Design and verification of a satellite-terrestrial integrated IP network model for the Global Energy Interconnection

1 Introduction

In the future,electric power will be used as a unified medium by implementing the interconnection of multiple energy networks.The interconnection of different energy networks based on power grids is an important basis for the construction of the Energy Internet [1,2].In order to build the Global Energy Interconnection (GEI),establishing a unified communication network model is crucial [3,4].The power system has been built in the form of a largescale communication network.However,the cost of full coverage through the terrestrial network is too expensive,and the terrestrial network cannot be extended to foreign countries.By contrast,satellite communication technology supporting the IP network used for the measurement and control of the Energy Internet has developed rapidly.This makes the construction of a satellite-terrestrial integrated communication network even more essential [5].

A satellite communication system that has the characteristics of high coverage,high transmission rate,and high dynamic adaptability plays an important role in information infrastructure.At present,the data transmission based on satellites mainly adopts the distribution mode based on the application layer,which cannot support the direct interconnection of the satellite segment and the terrestrial segment in the network layer.It will be difficult to meet the needs of this system in the future.Furthermore,with the rapid growth of the number and type of clients using the GEI,the data transmission rate will improvement continuously.The traditional applicationlevel data distribution mode will lead to the continuous increase of system complexity and maintenance costs,which will bottleneck the data distribution capacity and system efficiency [6].In order to solve these problems,the interconnection model for IP-based satellite and terrestrial integrated networks is directly applicable.

This paper proposes a satellite-terrestrial integrated network model oriented to the communication requirements of the GEI.For this system,a data transmission protocol stack based on IP was designed.In addition,the result of a use case of IP interconnection that depends on geostationary satellite communication is presented.

2 Background

2.1 Energy Internet

The Energy Internet is a new type of information and energy integration wide area network based on Internet methodology [7-9].It is a new energy ecosystem based on electricity with a high penetration of renewable energy,high synergy of multiple energy types,high synergy of energy value chains from supply to demand and from planning to operation,and high participation of stakeholders [10].It uses the power grid as its backbone and uses micro-grids,distributed energy,and other energy units as the local area network.Based on open and peer-to-peer integration architecture,it will implement the two-way transmission and dynamic balance of energy,so that the access ability for new energy sources will be maximized.Under the Energy Internet,information systems and physical systems will penetrate into every device,and each participant will be able to obtain the necessary information through appropriate sharing methods [11-13].

In the GEI environment,with its wide application of communication and control technologies,operators have a clear view on the real-time state of the physical layer of energy grid and are able to operate it from the dispatch centers.The advent of the GEI with extensive information interaction capabilities yields new wide-area communication needs due to a high dependency on cyber information.The key challenge is to efficiently exploit communication and information infrastructure while ensuring that costs are kept relatively low.In future studies,more factors should be taken into consideration,such as the depth of the coverage of the wireless communication and the level of communication congestion or delay [14,15].

2.2 Satellite-terrestrial integrated IP network

The composition of the satellite-terrestrial integrated IP communication system is shown in Fig.1.This system is composed of geostationary satellites (GEOs),medium earth orbit satellites (MEOs),low earth orbit satellites (LEOs),and a ground station.These satellites are connected through an Inter-Satellite Link (ISL),which is established in two or more layers of the orbital plane.In recent years,more and more non-geostationary earth orbit (NGEO ) satellite systems are being built.According to the rules of radio spectrum allocation,GEO and NGEO satellite systems share the same frequency bands,but GEO satellites have priority.Therefore,international telecommunications unionradio communications sector (ITU-R ) proposed several interference mitigation strategies to support the frequency sharing between NGEO and GEO FSS systems [16-18].Data centers are connected through ground IP networks to form the structure between the spacecraft and the ground.A gateway is installed in the ground station to implement communication protocol conversion and performance optimization [19,20].In the satellite-terrestrial integrated IP network,the business data can be routed and distributed directly based on the network layer,and does not need to be processed in the application layer.

