The Evolution of Satellite Network Architectures: Topology, Connectivity, and Service Delivery

Rajesh Uppal November 23, 2024 Comm. & NW, Space Comments Off on The Evolution of Satellite Network Architectures: Topology, Connectivity, and Service Delivery 463 Views

In an era where connectivity is paramount, satellite networks play a critical role in bridging communication gaps across vast distances and challenging terrains. This article delves into the intricacies of satellite network architectures, exploring their topology, connectivity, and the types of link routing and switching. We will also discuss transparent and regenerative processing network layering models, protocols used for service delivery, Internet Protocol (IP) networks based on DVB-S and DVB-RCS, and enhancements to the Internet Transmission Control Protocol (TCP) for satellite networks. Finally, we will touch upon the implementation of IPv6 over satellite networks.

A satellite is an artificial or man-made object that revolves around Earth. Satellites are relay stations in space for the transmission of voice, video and data communications. They are ideally suited to meet the global communications requirements of military, government and commercial organizations because they provide economical, scalable and highly reliable transmission services that easily reach multiple sites over vast geographic areas.

Satellite communication offers a number of advantages over traditional terrestrial point-to-point networks. Satellite networks can cover wide geographic areas and can interconnect remote terrestrial networks (“islands”). In case of damaged terrestrial networks, satellite links provide an alternative. Satellites have a natural broadcast capability and thus facilitate multicast communication. Finally, satellite links can provide bandwidth on demand by using Demand Assignment Multiple Access (DAMA) techniques.

Satellite communications involve four steps: An uplink Earth station or other ground equipment transmits the desired signal to the satellite; The satellite amplifies the incoming signal and changes the frequency; The satellite transmits the signal back to Earth, and The ground equipment receives the signal.

A satellite contains multiple channels, called transponders, that provide bandwidth and power over designated radio frequencies. The transponder’s bandwidth and power dictate how much information can be transmitted through the transponder and how big the ground equipment must be to receive the signal. In addition, the satellite’s antennas direct the signal over a specific geographic area.

Commercial satellite communications services are grouped into three general categories:

Fixed Satellite Services (FSS), which use ground equipment at set locations to receive and transmit satellite signals. FSS satellites support the majority of our domestic and international services, from international internet connectivity to private business networks.
Mobile Satellite Services (MSS), which use a variety of transportable receiver and transmitter equipment to provide communication services for land mobile, maritime and aeronautical customers
Broadcast Satellite Services (BSS), which offer high transmission power for reception using very small ground equipment. BSS is best known for direct-to-consumer television and broadband applications such as DIRECTV.

Satellite Network Components

A satellite network (also called a satcom network) comprises a set of satellite terminals, one or more gateways and one NCC that is operated by one operator and uses a subset of the satellite resources (or capacity).

The ground segment is composed of a user segment and a control and management segment. In the user segment, one finds satellite terminals(ST) connected to the end-user customer premises equipment (CPE), directly or through a LAN and hub or gateway stations (GW)

— CPE are also called user terminals (UT) and they include equipment such as telephone sets, television sets and personal computers. User terminals are independent of network technology and can be used for terrestrial as well as satellite networks.

— Satellite terminals are earth stations connected to CPE, sending carriers to or receiving carriers from a satellite. They constitute the satellite access points of a network
— The gateway earth station (GW) provides internetworking functions between the satellite network and the Internet or a terrestrial network.

The control and management segment consists of:
— a mission and network management centre (MNMC) in charge of non-real-time, high-level management functions for all the satellite networks that are deployed in the coverage of a satellite.
— network management centres (NMC), also called interactive network management centres (INMC), for non-real-time management functions related to a single satellite network.
— network control centres (NCC) for real-time control of the connections and associated resources allocated to terminals that constitute one satellite network.

