Virtual Price Based Policy Enforcement for Access Selection and Vertical Handover in Multi-Access Networks

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1 Virtual Price Based Policy Enforcement for Access Selection and Vertical Handover in Multi-Access Networks Fanchun Jin 1, Joseph S. Gomes 1, Yu Zhou 1, Hyeong-Ah Choi 1, Jae-Hoon Kim 2, JungKyo Sohn 3, Hyeong In Choi 3 1 Department of Computer Science, George Washington University, Washington, DC 2 Access Network & Mobile Terminal R&D Center, SK Telecom, Seoul, Korea 3 Department of Mathematics, Seoul National University, Seoul, Korea {jinfc joegomes,yuzhou hchoi}@gwu.edu, jayhoon.kim@gmail.com, {jgsohn,hichoi}@snu.ac.kr Abstract In multi access network environments, mobile stations may encounter multiple choices for selecting an access network. Carefully designed access selection schemes can provide not only mobile users with better services but also network operators with better resource utilizations. It is also envisioned that further improvements can be achieved by redistributing mobile stations from one access network to another (i.e., vertical handovers). Such decisions should be made by following carefully designed, yet simple to implement, protocols. As a part of SK Telecom s convergence network initiative, we have developed two radio resource management technologies using the concept of virtual price and evaluated performance of one of the techniques by creating multi-access network simulation environments in OPNET. The following components were implemented. 1) Multi-mode mobile station with Layer 2.5 to allow switch among different RATs. 2) Inter-network communication component. 3) Common radio resource management component to compute virtual prices and support access selection and VHO. 4) CDMA1X, EvDO, and HSDPA modules. Introduction Wireless data communication is experiencing explosive growth in last decade, leading to fundamental changes in the wireless networking paradigms. While a large number of wireless protocols exist to provide services to a variety of users with different traffic characteristics and hardware capabilities, it is expected that new radio access technologies (RATs) will be developed and deployed in the future, but most likely the existing RATs will not be completely replaced by new RATs [1]. Each RAT operates on a different spectrum band, occupies different bandwidth capacities, and supports a specific type of mobile stations. While some RATs may suffer from high traffic load, other RATs may experience low traffic load because of uneven distribution of mobile stations over different RATs. A multi-mode mobile station is equipped with multiple communication modems that are capable of supporting multiple RATs. In other words, a multi-mode mobile station may only be connected to a single network at any specific time, but it can switch to another network to obtain better services. The introduction of multi-mode mobile stations makes it possible for resource sharing among different RATs. In this context, the network operators are seeking a method to support the mobile users with Always Best Connected (ABC) services [2, 8], while trying to make maximum revenue. Numerous efforts are going 1 on worldwide to guide multi-mode mobile stations to make access selection and vertical handover decisions. The next generation of wireless networks, commonly referred to as Beyond 3G (B3G) networks, is envisioned to support higher bandwidth requirements on fully digital, all IP based networks that use a common frequency band across all providers and regions. There are many world-wide research projects toward B3G networks in various stages. The European Union 6th Framework Program (FP6), FuTURE in China, Future Wireless World launched by DoD in North America, NGMC Forum in South Korea, and Mobile IT Forum (mitf) in Japan are a few of them. Significant research has focused on access selection schemes that incorporate user-, network-, and service-specific information. Most of the works in this effort are at a rather conceptual stage, while some concentrate on performance gains using simulations or experiments in ad-hoc manners. The Ambient Networks project, a part of the FP6, is an integrated project in most advanced stage among the existing B3G projects and has developed a generic multi-radio access architecture [3] and a conceptual framework for access selection and resource allocation by joint radio resource management across different RATs [4]. In [5], the authors consider the problem of allocating user connections to a set of base stations to reduce power consumption of mobile stations and maximize admitted traffic flows while following access preferences of users. The authors used heuristic algorithms of bin packing problem to solve the problem, but do not consider the load balancing issues where different networks may have different capacities for traffic flows. Another