Mobility aware Multimedia X.400 e-mail:
`A Sample Application Based on a Support Platform for Distributed Mobile Computing
`
`Alexander Schill, Sascha Kümmel, Thomas Ziegert
`Dresden University of Technology, Faculty of Computer Science, 01062 Dresden
`e-Mail: schill@ibc.inf.tu-dresden.de
`
`Abstract
`The paper describes the design and implementation of a distributed system to enable the use of
`X.400 e-mail with multimedia extensions in an mobile environment. This system is a example
`for how to extend standard applications with mechanism for handling special problems of
`nomadic computing and mobile communications. This paper outlines the need of a generic
`software support for „mobile applications“. Based on these considerations, we describe a
`model for a generic software support platform for distributed mobile computing. It addresses
`the specific aspects of mobility by providing application-independent and reusable support
`services. In particular, it offers a framework for organizing distributed mobile applications into
`manageable domains, it equips mobile stations with enhanced functionality for location,
`resource and bandwidth management, and it uses industry standard RPC communication
`facilities for enhanced portability. The implementation of the mobile X.400 e-mail system is
`the first comprehensive validation of the support platform. First experiences are discussed at
`the end of this paper.
`
`Keywords: Mobile computing, mobile communication, e-mail systems, multimedia
`transport systems, distributed systems, distributed applications
`
`1. Introduction
`Electronic mail is a rather traditional kind of application that has been in use for several
`decades. However, in the context of mobile computing, a number of interesting new aspects
`and requirements are associated with it:
`• Bandwidth management: E-mail has recently been augmented with multimedia extensions
`within dedicated X.400 extensions, for example within the MMM (Multimedia Mail)
`project of DeTeBerkom /ADH93/. The transfer of the respective multimedia body-parts
`such as audio files, images, and video clips in a mobile environment requires careful
`consideration of bandwidth management aspects. This can be supported by dedicated
`information management support, in particular by separating body-parts and by applying a
`variety of compression techniques.
`• Disconnection: Due to the widespread use of e-mail, it is a key acceptance criterion to
`provide rich e-mail functionality on mobile stations, also in the context of disconnection.
`When a mobile station is operating in disconnected mode, the user expects at least an
`emulated e-mail functionality. Moreover, upon reconnection, transparent access to
`received e-mails as well as transparent transfer of sent e-mails is required.
`• Dynamic configuration: Mobile hosts dynamically move between various subnetworks
`and organizational domains. Therefore, frequent changes of the overall system structure
`result /KAT94/. A mobile e-mail application must reconfigure itself under consideration of
`the actual system and network environment. The users should not be involved in this
`process.
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`• Quality of communication mechanisms: In mobile environments, the quality of
`communication facilities also varies significantly /MEW93, DPB94/. The throughput,
`delay and costs of cellular WANs like GSM, wireless LANs and conventional networks
`are very different. The e-mail system software should incorporate these aspects as a base
`for optimizing the message transport /ATD93/.
`• Security: Finally, advanced security requirements arise from mobile computing /AZD94/.
`While the problems of link tapping have been solved to a high degree by encryption
`techniques and spread spectrum technologies, dynamic attachment to different networks
`imposes new requirements concerning account and password management and
`authentication /MST94/. Therefore, solutions for cross-domain account management are
`required. Moreover, cached data on notebooks are much more sensitive to loss or theft of
`such equipment; that is, additional security techniques are recommended.
`
`One way to consider these problems is the implementation of a new e-mail system from
`scratch. But this could lead to a very low user acceptance. That’s why we prefer the extension
`of existing standards and standard applications by introducing new functionality. To support
`existing applications without changing the binaries, a midleware concept is recommended.
`Due to the similarity to other applications concerning disconnection and bandwidth
`management, mobile security and related issues, it is important to build the components of a
`mobile e-mail support in a reusable way. For example, mobile database access, mobile
`message passing support, or mobile document management can make use of the generic
`functionality discussed above. Based on this requirement we developed a model for a generic
`support platform for mobile applications discussed below.
