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DTN and NGU

DTN and NGU. Scott Burleigh, NASA/JPL May 16, 2011 Berlin. Overview. DTN LTP insulates DTN from differences between TC and NGU. DTN LTP block aggregation solves the CFDP return traffic problem, but high volume uplink is still important.

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DTN and NGU

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  1. DTN and NGU Scott Burleigh, NASA/JPL May 16, 2011 Berlin

  2. Overview • DTN LTP insulates DTN from differences between TC and NGU. • DTN LTP block aggregation solves the CFDP return traffic problem, but high volume uplink is still important. • “Red” LTP instead of COP? Enables focus of uplink to be on coding and modulation. • NGU enables more powerful DTN-based applications. • Summary: NGU and DTN are complementary. Spring 2011 CCSDS SLS-NGU Meeting - Berlin Germany

  3. LTP Abstraction Under BP User application, e.g., data manager AMS messaging CFDP (unacknowledged mode) Remote AMS bridging UT adapter BP DTN routing Convergence layer adapters DTN LTP TCP, BRS, UDP, DGR encapsulation packets IP Internet routing AOS Prox-1 802.11 Ethernet TM TC NGU R/F, optical wire Spring 2011 CCSDS SLS-NGU Meeting - Berlin Germany

  4. CFDP Return Traffic Problem (1) • When acknowledged CFDP downlink runs over a TM/TC UT layer, the volume of CFDP acknowledgment traffic can be so high that it consumes all available uplink bandwidth. • Especially troublesome for small files: CFDP acknowledgment is at file granularity. • This was one of the motivations for NGU. • DTN solution: • Use unacknowledged CFDP over a DTN UT layer. • DTN LTP does selective negative acknowledgment just like acknowledged CFDP, but acknowledgment is at block granularity rather than at file granularity. Spring 2011 CCSDS SLS-NGU Meeting - Berlin Germany

  5. CFDP Return Traffic Problem (2) • CFDP files are fragmented into PDUs which are sent in bundles, but LTP can aggregate any number of bundles into a single block. Since block size is configurable, acknowledgment rate is configurable and uplink utilization is limited. • But increasing uplink bandwidth is still important: • Increasing block size to reduce the ack rate increases latency in closing LTP transmission sessions, which increases storage resource consumption at the sender and data delivery latency at the receiver. • Also increases likelihood of an error in any given block, which increases the number of sessions that can’t close in a single round trip, which further increases session closure latency. • Higher uplink data rates will enable blocks to be smaller, improving data delivery performance and reducing the storage load. Spring 2011 CCSDS SLS-NGU Meeting - Berlin Germany

  6. Omit COP from NGU? • LTP provides retransmission of lost content, using selective negative acknowledgment, for arbitrary data – not just for files, as CFDP does. • So use it for reliable transmission of spacecraft commands? • Some extra latency: commands would travel in LTP blocks, possibly aggregated, and the contents of an LTP block aren’t delivered until the entire block has been received. • Some extra overhead: • One LTP segment header plus one encapsulation packet header per command. • At least one report segment (and one report ack segment in the opposite direction) per LTP block. Spring 2011 CCSDS SLS-NGU Meeting - Berlin Germany

  7. Application Support • DTN is engineered to support any number of applications running on remote devices. • DTN includes open API definitions for development of new applications, e.g., FPGA upgrades, software loads, applications to support crewed vehicle operations, etc. • High data volume of these applications will require NGU. Spring 2011 CCSDS SLS-NGU Meeting - Berlin Germany

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