The use of IPv6 is the distinctive element of the GEI with respect to machine-to-machine (M2M) communication.The huge increase in the address space of IPv6 is an important factor in the development of the GEI for the purpose of assigning an IP address to billions of future simultaneously connected objects.For example,the widely used DVB-RCS2 protocol that is a broadband satellite multimedia communication protocol provides the functions for IPv6.The generic stream encapsulation (GSE) is used to reduce the levels of encapsulation and improve efficiency [21].In the DVB-RCS2 standard,IPv6 packets received on the LAN interface of a return channel satellite terminal (RCST) are forwarded according to the RCST IPv6 routing table.Packets may also be redirected to an internal agent for processing prior to transmission.Packets for transmission over the air interface are forwarded to the quality of service (QoS) module for transmission to the satellite.

One of the challenges in evolving from current IP networking via satellite toward the next generation of satellite-based IP networking.One common solution is tunneling.Tunneling IPv6 in IPv4 is a technique used to encapsulate IPv6 packets into IPv4 packets.The tunnel endpoints take care of the encapsulation through a dual stack,and this process is transparent to the intermediate nodes.

Fig.1 The composition of satellite-terrestrial integrated IP communication networks

3 Protocol structure

Among many space communication protocol stacks,the most important are the Consultative Committee for Space Data Systems (CCSDS) protocol and the delay and disruption tolerant network (DTN) protocol.According to the latest development of space data transmission technologies,the TCP/IP protocol system in satelliteterrestrial integrated networks have been proposed as an extension of the Internet in space [6].The protocol stack is optimized by referring to CCSDS and DTN.A typical protocol stack is shown in Fig.2.

Fig.2 The protocol stack of satellite-terrestrial integrated IP communication networks

3.1 Application layer

At the application layer,the electric power communication protocols,such as IEC61850,as well as MQTT,HTTP,and other standard Internet of Things (IoT) communication protocols were supported.At the same time,the dedicated application layer protocols that are oriented to the communication requirements of the Energy Internet can be designed and implemented.

3.2 Transport layer

The performance of traditional TCP protocols in the satellite-terrestrial network will be seriously reduced due to many complicated technical factors,such as long time delays in the link,high symbol error rates (SER) in the process,and different bandwidths between upstream and downstream links in the GEO satellite-based network.Therefore,while using the standard TCP protocol,the transport layer adopts TCP segmentation technology to enhance the performance of the transport layer for applications requiring reliable transmission performance [22,23].The main idea of TCP segmentation technology is to divide TCP connection into two or more parts.The proxy device is be used to isolate the long delay and high SER components from the rest of the flow.The dedicated protocols such as SCPSTP are used to enhance transmission performance in the components experiencing long time delays and high SERs.The conversion between TCP and SCPS-TP protocols is implemented in proxy devices.

3.3 Network layer

IP protocol is used in the network layer.IP protocol data units (PDUs) can be encapsulated into the CCSDS package based on the Internet Protocol Extension (IPE) protocol.Then the encapsulated package can be transmitted by one or more space data link protocol (SDLP) frames [24,25],as shown in Fig.3.

For other protocols such as DVB-RCS2,similar encapsulation methods are provided.

Fig.3 IP over a CCSDS space link

3.4 Data link layer

Referring to the CCSDS protocol stack,the Ethernet data link layer protocols are used in the network of the terrestrial segment and the subnet in spacecraft,and the SDLP protocols are used in the satellite segment [26,27].Since different DDL protocols are used in the satellite and terrestrial segments,it is necessary to complete the conversion in DDL protocols before transmission through inter-satellite or satellite-ground links.

4 Experimental evaluation

4.1 The process flow of protocol stack

The process flow of satellite-terrestrial integrated IP network protocol stacks based on GEO satellites may be illustrated through an example that supports the interconnection between the monitoring system with the IP subnet and the data center.

TCP protocol is used for business messages on the transport layer of the monitoring system,which includes the Ethernet-based intranet.The destination address of the message is the IP address of the corresponding information processing equipment.Before sending a transmission through the GEO satellite link,the protocol of the transmission layer is converted by a gateway,and the message encapsulation can be implemented by IP with CCSDS [28].The gateway of the ground station makes the corresponding conversion between the data link layer and the transmission layer,and then transmits the IP message directly through the data communication network.Finally,the router forwards the messages to the data center according to the destination address of the IP message.The process flow of a protocol stack is shown in Fig.4.

