PERFORMANCE EVALUATION OF TCP BASED APPLICATIONS USING DOUBLE CONSTELLATION GEO/LEO ARCHITECTURE M. Luglio(1), J. Stepanek(2) and M. Gerla(2) (1)Dipartimento di Ingegneria Elettronica, Università di Roma Tor Vergata Via del Politecnico 1, 00133 Rome, Italy E-mail:
[email protected] (2)Computer Science Department, University of California Los Angeles Boelter Hall, Los Angeles CA, 90095 USA E-mail:
[email protected],
[email protected]
Abstract The implementation of an IP based worldwide network needs the availability of a very efficient satellite segment in order to achieve real global coverage. To this aim the satellite component shall provide flexibility and high link availability. Satellite systems based on the use of geostationary (GEO) or Low Earth Orbit (LEO) constellations suffer from their intrinsic limitations in terms of coverage or flexibility. Traffic requirements concerning broadband services are expected to be very uneven both in time and in space. To match this requirement an innovative satellite system architecture, composed by a LEO segment to complement a GEO segment, has been proposed. In this scenario, the implications of using TCP/IP protocols must be evaluated to achieve real integration in the IP network. The paper, after describing the scenario and the main features of TCP/IP protocols over classical satellite links, will theoretically approach the implications at the TCP level due to the switching during data exchange between to segments characterized by different delays. Performance evaluation will be carried out for different system configurations utilizing ns-2 simulator where procedures representing this scenario have been modeled. Keywords Hybrid constellations, TCP/IP Acronyms GW Gateway GEO Geostationary Earth Orbit HO Handover ISHO Intersegment Handover LEO Low Earth Orbit MT Mobile Terminal Introduction Many operational (or planned) satellite systems aim to provide either global coverage or large bandwidth. Genuine ubiquity of broadband services may take some time to be achieved, especially in very remote areas. In this scenario, the hybrid constellation concept can be introduced, foreseeing the utilization of a LEO satellite component and a GEO satellite component cooperating to offer really global coverage. In fact, each component can provide a backup or complementary coverage for the other component in case of need. Furthermore, the availability of alternative access
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points in case of shadowing occurrence and satisfaction of extra capacity in case of special events increases the value of such architectures. This concept will be validated in an Italian Space Agency (ASI) funded project [1], in which the two components are conceived to connect through Inter Orbit Link (IOL), and has been approached in previous papers [2, 3, 4]. Alternatively, in this paper the two segments are considered alternative point of access to the network, working on a complementary basis. To pursue real integration the exploitation of effective handover (HO) procedures between the two different segments, part of a global telecommunication system, presents a key challenge. In fact, the lack of connectivity and interoperability between the two different segments hampers the ability to achieve global service. The framework of the activities carried on in SUITED (multisegment System for broadband Ubiquitous access to InTErnet services and Demonstrator) project [5] addresses the implementation of ISHO between the geostationary satellite segment (GEO) and Low Earth Orbit (LEO) or Medium Earth Orbit (MEO) segment. Details concerning such procedures along with their performance in terms of execution delay are presented in [6]. This paper investigates the problem of integrating such an architecture in the global IP network. In particular, the implication of using TCP protocol will be evaluated along with performance of typical TCP based applications in terms of throughput and delay. Ad hoc procedures have been implemented to upgrade the ns-2 simulator [7] to suitably model the described scenario. Sections 1 addresses the system architecture, section 2 briefly presents the TCP issues over classical satellite link, in section 3 the simulation scenario will be presented, in section 4 the simulation results will be shown and finally conclusions will be drawn. 1
System Architecture
The system architecture is composed of a GEO component and of a LEO component. The GEO component provides service over a typical continental coverage area (as one of the advanced high capacity systems, which are being to be implemented [7]). The LEO segment aims at providing an alternative access point to the network for those users located in areas where GEOs suffer from their typical limitations. For example where their coverage is completely absent (poles) or could perform not at their best (high latitudes, mountain regions, shaded and urban areas) or might not be economically convenient (deserts, oceans) or might not satisfy temporary extra traffic (when occasional political, sport or natural events occur). Both GEO and LEO component, being alternative access to the network, may include more than one unit, as depicted in Figure 1. In the above-described scenario, characterized by two inter-working segments, a suitable ISHO procedure [6] allows to switch the communication from GEO segment to LEO segment and vice-versa (see Figure 2). 2
TCP over Satellite
When considering the performance of real time (e.g. voice and video) applications the large propagation delays represent a critical issue [9]. The problem is particularly acute in TCP applications over links with large bandwidth-delay products, where a large TCP window is required for efficient use of the satellite link. A lossy satellite link can cause frequent “slow start” events, with a significant impact on throughput. In the case of LEO networks, a delay variation is introduced due to the fast movement of satellites, which may have an impact on TCP. In addition, such movement of satellites will cause connections to be handed off.
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Figure 1: System architecture
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Figure 2: ISHO overview In summary, the key satellite network features that need to be considered in order to evaluate the impact of satellite on internet application performance are: propagation delay, DelayBandwidth Product (DBP), frequent handover, signal-to-noise ratio (SNR), satellite diversity, routing strategy. In order to evaluate performance of TCP/IP over geostationary satellites, some previous work [10-13] has been mainly focused on evaluating performance in terms of throughput of different TCP schemes (Tahoe, Reno, New Reno, SACK), where the connection between two fixed stations goes across a GEO satellite. In this paper, we also address other TCP schemes (Westwood [14]) and evaluate TCP performance when terminals are mobile. Such an environment creates a very realistic scenario for the evaluation of performance in a satellite environment.
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3
Simulation Scenario
To investigate the efficacy of our hybrid constellation scenarios, we employed the wellknown ns-2 (Network Simulator) package [7]. While this simulation tool already includes support for satellites, we have added several enhancements to the channel model to support mobility and shadowing. Signal-shadowing remains the dominant effect influencing land mobile satellite (LMS) systems’ performance and availability. Unlike multipath fading, using fade margins and advanced transmission techniques fail to abate the effects of blockage and shadowing. As a result, shadowed links suffer from protracted high error-rates and even temporary unavailability. In addition, the probability of shadowing events increases with lower elevations, that is, higher latitudes in the case of GEO systems and lower elevation angles in the case of LEO/MEO systems. Including shadowing and mobility remains vital to our study by providing the opportunity for the shadowed terminal to handoff between constellations. Without shadowing, our terminal would never need to handoff between LEO and GEO satellites. For this reason, we use a physical-statistical land mobile satellite channel model as described in [15]. This model uses the geometrical projections of buildings surrounding the mobile terminal. In the model, height and width statistical distributions describe these projections [16,17], and the existence or absence of the direct ray defines the state of the channel. This is either line-of-sight (LOS), when a ray exists, or shadowed, otherwise. The mobile terminal employs a simple algorithm to perform handoff between satellites. When the mobile finds the link shadowed, it searches for the visible satellite with the highest elevation. Once the mobile finds a suitable link, it holds this link until it becomes shadowed, even as other satellites become available. At the other end of the TCP connection lays an unshadowed stationary gateway node. This node maintains links with multiple satellites including both LEO and GEO. As a result, the gateway can reach the terminal regardless of which orbits it uses. The simulations scenarios of this paper use two kinds of LEO orbits and one type of GEO. One of the LEO orbits consists of a satellites arranged in an “Iridium-like” configuration, that is, polar orbits with 66 satellites. The other type of LEO uses a “Globalstar-like” configuration with inclined orbits of 48 total satellites. Finally, the GEO satellite is located at –95 degrees longitude, and the mobile terminal and gateway are located in the vicinity of New York City. Table 1 summarize the LEO constellations characteristics. Constellation Orbit Number of orbital planes Number of satellite per plane Phase among the orbit Inclination of orbital plane Period Minimum elevation degree Orbit height (km)
LEO Polar 6 11 2π/22 86.4° 100’ 28” 8° 780
LEO Inclined 8 6 2π/48 52° 114’ 12° 1414
Table 1: Orbital parameters To measure throughput, simulations were performed using 300 FTP sessions each lasting 11 seconds. These short transfers are meant to model something like an HTTP session. Five satellite configurations were considered, 1) GEO only, 2) polar LEO only, 3) hybrid GEO and 224
polar LEO, 4) inclined LEO only and 5) hybrid GEO and inclined LEO. Two types of shadowing were considered when measuring TCP throughput. In the first type the TCP sender suffers from shadowing and mobility, in other words, the connection is source-shadowed. In the second type the TCP receiver suffers from shadowing and mobility, and the connection is sink-shadowed. In addition to shadowing, various uniformly distributed errors were also added to the channel to represent fading Bit Error Rates (BER). 4
Simulation Results
Figure 3 and Figure 4 show the results of using a symmetric bandwidth between LEO and GEO of 144 kbit/s. In this case the handoff from GEO to LEO results in an increase in the window due to latency only. While the latency difference is substantial, the low data-rate results in less effect upon TCP. As a result, the handoff between LEO and GEO produces significant benefit, while not doubling performance. Figure 3 reports on the performance for source-shadowed connections, while Figure 4 the performance for sink-shadowed connections. From these figures, we see that TCP performs worse in cases where the TCP receiver suffers from shadowing. Shadowing may have a more detrimental impact upon the sink because as the receiver performs handoff between constellations, all the “in-flight” packets recently sent are lost. In fact, this effect should worsen with larger windows. In the future, we plan to examine using IOLs and routing to address this problem. For the short duration TCP sessions, we also notice no significant different between TCP Westwood and New Reno. 0.9
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Figure 3 TCP Throughputs for Source-Shadowed Connections with BER of 10-6.