`
`2. Generic System Support
`
`Overall Structure
`The distributed environment is divided into organizational units. We call them domains. A
`domain is a base for generic mobile application support and resource management. A domain
`may consist of various subnets and a number of fixed and mobile stations (fig. 1). However, it
`is important to note that a domain is a logical rather than a physical construct; therefore, it can
`also include widely dispersed subnets or stations if they are considered to be organizationally
`close. Each domain is controlled by a domain manager. This component manages an
`information base with:
`
`all stations that are currently members of the domain,
`•
`• global application-level services and resources of the domain that are offered via RPC,
`•
`an abstract representation of the network topology belonging to the domain,
`•
`and a dynamically adaptable list of other domains
`
`Each station comprises a station manager that is responsible for station-specific management
`tasks including planning of remote communication and resource access, caching of
`information, and interactions with the domain manager. (Fixed) stations provide services and
`resources to the distributed environment. This functionality is detailed in the following
`section. A station is a member of at most one domain. Although stations usually interact
`directly with each other via RPC at the application level (i.e. without involvement of the
`domain manager), communication with the domain manager is required for various specific
`tasks.
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`Fig. 1: Overall structure of the generic approach
`
`Stations are able to dynamically enter and leave domains and to locate services and resources.
`When a station enters a domain, it registers itself with the domain manager and transfers
`descriptive information about itself to it. Upon leaving a domain, the domain manager deletes
`the station from its list of domain members and adapts the corresponding resource
`information. The domain managers themselves interact via a domain interaction protocol. It
`implements the exchange of information about member stations which offer services and
`resources. Most of this information is only exchanged on demand.
`
`Station Structure
`Station management enables the applications on a given (fixed or mobile) station to make use
`of our mobile support environment. A major goal is to provide a relatively system- and
`application-independent generic support that is operational on a large number of platforms and
`for a large number of applications. Therefore, the station manager offers techniques that
`support mobile computing and communication in a generic way rather that implementing
`specific mobile application services (such as mobile e-mail as a self-contained service). The
`station manager is structured internally according to /SCK95/ and comprises the following
`components: Location service (LOC) and resource broker (RB), Bandwidth and cost
`management service (BCMS), Application data mobilizer and manager (ADMM),
`Disconnected operation handling service (DOHS), Registry service (RS) and authentication
`and encryption service (AES). The related data about users and applications are managed by
`an active database (ADB). It handles all queries of other components. A detailed description
`off all components can be found in /SCK95/.
`
`3. Example Implementation: Mobile X.400 e-Mail System
`
`In the following, we describe the architecture and implementation of a mobile X.400 system
`on top of our support environment (see fig. 2). We assume that the reader is basically familiar
`with the general concepts of X.400 e-mail.
`In the mobile system extensions, the so called UA-Proxy (UA-P) gains access to the stored
`messages within the X.400 Message Store (MS) in substitution for the original remote user
`agent (rUA). It resides within the home domain of the mobile user and acts as mediator
`between Message Handling System (MHS) and our platform. By using the MS, the messages
`are continuously available; so conventional rUA’s can retrieve them.
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`After retrieval, the UA-P copies the messages to the current location (domain) of the
`respective user. Therefore two cases possible:
`
`1. Message retrieval on demand: The user claims the UA-P to retrieve new messages after
`registration in the new domain.
`2. Look ahead delivery based on user specific mobility profiles: The UA-P delivers the
`messages to all possible locations of the specific user. After arrival in one of the predicted
`locations, the user can immediately read his or her mails. The garbage collection at the
`other locations belongs to the realm of the intelligent queuing service of our architecture. If
`the user relocates to a domain which isn’t in the delivery-list, case 1 occurs and after a
`timeout period all other copies will be discarded.
`
`Domain A
`
`MS
`
`UA-Proxy
`
`SM
`
`Conversion-
`Service
`
`Mobile Station
`
`QS
`
`SM
`
`Viewer
`
`QS
`
`copy
`
`Evaluation
`and
`Garbarge
`Collection
`
`QS
`
`DM
`
`X.400-Subsystem
`
`UA
`
`Cache
`
`Viewer
`
`QS
`
`DM
`
`Domain B
`
`Domain C
`
`Fig. 2: The mobile X.400 email example
`
`The queuing service as part of the disconnected operations handling also offers some other
`specific functionality. The forwarding of data (i.e. mails in our example) operates reliably;
`when a mail is passed from a sender queue to a receiver queue or to an intermediate queue at a
`domain manager´s site, explicit confirmations that are imbedded into a two-phase protocol are
`exchanged and enable recovery after transmission failures. Moreover, multicast queues are
`already supported by our implementation. Finally, priorities can be used to control the transfer
`of data between queues.
