Summary

Hypertext is a methodology for reading or writing in a non-linear environment. In general, all hypertext systems have two primary components, nodes and links. Nodes are "containers" for information (could be text, video, sound, picture, etc), while link represents connections between nodes.

In the existing hypertext system there are some problems that haunt users:

• Users can feel disoriented, in the sense of not knowing where they are in the network or how to go from the current place to the other.
• If there are too much data, users can suffer difficulties to decide which information is important.
• Sometimes it is too difficult to customize hypertext systems to meet the needs of particular application.
• The non-linearity of hypertext systems can present problems to the system designer or user or both. For example sometimes it is hard to break thoughts into discrete units.

People often get lost in an unfamiliar environment. In hypertext, due to the use of a nonlinear structure, unfamiliar user with the documents usually lost because the nonlinear structure does not have "clues/hints" for the user orientation such as pages, chapters, paragraphs, etc in a linear system. Not to mention if there is a relatively large number of data or paths that a user must read/choose.

To avoid the problems stated above, a data model for hypertext could serve as a basis for making the overall structure of the document clear to the user. Frequently the nodes and link structures are not enough; this simplest hypertext model lacks an aggregation mechanism that would allow designers to treat groups of nodes and links as unique entities.

Hypertext models

HAM model

The Hypertext Abstract Machine (HAM) is a general-purpose, transaction-based, multi-user server for a hypertext storage system. It is a low-level storage engine; it provides a general and flexible model that can be used in several different hypertext applications. The HAM level is near the host file system; therefore it is a low level machine. The connection between HAM level and application and user interfaces is looser than HAM level with host file system. The HAM stores all of the information it manages in graphs or databases on a host machine's file systems.

Trellis model

Trellis model is based on mathematical framework, which is called Petri Net. The Trellis model gives a formalism for describing and reasoning about the features of existing hypertext systems. Components of the actual hypertext system need to be fitted into the trellis model (r-model) in order to apply this model as a reference model. Because of the different components can only be described at certain levels of the model, this could lead into a bit of complication.

Dexter model

The goal of Dexter model is to provide a principle basis for comparing systems as well as for developing interchange and interoperability standards. The Dexter reference model can serves as a starting point for the design of hypertext system, as well as a principle basis for comparing system and for developing standards in interchange and interoperability. Instead of starting from scratch, a designer can start building a hypertext system by following the constructs shown in the Dexter model.

Tower model

The Tower data model is made of two layers. Nodes, links, and anchors are the objects which are defined by the lowest layer as the first class objects in a hyperdocument. The second layer is that of modeling constructors that build complex information representations out of simpler ones. The model named tower model because it is a construction of cities of towers by its three constructors which can be applied independently. The difference between towers and cities: a tower gives different information about one object, whereas a city gives different descriptions of basically the same information.

Lange's basic data model

Very few attempts have been made to provide a formal basis for the research field (Hypertext systems). Due to the lacking of means to interchange and communicate between existing hypertext systems, this model is developed. Lange data model emphasizes the data structure of hyperdocuments, hence it is called datamodel of hypertext. This model look deeper into the nodes of hyperdocument than the Dexter model. However, because the model focuses on the data model, the presentation of hypertext is excluded.