IP Paging Considered Unnecessary:

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1 IP Paging Considered Unnecessary: Mobile IPv6 and IP Paging for Dormant Mode Location Update in Macrocellular and Hotspot Networks James Kempf DoCoMo USA Communications Labs 181 Metro Drive, Suite 3 San Jose, CA, 9511, USA kempf@docomolabs-usa.com Pars Mutaf Planete Team INRIA Rhone-Alpes Grenoble, FRANCE pars.mutaf@inria.fr Abstract Cellular telephones save power by switching to a low power, or dormant, mode when there is no active traffic. In dormant mode, the telephone periodically checks for a beacon, and if the beacon indicates that the phone has moved to a new paging area, the telephone performs a paging area update. Paging area update signaling requires considerably less power than is required to move an active traffic channel. When a call comes in, the network pages the telephone by flood signaling the paging area on a special channel, and the telephone brings up an active traffic channel. Recent work has attempted to extend paging to IP networks, in particular, networks running the Mobile IP mobility management protocol. In this paper, we compare Mobile IPv6 and IP paging for dormant mode location updating. Simulations are presented for a typical macrocellular scenario and a hotspot scenario, where broadband microcells supplement the narrowband macrocells. The results show that Mobile IPv6 dormant mode location management is roughly comparable to IP paging. Keywords-Mobile IP, IP paging, Mobile IPv6, hotspot service, dormant mode, power saving I. INTRODUCTION Cellular networks use a mechanism to locate mobile devices called paging [1] [2]. When the device switches to low power, or dormant, mode, no traffic channel is active, and location updates are made by the device to the network only at long time intervals when the device moves outside a paging area. When a call comes in, the device is paged by broadcasting on all cells in the paging area. Moving between paging areas thus requires much less power than moving with an active traffic connection, and requires less signaling on the wireless access network and in the core network. The success of paging in cellular networks has led to proposals for an IP paging protocol. In Section II, we discuss previous work on IP paging. An alternative to IP paging is to use the Mobile IP protocol [3] [4] for location management in dormant mode hosts. Mobile IP is typically only discussed in the context of supporting active mode hosts. Previous work has led to the identification of two reasons why IP paging might provide advantages over Mobile IP for dormant mode IP hosts [5]. In Section III, we present simulations that compare signaling cost for Mobile IPv6 and IPv6. Finally, in Section IV draws some preliminary conclusions. II. PREVIOUS WORK The initial work on IP paging was done in the context of specialized routing protocols for handling host mobility without requiring that the IP routing address change. These protocols confine knowledge of movement between subnets to a localized routing domain near the network edge, and are called micromobility routing protocols. Paging is used by specialized routers to maintain location information about dormant mode hosts with a lower frequency of update than for active mode hosts. The two micomobility routing protocols that have examined paging are Cellular IP [6] and Hawaii [7] [8]. The work on paging for micromobilty routing protocols led to examining how paging could be incorporated into Mobile IPv4 [3], where a change in routing address is required. In P-MIP [9], each wireless cell exists in its own subnet, and paging areas cover multiple subnets, similar to how non-ip cellular networks are architected. Paging areas provide wide geographic coverage for dormant mode hosts, while subnets provide more limited geographic coverage for active mode hosts. P-MIP extends Mobile IPv4 by allowing the Foreign Agent to act as a Paging Agent. The Paging Agent maintains dormant mode location and initiates pages. Analytical and simulation studies found that the growth in signaling cost as the number of cells (and thus subnets) in the paging area increased was considerably less with P-MIP than with MIP, and that the signaling cost as the number of data sessions increased was less as well. In [8], the authors examined two different paging architectures that enhance Mobile IPv4, and compare them with paging for micromobility routing. One architecture is similar to P-MIP. It places Paging Agent functionality in the Foreign Agent. Another moves Paging Agent functionality to the Home Agent. In addition, the authors compare these approaches to using the micromobility routing approach of Hawaii. The authors simulated the three different approaches using a collection of movement traces from real mobile networks, but the authors do not provide a detailed description of the mapping between cells, subnets, and paging areas 1.No extended comparison was made to Mobile IPv4 without 1 The authors do indicate that the zones in the mobility traces they used can be mapped to base stations, and that IP handoff occurs across zones, but they do not indicate what mapping was used between zones, cells, and subnets.

