E bit transport higher level data delivery services

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Unformatted text preview: ng network, are completely dedicated to that mission. The lack of effective standardization among various missions forces the ‘multi-mission’ tracking network to implement the lowest level of telemetry transport service, i.e., bit transport. Higher level data delivery services, oriented more toward computer-to-computer transfers and typical of modern day commercial and military networks, had to be custom designed and implemented on a mission-to-mission basis. The intent of the CCSDS Telemetry System is not only to ease the transition toward greater automation within individual space agencies, but also to ensure harmony among the agencies, thereby resulting in greater cross-support opportunities and services. The CCSDS Telemetry System is broken down into two major conceptual categories: a ‘TM Space Data Link Protocol’ concept (references [2] and [7]) and a ‘TM Synchronization and Channel Coding’ concept (reference [3]). a) TM Space Data Link Protocol is a concept which facilitates the transfer of spaceacquired data from source to user in a standardized and highly automated manner. TM Space Data Link Protocol provides a mechanism for implementing common data structures and protocols which can enhance the development and operation of space mission systems. TM Space Data Link Protocol addresses the following two processes: 1) The end-to-end transport of space mission data sets from source application processes located in space to distributed user application processes located in space or on Earth. 2) The intermediate transfer of these data sets through space data networks; more specifically, those elements which contain spacecraft, radio links, tracking stations and mission control centers as some of their components. CCSDS 130.1-G-1 Page 2-1 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE The TM Space Data Link Protocol Recommended Standard contained in references [2] and [7] is primarily concerned with describing the telemetry formats which are generated by spacecraft in order to execute their roles in the above processes. b) TM Synchronization and Channel Coding (reference [3]) is a method by which data can be sent from a source to a destination by processing it in such a way that distinct messages are created which are easily distinguishable from one another. This allows reconstruction of the data with low error probability, thus improving the performance of the channel. The TM Synchronization and Channel Coding Recommended Standard contained in reference [3] describes several space TM Synchronization and Channel Coding schemes. The characteristics of the codes are specified only to the extent necessary to ensure interoperability and cross-support. Together, TM Space Data Link Protocol and TM Synchronization and Channel Coding services provide to the user reliable and transparent delivery of telemetry information. Figure 2-1 illustrates the CCSDS Telemetry System in terms of a layered service model. It should be noted that the CCSDS TM Space Data Link Protocol and TM Synchronization and Channel Coding Recommended Standards only address the five lower layers of this model. LAYER APPLICATION PROCESS LAYER PHYSICAL MEASUREMENTS SYSTEM MGMT LAYER SERVICE PROVIDED BY LAYER PROVIDES USERS A METHOD TO INVESTIGATE PHYSICAL PHENOMENA BY USING THEIR INSTRUMENTS IN SPACE FOR DATA COLLECTION AND THEIR APPLICATION PROCESSES FOR ANALYSIS. PROVIDES TRANSLATION OF PHYSICAL MEASUREMENTS INTO SETS OF APPLICATION DATA UNITS. TLM APPLICATION DATA PACKETIZATION LAYER PROVIDES END-TO-END DELIVERY OF APPLICATION DATA UNITS. PACKET SEGMENTATION LAYER (OPTIONAL) PREPARES LONGER PACKETIZED DATA UNITS FOR MULTIPLEXING AND TRANSFER THROUGH A SPACE DATA CHANNEL. SEGMENT TRANSFER LAYER PROVIDES RELIABLE TRANSFER OF PACKETS AND SEGMENTS IN A COMMON STRUCTURE FOR THEIR TRANSPORT THROUGH THE SPACECRAFT-TO-GROUND COMMUNICATION LINK. TRANSFER FRAME CODING LAYER PROTECTS TRANSFER FRAMES AGAINST ERRORS INDUCED DURING TRANSMISSION THROUGH THE NOISY PHYSICAL COMMUNICATIONS CHANNEL. TLM BIT STREAM PHYSICAL LAYER PROVIDES THE PHYSICAL CONNECTION, VIA RADIO FREQUENCY SIGNALS, BETWEEN A TRANSMITTING SPACECRAFT AND THE RECEIVING STATION. PHYSICAL WAVEFORM Figure 2-1: Layered Telemetry Service Model CCSDS 130.1-G-1 Page 2-2 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE 2.2 2.2.1 TELEMETRY SYSTEM CONCEPT GENERAL The system design technique known as layering was found to be a very useful tool for transforming the Telemetry System concept into sets of operational and formatting procedures. The layering approach is patterned after the International Organization for Standardization’s Open Systems Interconnection layered network model (reference [4]), which is a seven layer architecture that groups functions logically and provides conventions for connecting functions at each layer. Layering allows a complex procedure such as the telemetering of spacecraft data to the users to be decomposed into sets of peer functions residing in common architectural strata. Within each layer, the functions ex...
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