Design Architectures for 3G and IEEE WLAN Integration

Transcription

1 Design Architectures for 3G and IEEE WLAN Integration Abstract Wireless LAN access networks show a strong potential in providing a broadband complement to Third Generation (3G) cellular systems. 3G networks provide a wider service area, and ubiquitous connectivity with low-speed data rates as compared to Wireless Local Area Networks (WLANs). WLAN networks offer higher data rate and the easy compatibility to wired Internet, but cover smaller areas. Integrating 3G and WLAN networks can offer subscribers high-speed wireless data services as well as ubiquitous connectivity. The key issue involved in achieving these objectives is the development of integration architectures of WLAN and 3G technologies. The choice of the integration point depends on a number of factors including handoff latency, mobility support, cost-performance benefit, security, authentication, accounting and billing mechanisms. We review 3G-WLAN integration architectures and investigate two such architectures in the case when the Universal Mobile Telecommunications Network (UMTS) network is connected to a WLAN network at different integration points, namely the Serving GPRS Support Node () and the Gateway GPRS Support Node (). The evaluation of these integration architectures were conducted through experimental simulation tests using OPNET. 1. Introduction Mobile communications and wireless networks are developing at a rapid pace. Advanced techniques are emerging in both these disciplines. There exists a strong need for integrating WLANs with 3G networks to develop hybrid mobile data networks capable of ubiquitous data services and very high data rates in strategic locations called hotspots. 3G wireless systems such as Universal Mobile Telecommunication Systems (UMTS) can provide mobility over a large coverage area, but with relatively low speeds of about 144 Kbits/sec. On the other hand, WLANs provide high speed data services (up to 11 Mbits/sec with 82.11b) over a geographically smaller area. WLANs are generally used to supplement the available bandwidth and capacity of a 3G network in hotspot area such as railways and airports with high trafficdensities, without sacrificing the capacity provided to cellular users. The rest of this paper is organized as follows. Section 2 provides a brief background on 3G and WLAN networks. Section 3 describes the related research and contributions of this work. Section 4 presents a comparison of the various internetworking architectures. In section 5 we compare two integration architectures connecting UMTS and networks. Section 6 presents our simulation results. Finally in section 7 we present some concluding remarks and future works. 2. Background 2.1 WLAN Standards 82.11b [4] WLAN has been widely deployed in offices, homes and public hotspots such as airports and hotels given its low cost, reasonable bandwidth (11Mbits/s), and ease of. However, a serious disadvantage of is the small coverage area (up to 1 meters) [7]. Other standards include 82.11a and 82.11g which allow bit rates of up to 54 Mbits/sec G Networks ITU defines 3G as any device that can transmit or receive data at 144Kbps or better [6]. In practice, 3G devices can transfer data at up to 384 Kbps. As a comparison, Global System for Mobile Communications (GSM) data rates are up to 14.4 Kbps and General Packet Radio Service (GPRS) is around 53.6 Kbps used in 2G and 2.5G respectively. Two main proposed systems for 3G recognized by the International Telecommunication Union (ITU) are Code Division Multiple Access (CDMA 2) and Universal Mobile Telecommunication System (UMTS) [19]. CDMA2 1x is an evolution of cdmaone [6], and supports packet data service up to 144 Kbits/sec. CDMA 2 1xEV-DO introduces a new air interface known as High Rate Packet Data (HRPD) and provides up to 2.4 Mbits/sec on the downlink (from base station to terminal), but only 153 Kbits/sec on the uplink. CDMA2 1xEV-DV is expected to introduce new radio techniques and an all-ip architecture for radio access and core networks. It promises data rates up to 3 Mbits/sec. UMTS is composed of two different but related modes: Wideband CDMA also called Frequency 1

