International Journal of Arts and Sciences 3(9): 166-179 (2010)

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Architecture of Inner Planetary Satellite Network: Delay Components and Communication Protocol Ardian Greca, Georgia Southern University, USA Abstract: Communication satellites have been in commercial use for more than three decades. Early deployment supported data, military communication, international telephony, and broadcast TV. The system can be extended beyond the existing LEO, MEO and GEO configuration as future requirements for deep space scientific missions might evolve. The current trend of extending the use of TCP/IP traffic between distant satellite locations faces difficult challenges. TCP/IP suite can provide services when an end-to-end path between source and destination exists for the duration of communication session, the maximum round trip time over that path is not excessive and not highly variable, and the end-to-end loss is relatively small. Based on the current research activities, all of these requirements can in some ways be fulfilled for short distances such as satellites around the earth. However, on the deep space, factors such as long and variable delay, data rate asymmetry and packet loss and errors will make TCP/IP suite to suffer major drawbacks and be unfitted for communication. Thus it is of growing interest to adapt the existing architectures and protocols, and develop new ones that can be applied to these challenged networks. In this paper we propose a new architecture which extends the communication beyond the GEO layer. We introduce a new layer called BGO (beyond GEO) and analyze the delay and communication protocol issues related to this configuration. The idea is to develop a structure that envelops the intended space with a minimal number of satellites in a stationary orbit relative to the Sun and the planets themselves. This network is called inner planetary satellite telecommunication network (IPSTN). We then present a mathematical model to study the total delay on IPSTN. We also propose a new communication protocol called IP2X, to improve the network’s performance, and study its performance. The protocol uses point-to-point transmission to deliver messages end-to-end from source to destination. Simulation results confirm that the total delay will determine key features of future space exploration related satellite communication networks. Furthermore, performance issues regarding the proposed network architecture can be improved by the proposed protocol. Keywords: Satellite networks, communication protocols, delay model, satellite network architecture.

Introduction Due to the inability of terrestrial communication to cover large areas on earth and a large downstream population, satellite systems are seen as a great opportunity to transmit data and multimedia around the globe. The current organization of satellite communication is based on a three layer architecture [1]. Low-Earth-Orbit (LEO) satellites are stationed in the first layer. LEO satellites are organized in a grid architecture that stretches approximately 500 to 1,600 km (310 to 990 miles) above the Earth's surface. LEO satellites are based on satellite constellations with several periodic circular obits, i.e., since they are the closest to earth they move faster than

the earth and appear and disappear periodically [1]. Medium-Earth-Orbit (MEO) satellites, stationed in the second layer, are typically between 8,000 km and 20,000 km (4970 to 12430 miles) altitude, i.e., they are exactly between LEOs and GEOs. MEO system design involves more delays and higher power levels than LEO satellites, but they achieve a higher coverage of the Earth with fewer satellites [2]. A satellite in Geostationary-Orbit (GEO), which is the third layer, does not appear to be moving when seen from the Earth. Its velocity in space equals approximately 11,071.9 km/h (6880 mi/h). A geostationary orbit is a nonretrograde circular orbit, which means not having a direction of motion opposite to that of the earth on its axis or of the planets around the Sun. The satellite remains stationary in an apparent position relative to the Earth, about 35,784 km (22235 miles) away from the Earth if its elevation angle is orthogonal 900 to the equator. Given the diameter of the Earth and the distance from the satellite to the surface of the Earth only three GEO satellites are needed to have the full coverage of earth if placed in the right orbits [2]. Furthermore, the rapid growth of satellite communications is evolving the IP protocol suite to extend the Internet nfrastructure in the sky. Most of the conventional approaches to intercommunication between these layers involve the architecture and the networking possibilities. A hierarchical architecture and routing in satellite networks is proposed in [1]. The authors show that the satellite over satellite routing can achieve better performance in terms of cost and quality of service. Another approach maintains the initial paths as long as possible to minimize the signaling overhead [3]. Other methods involve signal processing techniques [4], [5]. All of these approaches are limited and depend on the architecture of satellites around the Earth. Their main purpose is to create an efficient Earth communications network. However, there is a growing interest to extend the existing satellite communication to the deep space as future requirements for scientific missions might evolve [12]. In this paper we propose a new architecture which extends the communication beyond the GEO layer. We introduce a new layer called BGEO (beyond GEO) and analyze the delay and communication protocol issues related to this configuration. The idea is to develop a structure that envelops the intended space with a minimal number of satellites in a stationary orbit relative to Sun and the planets themselves. This network is called inner planetary satellite telecommunication network (IPSTN). We then present a mathematical model to study the total delay on IPSTN and propose a new communication protocol to improve its performance. Simulation results show that delay determines the key features of the network. Furthermore, the proposed protocol better fits network requirements. The rest of the paper is organized as follows. In Section two the structure of BGEO, IPSTN, and network performance components are explained in details. Section three covers the simulation results for the delay and proposed protocol. Finally, Section four concludes the paper.

