Networked Mobile Gaming for 3G-Networks Amjad Akkawi1, Sibylle Schaller2, Oliver Wellnitz1, Lars Wolf1 1
IBR; Technical University Braunschweig; Mühlenpfordtstraße 23; 38106 Braunschweig, Germany {a.akkawi,wellnitz,wolf}@tu-bs.de 2
NEC Europe Ltd.; Kurfürstenanlage 36; 69115 Heidelberg, Germany
[email protected]
Abstract. Mobile devices offer the opportunity to play games nearly everywhere. Moreover, networked games allow individual players to interact with other people and to participate in a larger gaming world, which also provides for new business opportunities. Hence, we currently see an increased interest from game developers, providers and players in mobile games. However, due to the inherent nature of wireless networks, many challenges have to be addressed. 3G technology combines two of the world‘s most powerful innovations, wireless communications and the Internet. This paper proposes an architecture for multiplayer games in 3G networks. It uses the capabilities and functionality provided by the IP Multimedia Subsystem (IMS). A prototype implementation demonstrates the feasibility of our approach.
Introduction Until recently, most games where stand-alone applications designed for a single player only. This has changed lately, where many games are directed towards a multiplayer scenario where people can interact and compete. Today, most of these networked multiplayer games are developed for PCs connected to fixed networks. Especially in the early days games had to cope with severe bandwidth limitations, e.g. due to narrowband modem links. Nowadays, many computers have access to broadband services (e.g. via ADSL). Currently, game developers, providers and players get more and more interested in games that can be played everywhere. Thus, games must be available on mobile devices. Yet, in order to support games on mobile devices some challenges, which are due to the nature of wireless networks, have to be overcome. One issue is that in such a situation only limited bandwidth will be available. Additionally, because of the mobility of devices, locations can change frequently and introduce several according problems. Various technologies for mobile networking and applications on mobile networks have been developed. There is no single mobile network technology that can serve all possible needs. Each of these solutions has its optimal place and purpose. Communication range, speed, latency, coverage as well as cost determine which technical solution should be used.
To support games in public and wide-spread areas, a wide-area mobile communication system such as a 2G or 3G system provides advantages. Furthermore, considering the huge base of end-systems, using such networks is interesting for providers and for users as well. Worldwide, the 2G GSM standard is used most often. GPRS is an evolution of GSM networks towards packet-switched data services. The GPRS speed is about ten times faster than current circuit-switched data services on GSM networks. GPRS fully enables mobile Internet functionality by allowing interworking with the existing Internet. Standards organizations and other related bodies have agreed to co-operate for the production of a complete set of globally applicable technical specifications for a 3rd Generation Mobile System based on the evolved GSM core networks and the radio access technologies supported by 3GPP (Generation Partnership Project [1]) partners. The technical specifications will be developed in view of global roaming and circulation of terminals. The first release of the 3GPP 3G standard has stabilised, and the first 3GPP compliant networks are going into operation. Work is currently ongoing for 3GPP Release 6. [2] From 3GPP specifications, a complete solution for the support of IP multimedia applications (including voice communications) shall be available. The solution consists of terminals, GERAN (GSM EDGE Radio Access Network) or UTRAN (UMTS Terrestrial Radio Access Network) radio access networks and GPRS evolved core network. [3] UTRAN is a new radio network architecture. It provides services with more bandwidth for data and better quality for voice as compared to GSM and GPRS. The 3GPP IP Multimedia Subsystem (IMS) enables a “platform” with capabilities like presence, multimedia conferencing, messaging, support for QoS (Quality of Service) for data traffic, etc. Wireless LAN (WLAN) is another type of access network supported by the 3GPP IMS. WLAN according to the IEEE 802.11 standards family provides bandwidth from 2 up to 54 Mbit/s. The actual bandwidth available per device decreases proportional to the number of the connected, active devices. By use of IMS service capabilities and standards protocols a new QoS for games as well as better performance and scalability can be achieved. Complex services can be created and integrated into a game; based on simple IMS services such as Instant Messaging and/or special services such as a scoring service. The availability of information like location, terminal capabilities and presence status can significantly ease the development and enhance the features of games. In this paper we propose an architecture that enables games both to run on 3G networks and to use the capabilities and functionalities of the IMS. The focus is on networked, multiplayer games. A prototype implementation shows the feasibility of our approach. In the next section “A General Game Service” we briefly introduce aspects of multiplayer games, depict a general game service, and explain its components. In the third section “Architecture for the Integration of Games over the IMS”, we motivate the choice of the IMS for game services and introduce our game service architecture. After the description of our prototype implementation in section 4 “An Example Implementation”, we conclude the paper in section 5 “Conclusions”.