2 Division Duplex (FDD) and CDMA- Time Division Duplex (TDD). FDD mode is considered the main technology for UMTS. Separate 5 MHz carrier frequencies are used for the uplink and downlink respectively, allowing an end-user data rate up to 384 Kbits/sec. The TDD mode is Time-Division- Synchronous Code-Division Multiple-Access (TD-SCDMA). TD-SCDMA operates on low-chip-rate carriers and allows end-user data rates up to 2Mbits/sec G Network Architecture A 3G network consists of three interacting domains- a Core Network (CN), Radio Access Network (RAN) and the User Equipment (UE) also called a 3G mobile station. 3G operation utilizes two standard suites: UMTS and CDMA2 which have minor differences with respect to the components they have in the RAN and the CN. The main function of the 3G core network is to provide switching, routing and transit for user traffic. It also contains the databases and the network management functions. The core network is divided into Circuit-switched (CS) and Packet-switched (PS) domains. Circuit switched elements include Mobile Services Switching Center (MSC), Visitor Location Register (VLR), and gateway MSC. These circuit switched entities are common to both the UMTS as well as the CDMA2 standards. 3G Radio Access Network 3G Core Network RNC UTRAN Packet switched domain PDSN IP Network UE PCF MSC HLR GMSC Circuit switched network cdma2 RAN Circuit switched domain UMTS cdma2 umts and cdma2 : Base station; UE: User equipment; RNC: Radio network controller : Serving GPRS support node; : Gateway GPRS support node PDSN: Packet data serving node; MSC: Mobile switching center GMSC: Gateway mobile switching center; HLR: Home location register RAN: Radio Access Network UTRAN: UMTS Terrestrial RAN Figure 1: Components of a 3G Network The differences in the CN with respect to the two standards lie in the PS domain. Packet-switched elements in UMTS include Serving GPRS Support Node () and Gateway GPRS Support Node (). CDMA2 packet-switched component is primarily the Packet Data Serving Node (PDSN). Some network elements like Equipment Identity Register (EIR), Home Location Register (HLR) are shared by both domains. The main function of the MSC server is to handle call-control for circuit-based services including bearer services, etc. The MSC server also provides mobility management, connection management and capabilities for mobile multimedia as well as generation of charging information. It can also be co-located with the Visitor Location Register (VLR). is the gateway to external data networks. It supports control signaling towards external IP networks for authentication and IP-address allocation, and mobility within the mobile network. provides functions for forwarding and handling user information (IP packets) to and from external networks (Internet/ intranets). provides session management, i.e. mechanisms for establishment, maintenance and release of end user Packet Data Protocol (PDP) contexts. It also provides mobility management and supports inter-system handoff between mobile networks. also supports generation of charging information. The PDSN incorporates numerous functions within one node. Routing packets to the IP network, assignment of dynamic IP-addresses and maintaining point-to-point protocol (PPP) sessions are some of its main functions. It also initiates the authentication, authorization and accounting (AAA) for the mobile station. The radio access network provides the air interface access method for the user equipment. An UMTS RAN (UTRAN) consists of Radio Network Controllers (RNC) and Base Stations () or Node- B. The RNCs manage several concurrent Radio Link Protocol (RLP) sessions with the User Equipments 2

3 and per-link bandwidth management. It administers the Node-B for congestion control and loading. It also executes admission control and channel code allocation for new radio links to be established by the Node- B. A CDMA2 RAN consists of a base station and 2 logical components- the Packet Control Function (PCF) and the Radio Resources Control (RRC). The primary function of the PCF is to establish, maintain and terminate connections to the PDSN. The PCF communicates with the RRC to request and manage radio resources in order to relay packets to and from the mobile station. It also collects accounting information and forwards it to the PDSN. RRC supports authentication and authorization of the mobile station for radio access and supports air interface encryption to the mobile station. 3. Related Work and Contributions A lot of recent works have focused on the design and evaluation of architectures to integrate 3G and WLAN networks. Buddhikot et al. [9] described the implementation of a loosely coupled integrated network that provides roaming between 3G and WLAN networks. Tsao et al. [1] presented yet another method to support roaming between 3G and WLANs by introducing a new node called the virtual GPRS support node in between the WLAN and the UMTS networks. Their approach showed a reduction in handoff latency compared to the Mobile IP approach. Tsao et al. [12] evaluated three different internetworking strategies: the mobile IP approach, the gateway approach and the emulator approach with respect to their handoff latencies. Bing et al. [16] discussed mobile IP based vertical handoff management and its performance with respect to signaling cost and handoff latency. Matusz et al. [17] analyzed the performance of vertical handoffs between IEEE 82.11b and UMTS in the case of Mobile IPv4. They concluded that handoff packet delay and throughput depends upon the number of WLAN users and the traffic generated by them. All of the above works have focused on evaluating integration architectures in terms of the handoff latency experienced by the users when trying to move across 3G and WLAN networks while accessing a public network such as the Internet. In contrast to the above related efforts, our work mainly focuses on the endto-end delay experienced when users located in two different networks, namely the IEEE 82.11b and the UMTS communicate with each other directly. We describe two basic integration architectures that are used to connect these networks. Through simulations, the end-to-end packet latencies experienced by users when data is exchanged between these networks through the and the nodes are recorded and verified using various types of applications. The results demonstrate the feasibility of these integration architectures. 4. WLAN and 3G Cellular Data Network Integration Architectures- A Review Table 1 reviews and summarizes the various 3G-WLAN internetworking strategies and their features. The Mobile IP [12] internetworking architecture considers 3G and WLANs as peer networks. It allows easy but suffers from long handoff latency and might not be able to support real-time services and applications. The gateway approach [1] permits independent operation of the two networks and provides seamless roaming facility between them. It connects the two networks using a new logical node called the virtual GPRS support node. The gateway approach does not require the use of Mobile IP and has a comparatively lower packet loss during handoff. The emulator approach [12] is difficult to deploy since it requires combined ownership of the two networks but does yield low handoff latency and is thus well suited for real-time applications. Tight coupling [9] deploys WLAN as an alternative radio access network and uses a gateway between the WLAN and the 3G core network. It offers faster handoffs and high security mechanisms but also requires combined ownership of the two networks. Loose coupling [9] has low investment costs and permits independent and traffic engineering of WLAN and 3G networks. However it suffers from high handoff delays and service disruption periods. Finally, the peer-networks approach allows easy but also suffers from high handoff delays, thereby making it unsuitable for real-time applications. The choice of the integration architecture is important since multiple integration points exist with different cost-performance benefits for different scenarios. 3

4 Internetworking Approach Complexity of Handoff Latency Network Management and Ownership Authentication Billing Real time application support Mobility scheme Mobile-IP approach Gateway approach Emulator approach Tight coupling Loose coupling Peer networks approach Easiest Medium level difficulty. Very difficult Medium level difficulty High level of difficulty Very easy High Low Low Low Higher compared to tight coupling High Separate ownership of 3G and WLAN networks permitted with roaming agreements Permits 3G and WLAN networks to operate independently. Combined ownership required Generally requires both WLAN and 3G networks to be owned by same operator Permits independent of WLAN and 3G networks. Both, same or different operator ownership permitted Reuse 3G cellular AAA functions in WLANs Can provide both, separate WLAN security as well as 3G AAA. Uses 3G authentication mechanism 3G ciphering key used for WLAN encryption Cellular access gateway provides authentication Use of AAA functionality of the 3G UMTS network Uses 3G billing feature Can provide separate as well as combined billing mechanism. Uses 3G billing mechanism 3G accounting features reused Billing mediator to provide common accounting Billing feature of 3G UMTS network used No Yes Yes Yes Not very suitable Not very suitable Use of Mobile IP Intersystem roaming agreements between the networks through the use of a gateway. Mobility handled by UMTS Session management and GPRS Mobilitry Management. GPRS mobility management procedure used. Integration of Mobile-IP functionality in WLAN gateway. Mobile IP scheme used. Table 1: Comparison of various 3G-WLAN Internetworking Strategies 5. Simulated Architectures We evaluated via simulations using OPNET two internetworking architectures to interoperate the 3G (UMTS) and WLAN networks by connecting them at two strategic points- the node and the node as shown in figure 2. UMTS RAN RNC UMTSCore Network Internet UMTS RAN RNC UMTSCore Network Internet UE UE: User Equipment AP: Access Point MN: Mobile Node : Base station : Serving GPRS support node : Gateway GPRS support node VGSN: Virtual GPRS support node RNC: Radio network controller GW: Gateway AP Wireless LAN MN Flow of data UE UE: User Equipment AP: Access Point MN: Mobile Node : Base station : Serving GPRS support node : Gateway GPRS support node RNC: Radio network controller AP Wireless LAN MN Flow of data Figure 2: 3G-WLAN Integration - a) ; b) 4

5 5.1 UMTS-WLAN Integration at the node When the UMTS and WLAN networks are connected through the node (figure 2a), the WLAN network does not appear to the UMTS core network as an external packet data network. Instead, it simply appears as another radio access network. The WLAN AP in this case needs to have the capability of processing UMTS messages. This WLAN access point possesses the features of a 3G (UMTS) radio network controller. On one of the interfaces this access point supports the functions of a RNC and on the other interface it communicates with the WLAN Mobile Node (MN) present in the WLAN. Thus, whenever a Mobile Node in the WLAN network wants to exchange data with the UMTS User Equipment, it first needs to undergo the GMM attach procedure (figure 3). This procedure is required to notify the of the location on the communicating node and also to establish a packet-switched signaling connection. The WLAN AP is responsible for sending these request messages to the on behalf of the WLAN Mobile Node. The GMM attach procedure is a three-way handshake between the communicating node, the RNC and the. Upon completion of this procedure, the WLAN Mobile Node is authenticated into the UMTS network. The WLAN access point in this case is aware of the UMTS network and sends messages from the WLAN Mobile Node to the UMTS User Equipment by converting each packet into a UMTS format by adding the appropriate packet data protocol (PDP) headers. UE RNC/Specialized WLAN AP 1: GMM Attach Request 2: GMM Attach Request 4:GMM Attach Accept 3: GMM Attach Accept 5:GMM Attach Complete 6: GMM Attach Complete 7: PS Signaling Connection Figure 3: GMM Attach Procedure 5.2 UMTS-WLAN Integration at the node In this type of integration (figure 2b), whenever a Mobile Node in a WLAN network wants to communicate with a User Equipment in the UMTS network, it does so through the node. The User Equipment in the UMTS network first activates the Packet Data Protocol (PDP) context that it wants to use. This operation makes the User Equipment known to its and to the external data networks, in this case, the WLAN network. User data is transferred transparently between the User Equipment and the WLAN network with a method known as encapsulation and tunneling. Data packets are equipped with PS-specific protocol information and transferred between the User Equipment and the Mobile Node in the WLAN through the node. The protocol that takes care of this is the GPRS Tunneling Protocol (GTP). It tunnels user data between the s and the s. For this kind of internetworking configuration, the WLAN AP is a simple 82.11b access point and does not require any special capabilities to process UMTS messages. It simply forwards the IP packets from the Mobile Node to the node. The packets are tunneled by the and delivered to the UMTS RAN. 5.3 Simulation testbed Application QoS Class Measurement Parameter (seconds) Size Protocol FTP Background File download time 1-1 Kilobytes TCP FTP Background File upload time 1-1 Kilobytes TCP GSM encoded voice Conversational End-to-end delay 33 Bytes UDP GSM encoded voice Conversational Jitter 33 Bytes UDP HTTP Web browsing Interactive Page response time 3 Bytes TCP Table 2: Description of various applications tested 5

6 A network simulation model was constructed using OPNET 1..A [18]. OPNET is a discrete event simulator with a sophisticated software package capable of supporting simulation and performance evaluation of communication networks and distributed systems. The simulation environment we used had a UMTS network connected to a WLAN network. The UMTS network was composed of the radio access network with a (Node-B), RNC, and User Equipment and a packet-based core network with and nodes. In order to simplify the simulation process, only one UMTS cell was used. The WLAN network is composed of 82.11b wireless Mobile Nodes configured in IS (infrastructure basic service set) mode. In the integration case, a simple WLAN access point was used, while in the integration case, a different access point with additional capability of processing UMTS messages was employed. The simulation used a standard OPNET 82.11b PHY module with maximum data rate up to 11 Mbits/sec to simulate the wireless medium. The goal of the simulations was to compare the delays involved when user data is exchanged between the UMTS and WLAN networks connected via two methods, namely and. We assume no congestion or queuing delays occur at any of the other intermediate nodes. Data is exchanged between a single User Equipment and a WLAN Mobile Node for a duration of one hour. Different types of traffic was generated using four different applications including Voice over IP (VoIP) in GSM encoded format, FTP, and HTTP (web browsing) as shown in table 2. These applications correspond to the various UMTS QoS classes. Conversational class is intended to carry real time traffic flows such as VoIP. Interactive and background classes are meant for bursty packet data transmissions and include traditional Internet applications such as FTP, HTTP etc. Interactive class is more suitable for web browsing where quicker response times are expected as compared to FTP. Packet delay, jitter, upload, and download response times were measured. Other parameters associated with each application are summarized in table 2. FTP flows are configured with file sizes in the range of 1 to 1 Kilobytes with an increment size of 1 Kilobytes. Real-time VoIP flows were configured with a constant packet size of 33 bytes. Page size used for HTTP transfers was 3 bytes. 6. Simulation Results and Discussion Simulations performed for both UDP and TCP flows are presented. For the UDP flow (VoIP traffic), endto-end packet delays and jitter were measured. For TCP flows (FTP,HTTP) the upload/download response times were measured. FTP: File Dow nload Response Time FTP: File upload Response Time Download time (seconds) File Size (KB) Upload Time (seconds) File Size (KB) Figure 4: a) Download time; b) Upload time. 6

7 End-to-end delay (seconds) Voice Packet End-to-end delay Simulation Run-Time (seconds) Jitter (seconds) Voice Jitter Simulation Run-Time (seconds) Figure 5: a) End-to-end delay; b) Jitter. Figures 4a and 4b show the simulation run-times corresponding to the average file download and upload times experienced when transferring files of various sizes between the User Equipment and the WLAN Mobile Node under two different integration scenarios. We can see that the download/upload times are much lower for all file sizes when the transfer is done through the node as compared to when the networks are interconnected at the. In figures 5a and 5b the average delay and jitter for voice is presented. It is observed that both, delay and jitter values are much lower in the case. The average end-to-end delay is approximately.5 second (5 ms) in the case which is within the range of tolerable delay ( to 15 ms) [2] while delay for the case is much higher. Similarly, figure 6 shows the response time to access a web page of size 3 bytes. As figure 6 illustrates, the page response time is initially high and then decreases as the simulation progresses. We speculate that this reduction in the page response time may be because the web server s cache is initially empty and the first few page requests will cause the page to be fetched from the disk resulting in a high response time. As the more requests are generated with time, the cache is being filled and there is an increasing probability that one or more requests can be satisfied by the cache thereby reducing the overall page response time. Page response time (seconds) HTTP Page Response Time Simulation Run-Time (seconds) Figure 6: Page response time. The simulation results reveal that the application response time (delay) is consistently higher in the case where the UMTS and WLAN networks are connected through the node, as compared to the case where the two networks are connected at the. This higher response time can be attributed to the 7

8 additional processing time required at the WLAN access point in the first case. When the two networks are connected at the node, the WLAN access point performs the functions of a RNC on one interface and on the other interface implements the functionality of a WLAN AP. Therefore, it has to perform the additional initialization steps to authenticate the WLAN Mobile Node to the UMTS network, in the form of GMM Attach procedure and PDP context activations discussed in figure 4. Furthermore, internetworking through the requires the WLAN access point to be UMTS-aware and process each packet from the WLAN Mobile Node by adding the required packet data protocol (PDP) headers before it can be sent to the. This adds additional overhead to each packet. When integration is done at the node, the WLAN AP is a simple 82.11b access point and does not require any special capabilities to process UMTS messages. Data packets are transferred between the User Equipment and the WLAN network using encapsulation by the GPRS tunneling protocol. This reduces the packet latency as there is no additional delay due UMTS initialization procedures or packet conversions. The advantages, however, of using integration scheme include the reuse of UMTS authentication, authorization, accounting (AAA) mechanisms, usage of common subscriber databases and billing systems, increased security features (since the UMTS security mechanisms are reused), as well as possibility of having continuous sessions as users move across the two networks, since the handoff in this case is very similar to an intra-umts handoff as the WLAN AP appears as another RNC to the node. In the case of integration, since the WLAN is considered to be an external network, different billing and security mechanisms are needed. Service disruption is also possible during a handoff from one network to another. 