4G Mobile Network Architecture

4G Mobile Network Architecture Kalle Ikkel¨a, [email protected] Marko Myllynen, [email protected] Juha Hein¨anen, [email protected] Olli Martikainen, [email protected] May 28, 2001 Abstract Telephone TV Data ACCESS TECHNOLOGIES HORIZONTAL ACCESS The convergence of telecommunications, computing, and content industries has been one of the major trends during recent years. The convergence has lead to the creation of a wide range of multimedia services NETWORK TECHNOLOGIES IP BASED NETWORK which are available in digital form through the Inter- net. There is a need for user and terminal mobility supporting both personal communication and multi- NETWORK BASED SERVICES HORIZONTAL SERVICES media services on top of the Internet. To help meet this need, this paper presents ongoing research on the Fourth Generation (4G) roaming and mobility model. Figure 1: Vertical services’ “pipes” compared to hori- The 4G model is compatible with existing IP stan- zontal service structure dards. The basic elements of the model are described and the model is compared to the GSM (2G) model. vices easier and more widely applicable, but also create a platform for the integration of various new services 1 Introduction and applications into the same terminals. New terminal applications in horizontal networks Traditional telecom and content services are vertically can be divided into simple fixed-purpose termi- integrated. Each service depends on a dedicated net- nals and intelligent terminals. Possible fixed- work and corresponding terminals. Examples of such purpose terminals can be wearables (watches, eye- vertical services are the fixed telephone services, tra- glasses, clothes) or appliances (light switches, doors, ditional data services and GSM services. Internet micro-ovens). Intelligent terminals include Personal changes the vertical structure to a horizontal one: all Digital Assistants (PDA), Smart Phones or Media Ter- terminals and services will be Internet compatible. In- minals. Simple terminals will connect to Personal stead of vertical service ”pipes” there will be a hori- Area Networks (PANs) or Domestic Area Networks. zontal structure of services, network and access, as Intelligent terminals apply to Local Area Networks illustrated in Figure 1. (LANs) or public access networks. They will have The horizontal structure will change terminals, ser- software and content-defined functionalities that allow vices, and the way services are managed. The hor- various applications within one device. izontal structure will allow different combinations of Fourth Generation Mobile (4G) means broad- service functionalities in the terminal equipment as band mobile wireless services, which are based on depicted in Figure 2. The ovals show possible combi- IEEE 802.11 Wireless LAN (WLAN) [1] or Bluetooth nations of services in single terminals. (Bt) [3] access, IP mobility and Web type services. Horizontal networks will not only make existing ser- The radio access can be connected to private corporate  Games TV PDA PC Audio eBooks Mobile Devices HORIZONTAL ACCESS IP BASED NETWORK Figure 3: GSM network overview HORIZONTAL SERVICES several functional entities, whose functions and inter- faces are defined. The GSM network can be divided Figure 2: Restructuring to multifunction terminals into three parts: The Mobile Station (MS), carried by and horizontal services the subscriber; the radio link, controlled by the Base Station Subsystem (BSS) with the mobile station; the Mobile services Switching Center (MSC). The MSC LANs, public administration LANs, mobile WLANs is the main part of the Network Subsystem and per- installed in trains, airplanes, buses or cars and to forms the switching of calls and management of mobile single DSL connections. Corporate offices, shopping centers, hotels, airports, home networks, and personal services, authentication, for example. The operation area networks (PAN) will be the leading adopters of and setup of the network is managed by the Opera- these technologies. The client devices for the 4G ap- tions and Maintenance Center (OMC). Each compo- plications can be categorized as follows: nent dealing with mobility is described in more detail next. • Laptop PC with WLAN Mobile Station (MS): The mobile station is the user terminal, which consists of a radio transceiver, • Laptop PC with Bt signal processors, display, and a Subscriber Identity • PDA with WLAN Module (SIM card). The SIM card enables the usage of services and personal mobility. • PDA with Bt Base Station Subsystem (BSS): The Base Station Subsystem has two components, the Base • Dual-mode wireless phone with GSM and WLAN Transceiver Station (BTS) and the Base Station Con- or Bt troller (BSC). The radio transceivers are located in • Wireless phone with Bt BSCs. The BSC also manages the radio-link with MS. The Base Station Controller manages one or more • Other specialized Bt devices. BTSs and handles radio resources such as radiochan- nel setup, frequency hopping, and handovers. 