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SIMSCRIPT II.5 SOLUTIONS
Space Station Information System - SSIS
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National Aeronautics and Space Administration and The Mitre Corporation
As part of the Space Station Freedom Program (SSFP), NASA is developing the Space Station Information System (SSIS) to support the information processing and communications needs of the Space Station.
One aspect of the MITRE technical support and the MITRE Sponsored Research activity is the development of a discrete event simulation model of the SSIS baseline architecture. The portions of the SSIS included in the model are the Space Station Manned Base, the SSIS space-to-ground link, and the Ground Distribution System. MITRE has designed the SSIS Protocol Performance Model as a tool to be used in evaluating the ability of the SSIS to meet performance requirements.
The SSIS Protocol Performance Model is designed to model the basic functionality of the SSIS and to support the investigation of SSIS performance characteristics. It is an end-to-end model of real time data flow between the onboard application processes and the ground service access points on both the forward and return links. The data flow is modeled at the packet and data unit level to permit the collection of statistics on a packet and data unit basis. The architecture of the SSIS Protocol Performance Model is described in terms of the data structures representing packets and data units in the model, the processes and routines which generate the simulation events, the data input requirements for the model, and the performance measures collected from simulation runs of the model.
The model uses a combination of SIMSCRIPT II.5 processes and routines to represent the SSIS baseline architecture, to simulate the movement of packets and data units through the SSIS, and to collect performance measures during a simulation. The model contains a number of input parameters that can be set to reconfigure the environment for a specific simulation run. The parameters include:
- Packet and data unit attributes such as length and GOS
- Rates for space and ground segment packet data generation and for
bit stream data generation
- Delays for packet and data unit processing at each SSIS component
- Probabilities that data units are in error due to noise in the space
channel
- CCSDS, ISO 8473, and data unit protocol header and trailer lengths
- Parameters to set the length of the simulation run, define reporting
time intervals, and control the scaling of presentation graphics
Measures of performance are collected during a simulation to characterize SSIS performance and to support comparison of alternative proposed implementations and operating policies. These measures include queue length, component utilizations, delay and throughput, and latency response time.
The focus of the simulation analysis was the latency period or mean response time for ground-based telescience. The analysis was performed in three sets of simulation experiments designed to study the sensitivity of the model's outputs to SSIS component processing delays, priority processing, and packet lengths.
- SSIS component processing delay experiments were performed to
analyze the effects of SSIS components processing delays on model
performance measures. The results indicate that the size of the
queue at each SSIS component is directly related to the length or
amount of processing delay at the component, the traffic loading
assignments, and the SSIS data transmission rates on the forward
and return links.
- Priority processing experiments were performed to study how latency
is affected by priority processing of data units at the Virtual
Channelizers. Two cases were identified for consideration in this set
of experiments: steady state and transient priority processing. The
results show that, in the absence of large queues at the Virtual
Channelizers, priority processing of data units has little impact on
the end-to-end latency.
- Packet size analysis experiments were performed to determine the
effects of varying the lengths of packets on SSIS component
processing delays and latency times. These results imply that
response time in the SSIS will improve as packet size decreases.
However the overall throughput will decrease for packet data
because of the increased protocol overhead associated with smaller
packet sizes. This tradeoff will also depend to some extent on the
ratio of packet data to bit stream data in the system.
Customer Quote: "MITRE recommends the use of the SSIS Protocol Performance Model to support the analysis of the design and development of the SSIS."
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