operators will be able to select an access network on the arrival of a new call or ... Selecting the most optimal connection and controlling access to the available ... architecture using policy-based management. ... Intersystem mobility or a vertical handover occurs when a mobile device .... In doing so, the load is balanced.
Intelligent Access and Mobility Management in Heterogeneous Wireless Networks using Policy Ken Murray, Rajiv Mathur & Dirk Pesch Adaptive Wireless Systems Group Department of Electronic Engineering Cork Institute of Technology, Cork, Ireland E-Mail: {kmurray, rmathur, dpesch}@cit.ie
Abstract The next generation of mobile networks will utilise multiple radio access technologies. These heterogeneous wireless networks will enable the user to seamlessly roam between the different access technologies to maintain network connectivity and satisfactory QoS. Network operators will be able to select an access network on the arrival of a new call or handover request. Selecting the most optimal connection and controlling access to the available networks is an important consideration for overall network stability and providing guaranteed QoS. In this paper we propose an intelligent call admission control and mobility management architecture using policy-based management. A call admission policy admits a new user based on the current load and a predefined service mix. A network health monitor continuously updates the residual capacity by observing the quality of the current connections. The mobility management policy focuses on handover control between the available access networks. Mobiles can be selected for forced handover when a network reaches a congested state. A selection policy controls the mobile selection procedure.
1
Introduction
It is envisaged that next generation mobile networks will utilise several different radio access technologies such as WCDMA, EDGE and IEEE 802.11b/g WLAN integrated to form a heterogeneous wireless access network [1]. Each access network will provide different levels of QoS, capacity and coverage to the end user. Multi-modal terminals will seamlessly and dynamically roam between the different access technologies, so as to maintain the most optimal network connectivity for services requiring variable levels of QoS. Supporting this seamless mobility, deciding when a mobile should perform a vertical handover and the handover execution procedure are seen as the key issues in resource management for heterogeneous wireless networks [2]. The range of anticipated services in future generation wireless networks will introduce high variability in the required QoS and therefore, the most optimal network for a given connection can dynamically change. Call admission control is responsible for making access decisions in response to a user’s access request. The access decision can be based on criteria such as the required QoS level, bandwidth requirement, residual capacity in each available network, coverage and cost. Admission control facilitates high capacity and spectrum efficient network usage. An intelligent call admission control algorithm is required to maintain QoS contracts and user connectivity in an environment where users are dynamically roaming between different access technologies [3]. Intersystem mobility or a vertical handover occurs when a mobile device changes its point of attachment to a different access network. Choosing the correct time to initiate a handover request and selecting the best network to handover are important considerations for overall network stability in a heterogeneous network environment. Previous studies have only considered the handover management in heterogeneous
wireless network [1,4,5,6]. Such proposals fail to address the initial call placement and to which network should each call be assigned to optimise the overall network capacity and QoS. In this paper we present a call admission control and intersystem mobility management architecture for a heterogeneous wireless network. We examine the use of a policy based access management system to control user access requests and network/user-controlled movement of sessions between the available networks so as to continuously maintain a guaranteed QoS to the end user. We also propose an authentication policy to control user access rights when attempting to connect to a network, we show how such a policy can be integrated into the policy-based architecture.
2
Intersystem Handover
It has become widely accepted that future generation wireless networks will be IP based [7]. The different access technologies will be integrated to form a heterogeneous wireless network and will be used to access a common backbone IP network. Much work has been done with Mobile IP and Cellular IP in solving the issues surrounding the handover execution between IP based networks [8,9,10,11]. In order to make a fully functional heterogeneous network we need to address call admission control and mobility management. Such management is crucial prior to the network access and handover execution stage so as to achieve a balanced load and optimal spectrum usage across the network access layer. Policy can be used to decide at the call initiation stage which network can best support the requested service and in deciding when is the best time to implement a vertical handover to another available access network.
3
Policy Based Management
A policy based management system is one in which its operation is determined by a set of rules and instructions. Policy rules are evaluated when triggered by an event. An event could be the arrival of a new user into the network, a handover request or the degradation in the QoS. The policy rules define how the network should handle such events. The outcome of the policy evaluation is a policy decision, which is enforced on a specific network device, such as a base station, to admit or reject the new call arrival. Policy rules are declarative and so can be adapted at run-time to flexibly control system behaviour as network conditions change and are therefore becoming increasingly popular in adaptive, run-time configurable networks and information systems. To this end, we propose a policy system architecture for making access control decisions and network selection decisions in the network and mobile terminal respectively. The design of policy rules that achieve business goals has to be done carefully and with a good understanding of the networks operation. Conflicting policies can cause unexpected behaviour in the network and mechanisms are available for detecting certain conflicts before deployment [12]. The next two sections describe our proposed architecture for a policy based call admission and mobility management controller for a heterogeneous wireless network.