2 Architecture of GSM and Mobile services Switching Center (MSC): The Mobile services Switching Center is the central com- UMTS Networks ponent of the Network Subsystem. MSC is like a nor- mal switch in PSTN or ISDN. MSC also handles all An overview of a GSM network architecture is pre- the functions needed to manage mobile subscribers, sented in Figure 3. A GSM network is composed of including registration, authentication, location updat- ing, handovers and call routing. The roaming func- 2.2 Roaming and Handover tionality of GSM is provided by the Home Location Register (HLR) and Visitor Location Register (VLR) Roaming is the movement of the mobile terminal from together with the MSC. The MSC has no information one part of the network area to another part, while about particular mobile stations; this information is retaining the capability of making or receiving calls. stored in VLRs and HLRs. The handover is the action of switching a call in progress from one cell to another (or between radio Home Location Register (HLR): This func- channels in the same cell). The handover is used to tional entity is a data base in charge of the manage- allow established calls to continue when mobile sta- ment of mobile subscribers. A Public Land Mobile tions move from one cell to another. Network (PLMN) may contain one or several HLRs. The handover can be carried out in several ways: The number of HLRs depends on the number of mo- bile subscribers, on the capacity of the equipment and Intracell handover: The mobile unit is switched on the organization of the network. The HLR con- from one channel to another within the cell area. tains two kinds of information: subscription informa- BTS-BTS handover: The mobile station is tion and some location information that enables the switched from one Base Transceiver Station to another charging and routing of calls toward the MSC where under the control of the same Base Station Controller. the MS is located (e.g., the MS roaming number, the BSC-BSC handover: The mobile station switches MSC address). between BTSs as well as between the BSCs at the same Visitor Location Register (VLR): This controls time. The handover is controlled by the target BSC. mobile station roaming in the MSC area it is in charge MSC-MSC handover: When switching between of. When a mobile station enters a new location area it the BTSs and BSCs, the mobile station may also starts its registration procedure. The MSC in charge switch from one MSC (Mobile Services Switching Cen- of that area notices this registration and transfers the ter) area to another. The handover is controlled by the identity of the location area where the MS is situated target MSC. to the Visitor Location Register. If this MS is not yet registered, the VLR and the HLR exchange informa- 2.3 Packet radio (GPRS and UMTS) tion to allow the proper handling of calls involving the MS. GSM in phase 2+ will be capable of handling both the conventional circuit switched transmission already in- troduced in GSM Phase 1 and the packet switch trans- 2.1 Areas of the GSM network mission provided by the GPRS (General Packet Radio Service) [8]. GPRS and UMTS are both based on Different areas of the GSM network and the functions broadband radio technologies as distinct from GSM associated with them are described below. Phase 1. The evolution from GSM towards UMTS Location area (LA): The location area is defined (3G) happens slowly, with GPRS gradually adding as an area in which a mobile station may move freely SGSN (Serving GPRS Support Node) and the GGSN without updating the location register. A location (Gateway GPRS Support Node) elements to the GSM area may consist of several cells. architecture (Figure : 4). The circuit switched trans- Network coverage area: An area in which mobile mission path between the GSM BSS (Base Station stations are able to communicate with the network. Subsystem) and external networks is routed through Service area (SA): An area in which a mobile sta- the GSM network via the MSC (Mobile Services tion is obtainable by another PLMN, PSTN or ISDN Switching Center) and the GMSC (Gateway MSC), subscriber without the subscriber’s knowledge of the while the packet switched transmission is routed via actual location of the mobile station within its area. the GPRS components SGSN (Serving GPRS Sup- System area: The system area consists of one or port Node) and the GGSN (Gateway GPRS Support more service areas with fully compatible MS-BS inter- Node). The UTRAN (UMTS Terrestrial Radio Access faces. The location registers of the individual service Network) will be