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Figure 4 TCP Throughputs for Sink-Shadowed Connections with BER of 10-6. Conclusions In this paper we investigated the efficacy of using a hybrid constellation of LEO and GEO satellites to improve the performance of TCP. For this purpose, we have employed the ns-2 simulation package with enhancements to model shadowing and mobility. As a result, we have found that the hybrid constellations improves TCP performance even in the face of fairly dramatic changes in the size of the TCP window. We’ve also presented a methodology incorporating a number of critical modeling aspects for studying this problem. The results presented here focus upon using symmetrical bandwidth resources between orbits, but asymmetrical bandwidth may likely exist between LEO/MEO and GEO satellites. Satellites with asymmetric resources complicate the handoff procedures. However, a trade-off exists between utilization of resources and frequency of handoff that deserves further consideration. Using bandwidth on demand, circuit-switching, and on-board processing may further complicate and effective handoff algorithm. We look forward to exploring this issues in the future. References [1] M. Ruggieri, F. Vatalaro, A. Paraboni, C. Bonifazi, DAVID, A Small Satellite mission for data distribution, Proceedings of 4th International Symposium on Small Satellite Systems and Services, CNES-98, Antibes Juan les Pins, France, Sept. 1998, session 3. [2] M. Luglio, W. Pietroni, Design Methodology of Hybrid LEO-GEO Constellations for High Capacity Communications, International Conference on Telecommunications, ICT2000, 22-25 May, 2000, Acapulco, Mexico, pp. 1177-1180. [3] M. Luglio, W. Pietroni, The Use of Hybrid Orbit Satellite Constellations for High Capacity Communications,19th AIAA International Communications Satellite Systems Conference, AIAA19, session 2, vol. 1, April 17-20, 2001, Tolouse, France. 226
[4] M. Luglio, W. Pietroni, Optimisation of Double Link Transmission in case of Hybrid Orbit Satellite Constellations, accepted by AIAA Journal on Spacecrafts and Rockets. [5] P. Conforto, G. Losquadro, C. Tocci, M. Luglio, R. E. Sherif, SUITED/GMBS System Architecture, IST Mobile Communications Summit 2000, Galway, 1-4 October 2000, pp. 115-122. [6] M. Leo, M. Luglio, Identification and Performance Evaluation of Intersegment Handover Procedures for Hybrid Constellation Satellite Systems, accepted by Wireless Communications and Mobile Computing. [7] “Network Simulator (NS-2)”, www.isi.edu/nsnam/ns/. [8] F. Carducci, G. Losquadro, V. Podda, The EuroSkyWay worldwide system providing broadband service to fixed and mobile end users, Fourth Ka Band utilization Conference, November 2-4 1998, Venice, Italy, pp. 33-40. [9] INTELSAT Internet Technical Handbook, April 20th, 1998. [10] W. D. Ivancic, D. Brooks, B. Frantz, D. Hoder, D. Shell, D. Beering, NASA’s broadband satellite networking research, IEEE Communications Magazine, Volume: 37, n. 7, July 1999, pp. 40–47. [11] C. P. Charalambous, V. S. Frost, J. B. Evans, Performance evaluation of TCP extensions on ATM over high bandwidth delay product networks, IEEE Comm. Magaz., Vol. 37, n. 7, July 1999, pp. 57–63. [12] H. Kruse, S. Ostermann, M. Allman, On the Performance of TCP-based Data Transfers on a Faded Ka-Band Satellite Link, Proceedings of the 6th Ka-Band Utilization Conference, June 2000. [13] M. Allman, H. Kruse, S. Ostermann, A History of the Improvement of Internet Protocols Over Satellites Using ACTS, Invited paper for ACTS Conference 2000, May 2000. [14] M. Gerla, M.Y. Sanadidi, R. Wang, A. Zanella, C. Casetti, S. Mascolo, TCP westwood: congestion window control using bandwidth estimation, IEEE Global Telecommunications Conference, 2001, GLOBECOM '01, Vol. 3, pp. 1698 – 1702. [15] P. Loreti, M. Luglio, R. Kapoor, J. Stepanek, M. Gerla, F. Vatalaro, M. A. VazquezCastro, “LEO Satellite Systems Performance with TCP-IP applications”, Proceedings of Milcom 2001, Tysons Corner, McLean, VA, USA, October 28-31, 2001, session U24. [16] P. Loreti, M. Luglio, R. Kapoor, J. Stepanek, M. Gerla, F. Vatalaro, M. A. VazquezCastro, Throughput and Delay Performance of Mobile Internet Applications Using LEO Satellite Access, International Symposium on 3G Infrastructure and Services, Athens, GR, 2-3 July, 2001, pp.68-73. [17] M. Gerla, R. Kapoor, J.Stepanek, P. Loreti, M. Luglio, F. Vatalaro, M. A. VazquezCastro, Satellite Systems Performance with TCP-IP Applications, Tyrrhenian International Workshop on Digital Communications, IWDC 2001, September 17-20, 2001, Taormina, Italy, pp. 76-90.
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