XML and Information Visualization Application to Network Management Bénédicte Desclefs, Michel Soto Laboratoire d Informatique de Paris 6 University Pierre et Marie Curie Paris, FRANCE Email : blegrand@rp.lip6.fr Web page : http://www.lip6.fr/rp/~blegrand Abstract Complex information systems are very difficult to represent and users often have problems to find relevant information; this is the case with Web navigation: users feel lost. Data contained in such systems can often be organized hierarchically, thus it is possible to represent it as a tree. However, most visualizations are cluttered because of the amount of information to display. We propose to solve these problems by using Virtual Reality (VR) techniques. After reviewing different 3D visualization methods, we decided to use cone trees. Unfortunately, the enhancement provided by graphic methods is not sufficient when hierarchies are very large; in such cases, trees must be pruned thanks to filtering methods. We chose to structure and manipulate data with the extensible Markup Language [XML]. XML allows the description and organization of any type of information. In this paper, we show how we implemented these principles in the field of network management (for networks based on Synchronous Digital Hierarchy (SDH) equipment). The application we developed eases navigation in data extracted from the Management Information Base (MIB) and accelerates access to useful information. In the future, we will apply these results to Web navigation. Biographies Bénédicte Desclefs was born in 1975. She received her engineer diploma from the Institut National des Télécommunications in 1997 and is currently a PhD student at LIP6 (Laboratoire d Informatique de Paris 6) in the Network Department. Her research deals with Virtual Reality and its use for complex systems visualization. She is interested in markup technologies, especially XML; she attended XML 99 Europe Conference in Granada. Michel Soto is an associate professor at the LIP6 Laboratory (Laboratoire d'informatique de Paris 6), University Pierre et Marie Curie, Paris, France. His main research interests are currently virtual worlds interoperability and complex systems visualization. Keywords: XML, DOM, information retrieval, representation of hierarchies, 3D visualization.
1 Introduction This work aims at studying new 3D visualization tools for large information hierarchies which are very difficult to use and represent. The Web is a complex information system: the volume of data and its lack of structure make navigation and information retrieval extremely critical. Complex data can often be divided hierarchically and thus can be represented as a tree. We want to develop a tool to visualize any type of hierarchical data. Before dealing with navigation on the Web, we focused on a more limited problem: management of Synchronous Digital Hierarchy networks. We developed an application which will be described in this paper. Synchronous Digital Hierarchy (SDH) offers technical possibilities to build high-speed transport networks (10.5 Gb/s). Broadband network services rely on high throughput and high reliability. SDH is an ITU-T digital transmission standard [SDH 96] that defines common interfaces for vendors compatibility, a digital hierarchy for optical-fibre cable transmission and a frame structure for multiplexing. [Omidyar 93] presents the basic concepts of SDH and its frame formats. SDH based networks have many advantages over today s digital networks. They meet the requirements of the new broadband network services: operation and maintenance functions provide automatic restoration of services when equipment or links fail e.g. cable breakings. Each level of an SDH network has overhead and quality monitoring elements needed to exchange information about its operation and maintenance [Sexton 97]. We started from an existing network management tool that provides a representation of the network and the list of alarms concerning equipment. Visualized elements are SDH crossconnects, consisting of racks, subracks, boards, etc. As showed in Figure 1, equipment decomposition is hierarchical. We thus need to represent a logical tree. However, this structure is not totally hierarchical as SDH ports can communicate; these cross connections must appear on our visualization. In the existing tool, the network is visualized in 2D; when the user clicks on a node, its children appear in the same window. Because of the great number of nodes to display, the screen is rapidly cluttered. The user gets lost and cannot find the piece of information he is looking for. In this paper, we propose a new 3D visualization which represents more efficiently both the global hierarchy and the connections between different SDH communication ports. This speeds up access to relevant information and helps the user find his way within the hierarchy. 3D allows a more efficient use of screen space. In particular, links between nodes do not intersect. We used Virtual Reality (VR) techniques and data analysis methods. VR provides solutions in terms of representation and navigation. According to [Ware 94], graph visualization with VR is three times as efficient as with a simple 3D diagram, because VR allows interactivity, which is essential [Kurnar 94]. It makes abstract concepts easier to understand. Moreover, level of detail techniques optimize rendering by displaying details only when the visualized object is close. However, these representation and navigation methods cannot solve the whole problem: there is still a large amount of information to display. Therefore the initial hierarchy must be pruned thanks to filtering and aggregating techniques.