Related Work It seems that there is no other work directly related with the topic of the paper: an architecture for mobile, networked games over the IMS. This paper proposes an architecture that enables games to run on 3G networks and at the same time to use the capabilities and features of the IMS. Current related work focuses on providing powerful mobile devices with different features for mobile games and wireless connectivity. Mostly, wireless connectivity for multiplayer games is limited to local area networks (e.g. Bluetoth). The architecture described in this paper is not tied to any game specific hardware. Also we took the approach to base the architecture on standardized components and interfaces. The remainder of this section lists some commercial game platforms that allow for wireless networked games. Nokia 0[4] has entered the mobile game arena with its N-Gage in September 2003. The Nokia N-Gage platform was designed for both games devices as well as mobile phones. The Nokia N-Gage games deck allows users to play multiplayer games connected locally via Bluetooth and/or remotely over mobile networks. The N-Gage device is a tri-band GSM 900/1800/1900 mobile phone which has, in addition to the large screen (176x208 pixels) supporting 3D-graphic, digital music player MP3, stereo FM radio, as well as. The Game Park Inc device (GP32) [5] is officially available in Asia and parts of America. The device is equipped with USB and memory card slot and has a big screen. The GP32 is also equipped with a Radio Frequency Module enabling up to 4 players playing locally wireless games. B’ngo is a mobile phone [6] with handheld functionality providing wireless games via Bluetooth. The B’ngo device provides up to 8 players in a single match. Sony Computer Entertainment (SCE) described its Play Station Portable (PSP) as the "21st-century Walkman," showing confidence in the company's upcoming product, which utilizes a number of high-end components. In terms of communication, the PSP will come equipped with 802.11-standard LAN, IrDA, and USB 2.0. 802.11-standard. 7] SCE plans to release the PSP at the end of 2004 in Japan. Gaming-hardware maker Nintendo, which claims 98 percent of the handheld console market with the Game Boy, has had a limited wireless-gaming offering. Customers of some Japanese carriers, including NTT DoCoMo, can attach their Game Boys to cell phones, which then act as a modem for Web-based play. The Nintendo wireless adapter will be launched in Japan during the first half of 2004. 8]
A General Game Service After a brief introduction to aspects of multiplayer games, we describe a general game service and explain its components.
Aspects of Multiplayer Games Multiplayer games allow two or more people to play together or against each other in the same game. Networked multiplayer games are playable over a network (e.g. the Internet). Since the first video game for computer (Space war) in 1961 and the first multiplayer game in 1969, the multiplayer game world has faced many challenges. Despite both the network and hardware limitations, a variety of multiplayer games were produced. The communication range, speed, network coverage, bandwidth and latency, as well as parameters of the game client devices (processor, memory, graphics, etc.) have an influence on what kinds of multiplayer games can be developed. These technical issues can have a significant impact on the applicable techniques for specific types of games. Independent of the type of network that connects the players with the game, the physical limitation of the network cannot be ignored. Important limitations are the scarcity of resources, interferences, etc. on the radio link, leading to small bandwidth and high latency. Several communication models and design technologies deal with these limitations. The communication model can be Peer-to-Peer, Client-Server based, or Hybrid. [9, 10, 11] In the Peer-to-Peer model the game clients communicate directly with each other. In the Client-Server based model a game server is involved in all game-related communication. The game server also keeps the complete game state. In the Hybrid model a part of the game is implemented with the Client-Server model, especially the communication of the game clients with the server, and the remaining part with the Peer-to-Peer model. Table 1 shows the possible usage for the different communication models. Table 1. Possible Usage of Game Communication Models Number of Players Small 1-16 Large 16-200 Super Large up 200
Communication Model Peer-to-Peer Client/Server & Hybrid Hybrid
Components in a General Game Service Figure 1 shows the components used generally for game services. The Game Service is the sum of the contributions of all these components.
