Viewpoint: HSR, PRP and TSN – at the cutting edge of embedded military comms

Technology becomes more important to military and defence operations with each passing year. We need only look to the UK Government’s recent Defence Command paper and budget review to reaffirm this, with the country’s priority seemingly being to bolster technological capacity on the battlefield. However, military technology is about more than just swarms of drones or new Boxer armoured vehicles; it’s about the fast and effective relaying of information on the frontline.

The military and defence sector has a long-standing history of being at the forefront of technological advancement. Technologies that are commonplace today, such as GPS, drones and even microwaves, find their origins in the defence sector. It comes as no surprise, then, that at a time of continuous technological development, the UK Government has set its sights on modernising its armed forces and adopting newer technologies.

However, the real area of interest in the UK’s new Defence and Security Industrial Strategy is not the technology it is initially investing in, but rather the focus on research and development. In particular, the UK Strategic Command is expected to invest £1.5 billion in the next ten years to “build and sustain a ‘digital backbone’ to share and exploit vast amounts of data, through the cloud and secure networks”, according to Defence Secretary Ben Wallace MP.

Embedded military systems are the foundation of this data-driven approach to defence. These systems are vital in collecting, analysing and communicating data from the battlefield or outposts. This means they must meet rigorous regulatory standards, be physically ruggedised to endure harsh environmental conditions and support rapid, reliable and secure data communication.

It’s the latter requirement that poses the biggest challenge for military embedded computing, while simultaneously standing to benefit the most from increased investment in new technologies. Many of the newer defence applications that have arisen from advanced technologies, such as increased video processing to improve situational awareness, require a distributed military computing approach that combines networking with hardware-accelerated computing via field-programmable gate array (FPGA) devices.

Distributed military computing requires provide low-latency, high-availability, interoperability between networked devices and high-bandwidth to be effective. The challenge has traditionally arisen from the wide use of Ethernet for networking. Although cheap and ubiquitous, it is generally non-deterministic in nature — beneficial for reliability, but less than ideal for mission critical systems that require real-time data transmission.

Next generation Ethernet

Standard Ethernet functions by sending data packets in a way that is largely dependent on network loads, which is not suitable for networked devices that need high degrees of synchronisation. New standards are arising, such as IEC 62439-3 and TSN Time-Sensitive Networking- (a set of standards under development by the Time-Sensitive Networking task group of the IEEE 802.1 working group), which are helping to meet mission critical network demands. These standards, alongside development in this technological space, are driving innovation in military networks.

One such company at the forefront of this is System-on-Chip engineering (SoC-e), an expert in FPGA-based Ethernet solutions that Recab UK works closely alongside to offer high availability systems for defence Ethernet networks. SoC-e is also an active participant in many leading industry and academic groups that are developing and governing the standards for deterministic Ethernet. These groups have been active in the defining and development of new Ethernet-based protocols such as High-availability Seamless Redundancy (HSR), Parallel Redundancy Protocol (PRP) and Time-Sensitive Networking (TSN).

HSR

HSR is one of two protocols standardised in IEC 62439-3, alongside PRP. The key difference between these two protocols is that HSR focusses on providing seamless redundancy in time-constrained networks, while PRP is capable of providing seamless redundancy via two independent standard Ethernet networks. PRP introduces redundancy to the nodes.

HSR nodes can be arranged in a ring topology (only ring topology), with HSR providing redundancy by sending packets in both directions. A simple HSR network consists of doubly attached bridging nodes, each having two Ethernet ports. Both ports send the same data frame so that, in a fault-free scenario, nodes receive two identical frames. This means that even if connection is broken, there is no frame loss and communication continues among all network nodes.

However, the requirement of a ring topology can be limiting in the implementation of HSR in military edge computing. Similarly, HSR requires that all nodes either process or pass along all data packets, which can increase processing requirements and add to network latency.

PRP

The PRP protocol works by creating parallel paths in redundant networks for duplicate data packets to pass through. Dual attached nodes (DANs) are connected to two independent and standard local Ethernet networks (LAN A and LAN B), with identical frames sent over both networks.

The PRP operative ensures the receipt of all the information, even if one network fails.

Non-PRP nodes can ignore the frames, reducing processing requirements, and the PRP protocol is compatible with standard Ethernet hardware.

