IP-Based Routing Algorithms for LEO Satellite ... - Semantic Scholar

IP-Based Routing Algorithms for LEO Satellite Networks in Near-Polar Orbits Mauro De Sanctis, Ernestina Cianca, Marina Ruggieri Dpt. of Electrical Engineering University of Roma “Tor Vergata” Via del politecnico 1, 00133 Roma, Italy e-mail: [email protected] ; [email protected] ; [email protected] Abstract—In this work, several aspects on IP routing for LEO satellite networks are investigated. First, a proper downlink routing procedure is defined, which has the advantage to allow the identification of the egress satellite, without the need of information from GPS systems to determine the position of the destination mobile terminal. Furthermore, the performance of the routing algorithm that minimizes the number of hops are evaluated through simulations, for an Iridium-like and a Teldesic-like constellation in order to show the sensitivity of the routing algorithms to the availability of cross-seam ISLs. A modification of the “minimization of number of hops” algorithm is proposed, which allows to reduce the total path length, and hence, the end-to-end delay. The idea is to exploit as much as possible inter-plane ISLs at higher latitudes. The comparison between the two IP routing algorithms shows that the proposed algorithm can significantly reduce the end-to-end delay when the source interface and the destination interface are largely spaced in latitude and longitude. TABLE OF CONTENTS 1. 2. 3. 4. 5. 6. 7.

INTRODUCTION LEO-BASED SATELLITE NETWORKS WITH ISLS NETWORK TOPOLOGY ADDRESSING ISSUE AND DOWNLINK ROUTING ISLS ROUTING ALGORITHMS PERFORMANCE COMPARISON CONCLUSIONS

1. INTRODUCTION LEO satellite networks can play an important role in providing broadband integrated Internet services to globally scattered users. The main advantages of LEO satellite constellations, when compared with GEO constellations, are: I – Global coverage of the Earth. In particular, with respect to a GEO constellation, a better coverage is provided in polar regions. II – Lower propagation delay (the two-way round trip delay is about 40ms in LEO and 500ms in GEO constellations). III – Lower power margins. 0-7803-7651-X/03/$17.00 © 2003 IEEE

On the other hand, the design of a LEO constellation is much more complicated [1]. A large number of satellites is needed to provide global coverage. The satellites move with respect to a fixed position on the earth, thus requiring more complex intra-satellite and inter-satellite handover procedures. When satellites equipped with On Board Processing (OBP) capabilities are used, Inter-Satellite Links (ISLs) can be provided and the constellation becomes a switching network in the sky where each satellite is a router node. The provision of ISLs releases the need for a dense terrestrial infrastructure. A lower number of ground-stations are needed to support the communication. Moreover, only one ground-satellite-ground hop is required to provide communication between two users, thus reducing the average propagation delay. One of the technical challenges for LEO systems with ISLs is the design of efficient routing strategies tailored for the highly dynamic nature of the network topology [2-10]. The satellite movements and changes in the distance between satellites in different planes cause a constant and periodic change in the network topology. Routing algorithms that are designed for fixed networks may cause routing loops in a dynamic network topology. A datagram enters routing loop when it passes through one satellite more than one time; this problem may cause high transmission delay or, in the worst case, the lost of the datagram. Similar problems related to a dynamic topology are also found in terrestrial wireless adhoc networks. However, as opposite to ad hoc networks, the topology of a LEO-based network changes in a predictable way and efficient routing algorithms should take this into account. Basically, a routing algorithm for a LEO satellite constellation consists of three basic steps: 1) uplink routing: identification of the ingress satellite that should serve the source terminal; 2) downlink routing: identification of the egress satellite that should serve the destination terminal; 3) ISL routing: computation of the path between the ingress satellite and the egress satellite. In particular, the uplink and downlink routing algorithms determine the addressing scheme in the network. In [6], a logical-topology has been introduced, which overlays the LEO constellation and hides the dynamic topology from the routing procedure that is executed on the logical topology. Furthermore, a new static routing