In section 2, we explain how XML allows to organize, model and manipulate complex sets of data. Section 3 presents visualization techniques which have proven to be successful in building ad-hoc visualization tools for complex data. In section 4, we depict our tool applied to SDH network management using both XML and 3D visualization techniques coupled with filtering and aggregating capabilities. Finally, section 5 discusses the results and presents further work. 2 Organization and modeling of data with XML Data must be organized and structured so as to be processed later. Our tool will be fit for use with any type of information. We investigated the extensible Markup Language [XML 98] which the World-Wide Web Consortium (W3C) describes as "a common syntax for expressing data structure". XML promises to be the standard for Web development and for structured data transfer. Distributed applications have shown problems caused by the lack of a uniform standard to transfer structured data. XML was developed to solve these problems [Pontus 99]. It offers a structured and consistent way to describe and transfer data. XML maintains the separation of the user interface from the actual data. XML documents contain character data (content) and markup (providing structures for that content). Unlike HTML [HTML 98] tags, XML tags have no predefined meaning. Data and markup tags are organized in groups called elements. Every XML document consists of elements hanging together in a logical tree structure. Elements may also have attributes attached to them; they describe elements properties. It is possible to store information about the document s content; this information is called the Document Type Declaration ([DTD 98]). It defines which markup tags can be used in the document, what order they can appear in, what other tags they can contain, etc. A DTD is necessary to be able to check if an XML document is valid and/or well formed. The following source code is a part of our DTD: <!ELEMENT equipement (rack*)> <!ATTLIST equipement id ID #IMPLIED isalarmed (0 1) "0" number CDATA "0" > <!ELEMENT rack (subrack*)> <!ATTLIST rack id ID #IMPLIED isalarmed (0 1) "0" number CDATA "0" > <!ELEMENT subrack (carte*)> <!ATTLIST subrack id ID #IMPLIED isalarmed (0 1) "0" number CDATA "0" > An equipment is made of racks, and has the following attributes: isalarmed and number. For more details about attributes, see part 4.2. XML s real force is its interaction with the Document Object Model ([DOM 98]). The DOM is a platform - and language - neutral program interface that allows programs and scripts to access and update the content, structure and style of documents in a standard way. The DOM is a set of interfaces and objects designed for managing HTML and XML documents. The use of the DOM in our application is described in section 4.3.
3 Information visualization techniques Both representation and navigation are important in order to visualize information. First, the way nodes are displayed is essential. It becomes a critical issue when the size of the hierarchy increases: representations must deal with this problem. The techniques and tools for data representation provide the user with an overview of the hierarchy and enable him to visualize a specific node within the whole context. However, one must keep in mind that visualizations are used not only for information retrieval, but also for understanding the global structure of the hierarchy. The interface should allow the user to manipulate data, and organize it according to his will: this is the concept of dynamic visualization. While navigating within a hierarchy, one should be able to find a specific node and to visualize it within the global environment. 3.1 Representation model One of the critical problems with complex data visualization is to find efficient abstraction or metaphor enabling the user to get an overview of data space and to find rapidly relevant information. In the following we review main metaphors used for hierarchical information visualization: - Treemaps ([Asahi 95]) represent hierarchical information via a 2D rectangular map. It provides compact visual representations of complex data spaces through both area and color. We did not choose this metaphor, because we wanted to use 3D. - Information landscapes ([Andrews 98]) and CGI s File System Navigator ([FSN 95]) display information in 3D. Such visualizations are very intuitive, but they are rapidly cluttered and not fitted to large hierarchies. - Fisheye Visualizations ([Sarkar 93]): they show the center of interest in a large scale and with great detail, while areas further from the center get smaller and less detailed. - Hyperbolic visualizations ([Munzner 95]): hyperbolic geometry is able to display trees that grow exponentially, whereas Euclidian spaces representations are rapidly cluttered. The hierarchy is laid out in a uniform way on a hyperbolic plane, which is mapped onto a circular display. This representation is less esthetic than information landscapes, but many nodes can be displayed. - Cone trees ([Xerox]) are 3D interactive visualizations of hierarchically structured information. Each sub-tree is associated to a cone; the vertex at the root of the sub-tree is placed at the apex of the cone and its children are arranged around the base of the cone. According to [Tversky 93], it was difficult to find a specific node within the whole structure in the first versions. A good distribution of nodes is not enough to provide an efficient visualization for large hierarchies. SPAVIS implementation of the cityscape metaphor ([Keskin 97]) enhances cone trees by giving redundant information about hierarchical information. The importance of nodes is symbolized not only by their position within the structure, but also by their size: the more important the node is, the bigger it appears in the visualization. One can also use other visual cues, such as nodes shape,