Performance Objectives for Satellite Networks

Telephone Performance

According to ITU-R Recommendation S.522, the performance objectives for satellite telephone services are defined in terms of the Bit Error Rate (BER). The BER must meet the following criteria:

One part in $10^{-6}$ : This level should be achieved for a 10-minute mean value for more than 20% of any month.
One part in $10^{-4}$ : This level should be achieved for a 1-minute mean value for more than 0.3% of any month.
One part in $10^{-3}$ : This level should be achieved for a 1-second mean value for more than 0.05% of any month.

Data Performance

ITU-R Recommendation S.614 specifies the performance objectives for satellite data transmission at 64 kbit/s, particularly for links operating below 15 GHz as part of an integrated services digital network (ISDN). The BER must not exceed:

$10^{-7}$ : For more than 10% of any month.
$10^{-6}$ : For more than 2% of any month.
$10^{-3}$ : For more than 0.03% of any month.

Forward Error Correction (FEC): Implement stronger FEC coding schemes to detect and correct errors more effectively. This can be achieved by using longer codewords or higher code rates, although there’s a trade-off with throughput.
Adaptive Coding and Modulation (ACM): Dynamically adjust the modulation scheme and coding rate based on real-time channel conditions. This ensures optimal performance by using more robust schemes during adverse conditions (e.g., rain fade) and maximizing data rate when conditions are good.
Interleaving: Rearrange data packets before transmission to mitigate the effect of burst errors. This spreads errors across multiple packets, making them easier to recover by error correction techniques.

Availability Objectives

Availability refers to the proportion of time during which a service meets its performance specifications. This can be impacted by equipment failure and propagation conditions. ITU-R Recommendation S.579 outlines the following availability objectives for telephony services:

Annual Availability: The service must not be unavailable for more than 0.2% of a year due to equipment breakdowns, equivalent to less than 18 hours of downtime per year.
Monthly Availability: The service must not be unavailable for more than 0.2% of any month due to propagation issues.

Satellite Constellation Design: Utilize constellations with redundancy built-in. This ensures uninterrupted service even if individual satellites experience failures. Techniques include having overlapping coverage areas and on-orbit spares that can be activated in case of emergencies.
Redundancy in Ground Infrastructure: Implement backup systems for ground stations and network equipment to minimize downtime due to equipment failures.
Proactive Maintenance: Regularly monitor satellite health and network performance to identify potential issues before they cause outages. Predictive maintenance can prevent failures and improve overall system uptime.

Delay

Delays in satellite communication systems accumulate due to several factors:

Terrestrial Network Delay: Any delays encountered within the terrestrial network components.
Propagation Delay: The time it takes for the signal to travel over the satellite link, influenced by the distance between the satellite and the Earth stations.
Baseband-Signal Processing Time: The time required for processing the signal at the baseband level.
Protocol-Induced Delay: Delays introduced by the various protocols used in data transmission and processing.

Low Earth Orbit (LEO) Satellites: Utilize constellations of satellites in LEO. These offer significantly lower propagation delays compared to traditional Geostationary Earth Orbit (GEO) satellites.
Protocol Optimization: Employ satellite-specific protocols designed to minimize protocol overhead and processing delays. This can involve techniques like header compression or selective acknowledgments.
Content Delivery Networks (CDNs): Utilize CDNs to cache frequently accessed content closer to end-users. This reduces the distance data needs to travel, minimizing delay for users.

In summary, the performance of satellite networks is meticulously defined to ensure reliable and efficient communication services. These performance objectives for telephone and data services, availability standards, and delay considerations are critical to maintaining high-quality satellite communication systems. The adherence to these standards ensures that satellite networks can meet the demanding requirements of modern telecommunication applications.

Satellite networks are characterized by their topology (meshed, star or multi-star), the types of link they support and the connectivity they offer between the earth stations.

Satellite Network Topology

Satellite network topology can be broadly classified into three categories:

Star Topology: In this setup, all ground terminals communicate through a central hub or gateway station. The hub station is connected to the satellite, which relays information to other ground terminals. This setup is prevalent in VSAT (Very Small Aperture Terminal) networks, ideal for centralized control and management.
Mesh Topology: Each terminal can communicate directly with every other terminal via satellite without the need for a central hub. This topology provides greater flexibility and redundancy, reducing latency and increasing the network’s robustness.
Hybrid Topology: Combining elements of both star and mesh topologies, hybrid networks offer a balanced approach, leveraging the advantages of both configurations to optimize performance and reliability.

