7 Summary

Direct comparisons are always difficult since the different technologies often focus on different niche markets. Early ATM development was based on the B-ISDN standards and was intended as a wide area network protocol. Fibre Channel supports both peripheral communications and isochronous applications such as video. Both of these technologies have evolved to encompass switched local area networks.

ATM is flexible, provides an easy interface between local and wide area networks, and there is significant support for ATM in the marketplace. However, ATM is also very complex, and the standards are incomplete. Fibre Channel is versatile, and supports almost every application. However, Fibre Channel is very complex and the standards and available products are not as mature.

In the battle for the desktop network interface, there will not be a clear winner. ATM will not be ubiquitous, given that not all applications require such capabilities, and there will continue to be cheaper alternatives, i.e. Ethernet. Between the technologies covered in this report, Ethernet will have the largest share of the market, followed by ATM. Fibre Channel will have a niche market. Fibre Channel is becoming the next generation interface for mass storage systems. Fibre Channel is also utilized as a physical layer for other technologies (e.g. Gigabit Ethernet).

This paper has presented a technical overview of several network technologies for possible use in the HSFOTB program, including ATM, Fibre Channel, and Gigabit Ethernet. While each technology has both advantages and disadvantages, the recommended technology is ATM over Fibre Channel. This conclusion is based upon several factors:

1. ATM can scale to gigabit speeds.
2. ATM Supports a variety of interfaces (fiber optic as well as twisted pair).
3. ATM can operate in both local and wide area networks.
4. ATM has been proven to operate over satellite links
5. The anticipated development costs are lower for ATM due to its wide commercial availability.
6. Fibre Channel provides for a LAN topology that is easily adaptable to the spacecraft environment.
7. Fibre Channel provides the best solution for gigabit speed transfer of large data sets (50-150Mb).

Neither Fibre Channel by itself nor Gigabit Ethernet are designed for wide area communications such as satellite links and therefore would require a gateway (e.g. CSU/DSU) to interface between the on-board LAN and the downlink. If ATM is used as the downlink, no gateway device is required, thus providing seamless integration between the on-board network and the ground segment. However, the issue of integrating the data downlink into a WAN beyond the spacecraft is beyond the scope of the current effort.