Game Application
Game Application Game Provider(s)
News
Reports
Distribution
High Score
Competition
Community service
Player and Team States
Chat
Instant Message
Conferencing
MMS
Gaming Service
Game Application
Game Information Service
Figure 1. Components in a General Game Service The central component in the Game Service is the Gaming Service that is a serverside platform providing network connectivity and general support for gaming. A Game Provider is a human, an organization, or a collection of humans and/or organizations that own a game application, or have the right to use and publish a game application (e.g. Tetris, Quake, Age Of Empires). The Game Provider publishes and distributes games via the Gaming Service. Potential game players must install the game client(s) in their terminals. For mobile terminals, for example, this could be done with Over-the-Air provisioning. [12] An authenticated and authorized player can either join a (running) game of his choice or create a new game session. A game session is a single gathering of players to participate in an instance of a game. A game session lasts for some period of time. The Community Service consists of different services that enable the players to communicate. Examples for Community Services are: Instant/Voice/Multimedia Messaging, Chat or Conferencing. One or more of these services may be integrated in a game and the player has the choice to use, join or leave them without having to interrupt the game. The Game Information Service contains functionalities for the management of game related information. It may contain the information as listed in Table 2. For the management, collection and distribution of game information the Gaming Service may use a Presence Service. Game presence information is a set of attributes characterizing the current status of a game such as score(s), player(s) and team(s), competition information, etc. The Game Information Service manipulates game presence information. Players and watchers of games can subscribe to all or parts of this information and may also be notified of any changes. Game watchers are users that do not (yet) actively participate in a game, but are interested in obtaining game related information and/or using game community services.
Table 2. Game Related Information Type of Information Scores and Competition Players and Teams
Downloading and Distribution News Reports
Description Past and current scores, and competition information about past, current and future games. Each game may have one or more Score and Competition Information tables. Contains information about the game players and their teams. It may also contain information about a player’s profile. Information about new releases, new updates, demonstrations and levels. This information may also contain a download link. Headlines, messages-of-the day, previews, screenshots, advertisement, etc. A report may contain different information collected about games, e.g. during game launch. They give a game provider the possibility to make development and marketing decisions. Reports may contain statistical information about the players’ requests and interests.
Architecture for the Integration of Games over the IMS In this section we will briefly motivate the choice of 3GPP IMS for game services and introduce our game service architecture. Why Games over 3GPP IMS 3G combines two of the world‘s most powerful innovations, the internet and wireless communications. Having access to the Internet in mobile devices will be of great value to many users. The 3GPP IMS is a standardized infrastructure, able to run services of all categories while allowing ease of inter-working between (mobile) operators. [13] It will allow: Peer to peer, real-time and non-real-time services delivered over the packet switched domain Seamlessly combined services with different QoS categories Only one infrastructure is required for the support all types of services, keeping operators’ CAPEX and OPEX low. From a technical point of view, the IMS provides a horizontal approach to integrated, real-time, multiparty services for mobile networks. The Session Initiation Protocol (SIP) was chosen as the signalling protocol. It allows determining capabilities of user terminals, to negotiate QoS parameters, and to use and switch between media components as needed.