Generally, the hub is a large earth station (antenna size from a few meters to more than 10 m) with higher EIRP and G/T than the other earth stations in the network. A star network topology places fewer constraints on the EIRP and G/T of the earth stations than a meshed network topology relying on a transparent satellite, due to the fact that the earth stations communicate with a large earth station (the hub). This architecture is popular among networks populated with small earth stations (antenna size of about 1 m) called very small aperture terminals (VSAT). The link from any earth station to the hub is called an inbound link or return link. The link from the hub to the other earth stations is called the outbound link or forward link.

Satellite networks come in two main flavors: Geostationary (GEO) and Low Earth Orbit (LEO). Each offers distinct advantages and caters to different needs.

GEO networks utilize a handful of powerful satellites positioned high above the equator, staying fixed relative to Earth. This translates to wider coverage areas and superior data bandwidth. However, the vast distance (around 36,000 km) introduces significant latency, making them less ideal for real-time applications like voice calls. Additionally, GEO satellite terminals work best with a clear view of the southern sky, limiting their effectiveness in some locations.

LEO networks, on the other hand, leverage constellations of smaller satellites orbiting much closer to Earth (800-1,400 km). This proximity translates to lower latency, making them perfect for mobile applications and voice calls. Additionally, LEO satellites are constantly on the move, minimizing signal blockage by obstacles. The trade-off lies in coverage area, as individual LEO satellites cover a smaller region compared to their GEO counterparts. This necessitates a larger constellation to achieve global coverage. However, LEO satellite terminals are generally smaller and more affordable, making them a good choice for mobile users on the go.

Connectivity and Types of Link Routing and Switching

Satellite networks employ different types of link routing and switching to manage data flow:

Transparent Transponder (Bent-Pipe): This method involves minimal processing of the signal by the satellite. The satellite transponder receives signals from the ground, amplifies them, and retransmits them to another ground station. This approach is cost-effective and simpler to implement.
Regenerative Transponder: In contrast to the bent-pipe approach, regenerative transponders perform significant processing, including demodulation, error correction, and remodulation of the signal. This results in improved signal quality and better performance, especially in high-noise environments.
On-Board Processing (OBP): Advanced satellites with OBP capabilities can switch and route data internally, reducing the need for ground-based processing and enabling more efficient use of bandwidth.

Multibeam Satellite Systems: Enhanced Connectivity on Orbit

Imagine a satellite network as a complex highway system. Traditional satellite connections operate like single-lane roads, limiting capacity and flexibility. Multibeam technology comes in, transforming this network into a multi-lane highway, enabling efficient and targeted communication.

Understanding Connectivity Levels:

There are two key levels of connectivity to consider in satellite networks:

Service Level Connectivity: This refers to the network topology used for services like internet access (star/multi-star) or VPNs (mesh).
- Internet Access Services: These are characterized by a star or multi-star topology with multipoint-to-point connectivity. In this setup, customer traffic is routed through a Point of Presence (POP), and the Customer Premises Equipment (CPE) connects to the nearest POP of the user’s Internet Service Provider (ISP).
- Virtual Private Network (VPN) Services: These services utilize a meshed topology with point-to-point connectivity, allowing multiple points to connect simultaneously (multipoint-to-multipoint connectivity). This configuration enables the interconnection of different Local Area Networks (LANs) of a company, forming a unified LAN.
Onboard Connectivity: This defines how the satellite’s resources (beams, channels) are interconnected to meet service needs. It depends on coverage type (global vs. multi-beam) and on-board processing capabilities.
- Resource Organization: Satellite onboard connectivity defines how the satellite network resources are switched onboard to meet service-level connectivity requirements. This organization depends on how the satellite resources (beams, channels, carriers, etc.) are structured on both uplinks and downlinks.
- Coverage Type: In cases of global coverage, any user within the coverage area can potentially connect to any other user. For multibeam coverage, interconnecting users within different beams requires onboard interconnection of beams and their allocated resources. This approach enables the reduction of earth station sizes and costs and allows for frequency reuse, increasing capacity without needing additional bandwidth.