approach to utilize heuristic bin-packing algorithm for access selection is presented in [6]. In [7], the authors consider initial access selection problem between wireless LAN and HSDPA/Long Term 3G Evolved networks and propose simple heuristic algorithms. This paper presents two approaches regarding load balancing by developing virtual price based technologies. To examine these approaches, we construct multi-mode mobile stations, internetwork communication component, and common radio resource management component. The implementation of each specific network and vertical handover protocol is out of the scope of this paper. In this paper, we will focus on the policy enforcement techniques for access selection and vertical handover. The packet flow of the signaling messages is described, and methods of making handover decision are described. This paper is organized as follows. In section II, we introduce marginal cost function, which computes the cell s virtual price,

2 and in section III we describe network architecture, access selection and vertical handover decision. In section IV and V, we describe our OPNET implementation of the CRRM model and multi-mode mobile station model respectively. In section VI, we present simulation results followed by conclusion in the next session. Marginal Cost Functions We define a marginal cost function of each cell that computes its virtual price, taking the current cell load and the cell capacity as input parameters. Using these virtual prices, a new mobile station is admitted to a cell with the lowest virtual price among all accessible cells (i.e., candidate cells to which the mobile station has strong enough signal strength). The virtual price of each cell is periodically updated, and existing mobile stations may be forced to perform handovers to other cells of the same or different RATs. A handover to a cell with a different RAT is called a vertical handover. Depending on different RAT and admission control algorithm, the amount and unit of the cell load and cell capacity are different. In admission control, the RNC/BSC compares the capacity of the cell with the expected load to make a decision. The capacity could be power, number of users, data rate. In our implementation of the RATs, we use power as the capacity. We use x to represent the cell load, and c to represent the cell capacity. Network Architecture and Access Selection and Vertical Handover Decision Classifier HO UE RNC CRRM Broadcast BS load info BS virtual price Compute virtual price Figure 2: Mobile station based CRRM Protocol In the following subsections, we will discuss the network based CRRM and mobile station based CRRM respectively. Network Based Handover Decision In this approach, the handover decision is completely made by the network, i.e., the CRRM. 1. Each RNC reports the uplink and downlink load information of the base stations that are attached to the RNC to CRRM. CRRM computes the cost information of each base station, using the formula x u /c u + x d /c d, where x u and x d represent the load of uplink and downlink respectively, while c u and c d represent the capacity of uplink and downlink respectively. 2. Let C denote the set of all cells in the heterogeneous network. 3. While C is not empty, do the following: (a) Find two neighboring cells, i and j, in C with the largest difference of virtual prices. (b) Arrange handovers between i and j until they both have the same virtual price or no more handover is possible. (c) Delete i and j from C. We follow the OPNET UMTS network architecture with an additional module for inter network communication that we have implemented ourselves to enable the inter RAT communication. A Common Radio Resource Management (CRRM) module is also introduced to control/assist mobile stations to make access selection and vertical handover decision. The CRRM module is directly connected with SGSN in UMTS/HSDPA network and PDSN in CDMA1x/EvDO network. RNC/BSC periodically reports base station s uplink and downlink load information to CRRM module in every fixed time period. The CRRM then guides mobile stations to perform handovers using one of the two different approaches that we developed: network based and mobile station based. Figure 1 and figure 2 show the signaling flow in each protocol. Classifier HO UE RNC CRRM BS load info Handover Decision Compute Virtual price Calculate gradient Arrange HOs Figure 1: Network based CRRM Protocol 2 In this approach, each mobile station must periodically report to the CRRM the list of cells (of any RAT type) to which it has enough signal strength to be connected. While such an architecture is possible by following the IEEE Media Independent Handover [9] framework (in which each base station maintains the geographic information of its neighbor cells and their broadcasting information) and by making the current base station transmit the broadcasting information of the