Proposed Architecture for IPSTN Layered Structure The existing satellite communication (in the Earths' orbit) serves to connect parties on Earth, but the need for communication beyond Earth opens a new area for research. The need will involve different types of service requirements, such as telecommunication between space ships (i.e. at moving locations in space) and the earth stations. There will be permanent stations beyond the Moon and numerous travelers needing to communicate with both terrestrial and extra-terrestrial parties. While users and uses are hard to accurately predict, as they may go beyond space exploration, space tourism, space laboratories etc., we can postulate a minimal viable network configuration to enable effective telecommunication in space. In this framework a new layer which covers a wider space segment is required. This is called BGEO (beyond GEO) layer, which is extended up to and including the asteroid belt. The same architecture of Earth satellites (LEO, MEO, and GEO) is proposed for other planets and the Moon. All GEO satellites are in a triangular configuration and form the BGEO layer, enough to cover the whole area. Therefore, we extend the same idea to envelop the intended space with a minimal number of satellites in stationary orbits relative to the Sun to assure 100% coverage as shown in Figure 1. There are at least three satellites which are configured in a triangle structure in orbit around the planets (Earth, Venus, Mars, etc.) and the Asteroid Belt. Furthermore, for a fully deployed IPSTN, varying opportunities and benefits can be broadly outlined. Figure 2 displays two enabling communication capabilities. One is using satellites in the asteroid belt (Fig. 2a) and the other is using the spaceship satellite communication (Fig. 2b).

Fig. 1. The proposed satellite network in the IPSTN.

Fig.2 . (a) Communication via Asteroid Belt. (b) Spaceship satellite communication. Given that a full deployment itself can take a long period of time, the IPSTN has to survive and serve for a very long period of time. There are a number of important communication characteristics for this configuration. First, a communication path might never exist in real time between source and destination. Second, there is an extreme delay, which can be measured in terms of propagation delay, transmission delay and variable delay (jitter). Third, all connections are made of asymmetric links, i.e., there is a significant difference of load in each direction. Also, at times a reverse channel might not exist at all. Furthermore, there are limited and variable resources, i.e., network elements are characterized by different bandwidth, power, and other requirements. Finally, the configuration is a heterogeneous network supporting different services, both IP and non-IP connections, etc. All these reasons make conventional transport and routing protocols impractical. Furthermore, existing resources are limited and variable and the future will only compound this situation as there is no alternative to a gradual buildup of a network of required magnitude. Although this configuration has a number of benefits, there still are several problems that have to be solved in order to make it complete. The slow deployment and very high risk of technological degradation introduces a big challenge. Also, securing steady long-term financing source(s) with the uncertainty of whether or not there will be any direct benefits at all combined with high costs keeps most business from wanting to participate. However, our intention is to primarily extrapolate the current technologies without consideration for expected progress yet to be made in space exploration. Performance Issues for the Proposed Architecture The performance of the proposed IPSTN network can be evaluated in terms of both semantic transparency and time transparency. Semantic transparency determines the capability of the network to transport information accurately from the source to destination. The two most common expressions of semantic transparency are reliability and availability. Reliability is the likelihood that a network will remain operational (despite failures) for the duration of a mission. This means the probability of failure during the mission should be very low, not extending

