**2.1 Types of RNN according to the performance of NN**

There are many classes of RNN architectures. All architectures can be best described using the state-space model from systems theory. This state-space model is explain in many references (Cheron et al., 2007) (Cruse, 2006)(Dijk, 1999), the following architectures or classes of architectures are presented:


These architectures emerge by applying constraints to the general state-space model. The architectures have been investigated and tested in applications by many researchers. In the following subsections, these specific constraints will be listed and the resulting architectures will be discussed. Each class is presented together with a quick look at some properties and examples of their application.

The architectures treated can be ordered hierarchically since some architecture is special cases of more general architectures. This hierarchy is visualized in Fig.1. The most general architectures are at the left, specific architectures are at the right. The accolades show what architectures are parts of a more general architecture description.