2 paging, but the authors did find that paging of any sort considerably reduced the number of Home Agent updates. Based on this research work, a problem statement and set of requirements for a standardized IP paging protocol were developed in the IETF [5] [1]. Current practice in the cdma network [11], which uses Mobile IPv4 for mobility management, is to also use Mobile IP for dormant mode location update, and to identify the paging area and subnet but allow multiple cells per subnet, rather than restrict subnets to a single cell as in P-MIP. The problem statement examined what advantages an IP paging protocol would have over the currently deployed practice. The study found two possible advantages: Allowing an arbitrary mapping of paging areas to subnets could result in reduced signaling load for dormant mode mobile hosts in the core and wireless access network, Moving paging from a technology-specific to a technology-independent protocol such as IP could allow paging on multiple technology interfaces with a single location update management protocol. Despite the advantages seen by the authors for IP paging, these points have some distinct disadvantages. Regarding the first point, the number of subnets that can be mapped to a paging area is limited by the capacity of the paging channel. As more subnets are added to the paging area, the paging channels in all subnets must carry heavier traffic because more dormant mode hosts are covered. If the capacity of the paging channel is limited, delays in location are likely and the channel can overflow. The limitation on the paging channel is more serious than that for Home Agent binding updates because paging is broadcast, so a page for a single mobile host will occupy the channel in all subnets, whereas a single binding update is unicast and therefore consumes far fewer resources in the access network. Regarding the second point, in multiple technology networks, IP paging is expected to give an advantage because it can maintain location on two technologies at once. This argument really only applies if a dormant mode host is expecting to be paged on one of several wireless interfaces, and not if the host is just expecting to be paged on only one. If the dormant mode host is expecting to be paged on more than one interface, it must keep multiple interfaces active at a low power while in dormant mode. Having more than one interface active draws more power than only having one. A more sensible power saving paging policy is to select the interface that has the best chance of meeting some criteria, for example, the interface on which the mobile will receive the widest geographical coverage, and keep that interface active. III. COMPARISON OF MOBILE IPV6 AND IP PAGING In order to determine whether these advantages might be realized in practice, we constructed simulations for a macrocellular network and for a hotspot network. The macrocellular simulation is designed to test the basic comparative signaling performance of IP paging and Mobile IP, the hotspot simulation is designed to test the comparative signaling performance for multiple technology networks. We constructed the simulation for Mobile IPv6 [3] because Mobile IPv6 provides many advantages over Mobile IPv4. The paging protocol was similar to that in [9], except the paging agent was independent of the access router and could be positioned at an arbitrary place in the network topology in order to allow the paging area to cover more than one subnet. A. The Simulation The macrocellular network consists of by square shaped macrocells, each macrocell being 1 km 2. In the macrocellular simulation, the number of macrocells per subnet is varied to determine the effect on signaling performance. In the hotspot simulation, microcellular hotspots are added to the macrocells in two configurations: 1 hotspot cell per macrocell and four hotspot cells per macrocell. In both cases, all hotspot cells within a macrocell are grouped into a hotspot subnet, and the number of macrocells per macrocellular subnet is fixed at 9, with hotspot subnets and macrocellular subnets being separate. When IP paging is simulated, two configurations are used for the number of macrocellular subnets/paging area (pa): 4 subnets/pa and 9 subnets/pa. Hotspot subnets are included into the paging area of the macrocells that contain them. The total simulated time period for both simulations is eight hours, and mobile hosts are simulated. Reported data is the result of averaging the data for all the hosts. The timing of session arrival causing dormant mode host activation is driven by a Poisson process. For the base set of measurements, the average number of incoming sessions is fixed at 2 per day for the macrocellular simulation and 1 per day for the hotspot simulation. In the session sensitivity simulation, simulations were additionally preformed for an average of 1, 5, and 1 sessions per day for the macrocellular model. The host stays in dormant mode until an incoming session occurs. The random waypoint model was used to simulate host mobility. In the macrocellular simulation, the mobile host randomly selects one of four possible directions to move, a distance, and a velocity. The distance is chosen between and km. The velocity is chosen depending on the distance. If the distance is less than 5 km., the velocity is chosen less than kph., simulating a mobile host in a slow moving car, bus, or walking. If the distance is greater than 5 km., the velocity is between and 18 kph., simulating a mobile host in a fast moving car or a train. When the host stops moving, it pauses for a random time up to 1 hour. In the hotspot simulation, the maximum distance is decreased to 1 km. and the pause time is decreased to 1 min., in order to increase the probability that the host stops in a hotspot. In addition, for each experiment, the expected number of hotspots per day that the mobile node enters is fixed. After a mobile node finishes a large movement, a move into a hotspot is simulated. The hotspot simulation examined the comparative signaling performance of the two protocols in the wireless access network as a function of the expected number of entered hotspots per day.