predetermined values. Availability expresses the fraction of time a network is operational. This means that the network traffic will not be dropped even when a network failure occurs. It is important to note that a network with high availability may in fact fail. Among the clear and pressing requirements is availability (statistically over say 99.9999%) with delivery by relaying without resending it or using acknowledgments (which due to delays seems impractical. See protocols below). Traditionally coding techniques are used to send a message to the destination and then recover it free from errors. They improve network reliability. There are a number of codes introduced and applied so far, and each has its own features. This requires deeper analysis and is out of the scope of this paper. The time transparency can be defined in terms of delay and delay jitter. The delay jitter is referred as different parts of the information arrive at the destination with a different delay. It is a statistical variable and is left for future research. The delay is defined as the time difference between the sending of the information at the source and the receiving of the information at the receiver. In order to get a mathematical model of the delay, we first introduce the delay components. There are four delay types on each satellite. The processing delay is calculated as the difference between the time the message is correctly received at the link node and the time the message is assigned to an outgoing link queue for transmission. The queuing delay is difference between the time the message is assigned to a queue for transmission and the time it starts being transmitted. The transmission delay is the difference between the times the first and last bits of the message are transmitted. Finally, the propagation delay is from the time the last bit is transmitted at the source node of the link until the time it is received at the destination node of the link [7]. Note that, the following notations are used to model the end-to-end delay of the system with space based routing using inter-satellite links: (1) S is the set of all satellites. (2) Rsd is the set of inter-satellite links on the route from source s to destination d,

sd S. (3) nsd is the number of satellite on the route from source s to destination d, sd S. (4) tu is the ground transmission time plus ground-to-satellite signal propagation

time. (5) tg is the ground signal queuing and processing time. (6) tri is the transmission time for satellite i. (7) tij is the signal propagation time from satellite i to satellite j. (8) qi is the queue delay in satellite i. (9) pi is the processing delay in satellite i. The total delay for a given satellite i is:

Ti=pi+qi+tri+tij (1) and the total delay is:

Td=tu+tg+ (2)

Substituting (1) in eq. (2) the total delay is:

Td=tu+tg+ (3)

In eq. 3, the processing delay can be approximated by the average time needed to perform routing table lookup and switching. The queuing delay is determined from the length of the queue at the time of measurement, the average packet size, and link capacity [6]. The transmission delay also depends on the average packet size and link capacity. While the propagation delay depends on other factors such as distance between satellites, speed, etc., and has a big impact on the choice of an appropriate communication protocol. Communication Protocol for the Proposed Architecture In the IPSTN architecture the delay is mostly dependent on distance which influences the protocol used in inter-satellite links [3], [4], [5], [6], [7], [8]. Since the distance is very big, the delay is in order of minutes and the existing "handshaking" protocols are not suitable for the proposed architecture. In order to cope with this problem, we discus some of the existing protocols and then introduce a new one that better fits into the IPSTN architecture. Currently, there are protocols, such as TCPSAT, UDP, and DTN, in competition with each other to become the standard protocol for long distance satellite communication. TCP over Satellite (TCPSAT) protocol uses a single-path link, has a fixed congestion rate, and the distances between satellites terminals and ground stations are known [10]. This protocol is a three-way "handshaking" protocol and requires acknowledgements which are practically impossible due to the huge delay and long distances. Furthermore, it requires an end-to-end path between source and destination to exist for the duration of the communication session, which is improbable in a deep space communication. The protocol assumes a relatively small end-to-end loss and might fit, at most, in connecting LEO satellites with Earth stations. The User Datagram Protocol (UDP) sends packets without requiring acknowledgements, but it needs to update routing tables frequently to keep up with connectivity between satellites. Since this frequency depends on the distance and delay it introduces a high rate of data loss, which in turn might not be acceptable for communication, so UDP is also unlikely to be viable for IPSTN Network. Most recently proposed is a delay-tolerant network (DTN) protocol, which is a store-and-forward communication protocol. The DTN is designed to move data across rough networks, i.e., networks that have long delays and noisy connections. The central concept of DTN is "bundling" which is a mechanism for a space