7. Conclusions and Future Work In this paper we discussed the architecture and performance of a 3G-WLAN integrated system connected at two different points namely the and. The architectures were evaluated with respect to the end-to-end latency, jitter and download/upload times obtained when data is exchanged between nodes located in the UMTS and WLAN networks respectively. Our simulation results show that the overall delays are much lower when the data exchange is done through the node as compared to when the networks are connected through the node. Additional delay in the latter case is attributed to the extra processing required at the WLAN AP which needs to perform the functionality of both a RNC as well as an 82.11b AP. Traffic passage through the is faster due to simple encapsulation procedure employed by the GPRS tunneling protocol. However, integration has its own advantages of providing strong security, common billing, authentication, etc. which are not possible when networks are coupled at the. Our future work focuses on evaluating these integration schemes with respect to the handoff latency and the development of an architecture that provides seamless session mobility when Mobile Nodes move across the 3G and WLAN networks. References [1] S. Jun-Zhao, J. Sauvola,, D. Howie, Features in Future: 4G Visions from a Technical Perspective, IEEE GLOBECOM, November 21, volume 6, pages [2] U. Varshney, R. Jain, Issues in Emerging 4G Wireless Networks, Computer Communications, June 21, volume 34, Issue 6, pages [3] A. Miah, K. Tan, An Overview of 3G Mobile Network Infrastructure, IEEE Student Conference on Research and Development, July 22, pages [4] IEEE 82.11b, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications- Higher-Speed Physical Layer Extension in the 2.4 GHz Band,

9 [5] U. Varshney, The Status and Future of Based WLANs, Computer Communications, June 23, volume 36, Issue 6, pages [6] J.D. Vriendt, P. Laine, C. Lerouge, X. Xiaofeng, Mobile Network Evolution: A Revolution on the Move, IEEE Communications Magazine, April 22, volume 4, Issue 4, pages [7] H. Luo, Z Jiang, B. J. Kim, N.K. Shankar, P. Henry, Internet Roaming: A WLAN/3G Integration System for Enterprises, AT&T Labs- Research, [8] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, ANSI/IEEE Std (E) Part 11, ISO/IEC [9] M. Buddhikot, G. Chandranmenon, S. Han, Y. Lee, S. Miller, Design and Implementation of a WLAN/CDMA2 Internetworking Architecture, IEEE Communications Magazine, November 23, pages 9-1. [1] S. Tsao, C. Lin, VSGN: A Gateway Approach to Interconnect UMTS/WLAN Networks, The 13th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC 22., volume 1, pages [11] M. Jaseemuddin, An Architecture for Integrating UMTS and WLAN Networks, 8 th IEEE International Symposium on Computers and Communications, ISSC 23, pages [12] S. Tsao, C. Lin, Design and Evaluation of UMTS-WLAN Internetworking Strategies, Proceedings of IEEE Vehicular Technology Conference, VTC 22, volume 2, pages [13] A. Campbell. J. Gomez, S. Kim, A. Valko, C. Wan, Z. Turanyi, Design, Implementation and Evaluation of Cellular IP, IEEE Personal Communications, August 2, volume 7, No. 4, pages [14] A. Helmy, M. Jaseemuddin, Efficient Micro-mobility using Intra-domain Multicast based Mechanisms (M&M), August 21 USC-CS-TR [15] A. Salkintsiz, C. Fors, R. Pazhyannur, WLAN-GPRS Integration for Next-Generation Mobile Data Networks, IEEE Wireless Communications, October 22, volume 9, Issue 5, pages [16] B. Hongyang, H. Chen, L. Jiang, Performance analysis of vertical handoff in a UMTS-WLAN integrated network, Proceedings of 14 th IEEE conference on Personal, Indoor and Mobile Radio Communications, PIMRC, Sept. 23, volume 1, pages [17] P. Matusz, P. Machan, J. Wozniak, Analysis of profitability of inter-system handoffs between IEEE 82.11b and UMTS, Proceedings of 28 th annual IEEE International conference on Local Computer Networks, LNC, October. 23, pages [18] [19] TS V6.., Universal Mobile Telecommunications System, webapp.etsi.org/action%5cpu/ 2412/ts_125422v6p.pdf [2] B. Goode, Voice over Internet Protocol, Proceedings of the IEEE, September 22, volume 9, Issue 9, pages