4.2 Experimental setup

The application experiment was carried out at the satellite center of the China Mobile Communications Company at Liuhang,Shanghai.The composition of the IP interconnection test setup based on GEO satellites is shown in Fig.5.

The simulator of the state monitoring system included a data source,a gateway,modulation and mediation equipment,and a transmitting and receiving unit.The ground station,which was connected to the data center simulation system,included a transmitting and receiving unit,modem equipment,and a ground gateway.

The simulator of the state monitoring system was deployed in Chengdu,Sichuan.The ground station was deployed in Liuhang,Shanghai.The communication channel between the simulators of the state monitoring system and the ground station was based on the Asia 9 satellite,which uses the Ku band (14051-14055 MHZ) for microwave links because that band is less affected by ground disturbances.The frequency interference of NGEO satellites is not taken into account because only GEO communication systems were used in the experimental setup.The communication traffic included data,voice,and video.In the experiment,the protocol conversion between the link layer and the transport layer were tested.

Fig.4 The process flow of a protocol stack

Fig.5 The experimental setup

4.3 Experimental procedure

First,the band resource and background noise of the satellite link was tested by a spectrum analyzer,which recorded the connection status and performance of the satellite link.Then,the end-to-end communication quality performance test was carried out by a bit error ratio tester,spectrum analyzer,and other instruments.Hence,the performance test of the satellite-terrestrial integrated IP network in a realistic environment (abiding by the relevant management regulations of China Telecom and the State Grid) was completed.The experimental data such as delay,jitter,and SER were recorded,as shown in Fig.6.

The satellite communication network of China Telecom adopted the ViperSat system,which is capable of independent management and quality testing of the communication channel.Based on the configuration function of the gateway,the communication quality (which is determined by the error rate,delay,and packet loss) was tested by reducing the transmission power in order to simulate the channel error.

4.4 Results and analysis

In this paper,the test results,which included the protocol stack verification and application performance,were analyzed.

4.4.1 Verification of protocol stack

In order to verify the performance of single and multiple TCP connections between the monitoring system,GEO satellites,ground station,and data center in the real environment,we carried out an experimental test.We simulated the channel error and delay by reducing the transmission power.The results are summarized in Table 1.

Fig.6 The experimental procedure

The test results show that the RF bandwidth and the delay remained stable.In the experimental process,the bandwidth was 1,000 kHz,and the delay was 509 ms.Communication error codes and packet losses occurred only when signal-to-noise ratio (SNR) was less than 5.The adjustment ranges of the emission level ranged between -17 dBm and -30 dBm.

Table1 IP protocol interconnection test

RF bandwidth Emission level（dbm） SNR Delay Data package number Packet loss Packet loss rate 1 Transmitter 1,000 kHz -17 13.4 509 ms \ \ \receiver 1,000 kHz -17 13.2 509 ms \ \ \2 Transmitter 1,000 kHz -24 9.5 509 ms \ \ \receiver 1,000 kHz -24 9 509 ms \ \ \3 Transmitter 1,000 kHz -25 8.6 509 ms \ \ \receiver 1,000 kHz -25 8 509 ms \ \ \4 Transmitter 1,000 kHz -26.5 7.1 509 ms \ \ \receiver 1,000 kHz -26.5 6.7 509 ms \ \ \5 Transmitter 1,000 kHz -27.5 6.4 509 ms \ \ \receiver 1,000 kHz -27.5 5.8 509 ms \ \ \6 Transmitter 1,000 kHz -28.5 5.4 509 ms \ \ \receiver 1,000 kHz -28.5 5 509 ms \ \ \7 Transmitter 1,000 kHz -29.5 4.6 509 ms 263 5 1.90%receiver 1,000 kHz -29.5 4.2 509 ms 270 7 2.60%8 Transmitter 1,000 kHz -30 4.3 509 ms 138 19 13.70%receiver 1,000 kHz -30 4 509 ms 142 13 9.10%

4.4.2 Application performance test

The commonly used network functions such as voice,data,video,and multimedia information transmission were tested,and the results are shown in Table 2.The results show that the satellite-terrestrial integrated IP network can support common services,including multimedia conferences,voice calls,and data transmission.