`After delivery to the actual domain (Fig. 2: Domain B), the X.400 subsystem which resides on
`the mobile station is able to receive the messages and to hand them out to the user. A
`conventional rUA equipped with an additional viewer acts as a user interface. In the context of
`message delivery (to the destination domain and station) our architecture supplies assistance
`for the evaluation of the link-quality (cost and bandwidth evaluation). In the case of a high
`bandwidth and low cost link (e.g. Ethernet),all messages will be transferred without
`compression. In the worst case (expensive and low bandwidth link) only the textual parts of
`the messages and our special body part (which includes the description of the multimedia
`parts) will be fetched (preview). Afterwards, the multimedia body-parts (video, audio, etc.)
`can be transferred in a (not necessarily quality-preserving) conversion into a compressed
`format in order to reduce the amount of data. The compression method is chosen by the user
`(options dialog offered by our viewer). After the selection of a specific compression format a
`compression service encodes the data (/SCK95/). We currently support audio attachments in
`WAV format, pictures in JPEG format, video clips in MPEG format, postscript files and
`selected formats of DeTeBerkom MMM /ADH93/. The receiving station caches all
`transferred data.
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`4. Experiences
`Most of the required functionality maps rather naturally onto the generic components of our
`support platform. Of specific importance for the mail application is the disconnected operation
`service; specific instantiations of the caching and queuing service have been on top of this
`facility. The information representation concerning applications, users, available network
`bandwidth, and other system data is handled by the active database of the generic support
`architecture. The queuing service complies with all requirements of mobile multimedia
`transport systems.
`Major general experiences with this prototype implementation are as follows:
`
`• Distribution transparency: It became obvious that a very high level of transparency of
`distribution (as addressed by conventional client/server solutions) is not possible nor
`desirable anymore in mobile environments. However, flexible software support for mobile
`distributed applications with varying levels of distribution transparency is a central issue.
`The bandwidth and cost management component illustrates a possible solution.
`
`• Generic support: Explicit and generic support for bandwidth management, disconnection
`handling, and resource access and location management is therefore required. Our
`experiences confirm that it is possible and reasonable to imbed this into a generic support
`architecture so that it can be reused by several applications such as mobile e-mail and
`mobile database access.
`
`• Network information: It is sufficient to represent the network-related information at a
`rather high level as a basis for semi-automatic decisions concerning resource access (rather
`than using low-level network management information).
`
`•
`
`Standards. The use of established client/server-based industry standards, namely DCE, is
`crucial for achieving good acceptance of application developers and end users. The use of
`RPC has significantly simplified the integration of workstation and PC platforms based on
`the
`interoperability of DCE and Microsoft RPC using NDR (Network Data
`Representation). However, obvious potential for optimizing the performance of RPC
`exists based on dynamic adaptation of block sizes at the transport layer.
`
`5. Conclusion and Future Work
`This paper presented an overview of a support platform for mobile computing in conjunction
`with a mobile X.400 e-mail application. We outlined that generic support for mobile computing
`is desirable and technically feasible to a large degree. Moreover, we illustrated that
`conventional applications such as e-mail require significant enhancements in order to achieve
`proper functionality in mobile system environments. Currently, we are working towards the
`completion of our prototype, to be displayed at CeBIT ´95. Related research projects within our
`group are focusing on quality of service for multimedia transmission, enhancements of remote
`procedure call for mobile environments, and advanced client/server applications.
`
`The final paper will include performance data, a detailed description of selected parts of of the
`support platform which are most important for the e-mail application and it will introduce a
`model and toolkit for handling multimedial data under consideration of non-technical
`(subjective, human) parameters.
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`Acknowledgments
`We would like to thank all involved colleagues and students for their significant efforts in
`building the implementation of the described prototype. Moreover, we would like to thank
`Digital Equipment GmbH (CEC Karlsruhe) and Daimler-Benz AG (Research Center Ulm) for
`their generous support of our work.
`
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