The Power of Multibeam:

Multibeam coverage divides the satellite’s footprint into smaller, focused beams. This allows for:

Reduced Earth Station Size: Smaller, less expensive ground stations are possible due to more targeted communication.
Increased Capacity: Frequency reuse across beams enables higher capacity without needing more bandwidth.

Interconnecting Beams: Techniques and Trade-offs:

A satellite payload with multi-beam coverage must ensure interconnection across all network earth stations and coverage areas. Different techniques are employed depending on the on-board processing capabilities and the network layer involved:

Transponder Hopping: Utilized when there is no on-board processing. Beam switching through transponder hopping is practical with a low number of beams. However, with a large number of beams, this method becomes complex and heavy due to the increase in required transponders.
On-Board Switching: Employed when transparent and regenerative processing is available. This includes a programmable switching matrix with inputs and outputs corresponding to the number of beams. The matrix connects each uplink beam to each downlink beam via receivers and transmitters. This technique, known as Satellite Switched Time Division Multiple Access (SS-TDMA), is feasible only with digital transmission and TDMA access. Traffic is stored and transmitted in bursts when beams are interconnected.
1. Transparent Switching: A passive approach that connects beams through a matrix. It requires digital transmission and Time Division Multiple Access (TDMA) protocols.
2. Regenerative Switching: This actively processes and regenerates signals, offering more flexibility but requiring more processing power.
Beam Scanning: Each coverage area is cyclically illuminated by an antenna beam, controlled by a beam-forming network within the satellite’s antenna subsystem. At least two beams are required simultaneously—one for uplink and one for downlink. This approach reduces fixed simultaneous beams, thereby minimizing co-channel interference (CCI).

Choosing the Right Technique:

The optimal onboard connectivity technique depends on factors like:

Number of beams
Onboard processing capabilities
Desired network layer (physical vs. data link)

By leveraging multibeam technology and choosing the appropriate interconnection method, satellite networks can achieve greater efficiency, flexibility, and cost-effectiveness for a wide range of communication needs.

Network Layering Models and Protocols

Network protocols are essential for ensuring communication between devices. They establish rules and conventions for data exchange, which includes segmentation, reassembly, encapsulation, connection control, ordered delivery, flow control, error control, routing, and multiplexing. These protocols enable devices to understand each other and process the received information accurately. A protocol stack is a list of protocols (one per layer), and a network protocol architecture is a set of layers and protocols.

Layering Principle and Protocol Models

The layering principle is crucial for network protocols and reference models. The International Organization for Standardization (ISO) developed the seven-layer reference model in the 1980s. A significant trend in telecommunications is the shift towards IP network technologies, with satellite networks following suit. TCP/IP, the dominant protocol suite, comprises multiple layers.

Satellite networks utilize layering models and protocols to deliver various types of services to end users:

Link Layer: At this layer, protocols such as DVB-S (Digital Video Broadcasting – Satellite) and DVB-RCS (Digital Video Broadcasting – Return Channel via Satellite) are employed to manage the physical and data link connections. DVB-S provides the downstream link, while DVB-RCS handles the upstream link.

Network Layer: The Internet Protocol (IP) is commonly used for addressing and routing packets across the network. Based on a datagram approach, it provides best-effort service without guarantees of quality of service (QoS). IP routes packets from router to router using a destination IP address until they reach their destination. IP address management is handled by Internet authorities. IP networks based on DVB-S and DVB-RCS enable efficient data transmission over satellite links.

IP Network Layer:
Transport Layer Protocols:

Transport Layer:

The Transmission Control Protocol (TCP) is used to ensure reliable data delivery.Includes TCP and UDP, which manage end-to-end communication flows.