target base station to the mobile station, it would require a great deal of system overheads. Since the IEEE is not finalized yet, implementation of this approach remains as a future task. Mobile Station Based Handover Decision We now extend our CRRM algorithm to allow mobile stations to make final decisions when the network (or CRRM) does not exactly know which cells may be accessible by each mobile station (i.e., mobile stations do not report the signal strengths of their accessible base stations to the network), but each base station maintains geographic information of its neighbor cells as specified in the MIH protocol [9]. Suppose MS mj is connected to BS bi whose neighbor set is denoted by Si. Note that mj may or may not be able to connect to all base stations in Si, but the set of base stations to which the mj can connect must be a subset S0i of Si. The network (or the CRRM) then delivers the virtual price of each cell in the neighbor list Si of bi, and bi informs the virtual price of each cell in Si to mj. mj then make a final decision using the information on signal strength to each base station in Si. More precisely, mj select a cell with the lowest virtual price from S0i. The virtual price information will be obtained simultaneously by the mobile stations within any specific base station if the virtual price is broadcasted. Since each mobile

3 station makes decision independently, we may observe two situations. One is the mass handover, i.e. all mobile stations switching at the same time and second is the pingpong effect, i.e. the same mobile station switching back and forth. To avoid the two situations, we assign a timer uniformly chosen from 1 to 50 seconds, such that once a mobile station performs a handover, it cannot perform any additional handover during the timer period. modems to perform vertical handover. The Classifier also delivers incoming packets from upper layer to the appropriate modem, and delivers packets from lower layer to upper layer. The state transition diagram for the classifier is shown in figure 5. CRRM Model In this section, we describe our implementation of CRRM model. The state transition diagram of this model is as shown in figure 3. In Init state of CRRM, the state variables are initialized and the networks that are connected to the CRRM model are recognized. In Idle state, the machine waits for incoming events. The event could be a report from an RNC/BSC about the cell load information or the expiration of the clock timer. If the incoming event is from the network, it delivers the packet to Report state. Depending on the network that sends packet to the machine, Report state transfers the packet to one of the states: EvDO Report State, HSDPA Report State, UMTS Report State, and CDMA1x Report State. In these states, the virtual price for the cell is calculated using the formula given in previous section. If the event is self interruption, the machine triggers mobile station based handover. Figure 5: Classifier State Transition Diagram In Init state of Classifier, the state variables are initialized. In Idle state, the machine waits for an incoming event. The event could be either incoming packet from the upper layer, incoming packet from the lower layer (which includes the message from CRRM model), or the expiration of the clock timer. The Up state either delivers the packet from lower layer to upper layer if the packet is regular packet, or delivers the packet to Price update state if the packet is from CRRM model. The Price update state processes on the packet from CRRM model, and triggers a vertical handover. When the vertical handover is triggered, the machine enters the Attach state to register to the network by sending attach message to the new network such that the new network gets ready for the incoming mobile station. If the attach is successful, the machine enters the Detach state to detach from current network by sending detach message through the current active modem. After the detaching is finished, the mobile station continues communication through the new network. If the attach to the new network is failed, the mobile station does not perform vertical handover. Figure 3: CRRM State Transition Diagram Multi-Access Mobile Station Figure 4: Multi-mode Mobile Station Node Model As shown in figure 4, there are 4 modems in a mobile station to access to 4 RATs respectively. We add a Classifier layer between the modems and the Client layer. The Classifier layer receives virtual price information from the base stations within its communication range, and cooperates with the underlying 3 Our implementation of vertical handover in OPNET designates the target network instead of the exact cell, the attach messages only designate the target network. The modem that receives the attach message performs handover to the cell with the strongest signal in the target network. This may reduce the CRRM performance considering the following case. Suppose that there are two cells B 1 and B 2 in the target network. Cell B 1 has very high virtual price but higher signal strength from the mobile station that is going to have the vertical handover while cell B 2 has lower virtual price but have weaker signal strength. The CRRM requests the mobile station to perform vertical handover to cell B 2 but the mobile station will try to perform handover to cell B 1, according to our implementation of vertical handover. The details of implementing vertical handovers are out of the scope of this paper. The Down state delivers the packet from higher layer to an appropriate modem. Simulation Results