algorithm based on this topology has been presented. In [11], the Darting protocol has been proposed as suitable for LEO satellite networks. However, in [12], a comparison of the Darting protocol with the Extended Bellman-Ford algorithm protocol shows that both protocols have roughly equivalent end-to-end delay characteristics and the Darting algorithm induces much more overhead than its competitor. In [7], a distributed routing algorithm for IP traffic is introduced, which generates minimum propagation delay paths between source and destination. None of these works defines a specific downlink routing scheme. In this work, both downlink routing and ISLs routing are addressed. We refer to the logical-topology defined in [6]. Firstly, a downlink routing scheme that utilizes the logical location of the user is proposed. With respect to other downlink routing procedures [6], the proposed solution does not require the use of the Global Positioning System (GPS). Secondly, the routing algorithm with minimization of the number of hops is described and, through simulation, its performance in different satellite constellation are shown. In particular, the effect in terms of end-to-end delay of the presence of cross-seam ISLs in the constellation is highlighted. Moreoever, a modification to the routing algorithm with minimization of the number of hops is introduced and the performance improvement achieved by the proposed solution is shown. The paper is organized as follows. In Section 2, basic concept on LEO-based satellite networks with ISLs are introduced. The concepts of static topology are introduced in Section 3. The proposed downlink protocol is described in Section 4. The routing algorithm with minimization of the number of hops and its modification, which is proposed in this work, are described in Section 5. In Section 6, the performance results of the IP-routing protocols introduced in Section 4 are discussed. Conclusions are drawn in Section 7.

satellites located in adjacent orbits and at the same latitude. Finally, cross-seam ISLs are specific inter-plane ISLs between satellites in counter-rotating orbits.

Fig. 1 - View from the pole of the polar orbits. In this paper, we consider two LEO satellite constellations: Teledesic and Iridium. The characteristics of these two constellations are summarized in Table 1. The most important difference between Teledesic and Iridium is that Teledesic is provided with cross-seam ISLs as opposite to Iridium. Table 1 - Characteristics of the Iridium and Teledesic constellation. Iridium Teledesic Altitude (km) 780 1375 Number of satellites 66 288 Orbital planes 6 12 Satellites per orbit 11 24 Orbital inclination (degrees) 86.4 84.7 Orbital period (minutes) ~100 ~113 Cross-seam ISLs no yes

3. NETWORK TOPOLOGY 2. LEO-BASED SATELLITE NETWORKS WITH ISLS A LEO satellite constellation is formed by S satellites in N orbital planes; each orbital plane has M satellites at the same altitude of 500-2000 km. Circular orbits are the most important orbits; they can be divided into equatorial, polar and inclined. We will consider only polar (Fig. 1) and near polar orbits due to their importance in many envisaged applications of LEO constellations. Orbital planes pass through the north and south pole in case of polar orbits. In case of near polar orbits, orbital planes are inclined with respect to the equatorial plane of an angle δ ranging from 80o to 90o. Only satellite constellations in polar or near polar orbits can guarantee a global coverage of the earth. There are three types of inter-satellite links: inter-plane, intra-plane and cross-seam. Intra-plane ISLs provide connection between adjacent satellites located in the same orbital plane; inter-plane ISLs provide connection between

To solve loop problems caused by the dynamics of the network topology, a static topology is defined and routing algorithms are performed according to this topology [6]. Every satellite is provided with at least two intra-plane ISLs with satellites at lower and higher altitude; these links are permanent. Inter-plane ISLs are not permanent and they are turned off over the pole, beyond a critical latitude latmax . There is an unambiguous and dynamic correspondence between each satellite and one node of the network; this correspondence is periodic as the movement of satellites is periodic. The topology is fixed on the earth and the correspondence node-satellite depends on the position of the satellites above the earth. For each satellite, a geographic position and a logical position may be defined. The geographic position is identified by two coordinates (lon,lat); lon and lat are the longitude and the latitude of the satellite, respectively. The logical position is used to set the correspondence between a satellite and one node of the topology and it is identified by the couple of coordinates

(x,y), where x = 1, 2,…, N identifies the orbital plane and y = 1, 2,…, M is the position in the plane. This static network topology is shown in Fig. 2.

Fig.2 - Network topology. In the figure, the western hemisphere represents the coverage of the Earth surface from South to North, while the eastern hemisphere represents the coverage of the Earth surface from North to South.

