Service Maps for Heterogeneous Network Environments Dirk Kutscher Technologiezentrum Informatik (TZI) Mobile Research Center Bremen
[email protected]
Abstract Future heterogeneous wireless networks will encompass different link layer technologies and allow selecting the most appropriate network depending on different criteria. To support mobile nodes in the selection process, network information services are developed that provide the mobile node with sufficient information about its network neighborhood, typically focusing on the optimization of handover processes. In this paper, we take a more general approach towards network information services, which is needed to support mobile communications in the existing environments of WLAN hotspots and wide area mobile communications networks. We introduce the notion of “service maps”, a mobile data management approach allowing a mobile user to obtain a detailed view of available networks and the services they offer depending on the user context such as geographic position, mobility paths, and application requirements. We review our experiences with networking in highly mobile scenarios from which we derive a set of requirements for a general information service and present our system design and data model that also accounts for business and (incremental) deployment considerations.
1. Introduction In the emerging heterogeneous environments of wireless networks, mobile nodes (MNs) will be equipped with interfaces for different link layer technologies and thus be able to associate with different networks. Network selection may depend on numerous criteria such as radio signal strength, current network utilization, expected throughput, geographic position, and estimated trajectory of the MN, but also upon pricing and available higher layer services. The most important requirement to facilitate seamless mobility and efficient handover is to provide the MN with sufficient information about available networks and their characteristics before connectivity loss is experienced. Basically, two alternatives are available to obtain this informa-
J¨org Ott Helsinki University of Technology Networking Laboratory
[email protected]
tion: 1) The MN may sense its immediate environment and, after picking up a radio carrier, listen for information distributed within such a candidate network itself (e.g., beacon frames available in 802.11). 2) The MN may consult a network information service in a network it already uses, i.e., from outside a candidate network. This allows the MN to prepare, e.g., the selection of and association with a candidate network and to invoke the handover process. Approach 1) has the advantage of being self-contained but the disadvantages that a network must be in range, scanning multiple channels may be time-consuming (as for 802.11), and that devices moving between networks using the same link layer technologies require either two interfaces or need to interrupt communication in their active association while searching for candidates. While approach 2) allows overcoming these disadvantages, it requires a usable link being available in the first place and its operator’s willingness to distribute service information. With 1) being obviously necessary for stand-alone operation of a network but 2) allowing for the desired optimizations, hybrid methods are being pursued in which meta information about available networks are being distributed via other networks while the necessary low-level information needed for associating with each one is available from within. The information made available about a candidate network prior to association is typically rather minimal but sufficient to allow for network selection. Once a network has been chosen, additional configuration data, e.g., security and IP stack parameters, can be obtained to prepare an expected handover ahead of time, e.g., in Fast Mobile IP [7] scenarios. However, this dissemination of network parameters such as access router IP addresses and IPv6 network prefixes, is not sufficient for mobility in real-world heterogeneous networks. E.g., for supporting mobility in 3G/WLAN environments, more detailed information must be made available to MNs, requiring different approaches for management and distribution of network information. Today, the most common form of networking in heterogeneous environments is handover from 2G/3G wide area networks to local WLANs. Many mobile phones, laptops
etc. are equipped with interfaces to these networks and can be configured to switch between them, either triggered manually but also automatically. While the network association process for 2G/3G networks is sufficiently standardized and can thus be automated (once a policy decision for a network has been taken), WLAN association is more complex. As described in [23], authentication and association procedures can differ significantly, depending on the type of the installation, e.g., enterprise WLAN, private network, or public hotspot and even across different public hotspots.
operator may also form an important part of a user’s policy decision for network selection, hence such application service information needs to be made available in advance. This papers analyzes the additional requirements for generalized network information services (ISs) for heterogeneous networks and proposes an approach that can be used to transport basic network information as well as higher-layer service information, providing different interaction mechanisms in order to accommodate a broad range of networks and applications. We start by reviewing related work in section 2, focusing on the network IS domain. In section 3, we present the Drive-thru Internet environment as one extreme use case from which section 4 derives a list of requirements. We introduce the design of a generalized network IS in section 5 and conclude this paper with a summary and a discussion of future directions in section 6.
