  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 elements inside it,
in a similar way to 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 bitrates 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,
many technical solutions have been proposed and evaluated so far, in fields
like network architectures, data distribution and filtering, resource
balancing, computing models, predictive modeling and synchronization in NVEs.
This paper gathers and classifies most of 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 QoE is essential.
