are determined by a TCP Pareto distribution.
To also include cellular network node movement characteristics, routing and handover algorithm evaluation of the HWN* system implements an Attraction (Attractor) Points oriented Mobility Model (APMM) based on the random waypoint model. The algorithm first selects N attractors that are distributed at points where MTs will originate from or progress towards. Prior to heading for attraction points, nodes are grouped together using Cell Type Transition Probability, each subscriber selects a destination area with probabilities. In a typical MT movement, at the beginning, all MTs are scatted around in a metropolitan environment. After 100, 150 and 200 simulated minutes, the trend of MTs have been moved to several pre-defined attractor points which locate in the middle of northwest, northeast, southwest, southeast and the city centre. The northeast and southeast regions may have higher attractor probability than the rest ofhot spots and therefore more MTs gather at the right hand side. It is flexible to change the geographical location of hot spots by revising the attractor points. Meanwhile, with a speed control mechanism, the current MT speed is configured correlated to the previous speed value and a smaller sampling time makes the speed change more smoothly.
HWN* configuration deals with message exchange, core network layer structure and individual algorithm implementation issues. The multiple access systems of the cellular component in HWN* deal with the inter-cell and intra-cell interference caused by common access to a shared band of frequencies. The mutual interference happens between BS and MT, RN cellular interface and BS, and RN cellular interface and MT. A TDMA based standard interface has been modelled. Each session is assigned a time slot within a frame which it keeps until it is handed off. No other sessions within the same cell are assigned the same slot, and thus users within a cell do not interfere. The MANET component of HWN* employs the contention based Carrier Sense Multiple Access/ Collision Avoidance (CSMA/CA) for the multiple access, between MT and MT, and MT and RN MANET interface, as it is the most adopted access protocol for MANETs and 802. 1x networks. When regarding the RN MANET interface as a MANET node, if MANET node cluster head selection is performed and along with RN provide the time beacon for synchronisation, a syncronised CSMA/CA can support fast data transfer, adopts ACK for successful transmissions and implements the handshaking mechanism between RN and MT to reduce collisions.
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