3.4. Modeling the network behavior
Modeling comprehensive network topologies to connect simulated Cloud computing entities (hosts,
storage, end-users) is an important consideration because latency messages directly affect the
overall service satisfaction experience. An end-user or a SaaS provider consumer who is not
satisfied with the delivered QoS is likely to switch his/her Cloud provider; hence, it is a very
important requirement that Cloud system simulation frameworks provide facilities for modeling
realistic networking topologies and models. Inter-networking of Cloud entities (data centers, hosts,
SaaS providers, and end-users) in CloudSim is based on a conceptual networking abstraction. In
this model, there are no actual entities available for simulating network entities, such as routers or
switches. Instead, network latency that a message can experience on its path from one CloudSim
entity (host) to another (Cloud Broker) is simulated based on the information stored in the latency
matrix (see Table I). For example, Table I shows a latency matrix involving five CloudSim entities.
At any instance of time, the CloudSim environment maintains an m×n size matrix for all CloudSim
entities currently active in the simulation context. An entry eij in the matrix represents the delay that
a message will undergo when it is being transferred from entity i to entity j over the network. Recall,
that CloudSim is an event-based simulation, where different system models/entities communicate
via sending events. The event management engine of CloudSim utilizes the inter-entity network
latency information for inducing delays in transmitting message to entities. This delay is expressed
in simulation time units such as milliseconds.
It means that an event from entity i to j will only be forwarded by the event management
engine when the total simulation time reaches the t+d value, where t is the simulation time
when the message was originally sent, and d is the network latency between entities i and j .
The transition diagram representing such an interaction is depicted in Figure 5. This method of
simulating network latencies gives us a realistic yet simple way of modeling practical networking
architecture for a simulation environment. Further, this approach is much easier and cleaner to
implement, manage, and simulate than modeling complex networking components such as routers,
switches etc.
The topology description is stored in BRITE [18] format that contains a number of network
nodes, which may be greater than the number of simulated nodes. These nodes represent various
CloudSim entities including hosts, data centers, Cloud Brokers etc. This BRITE information is
loaded every time CloudSim is initialized and is used for generating latency matrix. Data centers