Table2 Application performance test

Source Destination RF transmitter frequency (MHz) 14,052 14,054 RF receiver frequency (MHz) 12,304 12,302 Bandwidth Occupancy on Satellite (MHz) 2 2 Emission level (dbm) -16 -16 Frame mode 12 12 SNR (db) 9.3 9.4 CNR (db) 16 16 SER 0.00E+00 0.00E+00 Satellite-terrestrial communication delay (ms) 509 509 Terrestrial communication delay (ms) 34 34

5 Discussion

If the cost of processing the data in the communication equipment itself is discounted,the delay is determined by the transmission distance.Therefore,the transmission delay can be considered as twice the interval of transmitting signals to the ground at the speed of light.The orbital altitude of GEO satellites is 35,800 km,and thus the delay is at least 239 ms.

In a satellite-terrestrial integrated network,the number of times that network switching occurs is reduced.Network switching is performed only once in the communication over thousands of kilometers.For the integrated transmission of video,voice,and data,we used a 2M bandwidth for the test,for which the delay was 509 ms.The test results show that the time delay remained stable and was not affected by the earth’s curvature.The range of emission level was from -17 dBm to -30 dBm.Transmission errors and packet losses only occurred when the SNR was less than 5.Accordingly,the length of the optical cable on earth was about 2,500 km,and the transmission delay was 34 ms after network switching occurred multiple times.

The synchronous digital hierarchy (SDH) system is widely deployed in power systems to transmit control signals.In communication processes,the main delay is due to optical fiber transmission because the delay of SDH equipment is very short.The delay can be estimated as

Here,c is the speed of light in a vacuum,n is the refractivity of optical fibers,which is about 1.44,and L is the length of the fiber.Thus,the transmission delay of control signals in an SDH system is approximately 12.5 ms,which is much lower than the delay of satellite-terrestrial integrated IP communication networks.

The test results show that the satellite-terrestrial integrated network can be used as a business channel for global energy interconnection,but the communication delay is much longer than that of optical fiber communication under the same conditions.Therefore,considering factors such as reliability and economy,satellite communication is more suitable for remote areas in order to implement communication access and equipment monitoring over wide areas.The control signals of power systems should still be transmitted through optical fibers because the delay of satellite-terrestrial integrated networks is not satisfactory for the requirements of real-time communication.At the same time,because of the differences in the satellite-ground link,it is necessary to limit the forward/backward data rate (which depends on the specific business characteristics of the routing or switching nodes) in order to avoid a situation in which individual applications occupy a large amount of bandwidth and to ensure the normal operation of the satellite-terrestrial integrated network.

6 Conclusion

With the continuous development of technology,the establishment of a satellite-terrestrial integrated Internet has become an inevitable trend in communication systems.The communication networks in the Global Energy Interconnection are typically wide area networks.Therefore,the communication process will directly participate in the automation of operations and affect the dispatch and control of the system [29].We propose an integrated IP network protocol stack designed to meet the communication requirements of Global Energy Interconnection,the practicability of which was verified by experimental study.

Satellite systems and energy network are important infrastructures.Through theoretical research,technical testing,and engineering applications,traditional energy data transmission modes will be transformed into a satelliteterrestrial data interconnection mode,which will play an important role in promoting the construction of the next generation energy network.

However,this paper described only the static IP satellite-terrestrial interconnection test based on one GEO satellite.In practical applications,multiple GEO satellites or LEO satellite constellations would provide communication services.Accordingly,mobility management and delay tolerance will become less problematic in satellite-terrestrial integrated networks.We will conduct further research and experimental studies to address these problems.

Acknowledgements

This work is supported by the State Grid Science and Technology Project (No.5455HT160004).