conditions, which deters the efficiency of the service delivery. This paper introduces a novel solution for the adaptation of the services to the momentary network ...
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE Globecom 2010 proceedings.
Enabling Ambient Aware Service Delivery in Heterogeneous Wireless Environments Marius Corici, Thomas Magedanz, Dragos Vingarzan, Peter Weik Fraunhofer FOKUS Institute Berlin, Germany {marius-iulian.corici, dragos.vingarzan, thomas.magedanz, peter.weik}@fokus.fraunhofer.de Abstract—With the development of novel access technologies e.g. LTE, WiMAX etc., the mobile devices are able to establish services over multiple wireless connections with different momentary capabilities. Being imported from the fixed line communication, the current services deployed are lacking an efficient mechanism of adaptation to the momentary connectivity conditions, which deters the efficiency of the service delivery. This paper introduces a novel solution for the adaptation of the services to the momentary network conditions of the mobile device which does not require a complete end-to-end re-signaling, by this reducing the communication over the wireless link and enabling efficient provisioning of the services and the development of new ones customized for the wireless environment. Furthermore, the concept is exemplified on the 3GPP Evolved Packet Core architecture and evaluated using the Fraunhofer OpenEPC testbed implementation. Evolved Packet Core; Heterogeneous Wireless Environment; Wireless Service Adaptation
I.
INTRODUCTION
With the development of a multitude of novel wireless technologies (e.g. WiMAX, LTE, Femtocell technologies etc.) a growing number of deployments are already using more than one access network in specific locations for delivering a mass wireless broadband connectivity to the multitude of mobile devices. These access networks complement each other in terms of offered throughput and operational costs, enabling an extended variety of connectivity parameters like maximum bandwidth allowed for a user and different delays to be available for the delivery of services to the mobile users. Due to the deployment of the new accesses as an over the top extension of the already existing technologies, in the near future it is expected to see a significant growth of the heterogeneous wireless environments in which mobile devices are able to connect and to exchange data over multiple access technologies at the same locations. On the other side, it is expected, due to the increased throughput capacity of the heterogeneous wireless environment that the number of users will increase in parallel with the development of novel applications especially targeting the mobile broadband communication. Current wireless applications (e.g. video on demand, file transfer, real-time video sessions etc.) consider that the connectivity of the mobile device is transparent to the service provider. The mobile device has the responsibility to transmit
notifications related to the resources that should be reserved for the specific services over the current access network. This implies that during service establishment procedure the parameters of the data session are negotiated between the mobile device and the service provider and then the resources are reserved at the level resulted from this negotiation. Whenever an event related to the wireless environment is detected by the mobile device (i.e. a handover, a modification of the momentary resources that can be allocated to the service etc.), it has to trigger a re-negotiation of the service parameters with the service provider in order to be able to adapt the data sessions of the service. The re-negotiation of the service parameters implies that a signaling session is established over the wireless link between the mobile device and the service provider, independent and after the actual event happened in the network similar to the establishment of the service. Although this mechanism offers a solution for service adaptation to the momentary context of the mobile device, it presumes that the mobile device is aware of the new service parameters that can be reserved in the new momentary context and that it is able to communicate this information to the service provider with a reduced delay not hampering the seamless characteristic of the communication. In order to optimize the procedures for the service adaptation this paper proposes a novel concept in which the network core takes the functionality of signaling the service provider on behalf of the mobile device. This enables a fast processing of events pertaining to the communication over the heterogeneous environment at a reduced communication over the wireless link. The procedures related to this new concept are then exemplified using the 3GPP Evolved Packet Core (EPC) architecture, a reference all-IP multi-access core network which integrates both 3GPP e.g. GSM, UMTS, LTE etc. and non-3GPP e.g. CDMA, WiFi, WiMAX access technologies. A main concept of the EPC architecture is the mobility of the users based on a coordination mechanism between link, network and application layer. The concept here presented integrates in the EPC as an extension of the policy based resource reservation functionality. The remainder of this paper is organized as follows: Section II provides the background of the proposed method. Section III describes the general concept and then exemplified on the 3GPP Evolved Packet Core architecture. Section IV describes
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the testbed used to get to the evaluation of the experimental results and in Section V conclusions will be provided. II.