2. Related Work
Figure 1. WLAN Hotspot information display
Figure 1 illustrates some of these problems: it depicts an information display at a public hotspot intended to inform users about the available Wireless Internet Service Providers (WISPs) at this specific location. Based on this information and personal service contracts, the user can decide if she wants to initiate network association — a process involving manual interaction typically following the webbased Universal Access Method (UAM, [2]) authentication procedure. This very common example illustrates that automatic network selection and association requires additional information services beyond basic handover optimization hints. This is especially true when we consider mobile scenarios, e.g., as in the Drive-thru Internet project (section 3) where hotspots are accessed from moving vehicles so that selection and association must be performed quickly and automatically. In such environments it is especially useful to have advance knowledge of hotspots and their locations, providers, tariffs, and access parameters. Even if the user is not moving fast, allowing for automated authentication based on preferences would reduce the setup hassle and improve the user experience noticeably. Finally, network access may not be the (only) service to base provider selection on: application services available in a hotspot, a WiMAX link, or a 3G service run by a particular
Today, ISs are developed with different objectives. As described in [27], the main focus is on avoiding connectivity interruptions as may be caused by handover in heterogeneous networks. IEEE 802.21 Media-independent Handover Services [1] defines Media-independent IS (MIIS) as part of a handover service including a command and event service for communication between MNs and network elements. MNs shall be able to acquire a global view of heterogeneous networks to facilitate seamless handover and to allow network selection according to the MN’s requirements. MIIS is intended for L2 information about network elements in the neighborhood but also about higher layers (Internet access parameters, descriptions about VPN and VoIP services, etc.) MIIS defines a data schema that allows representing this information in protocol transactions. It also has the concept of looking for network services in a geographic region, e.g., looking for available 802.11 networks using the current 3G link. IEEE 802.21 does not define the IS transport mechanism, but suggests to transport link layer information directly with the link layer protocol (which would require extensions to many link layer protocols). The intended interaction scheme for IS is request/response. In the IETF, ISs have played a role in different protocol developments. The Candidate Access Router Discovery (CARD) protocol (RFC 4066 [4]) can optimize next access router selection by reverse address translation, enabling an MN to map an observed access router (AR) layer 2 ID to its IP address by querying its current access router. RFC 4068 (Fast Handovers for Mobile IP [7]) defines off-link inverse neighbor discovery by which an MN can query its current AR for subnet information of neighbor access points in order to prepare handover. The discussion of IS in the context of IEEE 802.21 has led to an effort for developing an IS transport in the IETF
and some initial proposals were submitted. [6] performs an analysis of requirements for a media independent handover IS. [10] specifies a transport independent network information representation format for allowing a node to discover information about surrounding networks and is targeted at ISs for Fast Mobile IP and CARD scenarios. For commercial hotspots, the most common method is web-based login, recommended by the Wi-Fi Alliance [2] as Universal Access Method (UAM) and based on the notion of captive portals: a user is granted L2 access but, to obtain full Internet connectivity, a web-based authentication process must be performed first. We have discussed UAM in detail in [23]. It has the advantage that it imposes very little requirements on user equipment, but the authentication approach is clearly targeted at human users and difficult to automate, e.g., for automatic network association for mobile computing. It would be preferable to have the hotspot operator disseminate information about available services and providers, tariffs, and access methods using a standardized protocol and description language. The Wi-Fi Alliance suggests the service announcement-based smart client authentication protocol [2]. Other announcement-based approaches are discussed, e.g., in [3] and [11].1 The existing approaches do not consider network ISs and service location together in a holistic fashion. While some activities for IS such as IEEE 802.21 are considering distributing network information to an MN for not-connected networks, these approaches are limited to the handover optimization use case. On the other hand, service location approaches so far have mainly focused on locating and describing services directly available in the currently associated network, without really considering the requirements for infrastructures that can provide MNs with service information independent of the currently connected network.
3. Drive-thru Internet The Drive-thru Internet approach [17] aims at providing Internet services to mobile users moving at high speeds. We enable network access by exploiting connectivity from conveniently located WLAN hotspots next to the road while the user traverses a hotspot’s coverage area. The Drive-thru architecture allows existing and future applications to take advantage of such potentially short and unpredictable periods [18]. It relies on a connection splitting approach where a proxy in the fixed network maintains long-lived connections on behalf of mobile clients that would otherwise be affected by intermittent connectivity. The Persistent Connection Management Protocol (PCMP) is used for the com1 Independent
of specific wireless network access, a variety of service discovery protocols have been developed, most of which focus on “local” services within a connected network (e.g., [8] [5] [14]) but reviewing this entire landscape is beyond the scope of this paper.