orientation and texture. [Carriere 95] also enhances cone trees by using fisheye views, colors, shapes and text. Using shapes rather than texture is interesting as different shapes are easy to distinguish, even with a poor resolution. Text can be added to give more information about a node (for instance the title of the Web page). The use of different levels of detail improves the rendering and reduces visual cluttering. 3.2 Fundamental factors Structuring large amount of complex data is necessary to improve information retrieval and navigation nevertheless the way information is presented to the users is also a major issue. Fundamental factors for a good visualization interface are ([Kurnar 94]): An overview of the structure for a global understanding of the structure and of the relationships within the hierarchy, The ability to zoom and to select some nodes, Dynamic requests in order to filter data in real time. Another important feature is to allow the user to work with data according to his needs. Several ways to get a given result must also be provided by the interface to cope with the habits of different users and to quicken the learning of the tool. 4 Application to Network Management SDH networks managers have to explore a huge amount of data to obtain relevant information. This information is located in a widely distributed database named the Management Information Base [MIB]. All network equipment (host, switch, modem, etc.) maintain the MIB. At first, we chose to investigate this area to have a first experience in complex data visualization. Our work will then be generalized to the Web. There are many similarities between SDH network management and the Web: the amount of data to explore, the structure mainly hierarchical with potential cross connections between elements (cf. Figure 1). The main difference with the Web lies in data types. In SDH network management, there is a finite number of well known and standardized types[snmp], which is not the case for Web data types. Moreover, this problem is a concrete one, with well identified users enabling us to get rapid feedback about the efficiency of our approach. In the following we detail our approach and the prototype we developed.
4.1 Data model Raw data is SDH network management data extracted from the MIB. SDH equipment can be divided as follows: e q u ip m e n t X 2 3 r a c k X 6 s u b r a c k X 1 6 b o a r d X 1 6 X 1, X 3 o u X 2 1 l o g i c a l p o r t p h y s i c a l p o r t P D H p o r t S D H p o r t 2 M b i t/s 6 M b i t/s 3 4 M b i t/s 4 5 M b i t/s 1 4 0 M b i t/s S T M 0 S T M 1 S T M 4 S T M 1 6 S T M 6 4 1 5 5 M b it / s 6 2 0 M b it / s 2. 5 G b it / s 1 0 G b it / s X 4 X 1 6 V C - 4 X 3 V C - 3 X 7 A U - 3 X 1 A U - 4 X 6 4 T U G - 3 X 1 T U - 3 V C - 3 X 7 T U G - 2 X 1 X 3 X 4 T U - 2 T U - 1 2 T U - 1 1 V C - 2 V C - 1 2 V C - 1 1 S D H : S y n c h r o u n o u s D ig it a l H ie r a r c h y P D H : P le s io c h r o n o u s D ig it a l H ie r a r c h y S T M : S D H f r a m e V C : V ir t u a l C o n t a in e r T U : T r ib u t a r y U n it T U G : T r ib u t a r y U n it G r o u p C : C o n t a in e r A U : A d m in is t r a t iv e U n it X 1 C - 4 C - 3 1 4 0 M b i t /s 4 5 M b i t /s 3 4 M b i t /s C - 2 C - 1 2 C - 1 1 6 M b i t/s 2 M b i t/ s 1.5 M b i t/ s Figure 1: Hierarchical composition of SDH equipment SDH is based on STM-N frames which have a throughput multiple of 155.20 Mb/s. These frames are built in several steps: first, data is placed in containers (C). A Virtual Container (VC) is created when a pointer is added to each container. The association of a VC and a pointer leads to a Tributary Unit (TU) for low bit rates and to an Administrative Unit (AU) for higher bit rates. A TUG (Tributary Unit Group) is a group of TUs. 4.2 Raw data organization Data concerning equipment is stored in an XML file. Each rectangle in Figure 1corresponds to an XML element (there are 37 types of elements). Additional information about these nodes is contained in attributes. Each element has an identifier and a Boolean attribute (isalarmed), set to true if an alarm was detected. The Boolean attribute crossconnected indicates the existence of a connection between two elements. If there is one, a reference to the linked node is provided by the linked-node
