1 Introduction

1. Purpose

The need for higher speed (>1Gb/s) communication on spacecraft data networks is increasing. Instruments involving large array CCDs which generate pixel-per-bit data are capable of generating 10e7-10e8 Mbit/sec data streams. Simultaneously, other instruments are generating lower data rates capable of being transmitted over conventional wiring media. The ability to use wideband optical media capable of integrating the data and command transmission requirements for the entire spacecraft databus, could provide advantages in weight, power, signal routing, reliability and overall spacecraft/payload interface design.

Generally, spacecraft have utilized low speed interfaces such as RS-422, MIL-STD-1553 or MIL-STD-1773 for data communications. These basic systems are usually limited to bandwidths of 1 Mbps or less. Recent advancements in network technologies have dramatically increased available bandwidth for data communications; newer technologies can scale up to 2 Gbps and more. These improvements have a significant impact on the High-Speed Fiber Optics Test Bed (HSFOTB) architecture in terms of performance and cost. The HSFOTB network design will utilize new technology and provide for an easy migration path to faster data rates as technology matures.

The purpose of this investigation is fourfold:

  1. Present an overview of the selected network technologies
  2. Compare and contrast the capabilities of each
  3. Assess the general requirements of a network of payload instruments capable of transmitting Gb/s data rates; either aggregate data rate or individual data rates.
  4. Make recommendations for the elements of the HSFOTB architecture

1. General Requirements

Ground-based commercial fiber optic networks have existed for some time. These networks provide reliable connector-based and connector-less data transmission for a large number of users with heterogeneous (voice, data, video, high speed, low speed, etc.) data streams. The requirements for a spacecraft network system differ significantly from those of a commercial network. Table 1 below highlights the key differences.

Table 1

Category

Ground, commercial

Space, science

Cost

Cost dictated by revenue stream and business model. Ability to make in-situ upgrades to capabilities is important

No revenue stream. Science data needs are paramount. Design re-usability important. Initial design costs, manufacturing costs and availability of space qualified parts are drivers

Quality of Service

Function of competitive environment and customer ability to pay. Packet losses, loss of service and outage times are major factors which at times may be tolerated by the customer

Function of science and mission priorities. Data loss is not tolerable. Overall end-to-end reliability and availability are main factors.

Data

Voice, data, video. Classes of variable bit rate data. Traffic prioritization and bandwidth management are key factors.

No voice. Fixed number of users with fixed data rate requirements. Serial data from RS-422 type interfaces required. Available bandwidth will have margin over worst case traffic load.

Media and protocol

Mixed media and protocols. ATM, Ethernet, FDDI, Fiber Channel, etc. may be accommodated.

Connection-oriented, fiber network with a single protocol is possible.

 

Based upon these generalizations, the following top-level requirements for a high-speed fiber optic test bed for space applications can be defined.

  1. A connection-oriented network consisting of a small number of nodes and switches.
  2. Fixed traffic priorities with constant bit rate data.
  3. Full duplex operation for data acknowledgement, retransmit and status
  4. Local Area Network type operation
  5. High priorities on low-error rate and high-speed data transfer
  6. Low priority on bandwidth utilization and traffic management
  7. End-to-end fiber solution
  8. High priority on hardware modularity and low power

1.3 Technologies Covered

This paper covers four network protocols: (1) ATM, (2) Fibre Channel, (3) Gigabit Ethernet, and (4) FDDI as candidate technologies for onboard data communications. An overview of each protocol is presented, including protocol architecture and services provided by each. While technical details are presented, this paper is not intended as a tutorial in the technologies covered, nor is this paper intended to be an exhaustive survey of available network technologies. The identified protocols were selected based on two simple criteria; support of gigabit bandwidths and commercial availability.

Recommended sources describing industry standards include the Internet Engineering Task Force (IETF) Request For Comments (RFC) documents, and the various IETF working group drafts. The interested reader is referred to the appropriate sources listed in the bibliography for further information.

1.4 Executive Summary

This study recommends ATM over Fibre Channel as the network technology for the High Speed Bus. This conclusion is based on several factors. ATM can scale to gigabit speeds, supports a variety of interfaces (fiber optic as well as twisted pair), and can operate in both local and wide area networks. Moreover, ATM has been successfully tested over satellite links. While still relatively new, ATM is a proven technology, with many commercial vendors, thus enabling lower development costs. Fibre Channel provides a flexible network topology capable of supporting both ATM and other networking approaches. This flexibility will prove useful in evaluating competing network approaches.


Paper Outline


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