interconnected to this core network areas remain autonomous; updating of the location in- via two IWUs (Inter Working Units), one between the formation is not performed when roaming mobile sta- Iu interface and the GSM A interface, and another be- tion moves from one service area to another. tween the Iu interface and the GPRS Gb interface (see Figure 4). This architecture makes it possible for both functions as possible for terminals to execute. Ter- GSM and UMTS customers to be connected both to minals will cost less when no additional software and circuit switched networks (e.g., PSTN and N-ISDN) processing capability is required, and only servers will and packet switched networks (e.g., the Internet and need to be upgraded. intranets). Additionally, users should also be able to roam between GSM and UMTS networks. [8] As simple base stations as possible As some terminals, also base stations (access points) will have limited processing capability. Increasing processing capability would cause base stations to cost more, which is highly undesirable. Bluetooth and WLAN access points have maximum coverage of roughly about 100m, depending on power and sur- roundings. With such a short distance coverage, a relatively large number of base stations are needed. Thus, expensive base stations could dramatically in- crease the overall cost of a network. Figure 4: Evolution of GSM platform towards UMTS Standard RFC based protocols To avoid service dispersion and interoperability prob- 3 Requirements for IP Mobility lems all communications should be based on open, standardized, protocol definitions. Closed, propri- 4G terminals will be used to access various services, etary protocols could easily lead to a situation where including current Internet based services as well as a user cannot access all the services needed because new services targeted for 4G terminal users. A factor of communication protocol incompatibilities. This common to these services is that they all use IP. Thus, would cause additional costs for both service providers clearly, 4G terminals must be based on IP, too. (providers should support many protocols instead of Hybrid terminals, which support IP and some other just one global standard) and customers. Higher costs communications technologies, such as 3G UMTS, can and incompatibilities could reduce general interest in be used but because these hybrid terminals are also the whole 4G concept. able to communicate by means of IP, 4G service providers are required to offer only IP based access to their services. Scalability down to picocells and locations up 10 The problem with current IP schemes is that hosts to 10 users globally are expected to be stationary, using one fixed IP ad- New architecture should be scalable both downwards dress all the time. In the 4G networks terminals must and upwards. The less coverage one base station has, be able to move around without limitations set by the the more accurately the location of terminals is known. network architecture. The following sections describe Therefore, some operators use low coverage base sta- requirements and solutions for IP Mobility. tions even if high coverage base stations are available. This means that the new architecture has to operate 3.1 Goals for the New Architecture smoothly with both low and high coverage base sta- tions. No additional software required in terminals Although it is unfortunate, it is nevertheless a fact Some terminals will have limited memory and process- that at the beginning of the 4G era most of the pop- ing capability, and power consumption by any kind of ulation of the world are not immediately able to use terminal should be minimized by avoiding any unnec- the new technology. The new architecture should be essary activity. Therefore, all possible mobility related ready for them to avoid scalability and other problems functions should be executed by servers leaving as few in the future. VoIP and Web compatibility Outbound proxy intercept, terminals send registration messages to their home network but Outbound proxy New 4G terminals and the whole 4G architecture will intercepts these messages and changes the visitor ad- support both VoIP (Voice over IP) and current World dress to point itself. In the case of the User-initiated Wide Web (WWW) based services. Currently WWW proxy registration, the terminal recognizes it is visiting provides a vast amount of services that will not be a foreign network and sends registration messages to modified for 4G terminals. Therefore 4G terminals the Outbound proxy that then forwards the messages must be able to communicate with current protocols to the terminal’s home network. Thus, all incalls and to achieve Web based service access. outcalls involving a visiting user travel through the Most handhelds used today are cellular phones used Outbound SIP proxy. to direct conversation between humans. Undoubt- The use of Outbound proxy for all registrations al- edly, the need for direct conversations remain and 4G lows network operators to collect statistics and possi- should support this. This can be done with the help ble billing information and also to reduce the number of VoIP. of SIP registrations. In this way, several location areas can be combined to one 4G service area (4GSA). By using hierarchical registration [9], the home loca- 4 4G Architecture tion area SIP redirect server is updated only when a terminal moves from one service area to another. SIP The fourth generation (4G) of mobile networks will redirect server redirects SIP calls to the Outbound offer mobile services based on high-speed wireless con- proxy that will forwards calls to the terminal in its nections, IP mobility, intelligent terminals, and World current location in the visited network. Wide Web type services. 4G operators are the most With SIP, users can be reached globally. The same likely service and content providers to use different effect could be achieved using Mobile IP [5], but the kinds of radio access technologies. Radio access can triangle routing creates problems. The data trans- be based on private corporate LANs, public wireless ferred from a server to the user’s terminal is routed LANs or mobile LANs installed on trains, airplanes, via a home agent, which is a non-optimal situation. and so on. Handhelds, laptops, and mobile phones will The usage of SIP enables direct point-to-point data be used to access the Internet and local services. transfer. 4G location area (4GLA) diameter can be from 100m to 1 kilometer. Figure 5 presents the 4G net- work architecture. The idea is to use Session Initiation 5 Micro Mobility Protocol (SIP) [2]. Every home location area contains a SIP redirect server, which is responsible for main- The term micro mobility here refers to mobility inside taining the current location of users. The home SIP a single location area. Micro mobility is needed when redirect server is analogous to HLR in GSM network terminals move around using different base stations architecture. When a call is made, the home SIP redi- and IP subnetworks. The requirement for micro mo- rect server returns the current address of called party. bility means that the terminal is able to use the same The SIP client of the caller then makes another call IP address all the time to keep TCP (or other higher to this particular address (or addresses - SIP redirect layer) connections alive when the terminal moves to server can return several addresses). another base station cell or IP subnetwork. Micro mo- Every location area where visitors are allowed, con- bility is controlled by the gateway router responsible tain a visitor SIP proxy (Outbound proxy), which is for the location area. analogous to VLR in GSM network. Every visitor in In the 4G architecture we are working with, base a foreign network registers with the home SIP redi- stations are invisible to the IP layer of terminals. rect server if they want to be reachable. This regis- When a terminal sends an IP packet, it sends it to tration can be done in several ways as described in the gateway that all base stations are connected to. [7]. We propose that only Outbound proxy inter- Base stations only relay layer 2 packets between termi- cept and User-initiated proxy registrations are used. nals and the gateway. Link layer mobility is managed Both these registration methods use Outbound proxy by the device drivers of terminals and base stations. to forward SIP registration messages. In the case of Thus, if all base stations belong to the same IP sub-  SHOPPING MALL AIRPORT Wireless Network Wireless Network Wireless Network Wireless Network Wireless Network Wireless Wireless Network Network Wireless Network GATEWAY DHCP SIP GATEWAY PROXY DHCP SIP PROXY INTERNET GATEWAY SIP REDIRECT Wireless Network HOME NETWORK Figure 5: 4G network architecture network, no additional IP layer mobility management an IP address by the means of DHCP with short lease is required for any part of the network. However, we time (lifetime) when the terminal enters to the local are experimenting with multicast to neighboring base network. The terminal starts sending DHCP Renew- stations to ensure smooth handovers. ing Requests when more than half of the lease time is In the case of several IP subnetworks, additional mo- passed. Secondly, when the gateway receives Renew- bility management is needed. Several IP subnetworks ing Requests it updates its routing caches. When IP are required for scalability, thus using a single IP sub- packets destined to a terminal arrive to the gateway, network is not a solution in the case of large networks. packets are routed to the