The first release of 3GPP architecture, Release 99 (R’99), defines the basic architecture of the network. R’99 was designed to be backward compatible with the existing GSM circuit switched infrastructure. The IP Multimedia Subsystem (IMS) is not part of this and the following release. The so-called Phase 1 IMS is specified in 3GPP Release 5 and contains the basic mechanisms for multimedia session management. The 3GPP Release 6 adds many new capabilities to the system and the IMS. Examples are: Presence, Conferencing, Messaging, WLAN-Interworking, and Group Management. The IP Multimedia Subsystem (IMS) is an extension of the 3GPP packet-switched core network. It uses the Session Initiation Protocol (SIP) to setup, maintain and terminate voice and multimedia sessions. The 3GPP packet-switched core network is used to support the multimedia session bearer (data path) while hiding the terminal moves. IMS Game Service Architecture The IMS game service architecture was designed specifically for games based on the Client-Server communication model. Such games can be integrated with minimal effort. Peer-to-Peer or Hybrid games can also profit from some functions of the architecture. Figure 2 illustrates the IMS game service architecture. The central component is the Game Focus. It maintains a SIP signalling relationship with each player in the game. Via IMS interfaces towards application servers the Focus can obtain the user location and his terminal capabilities, as well as authentication information accessing the 3GPP core networks HSS (Home Subscriber Server).
SIP Service Provision Service logic & Policies
IMS IMSMessaging Messaging Service Service
( (
Proxy
Focus
PUA
Presence Presence Service Service
Gaming Service
IMS IMS Game Watcher UE Applications Application
Game Watcher UE Applications Application
Figure 2. Game Service over the IMS and Using IMS Capabilities and Functionalities
The game platform API is used for the information exchange between the focus and the respective Game Servers. This API is used to communicate the availability of game servers, accepted players, game sessions, etc. The game platform API is based on OMA Games Services version 1.0 (i.e. MGIF). [14] The Focus serves the user’s request, and forwards it to the “Service logic and policies” component. The game policy can be thought of as a database that contains policy information about how the focus should operate and may also contain player authorization information. It is the responsibility of the focus to enforce those policies. The focus also must both know about and eventually react to policy changes. Such changes might result, for example, in the termination of a dialog with a player by a sending SIP BYE message. Most policy changes will require a notification to subscriber(s) via the game notification service. The Proxy component is needed when, e.g., a player wants to join a game’s chat session. Thus the chat can be tightly integrated with the game, and the players need not to know/learn extra information (e.g. SIP-URI) about the chat service. In that case, the game service may also check the authorization. The Presence User Agent (PUA) component is needed to inter-work with the IMS Presence Service for game and player related presence information. The IETF SIMPLE protocol is used to publish and obtain presence information. [15, 016] The game client (devices) and the game focus manage game sessions with the help of the SIP protocol. [017] SIP is also the signalling protocol in the IMS, thus the exploitation of the full IMS capabilities is possible. The session signalling also includes information needed for the provisioning of QoS for the game data traffic. The IMS provides the game client with an authorization token that can be used to do network reservations for the games data path. The games data path is set up directly with the games server. All game servers in this architecture will accept only data connections to game clients, which have been accepted previously via SIP signalling with the focus.
An Example Implementation We implemented a prototype of the architecture described in the previous section (see also Figure 3). For our architecture two games were integrated: TetriNet and GNU VolleyBall. TetriNet was tested over GPRS as well as WLAN networks. TetriNet was playable over both types of networks. Because VolleyBall has most stringent requirements in regards to delay, the game was playable over WLAN, but not over the current GPRS network. In the GPRS network a delay is about one second was experienced. For our implementation we used NIST SIP stack implementation, which required Java Virtual Machine (JVM) 1.2. Due to restrictions of the JVM on the PDA, we had to modify the SIP stack to run on JVM 1.1. The original TetriNet is a real-time multiplayer game and supports up to 6 players in each game session. For the client device a PDA (HP Ipaq) was chosen. Due to the small screen size of the PDA, we limited the number of possible players to 4. Sample screenshots of the game client can be seen in Figure 5.
TetriNet
SIP Registrar
Game Server
/ '! "$#&%$' )(0+*,% -.