munication between the mobile Drive-thru client and the Drive-thru proxy, allowing for maintaining persistent transport layer sessions. [19]. Figure 2 depicts an overview of the Drive-thru Internet architecture. Drive-thru Proxies
Server
Internet ISP A
ISP B
Drive-thru PEP
Connectivity Islands
Drive-thru Client (DTC) + Application Clients (AC)
Figure 2. Drive-thru Internet Overview
Drive-thru Internet clients must be able to operate in today’s existing WLAN infrastructure that mostly consists of public hotspots. In [20] we discuss requirements for automatic hotspot association in detail and in [23] we describe a mechanism we have developed for automating the authentication process with different types of hotspots. The mechanism infers the type of hotspot and its authentication mechanisms and associates on behalf of the user, relying on some heuristics and user preferences[23]. Without any IS, the only way to find out details about access points is to actually move into the coverage zone and initiate a probing process, which involves detecting the fundamental WLAN parameters (open/closed system, ESSID, usage of WEP/WPA), comparing this information to an internal database for hotspot selection, and then performing the actual authentication. In case of a public hotspot with UAM authentication, the system has to find out about the WISP (e.g., by analyzing the UAM web page) and then perform the authentication by providing the pre-configured user credentials. Due to the necessary probing steps (some of them have to be performed in a trial-and-error scheme), this process can take two seconds or longer. The plurality of authentication schemes and the diversity even within the perceivedly straightforward web-based authentication procedures that resemble the UAM approach but, nevertheless, exhibit serious differences may prevent automated authentication altogether unless hotspot-specific authentication rules are implemented in the MN. From these considerations, two application areas for a network IS become apparent: 1) Learning about available Internet access services (hotspots) in the neighborhood including their access schemes and 2) learning about services
available at a specific hotspot. Advanced information about available hotspots per 1) is especially useful for vehicular networking because, in many cases, the path of the vehicle and its current position are known. Knowing about the positions of eligible Internet services allows the connectivity management subsystem and the applications (if they can be made aware) to schedule their behavior according to expected connectivity and thus achieve a better user experience. If several hotspot providers are available in the same area, the choice may depend on which of these offers Drivethru-specific services as add-on as can be determined per 2).
4. Requirements The Drive-thru Internet example above shows that Network ISs for real-world applications must be designed in a more general way. In fact, even if we only consider the handover-optimization objective, a more generalized approach is needed to enhance seamless connectivity in hotspot environments and nomadic computing scenarios. Specifically, it is important to allow for learning about a network’s services and configuration parameters without actually associating to the respective network, i.e., allowing for independent IS that can be accessed in advance, before network association. The 802.21 specification [1] proposes an IS architecture for handover optimization that already has the notion of access network independence. However, the concept is presently limited in scope in two important ways: it covers only lower layer services (i.e., network access) and access to the information is restricted to interactive retrieval.
Figure 3. Sample coverage scenarios
4.1. Network Association and Handover The discussion of mobile and nomadic hotspots usage in section 3 has already highlighted the need for detailed service descriptions, including operator details, supported authentication mechanisms, etc. Such information should
be augmented by further details such as tariff options (e.g., time vs. volume vs. flat-rate, accounting units, roaming charges). While this requirement particularly applies to today’s WLAN hotspots, we expect it to be also relevant for future deployment of public IEEE 802.16 services. [27] considers handover facilitation for different coverage scenarios: overlapping coverage (e.g., a hotspot within a 3G service area), tangential coverage (adjacent cell of the same of different L2 technology), and isolated coverage (gaps between cells) as depicted in figure 3. These scenarios pose different requirements on an IS. E.g., in the overlapping coverage scenario, an MN could actively scan for available points of attachment using two or more interfaces and detect candidate networks itself. In the non-overlapping scenario (tangential or isolated coverage), however, an MN has no possibility to learn of alternative points of attachment. When moving in the tangential scenario (with persistent connectivity), the MN can obtain information about alternative networks over its current connection. In the isolated case, connectivity will be intermittent (as in Drivethru Internet) so that the MN will not permanently be able to obtain information from ISs. The isolated case also represents bootstrapping after powering on an MN. In reality, the borders between these scenarios are likely to be blurred: mobility and radio attenuation/interference may easily turn overlapping or tangential coverage scenarios into isolated ones. This variety implies that an IS cannot rely on a single communication mechanism: instead, both network internal and external information sources must go together, the former for isolated cases and bootstrapping, the latter for handover optimizations. Moreover, handover optimization may require more than just layer 2 and 3 ISs. In heterogeneous networks that include WLAN hotspots, IEEE 802.16, and 3G networks, more detailed information is needed: about whole networks with wide-area coverage (such as a specific 3G network) as well as about regional or local connectivity islands such as hotspots and IEEE 802.16 zones. For mobile users in intermittent connectivity scenarios, such as Drive-thru Internet, the geographic position and the geographic scope of points of attachment are important parameters for selecting networks and for estimating future connectivity periods and possibly even influencing a user’s movement. One common scenario [1] is based on an MN querying an IS for specific information about a set of candidate networks or a specific point of attachment. However, this approach may become inefficient as the coverage area and the number of MNs grows. Furthermore, it is inherently limited to bidirectional networks and thus cannot exploit wide-area coverage as offered by (DVB-based) digital broadcast networks that also support data transmission to a large number of users at comparatively low cost. Often, these networks are even designed to accommodate mobile users, so they
could provide an alternative way to distributing network service information, especially when other alternatives are not available. Adding a push-based distribution would help dealing with unidirectional broadcast networks and also offer a scalable distribution mechanism. In order to provide MNs with relevant information for network selection and handover, it is required to allow for specific queries (and effective filtering operations). E.g., an MN should be able to query for specific available networks by specifying the network type (L2 technology), the geographic service scope and characteristics such as availability of IPv6 connectivity etc. The filtering requirements can depend on multiple factors such the MN’s current position, its path and speed (for geographic selection), but also on communication requirements, e.g., required bandwidth, application requirements (for certain services in the access network) and payment factors (depending on a user’s service contracts and roaming possibilities). Accessing ISs and filtering incurs power consumption as it require actions (including data reception) from the MN. Staying current on the surrounding network services requires permanent action on the MN. To optimize resource utilization, MNs should be able to move parts of the processing to the network services infrastructure. This can be achieved by the MN registering interest in certain types of network and/or locations and then being notified only about relevant service information, e.g., whenever something changes.
4.2. Higher Layer Services Higher layer ISs are important for both network selection and configuring application and protocol entities on the node itself. An example is a user’s VoIP-enabled mobile phone with multiple network interfaces. Before associating with a particular provider in a multi-operator hotspot, the phone could select the operator for the local SIP services (and determine whether such services are available at all) as this may influence the WISP choice. Furthermore, the MN may need the specific service configuration. With many conceivable services, clearly, the types of service and their individual configuration description need to be extensible in order to accommodate development of new services. Although, many applications will benefit from a standard vocabulary, provider-specific extensions must be possible. In addition, points of attachment may also offer local services such as access to multimedia content, regional information services etc. and advertise some free “walled garden” services to the user. In fact, any form of content provisioning may be viewed as a service that can be made accessible by distributing information through an IS. In this case, the service description needed to contain information about the content, i.e., meta-information, and information how to access the content, i.e., configuration information.
The filtering and distribution requirements apply here as well: assuming that higher level services change more frequently and are more complex, efficient distribution becomes crucial. The potential variety and complexity of services demands additional flexibility: To facilitate an MN’s network selection based on higher layer service descriptions, it is not required to deliver all configuration details, which would lead to scalability problems both on the MN and the IS infrastructure. Especially highly mobile nodes experiencing intermittent connectivity and scarce network resources must be able to obtain an overview of available networks and their higher layer services first, then perform network selection, and finally retrieve the configuration details only for the selected networks. This entails requirements for specializing coarse-granular service descriptions by one or more levels of detail.
4.3. Deployment Aspects Network ISs in heterogeneous environments also have important organizational and business-related aspects: a model where ISs are local services in the access network (per point of attachment or per single operator) is not viable in an environment of competing operators. An operator may have no incentive to distribute information about their competitors’ networks in the neighborhood as this could cause customers moving away to the competition. If such an operator only distributes information about own network services, an MN will only receive an incomplete service map of a certain region. This implies several requirements: most importantly, access to ISs must not be limited to the local access network so that a single operator may provide service information about its “neighboring” access points even if they are miles away. While, for efficient operation in handover scenarios, it should be possible to place the IS function on an L2 link, regional or wide area distribution based upon Internet technologies must be supported, too, thus enabling operator-independent ISs. Specifically, it must be possible to have local, operator-specific IS coexist with independent, general IS in the network. An MN should be able to obtain service descriptions from both sources for constructing a complete and consistent view of the network. This should also allow for third-party service brokers that collect and bundle service offerings from different providers. As information access is not geographically or topologically restricted, neither are service providers, and thus services can be made available from around the globe. Taking advantage of multiple IS sources is also a strong argument for a plurality of transport mechanisms and interaction patterns. In a WLAN, an IS may be queried for IP configuration information of surrounding points of attachments of a specific operator. This information will typically be obtained by query/response interactions. In addi-
tion, a public service might distribute operator-independent information over a unidirectional broadcast medium as an overview of available services in a region. Furthermore, the user may have subscribed to a web-based IS distributing detailed information for specific services in a publish/subscribe style. All three interaction patterns are useful for supporting network and service selection and should be supported. If we consider operator-independent IS sources, propagating service information from other providers must preserve authenticity and integrity of service descriptions even across multiple aggregation, filtering, and forwarding steps. Otherwise, forwarding false information may give room to denial-of-service attacks or allow biasing customers towards certain service providers.
5. Service Maps Based on these requirements we have designed a general-purpose distribution system for network service information at different layers general enough to support arbitrary types of services. The key elements are: 1) the notion of (geographic) positions and regional scopes for service provisioning; 2) support for different transport and interaction mechanisms: query/response, subscribe/notify and multicast distribution; 3) separation of the origin of service information from the distribution of service information; and 4) independence of the description format.
5.1. Main Concepts The fundamental concept of regional service maps is that receivers may obtain service descriptions from different sources, using different transport mechanisms and, potentially, different network interfaces. By evaluating some fundamental information about the advertised services such as their geographic (and/or network-topological) position and the type of service (e.g., Internet access, VoIP service), receivers can construct service maps that can be organized and searched by multiple ordering criteria. Based on their current disposition (geographic position, movement direction/speed, required service, existing service contracts, available network interfaces etc.) receivers are enabled to filter the service map in order to identify the relevant and most appropriate services. Conceptually, service maps are collections of capabilities describing services at different layers, their service providers, and relevant information for accessing them. Furthermore, service maps include location information describing where or how a service is available. The term location encompasses both geographic positions and civic locations, and descriptions may provide detailed information about a service’s availability. Thus services are described in an n-dimensional tuple space and different attributes may be
consulted to create a projection from this tuple space onto a particular map. This projection may involve a filtering process to limit the result to a certain “area”. E.g., civic information may be used to map available services to a particular building and geo-position coordinates may be used to identify wireless ISPs in a certain region. When moving around (physically and networktopologically), receivers will obtain new service descriptions over different channels, e.g., by receiving periodic broadcast/multicast announcements over a unidirectional network such as DVB data services, by querying local IS in the currently connected access network, or by relying on a public Internet-based IS that can be reached over any network link and providing a subscribe/notify service for specific types of service descriptions. The received service descriptions do not necessarily have to be mutually disjoint with respect to region, type and even the specific service that they describe. Instead, the same service description may be available over different channels. Service descriptions carry a globally unique identification, e.g., in order to detect duplicates. This unique identification is preserved over the whole delivery chain, i.e., it is a fundamental part of the service description and cannot be changed by intermediary systems such as service description aggregators and distribution systems. The identification concept also provides for versioning of service descriptions in two dimensions: 1) service descriptions may be updated and thus be available in different versions over time, and 2) service descriptions may occur in different variants with respect to their level of detail. For example, in order to get an overview of available Internet access services in a given region, using a low-bitrate, but ubiquitous and low-cost network link such as PHS in Japan, it is more appropriate to distribute rough service descriptions providing only the relevant information for service selection, and then have the receiver obtain a complete description of the selected services, e.g., in order to obtain the service configuration for actually using the service. To both concepts, consecutive versions over time and specialization, we apply the concept of delta encoding for distributing different versions of a base service descriptions. For example, if we consider a multi-channel broadcast distribution mechanism, we can apply concepts from scalable content distribution over unidirectional links and provide different repetition patterns for different variants of a specific base version: the base version could be distributed in longer repetition cycles, while deltas to that base version that can be used to construct more current variants, could be distributed more frequently. Similarly, specializations could have a yet another repeat rate. By applying these concepts, we meet the requirements for scalability both on the receiver and the distribution network side. The specialization concept is based on the notion of de-
riving a service description from a base version. The latter is explicitly specified in the derived specialization. Figure 4 shows an example: two WLAN hotspots are announced by broadcasting over DVB-T focusing on location and general provider information. In the WiMAX cells in the vicinity of the hotspot, further details for service selection and configuration for one hotspot are available. Within the hotspot itself, a link layer IS concentrates on the access parameters for efficiency while, after association, service configuration and access parameters are provided upon request. DVB-T
{ id:3361875, wlan hotspot, public (op-X), 802.11b, geopos (E … N …), 50m } { id:3734190, wlan hotspot, public (op-Y), 802.11g, geopos (E … N …), 200m }
WiMAX cell
{ id:3361875, wlan hotspot, public (op-X), 802.11b, geopos (E … N …), 50m, access { ch 1 }, auth { 802.11i, essid (op-x), realm (…), tariff { time (wlan123) 60s eur 0,10 } { flat (wlanworld), … } { service sip (op-A) } { service mp3 (op-C) } }}
WLAN
L2 beacon { access { ch 1 }, auth { 802.11i, essid (op-x), realm (…), … } { id:3361875, wlan hotspot, public (op-X), 802.11b, geopos (E … N …), 50m, access { ch 1 }, auth { 802.11i, essid (op-x), realm (…), tariff { time (wlan123) 60s eur 0,10 } { flat (wlanworld), … } { service sip (op-A), access (register, sip:…, …), access (message, sip:…, …)} { service mp3 (op-C), access (streaming, rtsp://..., …), access (download, …) } }}
Figure 4. Service description details
5.2. Platform: Internet Media Guides The framework for Internet Media Guides (IMGs) [16] defines an environment for delivering guides describing the availability of multimedia contents and programming from many originators to an arbitrary number of receivers. The IETF is in the process of defining a set of transport mechanisms based on existing Internet protocols, an envelope format, and a URN scheme for IMGs. All these mechanisms are independent of the metadata describing the programming information—which are imported from other specification such as SDP [9] or TV Anytime. The IMG transport offers three options: multicast-based push distribution (IMG ANNOUNCE) (via unidirectional networks) is supported to reach large receiver communities at minimal overhead [13], interactive inquiries via HTTP (IMG QUERY and RESOLVE) allow individual receivers to retrieve the portions they need [24], and subscription (IMG SUBSCRIBE/NOTIFY) may be used by receivers to obtain asynchronous notifications about changes [15]. IMG originators and receivers may interact directly or via one or more intermediaries (IMG transceivers); the latter may perform filtering and aggregation of IMGs, e.g., for regionalization or personalization purposes. Metadata comprising an IMG may be included in its entirety (Full IMG), in part following a differential encoding (Delta IMG), or by URI reference (IMG pointer). The type (full, delta, pointer) and the description format are identified in the IMG envelope that
also carries validity and version information and may further include authentication information [26]. While the original IMG idea focuses on describing multimedia content, we have generalized this notion to service descriptions [21]: access to a specific multimedia content implies a certain type of service, namely multimedia streaming, that can also be made explicit, thereby allowing additional services to be described as well. As the metadata format for IMGs is open and the envelope flexible, we can include, e.g., WLAN hotspot service descriptions and make use of the cross-network IMG transport mechanisms for advance distribution of network service information [22]. This provides the necessary mechanisms for Internet-wide delivery of service information represented as IMG metadata and also allows for localized information dissemination, e.g., by restricting the multicast scope.
5.3. Data Model The data model for service maps supports the notion of different specialization levels and extending coarsegranular descriptions into more detailed ones. A service description consists of the following components: 1) information about service providers (unique identification), 2) information about services (not-bound to locations), 3) location descriptions (geo-positions or civic addresses), and 4) specific service description instances (service descriptions bound to provider and location information). Figure 5 depicts this model conceptually.2 A service description can provide sub-structure, and for coarse-granular descriptions, not all elements of a parameter’s sub-structure must be present. Within a description instance, the different parameters can be identified uniquely, thus allowing for expanding an existing coarse-granular description by complementing it with a fully-specified parameter sub-structure that can be received independently from the coarse-granular description. Extending a coarsegranular description is implemented by adding a subtree of structured information to the original, partly-specified description. This means, there are several identifiers for a service description: 1) the globally unique identifier for the service description, 2) the version identifier, 3) identifiers for sub-elements within a single description. The data format for service descriptions is XML, and a single service description is represented by one XML document, providing child elements for every parameter. A parameter element may have arbitrary sub-structure (depending on application-specific data schemes), but there is a set of well-defined data types for leaf values, similar to the ses2 The
complete formal definition cannot be provided in this paper due to space constraints. The data model and other specification documents for service maps are available at https://prj.tzi.org/cgibin/trac.cgi/wiki/ServiceMaps.
sion description approach described in [12]. For specializing coarse-granular description, we use concepts from XML document tree manipulation and apply a mechanism similar to the one described in [25]. A single description document can contain services descriptions for multiple locations and from multiple different providers, e.g., when aggregating descriptions from different sources or when constructing a specific result set as a response to a client request. Service selection, filtering and aggregation can be performed either by client systems or by infrastructure services. Clients may include search parameters such as locations and service tags in their requests to the infrastructure (for QUERY/RESOLVE interactions). As depicted in figure 5, locations and service descriptions may be tagged with descriptive strings that are provided by the original service provider and allow users to identify interesting services. Announcement IMG Envelope • Source Identifier • Scope • Version •…
Locations
Services Local Access
Location reference
Refinement URI
Provider reference
Internet Access Tags: Internet
Tags: Bremen, Space-Park, building
Dependencies: Local Access
GEOPOS(53.11N, 8.74E)
Refinement URI
Reichstag, Berlin GEOPOS(52.52N, 12.38E)
Service #1
Tags: 802.11b/g, local-access
Space-Park, Bremen
Tags: Berlin, Reichstag, building
Service Instances
Service reference Refinement URI
Media on Demand Dependencies: Local Access
Providers Internet Access Refinement Authentication method: UAM
ID, Details UAM Details
Provider Y
for Berlin it is only reported which WISPs offer how many hotspots). For the city area of Berlin (50 km radius of the city center), precise hotspot locations and WISPs are reported but no further hot-spot details. For a central area in Berlin with 2 km radius the same (“part”) information or also additional services (“full”) are reported and, finally, for adjacent hotspots, further information (such as utilization, channel details, etc.) are included.
Tags: audio, video
Refinement URI
Provider X
Figure 6. Hotspots of a single WISP in Berlin
Tariff Details
ID, Details
Figure 5. Service description structure
5.4. Case Study: Drive-thru Internet Applying the above concepts to Drive-thru Internet, mobile users are primarily interested in finding suitable hotspots, i.e., visible from their intended path and accessible in terms of service contracts and suitable tariff scheme. Figure 6 depicts the hotspot distribution of a single WISP in the region of Berlin (yielding 453 hotspots in total). Secondarily, the mobile user may be interested in local services such as music content provisioning and local news updates. The information about hotspots, their locations, WISPs and further services are distributed by means of DVB-H broadcast channel for the entire region around Berlin. Table 1 summarizes the service map data to be distributed at different levels: For the entire country (“DE”), we assume that hotspot availability is grouped into regions and, e.g.,
DE Berlin 1000* 500 600 600 4 4 1000 1000 1000 500 500 500 1104 554 900 120 10 37 55% 25% 15 46 DVB-H 400 3.8% 11.5%
City area part full 50 50 600 600 4 4 1000 1000 200 200 500 2000 134 434 30 30 36 115 15% 15% 41 133 WLAN 1000 4.1% 13.3%
Local # Hotspots 10 Data (bytes) 600 # Providers 4 Data (bytes) 1000 # Instances 40 Data (bytes) 4000 Volume (KB) 170 Interval (s) 15 Rate (kbit/s) 91 L4 overhead 5% Total (kbit/s) 95 Distribution Rate (kbit/s) Utilization 9.5% * Hot-spots are aggregated into regions which are reported.
Table 1. Distribution rates for service maps Using our XML-based data representation, we calculated some 600 bytes of data for the hotspot location (identification, civic address, and GPS coordinates); 1000 bytes for per WISP and its service description, and some 500– 4000 bytes for linking WISP services and locations— depending on the details available at the different distribution levels. From the distribution of WISPs in Germany today, we obtain some 500 hotspots for the city of Berlin and we assume four equal-size WISPs offering access in 500 hotspots. This worst case estimate is also applicable to more
WISPs with different distributions as the number of WISPs has only a rather limited impact on the total data volume. We finally assume that the information about all WISPs are distributed within one service map. The table further lists the distribution intervals at each level and the resulting net data rate if always full service maps are announced. The transport overhead comprises the FLUTE protocol overhead (approx. 5%) [13] for the IMG ANNOUNCE operations and a variable FEC overhead (proportional to the announcement frequency). Applied to the net data rate, we obtain the gross data rate and set this into relation to a particular distribution channel: a single DVB-H service channel operating at 400 kbit/s and a WLAN multicast distribution operating at the lowest rate of 1 Mbit/s. As can be seen from table 1, the country-wide multicast announcement of WLAN service maps requires only some 15 kbit/s equal to less than 4% of a single DVB data channel that could be allocated for this purpose. This leaves room for expansion and for a variety of other services. For a specific city area, some 12% of a similar channel is required for distributing more detailed location information. For a dedicated city area, as little as 4% of a WLAN capacity is required when distributing only the location information— and a lower frequency would make this distribution suitable for cellular (MBMS) broadcasting as well. Including further information, in contrast, would require some 14% WLAN capacity and is hence not realistic; the same applies to frequently broadcasting full service information for adjacent WLANs taking 10% of the data rate. Therefore, we revert to partial multicasting peered with interactive distribution: as each MN knows when it is connected and which information it is interested in, it can inquire the relevant service map fragments directly taking into account its driving direction and applications—thereby reducing the interaction to a request of 1 KB or less and a response covering the selected subset (say, 10 %) of the hotspots or 40 KB. This information can be cached by the MN and need not be inquired again. The MN may subscribe to be informed about changes to the previously received information and will be updated in case of any changes. Very detailed configuration information will only be exchanged while accessing a particular hotspot. Assuming a usable coverage area of 1 km along the road in a rather optimal case [18], two times three lanes, and one car every 100 m per lane, at a velocity of 120 km/h each car remains within the WLAN hotspot for one minute and a total of 120 cars will pass through the WLAN hotspot per minute so that two detailed requests may need to be satisfied per second. This request applies to the current hotspot only and assuming 10 KB per request-response pair and ignoring caching, this incurs a load of 160 kbit/s that is easily accommodated in a WLAN. These mechanisms will ensure that, particularly for 3G service areas, the available bandwidth is not
unnecessarily filled up with service announcements. Only if the density of mobile users grows large and the number of interactive inquiries rises too much, the network IS may switch to broadcasting the most frequently requested parts of a service map—which can then be augmented by clients by means of interactive requests as needed.
6. Conclusions Network ISs are becoming an important functions in future heterogeneous wireless networks: they allow MNs to gain an overview of their environment, e.g., their direct network neighborhood, thus enabling several functions, from handover optimization to service location. When advancing from homogeneous, single-operator networks to heterogeneous multi-operator scenarios, ISs can relieve MNs from determining network characteristics and services by performing error-prone probing processing. They provide a more robust and more efficient mechanism, which is not only important with respect to MN local system architectures but also with respect to network resource utilization. We have shown that IS can enable many more functions than handover optimization. In fact, it is required to distribute more detailed information in many heterogeneous networking scenarios, e.g., for WLAN hotspots, as we have illustrated for Drive-thru Internet but as is also apparent from the large of number of different, currently incompatible, web-based information services (hotspot locator websites) that are intended for manual usage. However, when designing a broader, more generalized IS, several new requirements have to be considered, compared to the local-only IS environment, e.g., networkand operator independence, support for multiple transport mechanisms, scalability and extensibility of the description format. The main contribution of this paper is to discuss these requirements and to propose a corresponding solution, based on current standardization efforts in the IETF. The IMG-based approach we have described relies on the IMG distribution infrastructure for service descriptions, on which we base our concept of regional service maps for IS. The main notion of this concept is the idea of distributing network service descriptions where services are described in an n-dimensional tuple space, and different attributes may be consulted to create a projection from this tuple space onto a particular map. This projection may involve a filtering process to limit the result to a certain “area” — the “map” of interest for a particular receiver. Future investigation topics include the question of the relationship between our generalized, operator-independent IS approach and current IS approaches targeted at handover optimization such as IEEE 802.21 and the related IETF activities. Two different questions can be investigated: 1) Is it useful to employ a common, generalized approach for IS
(for specific handover optimization and for broader usage as described in this paper) or are these applications sufficiently different with respect to efficiency requirements (e.g., message size, implementation complexity)? 2) How can lowlayer IS benefit from a ubiquitous generalized IS, e.g., what are the interworking possibilities in order to allow low-layer IS to obtain handover-relevant information from a complementary, generalized IS and to distribute them in the most efficient way to MNs? In addition to these questions, our future work will include the development of specific data schemes for different application areas, including but not limited to public WLAN hotspot and application-specific higher-layer service descriptions. We have presented a new approach for mobile data management in future heterogeneous networks. Relying on the fundamental generalized IS approach and the results from our research agenda, we will move our generalized IS concept and its implementation to a large-scale deployment in order to demonstrate its usefulness for service discovery and utilization in heterogeneous networks. The ubiquitous availability of an network-independent IS and the concept of regional service maps can thus be leveraged for a better mobility support in future networks and for the development of completely new services in the future.
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