Study into MBMS as a Public Warning Technology

Transcription

1 Study into MBMS as a Public Warning Technology GUILLERMO ESTEVE, DAVID VALVERDE (1), ANTONIO PORTILLA-FIGUERAS, SANCHO SALCEDO-SANZ, JAVIER DIAZ-ESTEBARANZ (2) (1) Vodafone R&D (2) Universidad de Alcalá de Henares Abstract: - This report analyses the possibilities of supporting the Multimedia Broadcast Multicast Service (MBMS) as a technology to be deployed in a Public Warning System (PWS) using shared resources,. It also provides a general overview of MBMS as defined in the 3 rd Generation Partnership Project (3GPP) standards. Different MBMS modes of operation are exposed and information about how they work is provided. One of these modes is selected as the most suitable one and several enhancements to the standards are proposed. Key-Words: - Public Warning Systems, Multimedia Broadcast Multicast Service, emergency, dissemination 1 Introduction There are exclusive or dedicated networks to support the warning messages dissemination, as sirens in areas where floods may occur or electronic displays in roads. This report analyzes the possibilities of supporting Public Warning Systems (PWS) using shared resources, this is, how to use the new Multimedia Broadcast Multicast Service (MBMS) of the Mobile Telephony network as an additional mean to distribute warning messages. This would result in a cost optimization and quality enhancement of PWS, widening the reach and possibilities of those systems, which ultimately impacts in everybody s live with a safety improvement. The renewal of PWS is in the political agenda of many administrations and its support by public communication networks may become a requirement in Europe in the mid-term as it has happened in the United States of America [1]. 2 Description of MBMS MBMS is specified by the Third Generation Partnership Project (3GPP) in [2] and [3] in the Release 6 of the standards, later modified by Release 7 and Release 8. This chapter slects information from those to describe the use of the 3GPP standardized features to support PWS; both on the Technical Report [4] itself and the contributions that served as input for this study. 2.1 Summary of the MBMS capabilities MBMS is a unidirectional Point to-multipoint (PtM) bearer service in which data is transmitted from a single source entity to multiple recipients. Transmitting the same data to multiple recipients allow an efficient usage of radio-network and corenetwork resources, especially on the radio interface. It may be implemented over GSM EDGE RAN (GERAN) and/or UMTS Terrestrial RAN (UTRAN). MBMS feature is split into MBMS Bearer Service and MBMS User Service. The MBMS Bearer Service includes the Multicast and the Broadcast modes. MBMS offers up to 256kbps per MBMS bearer in UTRAN. The bearer capacity in GERAN depends on the actual network design. MBMS bearers may use up to 4 downlink timeslots per carrier. One of the advantages of MBMS compared to other UMTS services as Short Message Service (SMS) is that the MBMS Bearer Service shares transmission resources in the core and radio network. MBMS User Services offers two delivery methods: Streaming and Download. Streaming service best fits for those applications with tight constraints related to the size and/or the time delivery (e.g. TV or FM-radio like of transmissions). On the other hand, Download services are convenient for high quality applications or those which, due to its size, may be stored in the terminals and do allow certain delay before presenting the content (e.g. download and play services). A number of files can be delivered during a single MBMS transmission session. Both methods can be used on MBMS Multicast and MBMS Broadcast mode. In order to deploy MBMS on current mobile networks, it is necessary to add one new network entity called Broadcast Multicast Service Centre ISSN: ISBN:

2 (BM-SC). Moreover, the functional entities called Gateway GERAN Service Node (GGSN), Serving GERAN Service Node (SGSN) and User Equipment (UE) need updates, mostly on the form of software, to provide the MBMS Bearer Service. This is one of the strengths of MBMS compared to other PtM systems. There is no need to develop a whole new network making it a cost effective solution. Another strength is the possibility to transmit multimedia data (e.g. Text, audio, picture and video) rather than just text. There are other broadcast services available, in 3GPP; CBS is an existing broadcast service currently used for low bit rate services (messaging). However it does not allow multimedia delivery services, making it less attractive for users and operators. Furthermore Cell Broadcast messaging is not widely deployed today because most of operators have not deployed this technology in their networks either. In addition to that, MBMS enable handsets to receive multiple radio links of common channels from different cells and to combine them. This technique is known as Combining [5]. This method reduces the block error rate at the cell boundary and as a consequence, the cell capacity is increased due to the reduction in power transmission at each Node B. Ciphering for MBMS multicast data is done between the BM-SC and the User Equipment (UE). Within MBMS PtM data transmissions no radio interface ciphering is needed. The reception of MBMS must be set locally in the terminals for both Multicast and Broadcast modes. MBMS has the ability to send notifications regardless to the number of registered users to each service in any cell. 2.2 Modes of Operation There are three main modes of operation defined: - Broadcast mode. This is a unidirectional PtM transmission of multimedia data from a single source entity to all users in a broadcast service area. The broadcast mode is intended to efficiently use radio access and core network resources because data is transmitted over a common radio channel rather than PtP connections. Data is delivered in the Broadcast service area as defined by the network (Home Environment). However, when few users are subscribed to this service, it over uses the network and occupies a large amount of spectrum because it will broadcast in all cells of the Home Environment regardless to the number of users. On the other hand, all content provided over the MBMS Broadcast bearers will be available to all MBMS capable handsets. Therefore, it brings simplification since no specific requirement to activate or subscribe to the specific service is needed. - Multicast mode. As in the case of the Broadcast one, the Multicast mode allows the unidirectional PtM transmission of multimedia data from a single source point to a Multicast group, efficiently using the radio access and core network resources. The main difference with the Broadcast mode is that while the Broadcast mode delivers information to all user equipments in the coverage area with MBMS Broadcast capability configured on the terminal, the Multicast one will only transmit information to a certain group of users who previously subscribed to a specific Multicast group. One user may belong to different Multicast groups according to his/her interests. In the last 3GPP release (Release 7) a Counting mechanism which is used to estimate the number of users in a specific cell; this information is used to trigger a switch from a PtP to PtM bearer service in a per node basis. Due to the user s Subscription functionality the network can select which cells shall receive the MBMS content (i.e. where the interested users are). Those users shall belong to multicast groups which they have been previously subscribed to. Compared to the Broadcast mode, Multicast allows using radio resources in a more efficient way. Thus, if there are no interested users in a specific cell, no radio resources are consumed. It is fair to mention that Multicast mode implies an additional amount of signalling as well as additional delays compared to the Broadcast mode due to the Joining, Leaving and Subscription procedures that must be carried out. - Enhanced MBMS Broadcast mode This is as using Broadcast mode in the core network and Multicast mode in the radio network. It is a new mode introduced in Release 8. Broadcast mode underperforms in radio aspects because it uses, regardless of the real needs, the same amount of resources. Thus, in situations where few users are available, it comes up with poor trade-off between required resources and provided service. It takes the benefits of MBMS Multicast from a radio point of view; it carries a more efficient use of radio resources with the implementation of PtP and PtM bearer switching and UE Counting. Furthermore, it does not require the development of Multicast functions in the Core Network. Thus it leads to a faster service activation and session switching times due to Signalling is reduced; the Multicast procedures ISSN: ISBN:

3 removed are: Subscription, Joining and Leaving phases. 2.3 How MBMS works Multicast Mode MBMS Multicast service is enabled by a set of procedures. Subscription, Joining and Leaving phases are performed per user and the rest of the phases are performed solely by all users interested in a specific service. Some of them may run in parallel to other phases. - Subscription: This is the user s agreement to receive some MBMS service provided by any operator. This information is stored in the BM- SC. It can be done anytime before or after the service Announcement. - Announcement: Within this phase a UE retrieves a set of available MBMS services provided by an operator or an external authorized content provider (as News, Multimedia Downloads, etc ). It gets the set of services and their related parameters that may include a schedule of Session Start time before the services start. There are different service announcement mechanisms: via SMS to advertise MBMS Multicast and Broadcast services (i.e. CB-SMS), via MBMS Broadcast mode, PUSH mechanisms (WAP, MMS, SMS-PP), URL, etc [6]. - Joining: This is a specific Multicast mode phase. Once a UE has been subscripted to any Multicast service it can become a member of a group in order to receive MBMS Multicast data from a specific MBMS bearer service. A MBMS user service may be carried by more than one MBMS bearer service. The user decides when to join a Multicast group regardless to when the Service Announcement was. Joining time may be randomised according to some Announcement parameters in order to avoid Joining overloads. - Session Start: It is the trigger for bearer resource establishment for MBMS data transfer. At this point the BM-SC is ready to send MBMS data. When an MBMS user service is carried by more than one MBMS bearer service, a Session Start message is sent for each MBMS bearer service. - Notification: It advices UE about ongoing/forthcoming MBMS data transfers. - Data Transfer: MBMS data is transferred to user terminals. - Session Stop: At this phase bearer services are released. BM-SC determines that no more data will be sent for some period of time, long enough to justify removal of bearer resources associated with the session. When this idle period is not long enough, it is not efficient to do so and the connection is maintained. - Leaving: The subscriber does not wan to belong to a specific Multicast group so the subscriber deals with the end of a multicast group membership. This phase is Multicast specific Broadcast Mode As the operational issues is concerned, the main difference between Multicast and Broadcast modes is that the last one lacks on the Subscription, Joining and Leaving phases. This makes the Broadcast mode lighter and faster than the Multicast one since there is no signalling associated to these phases. However there is not that much control and information about the user as in the Multicast case Enhanced MBMS Broadcast Mode Enhanced-MBMS (E-MBMS) Broadcast is the best meeting both the radio and core network efficiency requirements. E-MBMS deploys in the Core Network the Broadcast mode since it lacks some phases compared to the Multicast one so signalling is simplified due to the removal of the Multicast Join and Leave procedures. In the RAN it applies Multicast mode due to its radio resources efficiency. Compared to the standard Broadcast implementation, it does not require any additional interface. E-MBMS Broadcast has clear benefits over the Broadcast-alone and Multicast modes for scheduled broadcast-type (e.g. Mobile TV: easy handling in BMSC, faster channel activation and switching times). 3 MBMS in PWS ecosystem 3.1 Usage Scenario The decision to implement a Public Warning System using shared resources must be taken between national or international regulators, safety professionals (police, fire brigade ) and technical parties as Operators and hardware manufacturers. Once taken, the regulation will foster the deployment and utilization of such services. The steps taken in distributing a warning message via PWS using shared networks should typically be as follows: - Some sort of disaster comes up. It is detected by the Warning Notification Provider (e.g. Local government, fire brigade ). - At this stage and according with its internal procedures, the information may be forwarded to some superior organisation (e.g. National government) transferring the leadership in the ISSN: ISBN:

4 crisis management and therefore the decision on the information to deliver to the citizens. - The entity taking the role of Warning Notification Provider should be able to select the specific area where the warning must be delivered through the PLMN. This notification shall be delivered to the citizens using the selected communication bearers. Different sort of notifications may be created and delivered depending on the location. For instance, the most damaged / threatened areas may receive a notification with content relevant to their status, as an alert to start evacuation as soon as possible. On the other hand, people in the areas in the boundary may receive different information. A reception reporting procedure is available for all MBMS delivery methods, which includes the user. A generic PWS architecture is depicted in Figure 1. The authorized agencies are connected to PLMN via a PWS gateway. The task of the PWS gateway is to translate the PWS message into MBMS specific format and protects the PWS distribution channel against un-authorized usage. Furthermore, the PWS gateway supports the authorized agencies to define the transmission area for PWS message. 3.3 Requirements There have been intensive works in the CHORIST project [7] gathering requirements from entities that may take the role of the Warning Notification Provider, this means be users of the PWS. As a result of this work we consider that: - Citizens expect a system that shall provide services across local, regional, national and international levels. - Self-configuration is and interesting feature because installation and configuration should require as less technical skills and human intervention as possible. - Authorities should be able to generate minor changes/updates in the system without requiring third party involvement (neither operators nor citizens). - The main information exchanges in the system shall use international standards. In addition to this first hand requirements, the 3GPP study for requirements of a PWS lists a number of aspects that should be considered when defining requirements in a PWS stage 1 specification [8]. The mentioned PWS stage 1 work is still to be done by 3GPP and it might happen that it is never specified if the partners in 3GPP do not pull from the issue 1. 4 MBMS as a PWS Technology The MBMS broadcast mode fulfills the localization requirements of a PWS. It can disseminate warnings to a specific area or group of users for the given session. Fig. 1 MBMS in a PWS 4.1 New Capabilities required for PWS New capabilities of MBMS that should be considered for PWS could be summarized as follows, based on PWS service aspects as listed in [4] Priority: - Functionality so the core network could determine the subscriber preferred service. - Functionality to deliver warning information to the preferred service. Support of roaming subscribers: - Setting a common PWS message identifier on MBMS for cross border PWS services. - Default character encoding. Congestion situation: - Priority control for PWS MBMS data. Enabling and disabling of PWS service: - Core network able to manipulate configuration information on the terminal. - User interface to configure the behaviour of the terminal. - Functionality to store, load and activate the configuration. - Functionality to utilize PWS identifier (type) in the roaming network. 1 And example of this is the NTT Docomo impulse to promote the recommendation [9] ISSN: ISBN:

5 - Functionality to identify whether the user device is in its Home Network or in a Roaming network in order to change the configuration. 4.2 Support of Pre-release 6 Handset Devices MBMS based PWS requires MBMS capabilities in the UE, (this is: the handset devices). There is no MBMS support in 3GPP pre-release 6 devices. At this moment (summer 2008), MBMS capable devices are not commercially available but some sample sets are being issued to Mobile Operators for testing purposes. 4.3 Possible MBMS standardization updates for PWS Recent work in 3GPP [9] has proposed E-MBMS as a suitable PWS technology without any change, but here we present a proposal of improvements from release 6 specifications. They will have the disadvantage of being applicable from the introduction of EUTRAN (or Long Term Evolution UTRAN) access only. (1) Interface specification for the gateway between the Message Dispatcher and the network entities (see Fig. 2). This protocol might be specified by 3GPP or by any other standards body, as OASIS. (2) Definition of the PWS service layer in the mobile terminals: The use of the MBMS service layer tools need to be defined in some additional details: - A specific behaviour of the terminals when receiving a PWS message is required. - A default configuration of the subscriptions to include a specific channel to receive the warning messages. This channel configuration should be mandatory 5 Conclusion MBMS is a very suitable technology for PWS. Its ability to reach a large amount of audience in an efficient way makes it a good solution fulfilling the most important user requirement. In order to get a more efficient usage it is possible to use the recently defined Enhanced mode which takes the best from Multicast mode in the RAN in order to use radio resources in an efficient way and Broadcast mode in the Core network to reduce signalling. To reinforce this conclusion the draft of the 3GPP [9] includes E- MBMS as a possible PWS. Furthermore, as it was previously remarked, MBMS can deliver multimedia information besides pure text, increasing the quality of information that can be delivered to the UE. Time delay is a main constraint in PWS. Do remain that the shorter it takes since the emergency comes up until handsets receive the notification, the more efficient the PWS will become. Compared to other PWS means, MBMS shall save up very valuable time. In terms of costs the MBMS architecture becomes a very interesting solution because it just needs one new entity (the BM-SC) and software updates in current mobile networks nodes. This would get them more affordable than dedicated PWS infrastructures starting from scratch. The main disadvantage for MBMS is the lack of installed base of UE because current handsets do not includes support for this feature. The replacement ratio of devices by the users is below 18 months in Western Europe, therefore MBMS as a PWS technology looks promising in the mid-term; from 2012 onwards. 6 Acknowledgements This project is funded by the Sixth Framework Program of the European Union under the project CHORIST and the by Comunidad de Madrid, Universidad de Alcalá and Ministerio de Educacion of Spain, through Projects CCG06-UAH/TIC-0460, CCG07-UAH/TIC-1894 and TEC References: [1] FCC. Commercial Mobile Alert System First Report and Order (CMAS First Report and Order), 9 April /FCC-08-99A1.doc [2] 3GPP TS : Multimedia Broadcast / Multicast Service; Stage 1. [3] 3GPP TS : "Multimedia Broadcast / Multicast Service (MBMS); Architecture and functional description". [4] 3GPP TR : Study for requirements for a public warning system. [5] 3GPP TS Introduction of the Multimedia Broadcast / Multicast Service (MBMS) in the Radio Access Network (RAN); Stage 2 [6]3GPP TS : Multimedia Broadcast / Multicast Service (MBMS); Protocols and codecs ISSN: ISBN:

6 [7] VV.AA. Report on user requirements and initial supporting cases (SP1.D4), CHORIST public deliverable available at [8] 3GPP TS : Study on requirements for a Public Warning System (PWS) service [9] 3GPP TR V0.1.0 Earthquake and Tsunami Warning System Requirements and Solutions (ETWS); Solution Placeholder (Release 8). ISSN: ISBN: