Merging WLAN with GPRS Architecture The Profitable Solution for Bandwidth Demanding Applications

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1 Dimitrios Karaboulas, Spiros Louvros, Gerasimos Pylarinos and Stavros Kotsopoulos Merging with Architecture The Profitable Solution for Bandwidth Demanding Applications Authors Spiros Louvros Cosmote S. A., Hellas Mobile: Dimitrios Karaboulas, Gerasimos Pylarinos, Stavros Kotsopoulos Wireless Telecommunications Laboratory, Department of Electrical and Computer Engineering,, Greece Wireless LAN () access technology provides broadband services with superior capacity compared to existing GSM and services in an indoor environment. This paper proposes an architecture that interconnects the radio access technology with core existing cellular network infrastructure, by using wireless ATM as a lower layer solution Introduction Today the ability to communicate while moving is a necessity. Current secondgeneration (2G) cellular networks have enabled a satisfactory level of mobility with high level of penetration in the market for voice and low-speed data services. Although 2G technologies are adequate in meeting voice communication speeds of a typical subscriber, their data communication capabilities are limited (< 9.6 kbit/s). High bandwidth demand of typical data applications often exceeds the transmission capacity of cellular networks. The pressing of the communication world for high-speed data rates pushed GSM technology to highspeed circuit-switched data (almost 54 kbit/s) and to general packet radio service () as the 2.5G evolution (approximately from 40 kbit/s up to 100 kbit/s). The last big evolution of cellular GSM networks was the third-generation (3G) cellular system with competitive rates of 300 kbit/s initially, with possibilities to upgrade up to 2 Mbit/s. However, due to delay of 3G cellular networks deployment and the large needed investment for new spectrum, mobile operators are looking for innovative ways to improve the current offerings to customers with services similar to 3G services. Today, cellular networks seem to offer data services to customers using the global and successful solution of. However, has the main disadvantage of the limited potential of supporting high data rates required in business and multimedia applications. Therefore, since 2.5G technologies are insufficient to meet market needs and 3G cellular data technology is not yet (with moderate future deployment) available, mobile operators are turning to the wireless local area network () technologies. This recent interest of mobile operators is justified because of the recent global evolution and successful deployment of networks. This worldwide approval of networks is a result of the very high data rates providing superior bandwidth compared to any cellular technology. As a result, IEEE b offers a theoretical throughput of 11 Mbit/s (typically 6.5 Mbit/s) with potential of 100 Mbit/s, while a handset offers a theoretical data rate up to 172 kbit/s (typically 42 kbit/s) and the third-generation terminal up to 2 Mbit/s (typically 144 kbit/s). The future mobile operator networks shall be a combination of several radio communication technologies, such as /UMTS or /. penetration is expected to be widely deployed in public locations such as hotels, commercial centres, airports, ports and enterprise buildings. The architecture of deployment is based on the hot-spot solution. In order to increase revenue, operators must provide a seamless experience among current cellular and access networks. The future mobile operator networks shall be a combination of several radio communication technologies, such as /UMTS or /. The target is to utilise infrastructure investments of existing cellular networks and interconnect the network architecture into the core cellular network. The network should maintain compatibility with the existing GSM/ mobility management procedures (location update, authentication, roaming and billing functions), minimising the investment in the merging network deployment. A GSM subscriber identity module (USIM) 1 is a natural choice for subscriber management since it is widely deployed and enables roaming to existing GSM/ handsets and networks. Moreover, subscriber identity should be used in all access networks to enable seamless service availability. 168

2 Interworking System Overview Presentation of the idea The general idea is that a cellular data customer can use the resources of whenever possible in order to increase its data rates and to make use of all the available multimedia services without delays. Of course the deployment of the network is based on hot-spots architecture. Hence the coverage of network will be limited to special places, where most of the customers can use a laptop or a PDA to use the offered services. Such places are commercial centres, city centre areas like cafeterias and hotels, enterprises buildings and airports and finally crowded domestic areas. There is no need for full coverage of the whole geographical area of a city or a country, since in the areas of no coverage the solution is enough to provide the mobility freedom to cellular customers with satisfactory moderate data rates. In such a case, the cellular customer might start a session (flow of packets between the end-user and the network) with the best network coverage, say in a hot-spot area. While the customer is waiting, he/she can take advantage of the high bandwidth of in order to gain access to the offered services. As the customer moves away from the coverage area of the hot spot, the enddevice detects the failure of service provision and seamlessly should switch the flow to the next available data network; that is,. It is important that this flow switching is seamless to the end-user and that, when the end-user gets back to a place with coverage, the flow should be handed back to the network. Generally speaking, the integration of network within cellular networks depends always on the ownership or management of the network. There are two general scenarios. The first case is that the cellular operator owns and is responsible for the management of the network. In such a case, the operator has the advantage of providing preferred billing and customer management policies. Additionally, operators can use the existing cellular authentication and billing mechanisms for customers that are using the access network. The other case is that the is not owned by the cellular operator but by the wireless Internet service provider (WISP), and there must be certain roaming contracts in order to allow the usage of the access network in certain hot spots. In this solution it is more difficult to implement the seamless session flow switching, as discussed earlier, since in most cases of roaming the -based session should be broke down and restarted. Interworking architectures The integration of with cellular networks has been specified by the European Telecommunications Standards Institute (ETSI) 2, 3 in two general approaches, the so-called loose coupling and tight coupling. With loose coupling the deployment of is a complementary access network to the network, using the subscriber databases without using the core network 4. In Figure 1, it is obvious that the loose coupling would be to the Gi interface. With this interconnection, the data bypass the network by accessing directly the external -based network. The tight coupling integrates network towards the core network in the same way as any other access network ( RAN or UMTS UTRAN). In this case, the data goes through the core network reaching the external -based network. From Figure 1 4 it is obvious that the tight coupling goes through the Gb interface to the. The current preferred solution by the operators seems to be the loose coupling by using a user services identidy module (USIM) based authentication and billing. With this solution, the customer may use his/her subscriber identity module (SIM) card to access a set of wireless data services over the network. There are also two main advantages for using the loose coupling instead of tight coupling. First of all the network may be owned by a WISP operator and in that case roaming/ billing agreements have to be enabled through a dedicated connection between the cellular operator and the WISP Figure 1 interfaces TE MT MSC/VLR BSS Gn Gb SMS-GMSC SMS-IWMSC Gs Gp operator. In that case, the interoperability is guaranteed and there is no need for a cellular operator to deploy also a network and to invest more for the services. On the other hand, tight coupling exists only when the network is owned and managed by the cellular operator since the integration is inside the cellular architecture. Second, it is not necessary to introduce cellular technology firmware in the network as it is with the tight coupling. In this paper, the loose coupling will be presented as a recommended solution and an overview of the system architecture will be discussed. Loose Coupling Solution Overview Figure 2 shows the integrated / (I) system architecture, consisting of the access network interconnected to the existing cellular core network through the Gi interface towards the backbone. A single subscriber module (USIM) should be used in all access networks to enable smooth roaming and seamless services availability. In order to use the potentials of the core network already deployed, authentication, billing, roaming and security issues of should be compatible. In that way the effort is to minimise the modifications in the network and to allow small infrastructure additions in the interworking boundary. The main design challenge is to transport standard GSM authentication signalling 5 from the customer terminal to the cellular core network using protocol. According to Figure 2, only the signalling messages (authentication, billing, roaming) are transported to the cellular core Gd Other PLMN Gn Gr SM-SC HLR Gf Gi EIR Ga CG PDN TE 169

3 Figure 2 The integrated / network architecture GSM Base Station RAN Public Points Controller GSM Base Station RAN Core network through the Gi interface. The traffic -based packets are routed from the access controller, directly to the WISP core backbone, to their destination. Hence endusers do not overload the cellular core network with their traffic demands. The authentication server (AS) is the major control point of the I subscriber management. A single entity in the network may support several access controllers (using RADIUS authentication protocol) providing authentication and billing services for thousands of roaming users in different radio access zones. When the user disconnects, the authentication server works as an interworking entity. It receives accounting data from the access controller, converts them into billing format 6 and issues the charging data records (CDRs) to the cellular billing system. The authentication server interconnects in a seamless way the core cellular infrastructure with the access network. It provides a gateway to the cellular core network elements (home location register (HLR) and charging gateway). The authentication server sends standard GSM authentication signalling to the HLR using Signalling System No. 7 (SS7) protocol that connects various operator networks together. The cellular network uses the standard identification procedure with GSM international mobile subscriber identity (IMSI) code stored in the USIM card. The authentication server always checks if the roaming user has subscribed to the service. The access controller (AC) provides an Internet gateway between the radio access network and the fixed (WISP provider or operator owned) core. It functions in the same way as the node in the network, allocating addresses to the mobile terminals and maintaining a list of the authenticated terminals addresses. The AC tracks the addresses of each incoming or Cellular Site Server MSC/ HLR Cellular Core Charging Gateway outgoing packet and discards the packets that come from a non-authenticated terminal. The access controller separates the mobile terminals using a terminal address and a unique link-layer-specific MAC address. The MAC address verification works as a filter for duplicate addresses that might be used by a hostile user. The AC gathers accounting information for billing purposes, forwarding them in the standard RADIUS protocol to the AS. The access point (AP) offers a wireless asynchronous transfer mode (ATM) or Ethernet link between the mobile terminal and the fixed LAN. The choice of ATM or Ethernet depends always on the operator who manages the network. The access points are the hardware infrastructure to provide the electromagnetic coverage to a certain access zone or a hot-spot area. During the network deployment, several APs are connected to the same core LAN with the AC. The typical coverage range of a single AP is m indoors or up to 500 m hot-spot outdoor coverage. The coverage can be extended by using directional antennas and radio network planning tools. An I customer terminal might be available for any terminal with radio access capability and a USIM reader. The customer terminal technology may develop either an integrated card with a USIM reader or a card and an external smart card reader. Laptop vendors might develop in the future some laptop models with PCMCIA integrated smart card readers in the future. The I terminal may detect the correct roaming network by using predefined network profiles that contain the list of the roaming partners radio network identifiers. When entering a new location, the terminal compares the names of available networks with the roaming profile and associates itself with the correct. The operator may distribute the profiles by using a web server. The I Network Operation Figure 3 illustrates the proposed protocol stack. Most of the signalling protocols are already known. The major issue to discuss is the ATM (wireless or wired) 7 or the Ethernet solution in the AP-AC interconnection or the AS-charging gateway (CG). Generally speaking, the solution depends always on the operator s transmission network. If the operator is simply a WISP, then the Ethernet seems to be the most scalable and cheap solution to expand its network in a large geographical area, using mostly TCP/ over Ethernet. On the other hand, the preferred solution, as stated previously in this paper, is the operator to be the cellular operator. As it was explained earlier, in that way there is no need to worry about roaming issues, and, in the case of coverage unavailability, the handover to should be seamless without any interruptions. It is a common issue for cellular operators to have a solid and scalable transmission network, based on a SDHthrough-ATM solution. With such a network, the cellular operator has the advantage to transport in one solution the circuit-switched traffic (GSM voice calls) and the data traffic ( or UMTS multimedia). For these reasons the authors believe that the TCP/ over ATM/SDH network is most preferred to deploy since, mostly in Europe, cellular networks have an increased penetration. procedure According to Figure 2, the authentication procedure is executed following the standard USIM-based authentication. The authentication is executed between the AS and the MSC/HLR cellular core elements, with the AC as a relay element. The procedure is explained briefly in three steps. In step 1, the terminal communicates with an AP and gets an address from the AC. It initiates then the authentication procedure by sending a specific authentication request message to the AC, seamless through the AP (same procedure between mobile terminal, BTS and in the network). In step 2, the AC proceeds the authentication message to the AS and the AS transforms the message into a SS7 message to communicate with the MSC/ HLR elements of the network. In step 3, the AS gets the authentication triplets from the HLR and performs authentication. 170

4 Figure 3 Signalling protocol stack in the interworking architecture Roaming Control Manager Controller MAP link to HLR Accounting SIM -SIM RADIUS CDR Signalling Transmission NAAP NAAP RADIUS RADIUS GTP GTP RADIUS Accounting TCP UDP UDP TCP TCP/UDP TCP/UDP ATM ATM ATM ATM ATM ATM Mobile Point Controller Server Charging Gateway Billing procedure Referring again to Figure 2, the billing procedure can be illustrated in a few steps. In step 1, the AC monitors the traffic (as the MSC/BSC does in GSM) and periodically sends the statistical traffic data to the AS. The AS checks the received accounting data to ensure that they come from an authorised subscriber. The AS works as an interworking unit for signalling, converting the accounting data into CDR format compatible with. In order to distinguish the CDRs from the CDRs, a special identifier is introduced in the CDR packet. In step 2, the AS acknowledges back the received data to the AC and it protects the received accounting data with the protection protocol RADIUS 8. In step 3, the AS delivers the generated CDRs to the charging gateway/billing centre. Roaming procedure Although the ISP has not seriously considered the roaming process, the cellular operators have developed the infrastructure to support roaming to different access networks (, GSM, UMTS) or to different operator networks. The idea is to use the already existing cellular node HLR 5 to perform the roaming procedure. The procedure starts in step 1 where a terminal moves to a new access zone of the network and communicates with the new AC. The terminal sends the authentication request to the new network through the new AC. The AC analyses the IMSI request and verifies the roaming service of the USIM. In step 2, the AS sends the authentication request to the HLR through the cellular operator core network. The HLR reads the subscriber s profile and responds to the AS the user profile and authentication triplets. In step 3, the authentication is proceeded and finally the AS sends the converted traffic accounting into CDR format to the new billing centre. The billing centres are exchanging information during the session lifetime, according to the roaming contracts of the operators. Summary In the past decade, customers demands have not been in voice applications but mainly in data applications and multimedia services. The cellular operators have failed to provide satisfactory data rates to the customers, even with the latest technological evolution of third-generation networks. The operators community is looking for the ultimate solution to increase the data rates offered to customers, hence increasing revenues. seems to be the most promising technology so far regarding data rates (11 Mbit/s). The main disadvantage of the network is the deployment of. Due to tremendous investment requirements, is initiated mostly as a hot-spot architecture in crowded areas and not as a wide area coverage network. Moreover. there is not so far a general strategy among the different WISPs to design a universal network with specific roaming procedures and interoperability capabilities. The most familiar idea recently to deploy coverage in a large geographical area is the merging of with different network technologies, as the cellular operators GSM/ network. By using the already existing infrastructure of cellular networks, all the procedures of mobility management, billing and roaming are integrated. The key idea is the use of USIM for both and networks. From the operator s point of view, this solution is feasible with the loose coupling since the infrastructure demands in both and are minimised and the revenues are increased. From the cellular subscriber s point of view, this is a very attractive solution, allowing entrance to the broadband access market using the same subscriber management agreements of cellular network. References 1 Louvros, Spiros, and Iossifides, Athanassios. The Evolution of the Smart Card in Mobile Communications. The Journal of The Communications Network, January March 2004, 3(1). 