Design and simulation of Satellites Communication Protocol

Wu Yangpu，Feng Wenquan

（Electronic Information Engineering Institute，Beijing University of Aeronautics Astronautics，Beijing，China）

Abstract： Due to long-link delay，high bit-error-rate，asymmetric bandwidth and time-varying topology，terrestrial network protocols cannot be applied in satellite network directly.In this paper，a protocol based SCPS protocols is proposed and a satellite network adaptable RTO（retransmission time-out）scheme is designed.By using off-line routing algorithm，dynamic routing is achieved.Simulaiton results show the high performance of satellite network data transmission.

Key words： satellite communication，dynamic routing，reliable transmission

1 Introduction

SCPS（Space Communication Protocol Specification）developed under the Consultative Committee for Space Data System（CCSDS）standards process，is a standardized protocol to solve space communication problems.SCPS focuses on the reliable transfer of information between space mission end systems and it has been utilized in the Department of Defense（DoD）community successfully for years.

In the paper a communication protocol suiting satellite network is designed according to SCPS-NP and SCPS-TP.This protocol has a high performance in high-bit-rate dynamic network and the performance improvements areValidated through simulation.The rest of the paper is consist of 4 parts----protocol design，simulation background，simulation results and conclusion.

2 Protocol Design

The protocol designed in the paper could support transport layer functions and network layer functions.

2.1 Transport Layer Design

Transport layer can provide reliable end to end data transmission.In this layer the main problem is to handle with packet loss.This design focused on dealing with two cases of packet loss：

（1）Packet loss caused by high error rate

（2）Packet loss caused by disconnection or dynamic changed in the network topology

The most difference between SPCS-TP and TCP protocol is the mechanism used to deal with packet loss.Network congestion is the default reason for packet loss in TCP protocol，when packet loss happens TCP protocol will start congestion control algorithms.But in space environment packet loss is mostly caused by bit errors rather than network congestion.Therefore the mechanism used in TCP protocol would reduce data transmission efficiency and lower end-to-end data throughput in space environment.In SCPS-TP a mechanism named SNACK is introduced to deal with this kind of packet loss.The principle of SNACK scheme is to remain data transmission rate and transmit the loss packet to the receiver again，so that the end-to-end data throughput would not be affected in the bad way.

The disconnection could be a short-term disruption cause by device switches or a long-term link failure due to satellite antenna failure.Disconnection and network topology dynamical changes will cause some momentary interruption of communication links，in this paper a satellite network adaptable RTO（retransmission time out）mechanism is designed for this situation.

In satellite network adaptable RTO mechanism，the sender satellites will stop sending data for a short time before the network topology changes.This can prevent packet loss due to dynamic changes in the network topology.Meanwhile，if the time of data retransmission using the same path exceeds a certain limit，cancel communication connection in the path and build a new communication link.The newly built connection could avoid the use of links used before，so that long-term failure link would be avoided effectively.Taking into account the short-term disconnection，waiting a certain time interval before data retransmission could wait for the connection re-established and avoid sending a large amount of data into the network in a short time.

2.2 Network Layer Design

SCPS-NP specifies a protocol to meet the needs of current and future space missions.It supports communication environments with static，highly managed communication through fully connectionless communication with dynamic routing.In addition，SCPS-NP provides alternative routing options and flexible routing table maintenance program and SCPS-NP has a good adaptability on dynamic topology network.

Many LEO/MEO offline routing algorithms，based on inter-satellite links，have been raised by domestic and foreign scholars who made use of the regularity of satellite movement .In the simulation，dynamic routing is achieved by using traditional off-line routing algorithm.The following characteristics of satellite movement are used：

（1）predictability and

（2）periodicity

（3）the number of satellites are fixed

Divide a satellite running period into a number of small time topology time slice，so the dynamic network topology could be considered as plenty of fixed topology.Use Dijkstra algorithm to calculate the routing tables corresponding to each short time topology.These routing tables are stored in each satellite，a new routing table is accessed when the topology changes.

However，the offline routing algorithm has its limitations which cannot be overcome.Because offline routing cannot adjust to current network stateVery well，when an unpredictable disconnection occurs a large number of network connection could interrupt and network performance could significantly drop.To deal with this situation and guarantee the data transfer performance，alternate routing tables are used in the satellite.

Besides，According to importance，data transmitted in satellite network can be divided into different priority.In network layer，in accordance with predefined priority，data transmission order is arranged.High-priority data always take precedence over lower priority data to be processed.

3 Simulation and Results

Simulation is completed inVC 6.0 environment，and data in table 1 were transmitted in the test.In the beginning of the simulation，choose a satellite to inject integrity information and EOP parameters to the whole constellation.After the completion of injection all the satellites would entry independent navigation mode and every single satellite would generate a set of navigation data per 60 seconds.During injecting and independent navigating real-time telemetry information generated by the satellites would be sent to ground station through 3 satellites.

Table 1 network data

In this study，delay is an important protocol performance criterion，for theValue can reflect whether the transmission strategies used in the protocol are effectively applied to satellite networks.The simulation results are shown in Figure 2 to Figure 5.

These experimental results showed that in the simulated space communication environment our protocol can still meetVarious data’s transmission needs and complete space communication tasks successfully.

4 Conclusion

The communication protocol presented in the paper can work well in the satellite network and the simulation results showed that the time delay requirement is satisfied and the protocol can fulfill space communication task successfully.It is meaningful for the design of space communication protocols.

作者简介

吴阳璞 女，1988年，四川，硕士，主要研究空间通信协议，E-mail：wuyangpu@hotmail.com