TCP: Ensures the correct delivery of data between client and server by detecting and retransmitting lost or erroneous data, providing reliable service despite potential underlying network unreliability.
UDP: Offers best-effort service without error recovery, suitable for real-time applications where retransmission delays are more problematic than packet loss.

However, standard TCP is not well-suited for the high latency and variable bandwidth of satellite networks. Enhancements such as TCP Spoofing, TCP Split, and Performance Enhancing Proxies (PEPs) are employed to mitigate these challenges.

Enhancements to TCP for Satellite Networks

The performance of TCP/IP over satellite links is affected by significant feedback delays compared to terrestrial links. TCP’s congestion control mechanism can drastically reduce data rates with packet loss, and the data rate increase is controlled by ACKs received from the source. Large feedback delays proportionally delay efficient use of satellite links. TCP enhancements, such as NewReno and SACK, have been proposed to prevent multiple reductions in data rates when few packets are lost. These enhancements, called End System Policies, improve end-to-end TCP protocol performance over satellite links.

To address the unique challenges of satellite communication, several enhancements have been developed for TCP:

TCP Spoofing: Intermediary devices intercept TCP connections and acknowledge packets on behalf of the receiving end, reducing the impact of latency on performance.
TCP Split: The end-to-end TCP connection is split into multiple segments, with each segment optimized for the specific network characteristics it traverses.
Performance Enhancing Proxies (PEPs): These proxies sit between the sender and receiver, optimizing the flow of data to improve throughput and reduce latency.

Application Layer: Various protocols and applications, including HTTP, FTP, SMTP, and DNS, operate at this layer to deliver diverse services such as web browsing, email, and VoIP. The use of standardized protocols ensures interoperability and seamless communication.

HTTP: For web browsing.
FTP: For file transfers.
SMTP: For email.
Telnet: For remote login.
DNS: For domain name services.

IPv6 Over Satellite Networks

As the world transitions to IPv6, satellite networks are also adapting to support this new protocol. IPv6 offers several advantages over IPv4, including a larger address space, improved security features, and more efficient routing. Implementing IPv6 over satellite networks involves:

Dual-Stack Implementation: Supporting both IPv4 and IPv6 on satellite network devices to ensure backward compatibility and a smooth transition.
Tunneling: Encapsulating IPv6 packets within IPv4 headers to facilitate transmission over existing IPv4 infrastructure.
Native IPv6 Support: Upgrading satellite network equipment to natively support IPv6, enabling more efficient and secure communication.

Latest Advancements and Future Trends in Satellite Networks

The satellite communications industry is undergoing rapid transformation driven by technological advancements and growing demand for global connectivity. Here’s a look at the latest breakthroughs and future trends shaping the future of satellite networks.

1. High-Throughput Satellites (HTS)

High-Throughput Satellites (HTS) represent a significant advancement, offering dramatically increased capacity compared to traditional satellites. Key features include:

Spot Beams: Utilizing multiple narrowly focused spot beams instead of broad beams to enhance capacity and reduce interference.
Frequency Reuse: Increased capacity through frequency reuse, where the same frequencies are used in different beams.
Applications: HTS are ideal for broadband internet services, in-flight connectivity, and maritime communications.

2. Non-Geostationary Orbit (NGSO) Constellations

Low Earth Orbit (LEO) and Medium Earth Orbit (MEO) satellite constellations are becoming more prominent due to their lower latency and potential for global coverage.

LEO Constellations: Companies like SpaceX (Starlink), OneWeb, and Amazon (Project Kuiper) are deploying large constellations of LEO satellites to provide high-speed internet access worldwide.
MEO Constellations: Systems like SES’s O3b provide lower latency than GEO satellites and higher capacity than LEO satellites, suitable for enterprise and government applications.

3. Advanced On-Board Processing and AI

Satellites with advanced on-board processing capabilities and artificial intelligence (AI) are enhancing network efficiency and service quality.