2 ETSI. Requirements and Architectures for Interworking between HERLAN/3 and 3rd Generation Cellular Systems. Technical Report ETSI TR GPP. Feasibility Study on 3GPP System to Interworking. Technical Report egpp TR v GPP. General Packet Radio Service (), Service description. Technical Specification 3GPP TS v Mouly, M., and Pautet, M. The GSM System for Mobile Communications. Telecom Publishing, 1992, pp ETSI TS Digital Cellular Telecommunications System (Phase 2+): General Packet Radio Service (); Charging v Karaboulas, Dimitrios; Louvros, Spiros; and Kotsopoulos, Stavros. Signalling Modification Proposals for Wireless ATM Networks. The Journal of The Communication Network, January March 2005, 4(1). 8 Rigney, C., et al. Remote Dial In User Service (RADIUS). IETF RFC 2865,

5 Biographies Dimitrios Karaboulas Dimitrios Karaboulas received his diploma in Electrical and Computer Engineering from the, Hellas, in Since 1994, he has been working as an external consultant-specialist in several telecommunication companies in Hellas and he currently runs and owns a company firm for ISO certification, supervision and technical solutions. He is currently a Ph.D. candidate in the Wireless Laboratory of Electrical and Computer Engineering Department,, Hellas, and his research interests are in the area of wireless ATM networking, mobile communications and telecommunication network planning. He has participated in several research projects regarding mobile communications and enterprise telecommunication solutions and has contributes to over 50 papers in conference proceedings. He is also an active member of the Technical Chamber of Greece. Spiros Louvros Cosmote S.A. Dr. Spiros Louvros received his Bachelor in Physics from the University of Crete, Hellas, and his Master in telecommunications from the University of Cranfield, UK, with a scholarship for graduate studies from the Alexandros Onassis Institution. In 2004, he received his Ph.D. from the University of Patras, Hellas, in mobile communications. He has worked for Siemens as a microwave engineer and for Vodafon-Hellas as a switching engineer. His current occupation is section manager in the Maintenance Department in Cosmote S.A. He has participated in several research projects regarding mobile communications. His area of interest is in mobile networks, telecommunication traffic engineering, wireless ATM and optical communications and he has contributed to over 30 papers in international literature and conference proceedings. He is a member of FITCE and Hellenic Physics Union and he holds a position of external researcher in the Wireless Telecommunications Lab of the Electrical Engineering department,. Gerasimos Pylarinos Gerasimos Pylarinos receieved the B.E. in Electrical and Computer Systems Engineering from Monash University, Melbourne, Australia, in 1992, and the B.E. in Electrical and Computer Systems Engineering from the University of Patras, Greece, in He received the M.Sc. in Data Communications Systems from Brunel University, United Kingdom. He is currently pursuing a Ph.D. degree at the University of Patras, Greece. He has worked at Philips Radio Communication Systems, Melbourne, Australia, developing hardware for mobile radio communication systems for 2 years. He subsequently worked as project manager in the Research and Development department at Intracom Radio Communication Systems, Greece, for 7 years. He is now manager of the Biomedical Engineering department of Kefalonia Hospital, Greece. His research interests lie in the areas of 3G and 4G wireless communications. Stavros Kotsopoulos Dr. Stavros Kotsopoulos received his B.Sc. in Physics in 1975 from the University of Thessaloniki, and received his Diploma in Electrical and Computer Engineering in 1984 from the. He complete his postgraduate studies in the University of Bradford in United Kingdom. He was awarded M.Phil and Ph.D. degrees in 1978 and 1985, respectively. Currently he is a member of the academic staff of the Department of Electrical and Computer Engineering of the and holds the position of Associate Professor. Since 2004, he has been the Director of the Wireless Telecommunications Laboratory. He teaches and carries out research in telecommunications, with particular interest in mobile communications, interference, satellite communications, telematics applications, communication services and antennae design. He is the (co)author of the book titled Mobile Telephony. His research activity is documented by more than 160 publications in scientific journals and proceedings of International Conferences. He has been the leader of several international and many national research projects. He is a member of the Greek Physicists Society and a member of the Technical Chamber of Greece. 172