SpaceFibre provides automatic fault detection, isolation and recovery. When a fault occurs on a SpaceFibre link, it is detected and the erroneous or missing information resent. SpaceFibre recovers from intermittent faults very rapidly, detecting faults, recovering and resending data faster than SpaceWire disconnects and reconnects a link. The retry mechanism does not depend on time-outs, naturally adapting to different cable delays.
Fault detection is provided by checking each 8B/10B symbol for disparity errors and invalid 8B/10B codes. SpaceFibre has selected the 8B/10B K-codes it uses to have enhanced Hamming distance from data-codes. This means that a single bit error occurring in a data-code cannot result in a valid K-code used by SpaceFibre. In addition each data frame, broadcast frame, Flow Control Token (FCT), Acknowledgement (ACK) and Negative Acknowledgement (NACK) are protected by a CRC. FCTs are used to manage the flow of data over a link and ACKs and NACKs are used to support the link error recovery.
Fault isolation is provided at various levels in SpaceFibre. AC coupling is used in the physical layer to prevent damage from faults that cause DC voltages exceeding the maximum permitted to appear on the transmitter outputs or receiver inputs. This feature also enables galvanic isolation to be implemented readily. At the Quality level SpaceFibre provides time containment, containing errors in the data frame in which they occur, and bandwidth containment, containing errors to the virtual channel in which they occur; an error in one VC does not affect data flowing in another VC. Babbling idiots are contained using the QoS mechanism described above.
Fault recovery is provided at the link level using a retry mechanism that resends data frames, broadcast frames and FCTs. The retry is very fast, uses a minimum amount of buffer memory, and adapts automatically to different link lengths. In addition to the retry mechanism the multi-lane functionality includes graceful degradation on lane failure. If a lane fails permanently, so that a retry or re-initialisation does not recover lane operation, a multi-lane system will continue using the remaining lanes available. This reduces the bandwidth available but does not stop the link operating. For critical operations an extra lane can be included and the graceful degradation will then provide automatic replacement of a faulty lane. The bit error rate (BER) of a lane is monitored and a lane reported as faulty if the (BER) is above a level which results in the effective link bandwidth being unusable. This feature allows lanes that can re-initialise successfully but which will not run for very long before having to re-initialise again, to be detected, isolated and replaced by a fully functional lane.
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Note: This information is taken from S. Parkes et al, “SpaceFibre: Multi-Gigabit/s Interconnect for Spacecraft On-board Data Handling”, IEEE Aerospace Conference, Big Sky, Montana, 2015.