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SpaceFibre Networks

A SpaceFibre network uses similar packet formats, packet addressing and routing concepts to SpaceWire. The main difference is that SpaceFibre includes virtual channels.

An example SpaceFibre router is illustrated in Figure 1.

Figure 1 SpaceFibre Router

The SpaceFibre router comprises a number of SpaceFibre interfaces and a routing switch matrix. Each SpaceFibre interface has several virtual channels (VCs). The VC number for each virtual channel can be configured, except for VC0 which is a virtual channel used for configuration, control and monitoring of the SpaceFibre network. When a packet arrives on a SpaceFibre interface it is placed in the appropriate virtual channel, i.e. the one with the same VC number as it was transmitted on. The leading data character of the packet determines which port of the routing switch the packet is to be forwarded through using either path or logical addressing. The port that it is to be switched to must have a VC configured with the same number as the VC that the packet arrived on. The packet is then passed through the routing switch matrix and placed frame by frame in the VC of the output port. The packet is then transferred across the SpaceFibre link, competing with other VCs in that port for access to the link medium according to their precedence [3].

If a packet arrives and the output port that the packet is to be switched to, does not have a VC with the same number as that on which it arrived, the packet is split and an error recorded.

Virtual channels can be used to construct virtual networks, where a single VC number is used for connecting to all or several of the nodes attached to the network. This is illustrated in Figure 2 where VC6 (blue) is used to connect all the nodes on the network. Using VC6 the Control Processor can send commands to Instrument 1 or 2 or the Mass Memory unit, setting their operating mode or reading housekeeping information, etc. This virtual network acts like a SpaceWire network.

Figure 2 Simple SpaceFibre Network

Figure 3 Realistic SpaceFibre Network

Virtual channels can also be used to construct virtual point-to-point links from one node to another. VC2 and VC4, in Figure 2, are providing virtual point-to-point links. VC2 provides a virtual point-to-point link between Instrument 2 and the Mass Memory Unit and VC4 between Instrument 1 and the Mass Memory. These virtual channels can be each allocated the bandwidth they need to send their data to the Mass Memory Unit. Once this bandwidth is allocated other virtual channels or virtual networks will not interfere with their operation.

Figure 3 shows a more realistic on-board network using SpaceFibre which includes a SpaceWire to SpaceFibre Bridge. Two high data-rate instruments (Instruments 1 and 2) have SpaceFibre connections. Four less demanding instruments have SpaceWire connections to the SpaceWire to SpaceFibre Bridge. Each instrument has a virtual point-to-point connection to the Mass Memory Unit and there is a virtual point-to-point connection between the Mass Memory and the Downlink Telemetry Unit. The Control Processor has a virtual network for configuring and controlling all devices on the network.

Figure 3 is solving a complex communication task with many separate, isolated virtual channels providing point-to-point links, and a virtual network being used to control the entire system. Figure 4 shows this same network with the virtual channels removed, revealing the simplicity of implementation of a complex communication task when using SpaceFibre.

Figure 4 Simple System Architecture with SpaceFibre

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