TSN

The technology that seems to be the most promising from a military perspective is TSN, which sits at the data link layer (layer two) in the open systems interconnection (OSI) model. TSN is defined by the IEEE 802 group of standards and provides determinism to enforce advanced quality-of-service policies, while also supporting the merging of real-time, reserved and best-effort traffic in the same network. TSN is also unique in that its streams are delivered with guaranteed bandwidth and deterministic latency.

There are several IEEE standards under development that outline specific features of TSN, including synchronisation (802.1AS), scheduled traffic (802.1Qbv), reserved traffic (802.1Qav), seamless redundancy (802.1CB), and frame pre-emption (802.1Qbu and 802.3br). Although not all standards are required to support TSN, networks and switches that can meet these standards offer a clear advantage in military environments. The selection of sub-standards above are among some of the most relevant for military networking.

The synchronisation standard (802.1AS) is, of course, imperative and arguably the most important in a military and defence context. Based on the IEEE 1588-2008 standard precise time protocol (PTP), the synchronisation in TSN allows devices in the network to share the same time reference within a nanosecond time range. This means its possible for Ethernet networks to provide a level of synchronisation that is comparable to GPS.

The high availability on TSN can be achieved by adding frame replication and elimination as defined in IEEE 802.1CB. In a similar way as defined for HSR, the frames include a sequence number and are replicated, with each copy sent through a different path in the network.

To achieve the lowest possible latency in engineered networks, the Time Aware Shaper functionality is introduced in the IEEE 802.1Qbv traffic scheduling standard. This works with applications where time-critical data is sent at regular periodic intervals and it is based on adding time gates on each queue on a port. This is complemented by the pre-emption supported by IEEE 802.1Qbu and 802.3br. A higher priority frame can interrupt the lower priority frame transmission, reducing the latency of time-sensitive streams.

Deterministic military Ethernet

Realising these new Ethernet technologies in defence applications requires the use of a system that provides the flexibility to support the right protocol for a given application. To this end, Recab has partnered with SoC-e, whose Relyum range offers an ideal all-in-one solution, to support military and defence original equipment manufacturers (OEMs) in the UK and Northern Europe. Products in this range are tested and certified to military standards including MIL-STD-810G & MIL-STD-461G.

Relyum by SoC-e is a range of military commercial off-the-shelf (COTS) managed 1/10G ethernet switch, router and edge computing equipment. The RELY-MIL-SWITCH-ROUTER platform supports up to 20x 1G copper and up to 6x 1/10G fibreoptic ports, with support for different media types and its distribution in the MIL-DTL-38999 connectors allowing for complete and cost-effective network infrastructures.

The product specifically boasts a Xilinx Ultrascale+ MPSoC device that includes six ARM CPUs, one GPU and a FPGA in the same integrated circuit. The switching and routing functions are accelerated by hardware in the FPGA section, which gives it the flexibility to meet various military requirements. This flexibility allows the Relyum to support HSR/PRP or TSN protocols with the same hardware. Fibreoptic rings combining these protocols are feasible thanks to the inter-switch coordination mechanism developed by SoC-e. Additionally, Relyum equipment support a wide set of security functionalities, in order achieve the strictest requirements in those kinds of applications.

The flexibility makes it an ideal option for numerous military applications, whether on land, in air or at sea. For example, military land vehicles benefit from zero-delay recovery time, deterministic Ethernet communication, and can also take advantage of the equipment to embed hardware-software microservices oriented to sensor data pre-processing. The RELY-MIL-SWITCH-ROUTER is also Generic Vehicle Architecture (GVA) compliant, making it an ideal fit for defence vehicles.

It’s innovations such as these that are driving a data-driven approach to military and defence, by connecting frontline devices and supporting the reliable, secure transmission of mission-critical data. Although overshadowed somewhat by armoured vehicles and drones in the UK Defence and Security Industrial Strategy, embedded military computing and networking are the true digital backbone of modern defence operations. Continued development and investment in this area will help the armed forces to continue crossing new technological frontiers in the years and decades to come.

Andy Conway is sales manager at military embedded system specialist Recab UK

Images: System-on-Chip engineering (SoC-e)