4 We have focused on the network performance due to vertical handovers. Accordingly, in our simulation environments, intra network handovers did not happen. The simulated network topology is shown in figure 6. The network size is 2000 m by 2000 m. The network is divided into 4 areas, and in each area, there are 4 base stations with 4 RATs respectively. There are 8 mobile stations in each area which are able to connect to the all 4 base stations in the area. The traffic configuration is same in all mobile stations, which is shown in figure 7. Among the 8 mobile stations in the left bottom area, 6 of them are initially connected to EvDO base station, while 2 mobile stations are connected to UMTS node-b. Obviously no throughput in HSDPA base station and CDMA1x base station will be observed, if there is no vertical handover. Figure 6: Network Topology Figure 7: Traffic Configuration The simulation time is 30 minutes, and we monitor the downlink and uplink throughputs. Figure 8 shows the downlink throughput of base stations in left bottom area, and figure 9 shows the uplink throughput of base stations in left bottom area. As the figures show, the maximum throughputs of base stations in CDMA1x and UMTS networks are around 20kbps to 30kbps, while it s more than 40kbps in HSDPA and EvDO networks which have more data rate capacity than CDMA1x and UMTS networks. Note that we have scale down the overall traffic in each base station due to the high computational overhead in using the OPNET. Despite the initial distribution of mobile stations over the 4 base stations, we observe that the throughput of each base station is distributed according to the data rate capacity with CRRM functions. Figure 8: Downlink Throughput in each base station in the left bottom area 4

5 Figure 9: Uplink Throughput in each base station in the left bottom area Conclusion In this paper, we presented two Common Radio Resource Management techniques: network based and mobile station based. We presented the OPNET implementation of CRRM model and multi-mode mobile station. The uplink and downlink throughputs in base stations from different RATs are monitored and presented. With refined vertical handover protocol and implementation we expect better simulation results. We expect further improvement if the network based handover approach is implemented. References [1] Prytz, M., Karlsson, P., Cedervall, C., Bria, A., Karla, I. Infrastructure cost benefits of ambient networks multi-radio access, In Proc. IEEE Vehicular Technology Conference. (2006) [2] Gustafsson, E., Jonsson, A. Always best connected, IEEE Wireless Communications 10 (2003) [3] Niebert, N., Prytz, M., Schieder, A., Papadoglou, N., Eggert, L., Pittmann, F., Prehofer, C. Ambient Networks: A Framework for Future Wireless Internetworking, In: Proc. IEEE Vehicular Technology Conference - Spring. (2005) [4] Berggren, F., Bria, A., Badia, L., Karla, I., emco Litjens, Magnusson, P., Meago, F., Tang, H., Veronesi, R. Multi-radio resource management for ambient networks, In Proc. of IEEE 16th International Symposium on Personal, Indoor, and Mobile Radio Communications. (2005) [5] Xing, B., Venkatasubramanian, N.: Multi-constraint dynamic access selection in always best connected networks. In: MOBIQUITOUS 05: Proceedings of the The Second Annual International Conference on Mobile and Ubiquitous Systems, Networking and Services, Washington, DC, USA, IEEE Computer Society (2005) [6] Mariz, D., Cananea, I., Sadok, D., Fodor, G.: Simulative analysis of access selection algorithms for multi-access networks, In WOWMOM 06: Proceedings of the 2006 International Symposium on on World of Wireless, Mobile and Multimedia Networks, Washington, DC, USA, IEEE Computer Society (2006) [7] Yilmaz, O., Furuskar, A., Pettersson, J., Simonsson, A. Access selection in wcdma and wlan multi-access networks, In Proc. IEEE Vehicular Technology Conference. (2005) [8] Fodor, G., Furuskar, A., Lundsjo, J. On Access Selection Techniques in Always Best Connected Networks, In ITC Specialist Seminar on Performance Evaluation of Wireless and Mobile Systems, Aug [9] Draft IEEE Standard for Local and Metropolitan Area Networks Media Independent Handover Services IEEE P802.21/S Technical report (2006) 5