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Network Architecture

Our network consists of three major segments: the ground network in California, the satellite link across the Pacific Ocean, and the ground network in Hawaii.

The ground network in California connects Caltech with JPL, the site of the ACTS ground station. This segment was established as part of Pacific Bell's extant CalREN fiber optic network and has proved to be the most reliable portion of our network.

The satellite connection was made available to us through a grant from NASA as part of the Gigabit Satellite Network (GSN) testbed program. NASA's Advanced Communications Technology Satellite (ACTS) was built to explore new modes of high speed data transmission at rates up to OC-12 (622 Mbit/sec). The 20-30 GHz frequency band has been employed for the first time by a communications satellite, with extensive rain fade compensation.

The ground network in Hawaii, which connects Keck observatory with the other ACTS ground station at Tripler Army Medical Center in Honolulu has been somewhat more complex in its evolution. This was primarily due to the relative inexperience of GTE Hawaiian Telephone and a lack of prior infrastructure in Hawaii. This segment initially consisted of a combination of underwater fiber, microwave antennae, and buried fiber. The higher bit error rates (BER) of the non-fiber segment produced noticeable instability in the end-to-end network. Fortunately, in January of 1997 this portion of the ground network in Hawaii was upgraded to optical fiber. The improved performance for high-speed data transfers of the final all-fiber network was immediately apparent.

In order to support standard higher-level (IP) networking protocols, we installed an Asynchronous Transfer Mode (ATM) network over this infrastructure. The transfer of 53-byte ATM cells is performed by hardware switches throughout the network, at speeds of OC-1 (51 Mbit/sec) and above. Several vendors have supplied the ATM switches and Network Interface Cards (NICs), providing a stringent test of compatibility in the relatively new ATM environment. Although we have encountered several interoperability problems, none have been serious, and the ATM and telephone vendors have been extremely helpful.

In order to facilitate reliable data transfer, as well as to allow the use of the wealth of software tools already available, we are running the standard IP protocols over ATM using a pseudo-standard implementation known as ``Classical IP''. This enables the use of the standard network-based applications that are in widespread use on the Internet. Tools such as ftp and telnet are part of every observing run, as are additional special-purpose applications, such as an audio conferencing tool (rat) and a shared whiteboard tool (wb).


 
Figure 2: Bandwidth test results between Keck Observatory and the Caltech campus in Pasadena, California, over the ACTS satellite network. TCP exhibits a remarkable dependence on the bit error rate.  
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next up previous
Next: Network Performance Up: Remote Observing with the Previous: Introduction
Patrick Shopbell
12/8/1997