4. ADDRESSING ISSUE AND DOWNLINK ROUTING The downlink routing procedure identifies the egress satellite that will deliver the data to the destination user terminal. The user terminal should be assigned an address that allows the identification of the egress node. If the Earth surface is divided in cells, the terminal can be identified by two positions: the geografic position given by two coordinates (lon,lat), and the logical position that specifies the cell where the terminal is located. Let us consider a partition of the Earth surface in square cells, shown in Fig. 3. The Earth surface is divided into square cells by P meridians and Q parallels.

Fig. 3 - Partition of the Earth surface in square cells. The size of the cells have to be equal or smaller than the size of the footprint of a single satellite beam, so that it is possible to serve each terminal by a single satellite beam. Each cell has an address (p,q), where p = 1, 2,…, P/2 is the logical longitude and q = 1, 2,…, 2Q+2 is the logical

latitude. The set of cells covered by one satellite is called macro-cell. Each satellite has to store the information about its own position (x,y) into the topology and the cells of its macro-cell. The information (x,y), which defines the correspondence node-satellite, is periodic with period T, where T is the orbital period. This information has to be updated every T/M minutes. The information about the macro-cell has to be updated every T/(Q+1), when the coverage is satellite-fixed cell and every T/M when the type of the coverage is Earth-fixed cell. The correspondence between macro-cells and satellites is periodic with period Tt, where Tt is the total period of the system. Due to both Earth rotation and satellite movement, the total period of the system, Tt, is the least common multiple between the Earth rotation period Tr (Tr = 24 h) and the orbital period T. While assigning a logical or geographic address to fixed terminals is not problematic, mobile terminals are unable to know their own geographic position without the help of a GPSs. On the other hand, the logical address can be distributed from the satellites of the constellation through an address distribution channel. A downlink routing procedure, which uses logical positions of the destination node instead of the geographic position like in [6], is described in what follows. The advantage of this procedure is that the use of GPS is not necessary. Let us assume that: 1) each satellite covers a number of α cells in horizontal direction and β cells in vertical direction; β is an even number and the macro-cells are not aligned in the horizontal direction, as shown in Fig. 4; 2) the datagram is positioned on the satellite corresponding to the node (x0,y0). The satellite covers the cells from p0 to pf and from q0 to qf; 3) the destination cell of the datagram is identified by the coordinates (p,q).

Fig. 4 - View of 4 macro-cells. Based on these assumptions, the position (x,y) of the egress node can be computed by:

p 0 ≤ p ≤ pf p > pf p < p0

 x0  x =  x 0 + 1 + ( p − pf ) / α   x 0 − 1 − ( p − pf ) / α   

(1)

if x – x0 is even:

q 0 ≤ q ≤ qf q > qf

 y0  y =  y 0 + 1 + (q − qf ) / β   y 0 − 1 − (q − qf ) / β   

(2)

q < q0

else if x – x0 is odd:

 y0  y =  y 0 + 1 + (q − qf ) / β   y 0 − 1 − (q − qf ) / β    where the symbol

q0 + β / 2 ≤ q ≤ qf + β / 2 (2’) q > qf + β / 2 q < q0 + β / 2

z  means the largest integer not greater

than z. Each satellite that receives a datagram can identify the egress satellite (or node) using equations (1), (2) and (2’), and, then, can send the datagram to the satellite identified by the ISL routing algorithms.

5. ISLS ROUTING ALGORITHMS In this Section, the ISL routing algorithm with minimization of the number of hops together with the proposed modification are described. In the Section, the transfer of a datagram from one satellite to another is referred as a “hop”. In order to determine the next hop of a packet, each satellite has stored a routing table. Routing tables are computed by the ISL routing algorithms; these tables are periodically updated. Since the network topology is static, the routing table of a logical node does not change in time. The correspondence node-satellite changes periodically, with period T/M. Therefore, routing tables stored on board of the satellites must be updated every T/M. In the description of the ISL routing algorithms, the following notation is used to identify the next hop of a datagram:

+ 1 : right_hop  d h =  0 : no_horizontal_hop − 1 : left_hop 

(3)