attribute. VC and AU elements have two more attributes giving information about performance: perf1minstarted and perf15minstarted. 4.3 Structured data processing Many XML parsers are now available; [Anez 99] reviews and compares some of them. We chose IBM s XML4J DOM Parser [XML4J], because of its good performances; its other advantage is that it is totally written in Java, which is interesting for us (we work with Java3D [Java3D 99], cf. end of paragraph). The analysis of an XML file leads to the creation of a DOM tree, which can be accessed and modified easily. There are methods such as getchildnodes(), getfirstchild(), getparentnode(), getnextsibling(), getnodetype(), getnodevalue(), getattribute(), etc. Nodes can be added (with the method insertbefore), suppressed (removechild) or replaced (replacechild). Thanks to the DOM, elements and their attributes are easy to access, so that the user can select relevant data. Our tool can also stress nodes where an alarm has been detected; we just have to locate the nodes for which isalarmed is true. The DOM tree is traversed to create a second tree, which is displayed in 3D : the Java 3D tree. Element nodes from the DOM tree can thus be visualized. 4.4 User interaction When our application is run, a 2D view of the tree and the corresponding XML source file appear. Then a 3D representation is shown, called global view, which contains the whole tree. The 3D representation model we used for our application is a cone tree. Nodes are spheres whose color changes according to the level in the hierarchy. The user should always have a global point of view, so that he knows exactly where he is within the hierarchy. Therefore, the trees displayed first represent the whole structure. Then, the user is free to manipulate data and to select the parts he is interested in. He must be able to focus on his center of interest. This is possible thanks to detailed views. It is possible to modify the 3D tree (node addition / suppression, modification of attributes value, etc.). These changes are reflected on the DOM tree. The user can perform the following actions on the tree: - Manipulation of the 2D tree: When the user clicks on a node in the 2D tree, the properties of this node appear, and the corresponding part of the XML code is stressed. - Manipulation of the 3D tree: The left button of the mouse makes the tree rotate. The middle button (or the arrows on the keyboard) allows the user to zoom. Two actions can be performed with the right button : If the mouse s pointer is not located on a node, moving the mouse translates the tree vertically and horizontally.
If the mouse s pointer is located on a node, a contextual menu appears, with the five following proposals: o Cancel, if the button was pressed by mistake. o Display in a detailed view: the sub-tree whose root is the selected node is displayed in another window, called detailed view. The number of detailed views is limited only by the power of the computer. When the user moves in the detailed view, traversed nodes are stressed in the global view too, so that the user can locate the sub-tree within the whole hierarchy. o Mask: the tree can be pruned by masking the sub-tree whose root is the selected node. o Develop: this performs the opposite of the preceding function. o Display connections: if the selected node is linked to another, the connection appears on the 3D representation. o Mask children: When a node is selected, this function masks all its children. The difference with the Detach function is that in this case, the parent node remains. In each 3D view (global and detailed views), there are two menus: Alarms and Connections, to display / mask alarms and connections. By default, neither alarms nor nodes are displayed. Figure 2 represents an example of global view: Figure 2 Global view of a hierarchy
The following figures are other examples of visualizations: Figure 3: 2D tree and XML source file Figure 4. Zoom on the 3D tree 5 Conclusions and future work We presented issues dealing with information representation and retrieval in large hierarchical structures, in particular in a tree containing data for network management. We approached this problem by organizing raw data with the XML language and the Document Object Model which allows an easy access to information. Cone trees were chosen as 3D-representation model. However, they are not sufficient when the hierarchy is very large. Users should be able to find information quickly. Therefore data must be filtered and special attention must be given to navigation. The prototype was shown to SDH network managers whose reactions are positive. In particular, users greatly like the 3D display that represents all supervised network elements since this is not possible with the 2D tool they currently use. Filters which highlight alarmed network elements and cross connections between SDH ports are appreciated too. Moreover, managers must frequently supervise and establish new cross connections in most equipment failure scenarios. These cross connections are difficult to locate with the current tool. In the future, we will first enhance this application by extending the panel of users actions. For instance, users asked us to provide the tool with the capability to add or remove connections between nodes using the graphical representation (and not only from the source file). Currently, they use the different interfaces of each network element to do this. Second, we will propose other 3D representation models (eg hyperbolic trees). Then, we will provide a history of navigation so as to allow the "back" function. Finally, we will apply these results to navigation on the Web that raises the same problems - disorientation and inefficiency. The nodes will symbolize Web pages or fragments of Web pages. It will be possible to filter data thanks to an intelligent information retrieval tool which will use keywords and DTD, for example.
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