BACKGROUND
The current state of the art separates the service adaptation from the changes of the access network context ([1], [2]). The service modification procedures are now triggered solely by the mobile devices or by the service platforms ([13], [14]), either at the request of the user or automatically by renegotiation of the service parameters. When the access network context of the mobile device changes (e.g. a vertical handover to another access network with different characteristics), the mobile device has to initiate on its own the service modification procedures including the resource negotiation with the service provider and the resource reservation. This solution is suitable for deployment environments which do not presume that the vertical handovers require frequently service modification, thus this procedure can be integrated with the user requesting modification of the service. In contrast, in dense wireless environments the mobile device is expected to roam through multiple accesses and to enable active services like real-time video streaming which may be adapted to multiple levels of resources (e.g. different throughputs and different delays) according to the requirements of the users and to the momentary network conditions. Instead in these environments, the service modification is less triggered by the requests from the user of the mobile device and more by the changes of the network conditions.
Figure 1 Conceptual Network Architecture
As depicted in Figure 1, currently, the service modification procedures are following the same path with the ones for the service establishment. When the mobile device executes a handover the conditions in which the service may be sustained change. The target access network has a different delay and may guarantee a different level of resources for the service at a different operational cost. Thus after the handover occurs, the mobile device re-negotiates the service parameters with the service provider. The newly agreed level of resources is then transmitted to the wireless core network which then executes the reservation procedures. The service adaptation procedure considers that the user of the mobile device may require a different level of resources for the services. However for the case of future vertical
handovers the users may set the parameters between which the services are to be delivered from the establishment of the service (like throughput or maximum accepted delay), and then the modification procedures are automatically triggered to select from the settings the level suited for the current network context. This automatic processing allows a fast adaptation to the wireless environment of the services. However it presumes that the services are adapted after the event of network context change is determined by the applications of the mobile device and presumes that a complete negotiation of parameters is executed with the service provider. As example network architecture, the 3GPP Evolved Packet Core (EPC) ([12], [1] and [2]) was chosen, due to its capability of providing all-IP network convergence between different types of accesses by offering a single unified mechanism of mobility management and resource reservation. As depicted in Figure 2, the EPC architecture includes a set of gateways which are transparently unifying the parameters of the different access technologies like LTE, UMTS, WiMAX, CDMA 2000, WiFi etc. The Serving GW (S-GW) and the evolved Packet Data Gateway (ePDG) are responsible for the 3GPP and untrusted non-3GPP technologies while other technology specific gateways are used for trusted non3GPP accesses e.g. ASN GW for WiMAX and HRPD GW for CDMA2000. The Packet Data Network Gateway (PDN GW) is the central node for the data traffic. The gateways ensure the mobility and the resource reservation based on the requirements of the mobile device, named User Equipment (UE). The EPC uses a Home Subscriber Server (HSS) for maintaining the subscription information for each UE. This information is used by the AAA Server for authentication and authorization in non-3GPP accesses and as part of the resource reservation procedures. The Policy and Charging Rules Function (PCRF) ([3] and [4]) makes policy based decisions for access control and resource reservations based on the subscription profiles and on the information received from the services providers generically named Application Functions (AFs) over the Rx interface ([4]). These decisions are then enforced in the gateways over the Gx and Gxx interfaces ([10]). For example, when a mobile device executes a handover, the PCRF makes the decision which is the level of resources to be enforced on the target access network. As this decision is taken during the handover procedures, the PCRF may reserve the same level of resources (at least the same throughput) for the active services. However, the user may require that the service is adapted to the target access network. For this the service is re-signaled and a new level of resources is then obtained. The correspondent AF requests the new resources to the PCRF which makes a new decision and executes new enforcement procedures with the gateways. Because no interaction is considered between the resources which are reserved during handover and the resources that are reserved during the service modification, the PCRF is required to make two decisions for a service to be adapted to the
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network context, one during the handover to the target access network and one when the new level of resources is requested by the AF. For the EPC example, this article addresses the problem on how to integrate these two reservation procedures into a single one and by this reducing the communication over the wireless link and the processing in the network for service adaptation.
Figure 2 3GPP Evolved Packet Core
III.