Fig. 3. The flowchart of the proposed IP2X protocol

network's nodes - probes, relay satellites, and the like - to hold data if the next hop in the network is unavailable [11]. The bundle layer resides between the application and lower layers and in terms of the OSI model has a combination of transport, session and presentation layers functionality. The bundling realization is yet underway and might not turn out to be very useful. Nevertheless, certain aspects of DTN may prove to be crucial. To cope with the above problems and to increase network efficiency in a completely covered spherical space with triangulation of main routers, we propose a two path sending protocol. This protocol is called IP2X and it consists of sending the same message to two visible satellites. The flowchart of the protocol is given in Fig. 3. The origin satellite forwards its message to two visible satellites. Each of them then checks whether they have received this message before or not. If they have received it before they just discard it. If not they have to check if the message is intended for them. If it was intended for them, they have to start a timer and will wait for a set amount of time before processing the message recover step. The time they have to wait will be approximated to the longest time expected in the network. If it was not intended for them they have to forward the message to two other visible satellites. And this process is repeated until the message arrives at the destination satellite or time expires. If at the destination one or more copies of the message arrive, the satellite will recover the original one by comparing the copies and running the recovery codes. There is a chance that the message will never reach its desired destination and it will be lost and never recovered. In any case sending the message over different paths statistically adds delay especially when the longer path happens to be more effective. Our proposed IP2X, two path, protocol is envisioned as highly redundant in order to enable survivable communication and will specifically use multi channel TDM to

Fig. 4. The finite state machine model for the proposed IP2X protocol.

send coded signals in parallel, compensates for block redundancy (after first signal segment is accumulated). The level of parallelism, as bandwidth, is presumed not to be the main problem (just survivability of a signal) and can be further increased to cover for resending the message. The resending also benefits from the intermittent nature of most IP communication, by using idle intervals. The steps required to handle the connection for the proposed IP2X protocol can be represented in a finite state machine as shown in Figure 4. In each state certain events are legal. When a legal event happens, some action may be taken. If some other event happens, an error is reported. Each handling process for the connection starts in the IDLE state. It leaves that state when a message arrives at a satellite. It will go to the STEADY ESTABLISHED state (by msg_arrive signal) which handles and controls other events. When there are no more actions taken for the message it returns to IDLE state by a Listen signal. When a message arrives, the action to go to the RECEIVE state is carried out by the rec_req signal. At this state the message is checked for errors that might have happened during propagation through space. If the message is corrupted, it will be send to the CORRECT state via the msg_corrupt signal. This state will handle all necessary actions to correct the message, and if it succeeds, the signal msg_rebuild will take it to the DETECT state, otherwise the signal msg_destroy will return to the STEADY ESTABLISHED state. The DETECT state is responsible for detecting the previous handling of the same message. If the message was previously handled the msg_clear signal will take it back to the STEADY ESTABLISHED state. If it was not handled before, the msg_new signal will take it to the QUEUE state. The QUEUE state will handle all storing and queuing of messages that are ready for processing. At the same time it will activate a timer which will be handled by the WAIT state (by init_timer signal). The WAIT state is responsible for controlling all timers set for different messages. If the message was previously handled, the time_out signal will take it back to the STEADY ESTABLISHED state. It will also

set the timer to a value of double the time needed for the longest communication distance to the farthest satellite. The need for the timer is not evaluated in this paper. Its necessity in our model is justified because all messages should be kept for comparison with the arrival of new messages. This issue can be solved in two ways, one by having a sequence number for all messages, and the other by storing the whole message. The first option requires less memory but needs to keep an ordering for numbers which might become very large. The second needs more memory but can be helpful for other issues that might arise concerning messages (such as retransmitting, etc.). However, this is an open issue for future research. Each message will be stored and kept in the QUEUE state and will go to the PROCESS state via the msg_process signal. When the message has been processed it will return to the STEADY ESTABLISHED state via the rec_complete signal. The PROCESS state will be responsible for handling the message to this satellite if it is the destination for the message or responsible for handing the transmission to the TRANSMIT state. If the message needs to be transmitted further, then the msg_transmit signal will go to the TRANSMIT state which will handle the transmission of the message to other two visible satellites. When the transmission is completed the from_complete signal will return it to the STEADY ESTABLISHED state.

Simulation Results In this section we present the results from the delay models and the efficiency of the proposed IP2X protocol. Total Delay for Satellites Communication The simulation model for a satellite network communication can either involve an Earth station, one satellite, and another receiving station somewhere else, or it can involve an Earth station, many satellites, and a receiving station. These two models can be named: 1-1-1 or 1-M-1 and are shown in Figure 5(a) and 5(b) respectively.

Fig. 5. Simulated delay models.

