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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 the 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 network architecture is composed of space segment, ground segment, control and management segment, and user segment. The space segment comprises satellites organized in the constellation and supports routing, adaptive access control, and spot-beam management. The ground segment consists of satellite gateways (SGWs) interconnected by optical backbone networks and satellite terminals (STs) that provide connections for end-user devices. The backbone network connects to external networks (e.g., Internet or corporations) through some point of presences (PoPs). The SGWs and STs are interconnected through the space segment.
The control and management segment is made up of network control centers (NCCs) and network management centers (NMCs). NCCs and NMCs provide real-time control and management functions for satellite networks. They perform the establishment, monitor and release of connections, admission control, resources allocation, the configuration of satellite network elements, and the management of security, fault and performance. The co-located SGW, NCC and NMC are commonly referred as satellite hub.
The user segment comprises all the end-user devices that are used by end users to consume satellite-based services, fixed or mobile. They access satellite networks directly or through terrestrial access points.
Compared with traditional satellite networks, next-generation satellite networks are characterized by onboard processing, affordable tracking antennas, and inter-satellite links. They prefer utilizing the satellites orbiting at low altitudes to reduce propagation delays, which enables real-time communications. Moreover, the transport services with quality of service (QoS) provision can be offered in the next-generation satellite networks by using the technologies of addressing, routing, etc. .
Current Satellite Network Challenges
Computer networks are typically built using different devices such as switches, routers, firewalls, and load balancers which communicate through various standard protocols. Network administrators are responsible for setting appropriate policies and managing all network devices in order to respond to a wide range of network events. Usually, these challenging tasks are performed manually with a rather limited number of tools available. Consequently, network management and configuration along with tuning network performance are quite tedious and potentially error-prone tasks.
However, existing satellite networks upgrade hardware/software inflexibly and depend on the closed and planed architecture. Although there are many advances in satellite communication technologies, satellite networks still suffer some limitations due to the traditional system design.
The communication technologies, networking protocols and satellite services are vendor-specific in the current satellite networks, so that the interaction between different satellite systems is considerably difficult.
Satellite networks are different from terrestrial networks since they inherently confront the challenges of high propagation delay, dynamical topology and limited resources. As a consequence, mechanisms designed for terrestrial networks are unsuitable for satellite networks of which settings are specified. In addition, the development of satellite communication technologies has not evolved at the same speed as terrestrial networks. These bring huge challenges to the integration of satellite networks with terrestrial networks
Since the manufacture and launch of satellites spend a lot of money and time, satellite networks usually use the static and scheduled configuration. As a result, the update and reconfiguration of satellite networks are inflexible. The maintenance cost of satellite networks is very high. The satellite resource provision for users is essential since it has a significant influence on user’s QoE. However, the scheduled and static satellite resource allocation makes it inflexible to optimize resource utilization and satisfy user demands.
It imposes great challenges for rapid introduction of new communication and networking technologies, puts a brake on really differentiated services provision for the highly various and increasing satellite network applications, brings large obstacles to the interoperability between satellite communication devices provided by different operators (or based on various communication technologies), and hinders the seamless integration of heterogeneous satellite and terrestrial networks
Software-Defined Networks (SDN)
SDN, a promising networking paradigm, receives increasing attention from industry and academia. The main idea behind SDN is to allow a logically centralized software-based controller (i.e. control plane) takes care of network intelligence and decision making, while data plane is responsible for traffic forwarding tasks.
Its main ideas are (i) the separation of control plane and data plane, (ii) the centralized control model of network states, and (iii) the deployment of novel network control and management functions based on network abstraction.
The means of implementing SDN are (i) to decouple control decisions from hardware infrastructure, (ii) to incorporate programmability into hardware infrastructure by using standardized interfaces (e.g., OpenFlow), and (iii) to exploit one physically or logically centralized network controller to determine network management policies and define operation for the whole network. SDN has efficient network resource utilization, simplified network management, cost reduction, and flexible deployment of novel services and applications.
NFV, a new approach to design, deploy and manage network services, whose essence is resource sharing, is designed to solve the ossification problem of existing network systems. The main ideas of NV are (i) the separation of virtual network and physical network, (ii) the coexistence of multiple heterogeneous virtual networks, and (iii) the independent deployment and management of virtual networks.
The means of implementing NV are (i) to abstract links, devices, and services from the physical network, (ii) to create logical virtual networks on the shared network infrastructure, and (iii) to allocate the network resources using hypervisors . With NV, customers can customize their private networks. NV improves infrastructure resource utilization and promotes innovations and diversified applications.
The major benefits of NFV are (i) to run and create network services with high flexibility by adaptively assembling and chaining software-based network functions without changing network architecture, (ii) to lower capital expenditures (CAPEX) and operational expenditures (OPEX) by using the centralized servers instead of installing the specialized hardware equipment for new services, and (iii) to facilitate controlling and managing the network globally and optimally by implementing the software-based network functions in centralized network servers.
To date, the basic idea of SDN has been integrated into many new networking paradigms and techniques, such as academic campus networks, data center networks, 5G systems, underwater communication systems, and NFV thus providing promising solutions to the specific issues in networking.
The newly emerging studies on SDN/NFV-enabled satellite networks mainly aim at the softwarization and virtualization in the ground segment.
SDR is a collection of hardware and software technologies. Its main idea is exploiting software to perform partial or total physical layer functions in radio, such as modulation/demodulation and signal processing. SDR is achieved by implementing the operating functions of radio through the modifiable software or programmable hardware. The SDR is flexible and reconfigurable. It creates the adaptability to new communication protocols and channel assignment policies without hardware changes.
To satisfy the requirements for more fine-grained switching, more flexible configuration, and management , Software Defined Networking (SDN) was introduced to satellite networks. The proposed architecture included Software-Defined Satellite (SDS), Software-defined satellite networks (SDSN), OpenSAN and SDN/NFV-based satellite networks, which aimed at
solving such problems as inflexible configuration and update, complicated management, and poor versatility of satellite networks. Since OpenFlow protocol is one of the most important protocols used widely between a controller and an SDN-enabled switch, its investigation is therefore
inevitable and significant.
Consequently, major network industry parties have set up the Open Networking Foundation (ONF) to promote SDN and to standardize the OF protocol. This has caused an intense adoption of SDN in almost every field of networking, from Data Center (DC) and cloud networks to Wide Area Network (WAN), wireless and recently 5G. Hence, both industry and academia are spending considerable amount of time and money to embrace SDN as a prevailing networking paradigm.
Accordingly, SDN market is expected to witness a substantial growth from USD 8.8 billion in 2018 to USD 28.9 billion by 2023, at a Compound Annual Growth Rate (CAGR) of 26.8% during this period
Challenges
SDN security is another issue which needs to be addressed. Although SDN market is flourishing and Tech Giants such as Google embraced it as the early adopters, security is still a major challenge for small to medium-size enterprises. While SDN is getting mature and being widely deployed, it definitely becomes an attractive target too. In addition, SDN introduces new attack vectors which did not exist in traditional networks.
Complexity is another concern. Although SDN is an absolutely competitive advantage for Big Techs, it can impose unnecessary burden on technical staffs in terms of operational complexities. These complexities possibly arise in implementation, deployment, and even administration of the networks. Top-tier providers, however, benefit from vast technical resources to tackle these difficulties while their smaller rivals simply cannot.