Adaptive Beamforming: AI-driven adaptive beamforming optimizes the coverage and capacity dynamically based on demand and user location.
Autonomous Operations: AI enables autonomous satellite operations, reducing the need for ground-based control and enabling quicker response to changing conditions.

4. Inter-Satellite Links (ISL)

Inter-Satellite Links (ISL) allow satellites within a constellation to communicate directly with each other, creating a mesh network in space.

Reduced Latency: ISLs reduce latency by minimizing the need for data to be relayed back to Earth before being forwarded to the destination.
Enhanced Reliability: They provide multiple paths for data to travel, increasing network reliability and resilience.

5. Integration with 5G Networks

The integration of satellite networks with 5G is a significant trend, promising seamless global connectivity.

Backhaul Solutions: Satellites provide backhaul for remote and rural 5G networks, extending coverage to areas lacking terrestrial infrastructure.
Direct-to-Device Communication: Future advancements may allow direct satellite-to-device communication, eliminating the need for ground stations or intermediary hardware.

6. Miniaturization and Cost Reduction

Technological advancements are driving the miniaturization of satellite components, reducing launch and manufacturing costs.

Cubesats and Smallsats: Small satellites, including cubesats, are being used for a variety of applications, from Earth observation to communications, at a fraction of the cost of traditional satellites.
Reusable Launch Vehicles: Companies like SpaceX with their Falcon 9 and Starship rockets are reducing the cost of accessing space, making satellite deployment more affordable.

7. Quantum Communication and Encryption

Quantum communication technologies are being explored to enhance the security and efficiency of satellite communications.

Quantum Key Distribution (QKD): Provides theoretically unbreakable encryption by using quantum mechanics principles to distribute encryption keys securely.
Entanglement-Based Communication: Research is ongoing into using quantum entanglement for instant and secure communication over long distances.

8. Environmental Sustainability

With the increasing number of satellites, the focus on sustainable practices and space debris management is growing.

Debris Mitigation Technologies: Developing technologies to actively remove space debris or deorbit defunct satellites.
Green Propulsion: Advancements in eco-friendly propulsion systems to reduce the environmental impact of satellite launches and operations.

Future Trends

1. Expansion of IoT via Satellites

The Internet of Things (IoT) will see significant growth via satellite networks, connecting billions of devices in remote areas.

Agricultural Monitoring: Satellites will provide connectivity for agricultural IoT devices, enhancing precision farming and resource management.
Logistics and Fleet Management: Improved tracking and communication for global logistics and transportation industries.

2. Space-Based Data Centers

Space-based data centers are a futuristic concept being explored to leverage the advantages of space, such as cooling efficiency and security.

Data Storage and Processing: Offloading intensive data processing to space-based data centers could revolutionize data management and reduce the burden on terrestrial infrastructure.

3. Enhanced Earth Observation and Remote Sensing

Advancements in sensor technology and data processing will enhance Earth observation capabilities.

Climate Monitoring: Improved sensors will provide more accurate data for climate monitoring and disaster management.
Urban Planning: High-resolution imaging for better urban planning and infrastructure development.

4. Multi-Orbit Network Integration

Future networks will likely integrate GEO, LEO, and MEO satellites, providing a seamless communication experience.

Dynamic Routing: Networks will dynamically route data through the most efficient orbit, balancing latency, bandwidth, and coverage.

The satellite communications industry is rapidly evolving, driven by technological innovations and increasing demand for ubiquitous connectivity. These advancements and future trends promise to enhance global communication infrastructure, bridging the digital divide and supporting the next generation of internet applications.

Conclusion

Satellite networks are a cornerstone of modern communication, providing connectivity where traditional terrestrial networks fall short. By leveraging advanced topologies, sophisticated routing and switching techniques, and enhanced protocols, these networks deliver robust and reliable services to users worldwide. As technology continues to evolve, satellite networks will increasingly integrate IPv6, ensuring they remain at the forefront of global connectivity solutions.