CONCEPT
This article proposes a novel method for service adaptation in which the actual service adaptation decision is included in the decision for resource reservation executed during the change in the network context. A special case of the network context change considered is the handover between access networks of various technology types (vertical handovers). As depicted in Figure 3, the method presumes a functional addition to the interface between the service providers and the resource reservation functionality in the core network and two procedures: one executed during service establishment and one executed when the network context is modified. Using these extensions to the architecture, the following general procedure for service establishment is considered: Step 1.1: The mobile device initiates the service establishment procedures with the service provider. During the negotiation multiple profiles on which the session may be delivered to the mobile device are determined. From these profiles one is chosen as a primary profile to be enforced on the current communication path. The rest are considered secondary session profiles. The profiles of the session may contain different levels of throughput, maximum and guaranteed bit rates, codecs, admissible delays etc. Step 1.2: The multiple profiles are transmitted to the resource reservation functionality. Compared to the state of the art, also the secondary session profiles are transmitted to the wireless core network. This enables the resource reservation functionality in addition to the already existing decision making and enforcement of the resources based on the primary session profile, to store the other session profiles which may be used in case the network context of the mobile device changes. Step 1.3: The policy based decision is made if the primary level of resources can be supported by the communication path of the mobile device and then they are enforced. Compared with the state of the art resource reservation procedures ([3]), the here presented procedure enables the
resource reservation functionality of the core network to store for each session not only the profile that is enforced on the data path, but also other secondary profiles which may be enforced in case the network context of the mobile device changes. No new messages are exchanged neither between the mobile device and the service provider nor between the core network and the service provider which makes this procedure to introduce a minimal delay to the overall service establishment (due to the storage of more than one session profile in the resource reservation functionality). When the context of the mobile device changes, the novel method proposes the following procedure to be executed: Step 2.1: The network context of the mobile device changes. This can happen because of a vertical handover or because the wireless access or core network become congested and the service cannot be offered anymore using the same session profile. Step 2.2: When the resource reservation functionality of the core network receives the notification on the context modification event (may it be a vertical handover), it selects one of the secondary profiles that can be guaranteed for the service considering the new network context. This session profile is then enforced using the already standardized mechanisms ([10]). Step 2.3: The service provider is notified on the level of resources that are currently reserved in order to adapt the active session to it. This notification may happen before the new session profile is enforced in case the resources allocated are lower than the resources of the previous profile or it may happen after the enforcement in case the resources allocated are higher. This allows that the data lost during the session adaptation to be minimal. No notification has to be transmitted for the adaptation of the upload data traffic as the mobile device is aware that the network context was changed. e.g. the mobile device is the one that triggers a vertical handover).
Figure 3 Concept
In the EPC, the role of the resource reservation decision function is taken by the PCRF while the role of the service provider is taken by the generic Application Function (AF) functionality. In this context, the concept here presented is translated into an extension of the existing Rx interface ([4]) between the PCRF and the AF and an extension of the decision functionality of the PCRF to store the secondary profiles during the service establishment and to make new
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resource reservation decisions based on them when the context of the mobile device changes. The concept here presented does not introduce any new interfaces, making it easy to be integrated in the overall architecture, as depicted in Figure 4.
Figure 4 Concept translation to the EPC
Multiple example procedures may be given for the service establishment and adaptation considering also exception cases. In order to showcase the concept introduced in this paper, the role of the AF is taken by a basic SIP video server similar to the testbed implementation described in the next section. The SIP video server is able to transmit downlink data streams to the mobile device using multiple levels of throughput e.g. 1Mbit, 500kbit, 300kbit, 50kbit etc. As depicted in Figure 5, the example EPC setup deploys as mobility protocol Proxy Mobile IPv6 (PMIPv6) ([5], [6]). The User Equipment (UE) is already connected to the service through a 3GPP access network (e.g. UMTS) through the SGW and through the PDN GW between which a PMIP tunnel is already established.