3.1
Call Admission Control using Policy
Policy system architectures tend to focus on the relationship between the points where the outcome of a policy is enforced, i.e. the Policy Enforcement Point (PEP), and the point where the decision on whether a policy decision is satisfied is taken, i.e. the Policy Decision Point (PDP). The PDP for network access control is implemented on a server within the network and uses an open communication protocol with enforcement points [12]. This approach allows implementing the PDP at a location where network information such as data rate, network coverage, mobility support and current load are available, e.g. a base station controller in GSM/GPRS, radio network controller in UMTS or gateway router in WLAN. Data required by the admission controller policy engine, such as link level QoS parameters for currently active connections and bandwidth availability is maintained in a policy repository. The policy repository makes available these policy parameters to the PDP in the decision making process. This architecture also facilitates the updating of policy rules throughout the network. For the user the enforcement point is their mobile terminal, which is typically powerful enough to run a PDP. The policy system architecture for both network and user is shown in Figure 1. The policy architecture has two main components, the network policy engine and the mobile terminal policy engine. The network policy engine is responsible for selecting an access network on the arrival of a new call or handover request from a mobile and for making call admission control decisions. These decisions are made in the PDP, which contains the network selector. The network selectors function is to assign an access network for the connection request. The policy in the network selector is to choose the access network that is currently the least loaded for the given cell. In doing so, the load is balanced between the access networks and avoids one network becoming excessively loaded. The network selector determines the residual capacity in the current cell for each network via the cell capacity variables stored in the policy repository. The cell capacity is the number of users that each cell can support, irrelevant of the services they are using. The network health monitor continuously updates this value as network conditions change. It does so by reporting the network stability at the link level. It uses link layer performance statistics in the policy repository to determine a network health value between 0 and 1 for each cell. The performance data includes the fraction of streaming connections that have a video frame drop rate, FDR, above a threshold (1%), the fraction of web users that have a block error rate, BLER, above a threshold (10%) and whether the voice call blocking is above a predefined threshold (2%). These thresholds are chosen so that each user has a satisfactory QoS. The FDR and BLER measurements are then mapped using the mapping functions in Figures 2 and 3 respectively. We have chosen these mapping functions to stress the importance of block errors and video frames dropped to the health of the wireless network. The network health is finally determined using the following formula, Network Health = 1 – (Voice Blocking * W1 + FDR * W2 + BLER * W3) Voice blocking is 1 if above the threshold and 0 otherwise, W1, W2 and W3 are weightings that highlight the importance of each variable, which sum to 1. If the quality of ongoing connections is more important than that of not receiving a voice connection at call set-up, then W1 should approach 0. Based on the network health value reported by the network health monitor, the cell capacity values are altered so as
to admit more or less users into each cell of each network. Once a network is selected, our initial policy is to allow 60% of all users to be voice, 20% to access web services and 20% to receive video streaming service. Defining a service mix in this way allows a network operator to prioritise services and maximise revenue by defining the preferred load for each service type. The service mix can be adapted at run-time by varying the policy parameters. The decision from the PDP is sent to the PEP, which informs a requesting mobile whether its connection request is granted and to which network it should connect. This approach should have the effect of balancing the load between the available access networks, with each network carrying the optimal service mix with a guaranteed QoS. The mobile terminal policy engine is responsible for mobility management procedures and is discussed in the next section.
Network Health Monitor
Network Policy Engine PEP
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Policy Repository UMTS Capacity EDGE Capacity WLAN Capacity FDR BLER Call Blocking
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Network Selector Policy Rules Access Request from user Policy Decision Exchange/Arbitration
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Figure 1.
Mobility Management Policy
Policy System Architecture
User Preference Cost QoS Network Measurements
Reduced QoS Network Unavailable
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Figure 2.
3.2
FDR Mapping
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Figure 3.
BLER Mapping
Mobility Management using Policy
One of the tasks of mobility management in a heterogeneous wireless network is concerned with deciding the correct time for a user to request a vertical handover to another available network and to which network should a mobile terminal change its point of attachment. The PDP for such decisions will be implemented within a mobile terminal. The PDP will have access to link level performance statistics measured by the mobile terminal during a session and stored in a policy repository. Data stored in the repository will include QoS parameters, signal strength measurements, whether a signal from a WLAN access point is being received or not, user preferences such as cost of connections, minimum QoS required for each service type etc. This policy repository will exist in the mobile terminal. If the perceived QoS falls below a predefined threshold for a particular service type due to lack of coverage or increased interference, the PDP will be enacted to make a decision whether a handover request should be made at this time or not. It may be the case that a signal is been received from a WLAN access point, but the signal strength is fading fast, it may be advisable under such circumstances not to change the current connection to WLAN but remain with the current point of attachment. The handover request is sent to the network PDP, as shown in Figure 1. A handover request is handled in much the same way as an access request, as described in the previous section, but may have different priorities attached. Similar handover control functions of the mobility management system can be implemented on the network. A handover selection policy can be used to select mobiles to handover to another network when the network reaches a congested state. Such a system is useful if mobiles experience reduced QoS but have no multi-modal capabilities. Another mobile(s) can then be selected by the network to handover in an attempt to increase the QoS for other users. The selection policy can be based on the last call arrived – with dual mode capabilities, bandwidth requirements, session type, user mobility etc. A return policy will decide if a mobile should return to its original network after a network has returned from its congested state. An authentication policy can also be included in the mobility management system. This policy will authenticate users as they attempt to connect to a network. Such a policy will be useful to control the number and role of users gaining access to a privately owned WLAN such as those installed in airport terminal buildings, hotels etc. The authentication policy will be implemented in the PDP of the network policy engine and will access a user authentication list in the policy repository.
4
Conclusion
This paper has demonstrated how policy based management can be used for call admission control and mobility management in a heterogeneous wireless network. We have presented a policy-based architecture in which call requests and reduced levels in QoS trigger policy engines located in the network and mobile terminal. Policy rules are evaluated using link level statistics stored in policy repositories. It is expected that the policy based call admission and mobility management system will achieve a balanced load between the available access networks and provide acceptable levels of QoS for user applications. Future work will investigate the performance of the proposed system via a computer simulation model of a heterogeneous wireless network. Performance will be evaluated in terms of QoS offered to the end user and the degree of load balancing between the available networks.
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