+ 1 : up_hop  d v =  0 : no_vertical_hop − 1 : down_hop 

(4)

where dh is the direction of the horizontal hop and dv is the direction of the vertical hop. Furthermore, let us denote with nh the number of horizontal hops and nv the number of vertical hops; nt = nh + nv is the total number of hops. n0 = (x0,y0) denotes the position of the node from which the datagram is being forwarded and nf = (xf,yf) the final destination node (the egress node). Let us note that vertical hops are always possible while horizontal hops are possible only if the position of the source node is out of the polar regions. Moreover, if the constellation is not provided of cross-seam ISLs the horizontal hops are not possible between nodes in different emispheres. The ISLs routing algorithm with minimization of the number of hops is very simple in an Iridium-like constellation that it is not provided of cross-seam ISLs. The values of dh and dv for this kind of constellations are shown in Tab. 2. Table 2 - Directions of movement in an Iridium-like constellation x0 < xf

x0 > xf

x0 = xf

y0 < yf

x0 > yf

y0 = yf

dh = +1

dh = -1

dh = 0

dv = +1

dv = -1

dv = 0

The steps of the algorithm are listed below: 1) if the position of the source node, n0 is equal to the datagram destination node, nf , the datagram must be sent down to the destination terminal; 2) if dh = 0 or if the position of the source node is in a polar region, the datagram must be sent in vertical direction (dv in Table 2); 3) if the source node and the datagram destination node are in the same polar region, but in different planes, the packet must be sent vertically, out of the polar region; 4) in all the other cases, the packet must be sent in horizontal direction (dh in Table 2). For a network provided of cross-seam ISLs (Teledesic-like constellation), the algorithm with minimization of the number of hops become more complicated [7]. For each couple of nodes (n0,nf ), two different routes, which minimize the number of hops, must be considered: the first one does not cross a polar region (RH) and the second one crosses at least one polar region (RV). Finally, the route that has the lowest number of total hops is chosen. When the ingress node and the egress node are in different latitudes and longitudes, the algorithm with minimization of the number of hops can be implemented by choosing either the horizontal or the vertical direction as the first direction of the hop. We propose to improve the algorithm with minimization of the number of hops by choosing the route with lower length of the total link. The length of an interplane ISLs is given by:

LISL = 2 ⋅ (R 0 + h ) ⋅ sin (π / 2 N ) ⋅ cos(lat )

(5)

where R0 = 6371 km is the average Earth radius and h is the altitude of the satellites. Note that this lenght is shorter at higher latitudes. Therefore, the steps of the proposed algorithm are the same as for minimization of the number of hops algorithm, besides when n0 and nf are in different longitudes and latitudes and n0 is out of the polar region. In the latter case, the algorithm works as follows: -

Tup _ down is the propagation delay of the uplink and downlink;

if the source node is at higher latitude with respect to the datagram destination node, then the datagram must be sent in horizontal direction; otherwise, the datagram must be sent in vertical direction.

N

TISLs = ∑ LISL(i ) / c i =1

is the propagation delay due to the use of the ISLs for N hops of the datagram; LISL(i ) is the length of the link and c is the speed of light; N +1

∑T

proc

(i )

i =1

is the delay due to the processing of the datagram by the N+1 satellites; N +1

∑T

6. PERFORMANCE COMPARISON In this Section, the previously described IP-based ISL routing algorithms are compared in terms of end-to-end transmission delay, for both Iridium-like and Teledesic-like constellations. In Fig. 5, the end-to-end delay vs. the distance between the terminals is shown for the routing algorithms with minimization of the number of hops in an Iridium-like constellation.

queue

(i)

i =1

is the delay due to the waiting queue in each satellite. The presence of an high number of peaks in the delay is due to the unavailability of cross-seam ISLs. In fact, when the ingress satellite and the egress satellite of a datagram are in different emispheres, then the datagram must pass through a polar region, increasing the end-to-end delay. Fig. 6 shows the variation of the delay vs. the distance between the terminals for a Teldesic-like constellation. By comparing Fig. 5 and Fig. 6, the performance improvement in terms of delay, due to the use of cross-seam ISLs, is highlighted. When distances between terminals are short, high delays can be achieved since source and destination satellites are positioned in the same polar region, where it is not possible to use inter-plane ISLs.

