MPLS/RSVP-TE-BASED FUTURE UMTS RADIO ACCESS NETWORK

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1 12th GI/ITG CONFERENCE ON MEASURING, MODELLING AND EVALUATION OF COMPUTER AND COMMUNICATION SYSTEMS 3rd POLISH-GERMAN TELETRAFFIC SYMPOSIUM MPLS/RSVP-TE-BASED FUTURE UMTS RADIO ACCESS NETWORK René Böringer*, Markus Bauer, Andreas Mitschele-Thiel*, Michael Söllner *Technical University of Ilmenau, P.O. Box , Ilmenau, Germany {rene.boeringer Lucent Technologies Network Systems GmbH, Nürnberg {mgbauer Abstract Currently we see a discussion on the transition from ATM-based transport to IP transport in the UMTS Terrestrial Radio Access Network (UTRAN). Issues that result from the transition to IP are QoS, esp. delay, jitter, loss, bandwidth guaranties and in sequence delivery of packets for voice traffic. The paper compares two solutions for future UTRANs, native IP multi-path transport and MPLS (Multi-Protocol Label Switching) transport. MPLS employs RSVP-TE (extension of the Resource Reservation Protocol for signaling in MPLS) as reservation scheme. We have developed an extension for the network simulator 2 that allows for comparing delay, jitter and loss of these two approaches. Our studies show that MPLS/RSVP-TE is able to deliver very stable QoS for a wide range of traffic mixes and traffic loads even for network utilizations beyond 90%. Thus, MPLS provides extra flexibility to the network operator and can help to reduce the network cost. Keywords IP-based RAN, MPLS, QoS, simulation 1. INTRODUCTION The current UTRAN has inherited network architecture, functions and the structure from the 2nd Generation of mobile networks, in particular from the Global System for Mobile Communication (GSM). GSM was actually designed to provide circuit-switched voice services. With the success of packet-based services driven by the Internet and the resulting demands, the GSM system has been enhanced by the General Packet Radio Service (GPRS) with a separated packet data core network. Until now, these separated domains for circuit-switched services and for packet-switched services share the same access network, the UTRAN. ATM (Asynchronous Transfer Mode) is currently the most common transport protocol in this domain because of its connection-oriented facilities. In the case of UMTS, this system is suboptimal with respect to IP services, which will be the dominating services in all public mobile networks in the near future. In addition, ATM equipment is expensive and mobile operators cannot benefit from the dynamics of the IP world, e.g. from short time-to-market periods of network management software. Different proposals for future UTRANs [1, 3, 5, 13] aim to replace ATM by IP to overcome the issues mentioned above. Thus, it is possible to use a wider range of

2 René Böringer, Markus Bauer, Andreas Mitschele-Thiel, Michael Söllner logical network topologies which may increase the reliability of the network and may decrease the cost of the network by applying dynamic network optimization. However, IP also has several drawbacks. It is known as unreliable, the support of mobility is only possible by using additional protocols and a dedicated bandwidth demand can only be approximately provided by over-provisioning. A pure IP-based solution is not able to replace all the functionality currently offered by ATM such as in sequence delivery of packets, explicit paths and QoS guarantees. 2. MPLS/RSVP-TE-BASED UTRAN We use the Multi-protocol Label Switching (MPLS) protocol, which combines both the flexibility of the IP world with the facilities of a connection-oriented protocol like ATM. MPLS uses short labels of fixed length within the forwarding process. The expensive lookup of IP routing tables is only necessary once at the boundary of the MPLS cloud. Furthermore, MPLS allows for binding traffic to explicit paths (Label Switched Path, LSP) by providing label tables at each network node which are maintained by a smart signaling protocol like RSVP-TE. RSVP-TE is an extension [10] of the Resource Reservation Protocol (RSVP) used to allocate network resources in IP networks in a host-to-host manner. Different proposals of an IP-based UTRAN are currently discussed in 3GPP Release 6. Based on one of them, the DRAN (Distributed RAN) approach proposed by Lucent Technologies [3], we have identified the requirements of the future UTRAN. Due to the changes to the topology and the functions assigned to the different network elements, the requirements for the transport system differ between the current UTRAN and the DRAN approach. The main idea is to connect the base stations via IP in native mode [4] and to route the user traffic directly from the Core Network Gateway to the NodeB, where MPLS/RSVP-TE is used to manage the paths and to provide the required QoS. Further work on MPLS addresses special issues like MPLS micro-mobility [4], advanced routing methods [6, 7, 8, 9, 12] to determine optimal explicit LSPs under different constraints, or the optimal granularity of LSPs [11] to reach a high overall utilization of the network. However, these approaches address neither MPLS with respect to future UTRAN topologies nor the special requirements like QoS of such an UTRAN. The focus of our study is the performance of MPLS and native IP in the future UTRAN. We especially examine the case that the traffic mix changes and that the traffic load in the system moves from one area of the network to another. 3. STUDIES By means of our MPLS/RSVP-TE extension to the network simulator 2 two different scenarios have been studied, pure IP-based multi-path transport and MPLS-based transport. We have studied the impact of dynamic changes of the traffic on the QoS. We address changes to the traffic load which may be caused by the movement of large

3 MPLS/RSVP-TE-based Future UMTS Radio Access Network user groups, for example in the evening after work or after the end of a soccer game. Another observed change is the traffic mix resulting from the different service demands during the day. The topology being studied is depicted in Figure 1. The network comprises six meshed traffic concentrators. Three of the traffic concentrators are additionally connected to 50 access nodes each (50/50/50 configuration), which represent the base stations (inodebs). Each access node hosts one data traffic source and up to 151 voice traffic sources based on an Adaptive Multi Rate (AMR) [2] user model (this results in a simulation of nearly voice users). The data traffic users generate the background traffic to ensure the high utilization of the network. Changes to the traffic mix were simulated by varying the amount of traffic between voice and data traffic (data/voice, e.g. 400/1580 kbps). Different traffic loads were modeled by the reallocation of some access nodes from one concentrator to another (e.g. or ). For the simulations, the following assumptions have been made: Figure 1 Simulation topology based on DRAN; concerning the user plane, most functionality of the RNC is moved to the access nodes (inode Bs) one LSP for data traffic and one LSP for voice traffic from each access node to the gateway (capacity is adjusted to the aggregated traffic of voice and data, respectively), the paths of voice LSP and data LSP are independent, the size of data packets is 1500 bytes (constant bit stream), the size of voice packets differs from 6 to 30 bytes without overheads (transmission time interval of 20 ms), voice users are always active (worst case scenario),

4 René Böringer, Markus Bauer, Andreas Mitschele-Thiel, Michael Söllner constant link delay of 1ms, delay incurred by network elements is due to queuing only. For the IP-based scenario, IP multi-path routing is assumed, i.e. all paths with the length of the shortest path are feasible and used with a uniformly split traffic load. In the MPLS-based scenario, RSVP-TE uses constraint-based routing, i.e. all paths providing sufficient bandwidth are allowed. 4. SIMULATION RESULTS In the case of a symmetric network (i.e. 50/50/50 configuration, same number of access nodes on each concentrator) with evenly distributed traffic sources, the IP multi-path scenario offers a slightly better performance, which means that delay, jitter and loss are lower than in the MPLS transport scenario. The reason is that an LSP with bandwidth reservation behaves as a physical link showing the slow link problem. Unlike MPLS, pure IP can profit from the multiplexing gain, which leads to better performance in this case. The MPLS scenario performs better in comparison to the IP scenario if the traffic sources are unevenly distributed and if changes in the traffic mix are assumed. This effect is caused by the ability of MPLS in connection with RSVP-TE to use unused capacities of links, which are not part of the shortest path (constraint-based routing). The impact of changes of the traffic mix is decreased by the separation of voice and data traffic in MPLS (each class uses different independent LSPs). Figure 2 and Figure 3 depict the differences between both implementations with respect to the stability of QoS if the traffic load or the traffic mix changes. In order to reduce the complexity of the figures, we have adapted the size of the queues for the different configurations such that the delay figures remain unchanged. This allows for a direct comparison of the loss figures of the different configurations. The MPLS scenario also provides very stable values of jitter and loss. (The outstanding loss value for the 400/1580 traffic mix in Figure 3 results from the illustration of the difference in percent in the case that the absolute values are very small.) This allows a high utilization of the network and offers a lot of flexibility to the operator. In contrast, the IP scenario shows changes up to several 100%. Thus, it is necessary to adapt the admission control resulting in a reduction of voice users or to apply over-provisioning. Both lead to higher cost either of link fees or of management cost. MPLS/RSVP-TE can save link cost by reducing the over-provisioning and administration cost due to a reduced need to monitor and to adapt the network configuration. A pure IP-based network always needs the support of other protocols to ensure the fulfillment of the requirements of a UMTS system and especially those of the UTRAN. Some of these requirements are the in-sequencedelivery of packets, the identification of flows and predictable delay, jitter and loss values. MPLS in combination with RSVP-TE and appropriate traffic scheduling can fulfill the requirements without other protocols, which would add additional overhead.

5 MPLS/RSVP-TE-based Future UMTS Radio Access Network change [%] / / / /380 voice mean jitter voice mean loss data mean jitter data mean loss Figure 2 Changes of jitter and loss in percent within the IP multi-path transport scenario if the traffic load (50/50/50 to ) changes in different traffic mixes (voice/data kbps). 100% - no differences; < 100% - value decrease; > 100% values increase change [%] / / / /380 voice mean jitter voice mean loss data mean jitter data mean loss Figure 3 Changes of jitter and loss in percent within the MPLS transport scenario if the traffic load (50/50/50 to ) changes in different traffic mixes (voice/data kbps). 100% - no differences; < 100% - value decrease; > 100% values increase

6 René Böringer, Markus Bauer, Andreas Mitschele-Thiel, Michael Söllner 5. CONCLUSIONS We have shown that in an UTRAN scenario, MPLS/RSVP-TE is able to deliver very stable QoS in wide ranges independent from changes of the traffic mix and traffic load. This means that a MPLS/RSVP-TE transport solution offers a lot of flexibility to operators and network designers. The results are generated by a simulator implemented for these measurements and based on the DRAN approach for a future UTRAN. Future work will aim on enhancements of the simulator to also allow the examination of MPLS micro-mobility. Furthermore, additional approaches for MPLS network optimization like advanced routing, LSP granularity and network reliability will be studied by means of the simulator and with respect to the requirements of a future UTRAN. BIBLIOGRAPHY [1] 3GPP, IP transport in UTRAN (Release 5), 3rd Generation Partnership Project Std., [2] 3GPP, Adaptive Multi-Rate (AMR) speech codec, Interface to Iu, Uu and Nb (Release 5), 3rd Generation Partnership Project Std. TR , v5.2.0, 2003 [3] Bauer M., Schefczik P., Soellner M, Speltacker W., Evolution of the UTRAN Architecture, Fourth International Conference on 3G Mobil Communication Technologies, 2003 [4] Chuissia F. M., Khotimsky D. A., Krishnan S., Mobility Management in Third Generation All-IP Networks, IEEE COMMUNICATION MAGAZINE, Sept [5] Kempf J., Yegani P., Openran: A New Architecture for Mobile Wireless Internet Radio Access Networks, IEEE COMMUNICATION MAGAZINE, pp , May 2002 [6] Kodialam M., Lakshman T., Minimum Interference Routing with Applications to MPLS Traffic Engineering, [7] Kowalik K., Collier M., QoS routing as a tool of MPLS traffic engineering, [8] Long K., Zhang Z., Cheng S., Load balancing algorithms in MPLS traffic engineering, High Performance Switching and Routing, pp , 2001 [9] Ma Q., Steenkiste P., On path selection for traffic with bandwidth guarantees, IEEE International Conference on Network Protocols, 1997 [10] RFC 3209, RSVP-TE: Extensions to RSVP for LSP tunnels, 2001 [11] Rouskas G. N., Jackson L. E., Optimal Granularity of MPLS Tunnels, 18th ITC, Providing Quality of Service in Heterogeneous Environments, 2003 [12] Suri S., Waldvogel M., Warkhede P. R., Profile-based routing: A new framework for MPLS traffic engineering, [13] Uskela S., Key Concepts for Evolution toward beyond 3G Networks, IEEE Wireless Communications, pp , February 2003