Fig. 6. Total delay for 1-1-1 Model

Based on the delay model discussed in Section 2.2, we simulated the existing three layer (LEO, MEO and GEO) structure. The queuing delay is considered to be triangularly distributed with minimum value of 30 ms, most likely value of 60 ms, and maximum value of 90 ms. The transmission delay, normally distributed with mean 25 ms and standard deviation of 0.2 ms. The processing delay is considered to be normally distributed with mean 50 ms and standard deviation of 5 ms. And, the propagation delay is considered exponentially distributed with mean of 120 ms. These values are chosen based on the other studies [8], and are quantifiable. Thus they do have a minimum and a maximum and therefore can be summed using the total delay equation. Results in Fig. 6 show that the average delay for LEO layer is about 240 ms, it is about 425 ms for the MEO layer and about 700 ms for the GEO layer. As expected, the total delay increases as the signal is transmitted to upper layers of the model. It also increases with the increase in the number of satellites involved for transmitting the message. Figure 7 shows the contribution of each delay component for three layers. The queuing delay, transmission delay and processing delay have almost the same values for the three layers, but the propagation delay increases almost exponentially. This increase is proportional with the increase of the distance of communication and becomes more dominant in the total delay.

Fig. 7. Components of the total Delay.

Fig. 8. Efficiency vs. message loss for TCP protocol.

Efficiency of the IP2X Protocol We simulated both TCP and IP2X protocols using C++ programming language and studied their efficiency. TCP was run using an acknowledgement three way handshaking protocol, and code from literature [10] was adopted for our requirements. The efficiency is measured in terms of packet delivery for a given set of timeout intervals, message loss rate and error rate. The results are shown in Figure 8. In this figure S1 represent the case of changing the timeout interval and keeping the message loss at 60%, and error rate at 40%. S2 represent the performance by changing the message loss for a time out interval of 60% and an error rate of 40%. S3 represent the case of changing the error rate for a timeout interval of 60% and message loss of 40%. As depicted in this figure, the efficiency of the TCP protocol degrades and performs very poorly. Thus the TCP protocol cannot be used for satellite networks. We also simulated the proposed IP2X protocol and studied its performance. The reaching ratio for each satellite is normally distributed with the error rate at 60%. Two scenarios are considered, one in which each satellite can reach any other satellite with the same probability and the other in which each satellite has a calculated map and has a number of satellites in its vision (can reach). In our simulations we used a binomial distribution to calculate the number of satellites in the vision for each satellite. This can be adjusted in practice since satellites are moving in space and some of them might not be reachable during the window time a satellite wants to transmit a message. Other reasons might be due to long delays, power availability, etc. For each scenario results are calculated for 10 sets of 100 simulations each and are shown in Figure 9. The performance of the IP2X protocol is increased for the second scenario where satellites in vision are considered for forwarding messages, compared to the one where there is no such a rule. This happens because satellites in the vision are usually satellites close to the one in consideration and the propagation delays are smaller. Also the number of paths that a message can travel to reach the destination increases due to the fact that each satellite will have different ones in their vision, thus the possibility that a message can pass through the same satellite more than once is reduced.

Fig. 9. Simulated IP2X protocol.

Fig. 10. Efficiency performance of IP2X protocol vs. simple one-way protocol. Figure 10, shows the comparison between IP2X protocol versus the simple one-way protocol, where the message is transmitted to only one satellite in their vision. As shown, IP2X outperforms the simple one-way protocol. This happens because the simple one - way protocol has a limited number of paths to reach the destination. We also compared the performance in terms of delay and results show that by decreasing delay, or in other words for satellites closer to each other, the performance of IP2X is increased. Based on these results, the proposed IP2X protocol can be used for future satellite networking.

Conclucions In this paper we introduced a new network architecture for future deep space intra-satellite communication networks. It is called IPSTN and uses the BGEO layer to make possible the covering of a huge space that extends up to the asteroid belt. The structure and performance parameters were discussed in detail. A mathematical delay model and better fitting communication protocol were introduced. This protocol is based on the transmission of messages to two visible satellites. It uses point-to-point transmission to deliver messages end-to-end from source to destination Simulation results show that delay plays an important role in key features of the network, and the proposed protocol can increase the efficiency of the network. It is also shown that the existing TCP protocol does not fulfill the performance requirements due to large delays.

For future work, the authors' plans are to:

• elaborate details regarding the proposed IP2X protocol • explore redundancy coding schemas fitting the future communication within

our solar system, • explore implications regarding the gradual growth of the network, traffic

expansion, and assess other alternatives of interest to the future IPSTN.

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