Fig. 5 - Delay vs. distance for the algorithm with the minimization of the number of hops (Iridium-like constellation). In Fig. 5, it is assumed that 10000 datagrams are transmitted between couples of terminals uniformly positioned on the Earth surface. For each forwarded datagram, the point-topoint delay ∆T of the packet is computed by: N +1

N +1

i =1

i =1

∆T = Tup _ down + TISLs + ∑ Tproc (i ) + ∑ Tqueue(i ) where:

(6)

Fig. 6 - Delay vs. distance for the algorithm with the minimization of the number of hops (Teledesic-like constellation). A different type of simulation for the routing algorithm with minimization of the number of hops is shown in Fig. 7. It is

assumed that 300 datagrams are transmitted between two fixed points on the Earth surface, spaced of about 11000 km. The delay shows an approximative periodic behavior due to the periodic movement of the satellite constellation. The performance of the proposed algorithm with minimization of the total link length, in a Teledesic-like constellation, is shown in Fig. 8.

both the algorithms in terms of the mean value and standard deviation of the delay. Table 3 - Statistics of the end-to-end delay

Minimization of the number of hops (Iridium-like constellation) Minimization of the number of hops (Teledesic-like constellation) Minimization of the total link length (Teledesic-like constellation)

Fig. 7: Delay vs. transmission instant for the algorithm with the minimization of the number of hops (Teledesic-like constellation).

Fig. 8 - Delay vs. distance for the algorithm with the minimization of the total link length (Teledesic-like constellation). For sake of comparison, the same kind of simulation as in Fig. 7 has been performed for the ISLs algorithm with minimization of the total link length, assuming the same source and destination points. The result of this simulation is shown in Fig. 9, while Table 3 summarizes the results of

Mean value of the delay

Standard deviation of the delay

µ = 0.0679 s

σ = 0.0278 s

µ = 0.0601 s

σ = 0.0218 s

µ = 0.0589 s

σ = 0.0222 s

The comparison between Fig. 9 and Fig. 7 shows a good improvement in terms of reduction of the end-to-end delay. Note that these results have been achieved assuming that the source interface and destination interface are largely spaced in latitude and longitude. When the delay is averaged with respect to different possible configuration sourcedestination, the proposed algorithm performs slightly better than the ISLs routing with minimization of the number of hops, as it is shown in Table 3. For both the ISLs routing algorithms, a Teledesic-like constellation is characterized by lower end-to-end delay.

Fig. 9 - Delay vs. transmission instant for the algorithm with the minimization of the number of hops (Teledesic-like constellation).

7.

CONCLUSIONS

In this work, the problem of efficient routing algorithms for the dynamic topology of LEO satellite constellations has been addressed. First of all, a downlink routing algorithm has been proposed, which makes no use of GPS system to determine the position of the mobile terminal. Furthermore, performance of the routing algorithm that minimizes the number of hops has been evaluated through simulations, for an Iridium-like and a Teledesic-like constellation, which provides cross-seam ISLs. A modification to the above mentioned algorithm has been introduced. The proposed algorithm is characterized by a slightly smaller average end-to-end delay. However, when the source interface and the destination interface are largely spaced in latitude and longitude, the proposed algorithm can provide a significant reduction of the end-to-end delay. What has been proposed in the paper could constitute a basis for the proper definition and design of LEO-based satellite systems.

Workshop on Satellite-Based (WOSBIS ‘98), pp. 81-88, 1998.

Information

Services

[9] P. Narvaez, A. Clerget, W. Dabbous: Internet routing over LEO satellite networks. Third ACM/IEEE International Workshop on Satellite-based Information Services (WOSBIS ’98), October 1998. [10] A. Iera, A. Molinaro: Designing the internetworking of terrestrial and satellite IP-based networks. IEEE Communications Magazine, pp. 136-144, February 2002. [11] K. Tsai, R. Ma: Darting: a cost effective routing alternative for large space-based dynamic topology networks. Proceedings of the 1995 IEEE MILCOM, pp. 682–687, 1995. [12] R. A. Raines, R. F. Janoso, D. M. Gallagher, D. L. Coulliette: Simulation of two routing protocols operating in a low earth orbit satellite network environment. Proceedings of the 1997 IEEE MILCOM, vol. 1, pp. 429–433, November 1997.

