  Thanks to the improvements experienced in technology in the last few years,
most especially in virtual reality systems, the number and potential of
networked virtual environments or NVEs and their users are increasing. NVEs aim
to give distributed users a feeling of immersion in a virtual world and the
possibility of interacting with other users or with virtual objects inside it,
like when they interact in the real world. Being able to provide that feeling
and natural interactions when the users are geographically separated is one of
the goals of these systems. Nevertheless, this goal is especially sensitive to
different issues, such as different connections with heterogeneous throughput
or different network latencies, which can lead to consistency and
synchronization problems and, thus, to a worsening of the users' quality of
experience or QoE. With the purpose of solving these issues, researchers have
proposed and evaluated numerous technical solutions, in fields like network
architectures, data distribution and filtering, resource balancing, computing
models, predictive modeling and synchronization in NVEs. This paper gathers and
classifies them, summarizing their advantages and disadvantages, using a new
way of classification. With the current increase of the number of NVEs and the
multiple solutions proposed so far, this work aims to become a useful tool and
a starting point not only for future researchers in this field but also for
those who are new in NVEs development, in which guaranteeing a good users' QoE
is essential.