3 9 2 8 Number of Binding Updates Number of Messages 1 MIPv6 IP Paging (4 subnets/pa) IPPaging (9 subnets/pa) MIPv6 IP Paging (4 subnets/pa) IP Paging (9 subnets/pa) Figure 1. Core Network Signaling Cost for Mobile IPv6 and IP Binding Update Figure 2. Paging Location Update and Paging The data collection was designed to answer two questions: What is the comparative core network signaling performance of IP paging and Mobile IPv6 for home agent binding update? What is the comparative access network signaling performance for local location update and paging? The first question required collecting only the number of Mobile IPv6 home agent binding updates in the core network for both Mobile IPv6 and IP paging. Binding updates are required for both because the mobile host uses the local IP paging agent as its care of address in dormant mode. The number of location area update messages to the paging agent was not measured. The signaling performance in the core network is reported as the number of binding updates per day per host. The second question required collecting data on local location updates and Layer 2 paging in the access network. The local location update consisted of a message to the access router when the host changed subnet for Mobile IPv6 and a location update message to the paging agent for IP paging. The Layer 2 paging message was the same in both cases. This is reported as the number of location update and paging messages per day per host. In addition, it was assumed that an L2 beacon message is available to both protocols to indicate when the host moved into a new paging area, though the beacon is not included in data collection. B. Macrocellular Simulation Results The results of the macrocellular simulation are shown in Figs. 1 and 2. Fig. 1 compares the average number of binding updates per host per day in the core network for Mobile IPv6 and IP paging as a function of the number of cells per subnet. The number of subnets per paging area was set at 4 and 9. As expected, IP paging reduces the number of binding updates because it aggregates movements between subnets and binding updates are sent only on paging area boundaries, rather than for every subnet, as in Mobile IPv6. The number of binding updates also decreases as the number of cells per subnet decreases, because a host can cover a larger geographical area without leaving the subnet or paging area. The dashed line shows how a fixed number of binding updates with Mobile IPv6 equivalent to IP paging can be achieved by adding more cells per subnet. On the dotted line, paging achieves about 25 binding updates per day for about 4 or 9 cells/subnet, while Mobile IP requires 36. Fig. 2 compares the number of access network location updates and paging messages per host per day for Mobile IPv6 and IP paging, with the same paging area configurations as in Fig. 1, as a function of the number of cells per subnet. The cost of IP paging is less than Mobile IPv6 when the number of cells per subnet is less than 16 (4 subnets/pa) or 9 (9 subnets/pa), but above that number, Mobile IPv6 signaling cost is less. This is consistent with the P-MIP results. Mobile IPv6 signaling cost reaches a minimum at 16 cells per subnet, while IP paging signaling cost reaches minima at 4 (4 subnets/pa) and 1 (9 subnets/pa). Again, as in the core case, the signaling cost of IP paging can be reduced by deploying more cells per subnet to achieve a broader geographical coverage. C. Hotspot Simulation Results Figs. 3 and 4 contain the results for the hotspot simulation. Both figures plot the average access network signaling cost per host per day against the expected number of hotspot visits per day. For Mobile IPv6, the number of messages sent over the air increases as the mobile host more frequently visits a hotspot. As a consequence, the Mobile IPv6 access network signaling cost increases with the number of visits to a hotspot. However, when there is a packet destined for the mobile host, the host

4 Number of Location Update and Paing Messages Expected Number of Hotspot Entries Number of Location Update and Paging Messages Expected Number of Hotspot Entries MIPv6 IP Paging MIPv6 IP Paging Figure 3. Paging for 1 Hotspot Cell/Macrocell Figure 4. Paging for 4 Hotspot Cells/Macrocell needs to be paged in only 1 hotspot subnet if the host is in the hotspot. If the host is not in a hotspot it needs to be paged in a macrocellular subnet, 9 cells in this simulation. For IP paging, no registration is needed upon entering and leaving a hotspot because the hotspot is part of the paging area. Therefore IP paging signaling cost does not change as a function of number of times a host enters a hotspot. However, when there is a packet for the host, the host needs to be paged in 1 macrocellular subnet covering 9 macrocells plus however many cells are in the hotspot subnet. For 1 hotspot cell per macrocell, there are 9 hotspot cells in the paging area, one per macrocell, for a total of 18 cells. For 4 hotspot cells per marocell, there are 36 hotspot cells in the paging area, four per macrocell, for a total of 45 cells. With 1 hotspot cell per macrocell, IP paging signaling cost is greater than Mobile