REFERENCES [1] L. Y. Chotikapong, H. Cruickshank, Z. Sun, “Evaluation of TCP and Internet traffic via Low Earth Orbit satellites”. IEEE Personal Communications, vol. 8, pp. 28-34, June 2001. [2] L. Wood, A. Clerget, I. Andrikopoulos, G. Pavlou, W. Dabbous: IP routing issues in satellite constellation networks. International Journal of Satellite Communications, vol.19 n. 1, pp. 69-92, 2001. [3] L. Franck: Routing algorithms for networks of intersatellite links, Ph.D thesis, Ecole Nationale Supérieure des Télécommunications, Paris, 2001. [4] T. R. Henderson: Networking over next-generation satellite systems. PhD thesis, University of California at Berkeley, 1999. [5] Y. Hu, V. O. K. Li: Satellite-based Internet: a tutorial. IEEE Communications Magazine, 39(3), pp. 154-162, March 2001. [6] Y. Hu, V. O. K. Li: Logical topology-based routing in LEO constellations. IEEE International Conference on Communications ICC 2001, vol.10, pp. 3172-3176, 2001. [7] E. Ekici, I. F. Akyildiz, M. Bender: A distributed routing algorithm for datagram traffic in LEO satellite networks. IEEE/ACM Transactions on networking, vol. 9 n. 2, pp.137147, April 2001. [8] Y. Hashimoto, B. Sarikaya: Design of IP-based routing in LEO satellite network. Proceedings of third International

Mauro De Sanctis received the Dr.Ing. degree in Telecommunications Engineering from the University of Roma “Tor Vergata” in 2002. He is currently a Ph.D. student of the Department of Electrical Engineering in the same University. He worked extensively on the development of a network simulator for LEO satellite networks. His main areas of interest are interworking and resource management satellite systems.

in

Ernestina Cianca received the "Laurea" degree cum laude in Electronic Engineering from the University of L'Aquila, Italy, in 1997. She got the Ph.D. degree from the University of Roma Tor Vergata, in 2001. She spent the last six months of her Ph.D. studies at the CPK, Center for Personkommunication, Aalborg University, Denmark. As a member of the WING (Wireless InterNetworkinG) group of CPK, her research activity was on IP-based data transmissions for future wireless systems focusing on the performance of TCP on wireless links (in particular satellite links) when CDMA-based air interface are considered. From Nov. 2000-to Apr. 2001 she was employed by Aalborg University as Assistant Research Professor. She is currently collaborating with the Communication Group of the Electronic Engineering Department of the University of

Roma Tor Vergata, working on the national project titled "Code Division Multiple Access for broadband satellite terrestrial integrated systems". Her main research interests are in the field of wireless access technologies, and in particular: resource management issues and power control in CDMA-based wireless systems (terrestrial and satellite systems); link ARQ techniques. Marina Ruggieri graduated cum laude in Electronics Engineering in 1984 at the University of Roma La Sapienza. She was with FACEITT and at GTC-ITT (Roanoke, VA) (1985-1986). She was Research and Teaching Assistant at the University of Roma Tor Vergata (1986-1991), Associate Professor at the University of L'Aquila (1991-1994) and Roma Tor Vergata (1994-2000). Since November 2000 she is Full Professor in Telecommunications at the University of Roma Tor Vergata. She has participated to International Committees for Professor Chair, Ph.D and Master degrees (Lund-Sweden, Delft-The Netherlands, Toulouse-France, TrondheimNorway, Aalborg-Denmark). In 1999 she has been appointed member of the Board of Governors of the IEEE AES Society (2000-2002) and re-elected for the period 2003-2005. Her research mainly concerns space communications systems (in particular satellites) as well as mobile and multimedia networks. She is the Principal Investigator of the ASI satellite communications mission DAVID and of a MIUR two-year national research program on CDMA integrated mobile systems. She is involved in the organisation of international Conferences/Workshops. She is Editor of the IEEE Transactions on AES for “Space Systems”. She is member of the Editorial Board of WPC Journal (Kluwer). She was awarded the 1990 Piero Fanti International Prize and she had a nomination for the 1996 Harry M. Mimmo and 2002 Cristoforo Colombo Awards. She is an IEEE Member (S'84-M'85-SM'94) and Chair of the IEEE AES Space Systems Panel.