correct IP subnet based on The problem of using several IP subnetworks is the fol- the updated routing cache. In the handover phase the lowing: when a terminal, using IP subnet A address packets can be multicasted to both old and new sub- moves to IP subnet B area, all IP packets destined to it networks. The address requested to be renewed will are routed to IP subnet A. Thus, the terminal will not be renewed even if the terminal has moved to another receive any of these packets. The IP address can not IP subnetwork. Thus, the terminal is able to use sev- be changed without breaking live TCP connections, so eral IP subnetworks without changing its IP address. we will need a solution which allows a terminal to use Lastly, when other hosts in the network are commu- an IP subnetwork address of IP subnet A in all other nicating with the terminal that has moved to another IP subnets, too. IP subnet, gateway uses Proxy ARP to relay ARP re- We are experimenting with a Proxy ARP [6] and quests to the moved terminal. Again, the terminal will DHCP based scheme. Firstly, a terminal is granted be able to communicate even if it has moved to other IP subnetwork. dress may be changed when roaming, no Mobile IP is This solution requires an intelligent gateway that needed. can perform these actions. The evaluation of the scal- ability of the proposed solution is work for the future. 7 Conclusions 6 Roaming In this paper we have presented our research on the fourth generation mobile network architecture. The Roaming to a new 4G location area requires several architecture is analogous to GSM. SIP is used to en- functions to be accomplished in order to be able to able incalls and outcalls even when the terminal is vis- use the services of the local network and the global iting a foreign network. Our future work will include network, the Internet. These functions are described scalability testing of the proposed micro mobility man- in this section. agement scheme and analyzing the issue of roaming As a new terminal roams to a location area, the ter- between different types of mobile networks. minal must acquire an IP address provided by the new network. This can be done, for example, with DHCP or Mobile IP. Without such an address a terminal can- References not communicate with the network. [1] ISO/IEC 8802-11:1999(E), ANSI/IEEE Std If the previous step succeeds and no additional re- 802.11, 1999 edition. strictions apply on the network, a user will then be able to use both global and local services as well as [2] M. Handley, H. Schulzrinne, E. Schooler, and to make VoIP calls to other parties (outcalls) on the J. Rosenberg, “SIP: Session Initiation Protocol”, local or other networks. In order to be able to receive RFC 2543, March 1999. incoming VoIP calls (incalls) and other SIP based con- tacts, the terminal must also be registered with the [3] Bluetooth SIG, Specification of the Bluetooth Sys- local SIP proxy. tem - Version 1.0 B, Specification Volume 1 & 2, A user with a terminal may roam to another lo- December 1999. cation area whilst having open TCP or SIP initiated [4] Droms, R., Dynamic Host Configuration Protocol, connections. A TCP connection is established between RFC 2131, March 1997. two IP addresses and is used to transmit data be- tween those two endpoints. If either of the used IP [5] Perkins, C., IP Mobility Support, RFC 2002, Oc- addresses becomes unreachable, then the TCP con- tober 1996. nection will die. Therefore, when roaming with open TCP connections, the terminal’s IP address must re- [6] Postel, J., Multi-LAN Address Resolution, RFC main unchanged in order to preserve the TCP connec- 925, October 1984. tions. This is usually done with Mobile IP. However, [7] Schulzrinne, H., SIP Registration, Internet Draft, in our experiment we manage IP addresses granted October 2000. by the location area DHCP so that they can be kept in the neighboring location areas. The smooth loca- [8] Martikainen O., Nyman E., R¨as¨anen J., Kopo- tion update is then carried out by multicasting the IP nen P., Terminal Mobility in Broadband Networks, packets arriving at the terminal to both location areas PWC’1998, 1998. involved. SIP initiated connections use a suitable protocol for [9] Wedlund, E., Schulzrinne, H., Mobility Support actual data transmission. For example, VoIP uses using SIP, Second ACM/IEEE International Con- Real-Time Protocol (RTP). In order to retain SIP ference on Wireless and Mobile Multimedia, Au- initiated connections when roaming, the data stream gust 1999. must be redirected to the new IP address of the ter- [10] Biggs, Dean, SIP Call Control: Call Handoff, In- minal with SIP handoff messages [10]. In the case of ternet Draft, January 2001. VoIP, for example, a new RTP connection is created using the terminal’s new IP address. Since the IP ad-