Figure 5 Sequence Diagram for Ambient Aware Service Delivery
Step 1: The UE sends a SIP INVITE request to the video server including all the supported codecs and parameters. Step 2: The AF sends a Diameter AuthenticationAuthorization Request (AAR) conformant to [4]. The AAR contains multiple Media-Subcomponents AVPs each one describing a possible session profile, ordered by the preference of the service provider and having the primary session first. It also contains a registration to an event on the modifications on
the network context of the UE, as for example an “IP-CAN type change” for receiving a notification when the UE changed the access network type. This addition enables the PCRF to receive the multiple session profiles. Step 3: The PCRF makes a policy decision on the session parameters to be enforced on the data path according to [3]. The resources are then reserved by the execution of an IPCAN session modification procedure according to [1], [2] and [3]. Step 4: After the resource reservation has succeeded, the Authentication-Authorization Answer (AAA) is sent to the AF confirming the resource reservation. Step 5-6: The SIP 200 OK and SIP ACK messages complete the SIP initialization of the session. The downlink data flow is transmitted by the video server in the conditions negotiated and with resource reservation. Step 7: When the UE selects a WiFi access network, it executes the attachment procedures including the authentication and authorization and the IPsec tunnel establishment according to [2]. It also executes the procedures for establishing the new PMIP tunnel (Proxy Binding Update/Proxy Binding Acknowledgement messages). When the PDN GW receives the request, it initiates an IP-CAN session modification procedure according to [3] in which the PCRF is requested to make a decision on the resources that have to be reserved on the target access network. After this procedure the UE establishes its connectivity on the target access network – the IPsec and the PMIP tunnels are completed. Step 8: During the decision, the PCRF selects a secondary session profile for which the resources are reserved according to the parameters of the target access network. After the successful reservation of resources, a notification in form of a Diameter Re-Auth Request (RAR) [4] is transmitted to the service provider. Step 9: The video server responds with a Diameter Re-Auth Answer (RAA) to the PCRF acknowledging that the session has to be adapted to the target access network. Step 10: The video server adapts the session to the correspondent parameters for the target access network and transmits the data stream accordingly. In comparison to the here presented method, in the current 3GPP standards, after the UE connects to the target access network, the service is adapted using the SIP and Diameter signaling having the same messages exchanged as in Steps 1-6 containing the new session profile parameters. The state of the art procedure consists of at least three more messages exchanged over the wireless link for the re-signaling of the session, the Diameter AAR/AAA and the decision making and the enforcement for the resources from the PCRF and the gateways. Using the functional extensions of the AF and of the PCRF as described here, the session can be adapted directly without requiring a re-negotiation of the parameters between the UE and the service provider. This reduces the signaling over the wireless link and thus reduces the delay of the service
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adaptation at the cost of a minimal extension of the policy based processing in the PCRF. As all the interfaces and the functionality here proposed is an addition to the existing standards, the optimization brought by this article can be easily integrated in the real deployments. IV.
TESTBED, RESULTS AND EVALUATION
To test the applicability of the proposed procedures in a real environment a testbed was based on the Fraunhofer FOKUS OpenEPC toolkit ([7], [11]). OpenEPC provides the basis functionality for mobility and resource reservation in both IPv4 and IPv6 environments, including a standard aligned Rx interface between the PCRF and the AF. This allowed that the enhancements brought by this paper to be easily integrated and tested using two access networks: a public 3G access network over which the service was established using the data connection and a local WiFi hotspot. Although the OpenEPC platform provides extension mechanisms for automated access network discovery and selection, in order to evaluate the solution here presented the access network reselection was executed manually from the UE. A UE able to connect to both the WiFi access network and to the public UMTS network was chosen. A tunnel was established using an IP over IP tunnel between the UE and the S-GW as the public operator is offering only private IPv4 addresses. Through this tunnel all packets transmitted by the UE are encapsulated and send through the 3G network which is not controlled by the testbed. Three gateways were deployed: a S-GW for UMTS and an ePDG for WiFi – both containing the PMIP Mobility Access Gateway (MAG) and the resource reservation and gating functionality according to the 3GPP specifications and are connected to DHCPv4 servers which offers the IP addresses to the UE. A PDN GW was introduced in the testbed which takes the role of the PMIP Local Mobility Anchor (LMA) making the mobility transparent to all the service platforms and the role of the resource enforcement function according to the 3GPP standards ([3], [10]) through Linux Traffic Control scripts [8]. The OpenEPC PCRF was connected to the gateways in order to receive events related to the connectivity of the UE and to enforce the resource reservation. The PCRF was connected over the Rx interface to the Fraunhofer FOKUS Media Delivery Function ([9]) which is able to establish video sessions with different throughput signaled over SIP. A counterpart minimal SIP client was integrated in the UE to be able to deliver the ambient aware service. The implementation of the communication between the network entities is based on the FOKUS Diameter Stack for Linux while the PMIPv4 implementation is based on Ubuntu Linux-based Operating System primitives. For the signaling part user-space raw sockets were used in both the PMIPv4 LMA and MAG. For the data tunneling, the standard IPv4 over IPv4 tunnels offered by the Linux kernel were considered. The scenario evaluated by this article is a simplification of the one presented in Section III as it does not include the authentication and authorization procedures as the IPsec
tunnel establishment. The throughput allowed to be used for the video session over the 3G interface was limited to 300kbit while the throughput over the WiFi was limited to 500kbit. Using SIP the UE initiates the service over the 3G access network which is initiated according to the scenario in Section III. Upon receiving the information, the PCRF stores the secondary session profiles and reserves the resources on the data path.