IPv6 until the number of hotspots visited increases beyond 26 per day. With 4 hotspot cells per macrocell, the signaling cost for Mobile IPv6 never exceeds IP paging, up to the limit of the number of visits simulated. The signaling cost for IP paging is greater because the network must signal in more cells with IP paging than with Mobile IPv6 when a session arrives for the mobile host. D. Session Arrival Rate Sensitivity In order to see if the session arrival rate affected the results, the macrocellular simulation was run with the average arrival rate set at 1, 5, and 1 incoming sessions per host per day, to supplement the 2 sessions per day used in the base simulation. The results confirmed the trend seen for 2 sessions per day in the base simulation. Fig. 5 contains a graph of the number of messages in the wireless access network with an average of one incoming session per day per host. Comparing Fig. 5 with Fig. 2, the minimum cost number of cells per subnet in Fig. 2 for paging is 1 while in Fig. 5 it is 4 to 9, depending on the number of subnets per paging area. In addition, the crossover point, where Mobile IPv6 dormant mode update becomes less costly than paging, is shifted to the left in Fig. 2, at about 18 cells per subnet rather than around 8 in Fig. 5. The reduced number of incoming sessions means that more signaling messages are required during dormant mode because dormant mode lasts longer, resulting in a higher cost for Mobile IPv6 when geographical coverage of subnets is limited; but, again, the same or lower cost can be achieved by expanding the geographical coverage of the subnets. IV. CONCLUSIONS The fundamental assumption underlying paging in cellular networks is that the signaling and power cost of dormant mode location update as a mobile device moves around a large geographical area is considerably greater than the cost of broadcasting a paging message in the cells covered by the paging area. In IP networks, this tradeoff is not so clear cut. Unlike cellular networks, IP networks are fundamentally connectionless so there is no need to perform expensive signaling and occupy a circuit when a dormant mode mobile host moves from one cell to another if the active mode location update protocol, namely Mobile IP, is used. The signaling cost of maintaining a precise subnet location of the mobile host is comparable with maintaining a less precise location using IP paging, especially if the number of cells per subnet can be varied. On the other hand, since paging maintains a less precise location, the signaling cost of actually paging the mobile host is much greater with IP paging because a broader geographical area is paged. If the number of cells per subnet is small, and thus the geographical coverage of the subnet is limited, IP paging has a smaller signaling cost than Mobile IP for dormant mode location update. If, however, the number of cells per subnet is larger, providing broader geographical coverage, Mobile IP provides equivalent or better signaling performance. While the work reported in this paper considered a case where the size of the paging area is fixed, it is possible that a sophisticated dynamic paging area management scheme could result in a more clearly defined benefit for IP paging. Further simulation work is certainly necessary to confirm these conclusions, but the initial results presented in this paper suggest that IP paging is unnecessary, because it provides only a slight performance gain in limited circumstances compared to Mobile IP, which must be deployed for active mode mobility management anyway.

5 Number of Messages MIPv6 IP Paging (4 subnets/pa) IP Paging (9 subnets/pa) Figure 5. Paging Location Update and Paging, 1 Session per Day ACKNOWLEDGEMENTS Pars Mutaf would like to thank DoCoMo USA Communications Labs for support while working on this project. The authors would also like to thank the reviewers for providing helpful comments on how to improve the paper. REFERENCES [1] Heine, G., GSM Networks: Protocols, Terminology, and Implementation, Artech House Publishers, MA, [2] Michael D. Gallagher and Randall A. Snyder, Mobile Telecommunications Networking with IS-41, McGraw-Hill, NY, [3] Perkins, C., "IP Mobility Support for IPv4," RFC 322, IETF, January, 2. [4] Johnson, D., Perkins, C., and Arkko, J., "Mobility Support in IPv6," IETF draft, work in progress. [5] Kempf, J., "Dormant Mode Host Alerting ("IP Paging") Problem Statement," RFC 3232, IETF, June, 1. [6] Campbell, A., et. al., "Design, Implementation, and Evaluation of Cellular IP," IEEE Personal Communications, June/July,. [7] Ramjee, R., et. al., "IP-based Access Network Infrastructure for Next Generation Wireless Data Networks," IEEE Personal Communications, August,. [8] Ramjee, R., et. al., "IP Paging Service for Mobile Hosts," Proceedings of ACM SIGMOBILE, July 1. [9] Zhang, X., Castellanos, J., and Campbell, A., "P-MIP: Paging Extensions for Mobile IP," ACM Mobile Networks and Applications, 7(2), March, 2. [1] Kempf, J., et. al., " Requirements and Functional Architecture for an IP Host Alerting Protocol," RFC 3154, IETF, August, 1. [11] 3GPP2, "P.S1-A v3. Wireless IP Network Standard," P.S1-A v3., July, 1.