Figure 3. A Prototype Implementation of the Gaming Service The following entities were implemented: 1. A game client manager and the TetriNet game client on the PDA with the Windows PocketPC 2002 operating system. A graphical user interface (GUI) manages all games installed in the PDA. XML files are used for the configuration. With this GUI the user (player) can register himself at the SIP Registrar and join/leave game sessions. Our game client manager on the PDA automatically starts the respective game client after the user successfully joined a game session. See also Figure 4 and Figure 5Fehler! Verweisquelle konnte nicht gefunden werden.. 2. The Focus of the Gaming Service on a PC running SUSE Linux 8.2. This component implements the functionalities of the Focus in the games architecture described above. The Focus manages the game sessions via SIP signalling with the game clients and with the game servers via the game platform API, enforcing the policies of the game provider. 3. The TetriNet game server on a PC running Windows XP. A so-called game agent supports the game server to register at the game focus. The game agent may serve all game servers on the host, and is able to trigger the initialisation of game servers. 4. Additionally a SIP proxy and registrar were needed. For this server we used the NIST SIP proxy/registrar on a PC running SUSE Linux version 8.2. In our testbed, the TetriNet game server registers itself in the gaming service. The gaming service allocates resources, creates a SIP URI for the game and registers it at the SIP Registrar. Several TetriNet game servers may publish game sessions at the gaming service at the same time. The client software on the IPAQ lists the published game session URIs, and then the user selects and joins a game session. Afterwards, the TetriNet game client starts automatically. It should be noted that the Focus is only involved with the game
session management. The data path for the game is set up directly between the game client and the game server.
Figure 4. Screenshot of the Game Client Manager GUI
Figure 5. Screenshots from the TetriNet Game Client
Conclusions In this paper we presented a brief introduction to aspects of mobile multiplayer games. We also presented a view of a general game service and explained its components. Then we introduced our architecture for games over the 3GPP IMS and provided information about our prototype implementation.
The prototype implementation of our architecture for Games over the IMS shows that games can be integrated relatively easily onto the 3GPP and IMS platform. The minimal changes that existing games will have to undergo are to enable them for SIP signalling to set up the game session before the data path for the actual game traffic is established. Besides the porting and deployment of existing games, the presented architecture is suitable especially for the development of new games, since different (multimedia) services can be integrated and used easily. Overall, the IMS capable to support mobile, networked games. Of course, mobile devices may have restricted capabilities for running game applications. Such limitations cannot be solved completely by the IMS but it can help to overcome critical issues The current prototype was evaluated on a SIP based network. The next step is to integrate the prototype with a real IMS system and to extend the prototype for using all available IMS functionalities (e.g. Presence Service, Conferencing). Security and charging issues are other topics that need more investigation.
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3rd Generation Partnership Project; www.3gpp.org 3GPP TR 21.902; “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Evolution of 3GPP System” 3GPP TS 22.228; “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Service requirements for the Internet Protocol (IP) multimedia core network subsystem; Stage 1” http://www.heise.de/newsticker/data/daa-11.07.01-003 http://www.gampark.com http://www.ttpcom.com/ttpcom/news/pr_03_bingo.htm http://www.gamespot.com/all/news/news_6072659.html http://www.nlgaming.com/nl/asp/id_1453/nl/newsDisp.htm Yu-Chen “Internet Game Design” www.gamasutra.com Joseph D. Pellegrino, Constantinos Dovrolis ”Bandwidth requirements and state consistency in three multiplayer game architectures“; NetGames 2003 Jouni Smed, Timo Kaukoranta, Harri Hakonne, “Aspects of Networking in Multiplayer Computer Games”; http://staff.cs.utu.fi/~jounsmed/papers/AspectsOfMCGs.pdf “Generic Content Download Over The Air Specification”, OMA-Download-OTAv1_0-20030221-C; www.openmobilealliance.org/tech/release.html “Mobile Evolution Shaping the Future”; a White Paper from the UMTS Forum; August 2003; www.umts-forum.org OMA Games Services version 1.0, Candidate Enabler Releases; www.openmobilealliance.org/tech/release.html 3GPP TS 23.141; “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Presence Service; Architecture and functional description (Release 6)” “SIP for Instant Messaging and Presence Leveraging Extensions (SIMPLE)”; www.ietf.org/html.charters/simple-charter.html “SIP: Session Initiation Protocol”; IETF RFC 3261