Figure 6 Testbed Architecture
Then when the UE selects the WiFi access network a notification message is transmitted by the ePDG to the PCRF which in turn selects the session profile for WiFi and enforces it on the data path. A notification on the access network modification is also send using the Rx RAR/RAA message exchange to the AF which in turn adapts the session to the 500kbit throughput. Based on this minimal scenario, the delay of a session renegotiation (execution of Steps 1-6 from the EPC procedure presented in Section III in the WiFi access network) was compared with the here presented solution. As preliminary results, it was observed that the resource reservation procedures, including the PCRF decision and the enforcement in the data path gateways over the Gx and the Gxx interface was part of the both procedures, thus the delay of the SIP signaling procedure together with the resource reservation procedures is highly increased compared to the here solution. This is due to the signaling over the wireless link of the session adaptation and to the signaling over the Rx interface. Considering that the resource reservation procedures have to be also executed in case of a vertical handover, this may be added to the overall delay of the state of the art procedure which includes the SIP signaling as part of the service adaptation. Also it is to be noted that the evaluation was performed for a wireless environment with a single UE, thus not providing a benchmark for the given solution. Multiple UEs may affect the resource reservation procedures or the SIP response of the video service request, increasing the delay of the overall procedures. A further evaluation with multiple mobile devices has to be performed.
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The results obtained depend on the implementation of the basic EPC procedures, like resource reservation and communication with the service providers and they should be regarded as guidelines for the optimization brought by the hereby presented method and not as a benchmark for the service establishment and modification. V.
CONCLUSIONS
This article presented a new concept for service adaptation which does not require a renegotiation of the parameters between the UE and the service provider. This novel approach is required because the services delivered over the heterogeneous wireless environment will not receive the same level of resources on the access networks to which the mobile device is connected to. From another perspective, this new concept allows the one-time signaling of the services which then adapt implicitly by network communication without requiring the UE interaction extending the capability of providing services over the wireless environment with a more flexible approach on the resources that have to be reserved on the data path. However, it presumes that due to the one time signaling, the multiple levels of resources that can be reserved for a specific service are maintained in the core network which highly increases the information stored for the data flows. In order to mitigate this issue, a mechanism of maintaining the multiple flow levels for a single application which addresses multiple UEs has to be considered. The concept was exemplified on the 3GPP Evolved Packet Core and evaluated and validated on a prototype implementation using the Fraunhofer OpenEPC toolkit. From the delay values obtained and from the reduction of the communication brought by this novel service adaptation method we can conclude that the solution is feasible to be implemented in real-life deployments as the functionality increase can be easily fit into the functionality already available in the standard version. The evaluation here presented referred only to the downlink data traffic of the UE from a network located Application Function which covers a wide range of applications including
video streaming, file downloading, centralized gaming etc. For the uplink data traffic a similar optimization as the one brought in the video server has to be considered as to be able to announce the applications from the network controller of the mobile device when the network context is changing. A similar approach can be also considered for the direct communication between multiple mobile devices. REFERENCES [1]
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