The Utilization Profiles of the CCSDS Unified Space Link Protocol (USLP)
Greg J. Kazz, Ed Greenberg · 2018 SpaceOps Conference · 2018
The purpose of this paper is to identify the utilization profiles for interfacing the Data Protocol Sublayer using the Unified Space Link Protocols (USLP) (reference 1) with the space link coding procedures as specified in the CCSDS Coding & Synchronization Blue Books (references 2 through 5), used in both telecommand and telemetry applications.This paper describes how the USLP Protocol utilizes the coding and synchronization sublayer to support: a. Direct to Earth (DTE) telemetry links for engineering and science data b.Direct to Earth (DTE) telemetry links for very high rate science data c.Direct from Earth (DFE) command, sequencing and flight software loads d.Space to Space Links (Proximity) utilized by orbiters for data exchange to/from surface bound assets The CCSDS has divided the functions of the Data Link Layer into two sublayers: the Data Link Protocol Sublayer (DLP-SL) and the Coding and Synchronization Sublayer (CS-SL).The Data Link Protocol Sublayer (DLP-SL) interfaces to the users, accepting the data that is to be transported, on the sending side of the link, and delivering that data on the receiving end.The Transfer Frame is the data unit that is transferred across the Data Link Protocol Sublayer and the Coding and Synchronization Sublayer boundary.The Coding and Synchronization Sublayer (CS-SL) provides the encoding, randomization, and frame synchronization functions that prepares the USLP Transfer Frame for transport across the space link.The CS-SL is divided into 2 processes: 1) The Frame Interface Processes (FIP) performs the interface functions required to prepare the data for delivery to the Coding/Decoding Process (CDP).This process includes prepending a Frame Start Marker to the provided frame, when management has designated that the frame is not to be aligned to the codeblock or when there is no block code used.2) The Coding/Decoding Process (CDP) performs the forward error correction processes that are used to optimize the performance of the link and minimize the error rate.The CDP creates the symbol stream that is delivered to the Physical Layer.The transfer of the USLP transfer frames across different types of space links is the focus of this paper.The Protocol Data Unit (PDU) that is passed in both directions between the Data Link Protocol Sublayer (DLP-SL) and Coding and Synchronization Sublayer (CS-SL) is the transfer frame.The USLP frame structure provides flexibility that can be constrained by the functions utilized within the CS-SL that prepare the transfer frame for transit.For example, the USLP transfer frame contains a length field that enables the frame to be of variable length but CS-SL under certain conditions may constrain the frame to be fixed in length.This paper describes 5 operational modes available for use by the Data Link Layer to provide data exchange across the USLP space link.These modes are different because different operational requirements apply to vastly different types of space links and thus the communications implementation requirements differ.The environmental issues include the power or energy available, the distance between the end points of the link, the complexity of the equipment available at those end points, the atmospheric conditions and 1 Group Supervisor/Principal Engineer, Project Software and End-to-End Information Group, MS 301-490. 2 Principal Engineer, Project Software and End-to-End Information Group, MS 301-490.