Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stecf.org/conferences/adass/adassVII/reprints/shopbellp.ps.gz Äàòà èçìåíåíèÿ: Mon Jun 12 18:51:49 2006 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 04:06:50 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: vallis

Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
Remote Observing with the Keck Telescope:
ATM Networks and Satellite Systems
P.L.Shopbell, J.G.Cohen, and L. Bergman 1
California Institute of Technology, Pasadena, CA. 91125
Abstract. As a technical demonstration project for the NASA Advanced
Communications Technology Satellite (ACTS), we have implemented re­
mote observing on the 10­meter Keck II telescope on Mauna Kea from the
Caltech campus in Pasadena. The data connection consists of ATM net­
works in Hawaii and California, running at OC­1 speeds (45 Mbit/sec),
and high data rate (HDR) satellite antennae at JPL in Pasadena and
Tripler Army Medical Center in Honolulu. The ACTS network is being
used to enable true remote observing, as well as remote eavesdropping.
The software environment is identical to that used for on­site observing
at the Keck telescope, with the added benefit of the software, personnel,
and other facilities provided by observing in a local environment. In this
paper, we describe our high­speed remote observing network, assess the
network's level of performance, and summarize the benefits and di#cul­
ties encountered in this project.
1. Introduction
Remote use of astronomical telescopes has been a topic of interest for many
years, even before space­based observing platforms (e.g., IUE) began to demon­
strate total remote operation out of sheer necessity. However, only very recently
are optical telescopes beginning to realize the benefits of true remote observing:
for example, observations with modest size detectors at Apache Point Obser­
vatory are being carried out remotely using the Internet (York 1995, BAAS,
186, 44.04). For this project, we have established remote interactive observ­
ing capabilities for Keck Observatory on Mauna Kea for observers at Caltech,
in Pasadena, California. The recently commissioned twin 10­meter Keck Tele­
scopes are the largest optical/infrared telescopes in the world and thereby typify
the data and network requirements of a modern observatory. In undertaking this
project, we were motivated by several operational and scientific advantages that
remote observing would o#er, including alleviating altitude­related di#culties,
saving time and money due to travel, enabling remote diagnosis of hardware and
software problems, and simplifying access to the telescopes for educational use.
1 Jet Propulsion Laboratory, California Institute of Technology
348

Remote Observing with the Keck Telescope 349
underwater fiber
GTE Hawaiian Telephone
LFN
Oahu
Keck HQ
Honolulu
Keck Observatory
(TAMC)
Hawaii
(OC­1)
LFN
JPL
Caltech
CalREN
Pacific Bell
(OC­3)
Pasadena, CA
Figure 1. Schematics of the final terrestrial networks in Hawaii and
California, as used for the Keck remote observing project. These net­
works were connected via NASA's ACTS satellite.
2. 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 OC­12 (622 Mbit/sec).
The 20--30 GHz frequency band has been employed for the first time by a com­
munications 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 infrastruc­
ture 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 non­fiber segment produced noticeable instability in the end­to­end network.
Fortunately, in January of 1997 this portion of the ground network in Hawaii
was upgraded to optical fiber. The improved performance for high­speed data
transfers of the final all­fiber network was immediately apparent.
In order to support standard higher­level (IP) networking protocols, we in­
stalled an Asynchronous Transfer Mode (ATM) network over this infrastructure.
The transfer of 53­byte ATM cells is performed by hardware switches through­
out the network, at speeds of OC­1 (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. Al­

350 Shopbell, Cohen, and Bergman
100 200 300
0
10
20
30
40
t (sec)
TCP (high BER)
TCP (low BER)
UDP
LRIS image ftp
Figure 2. Bandwidth test results between Keck Observatory and the
Caltech campus in Pasadena, California, over the ACTS satellite net­
work. TCP exhibits a remarkable dependence on the bit error rate.
though 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 pro­
tocols over ATM using a pseudo­standard implementation known as ``Classical
IP''. This enables the use of the standard network­based applications that are
in widespread use on the Internet. Tools such as ftp and telnet are part of
every observing run, as are additional special­purpose applications, such as an
audio conferencing tool (rat) and a shared whiteboard tool (wb).
3. Network Performance
The most important impact of a satellite component on a high­speed network is
the relatively large delay introduced by the round­trip signal travel time to the
satellite. In our network, this travel time is approximately 0.55 seconds, which
corresponds to 3.5 Mbytes of data at OC­1 speeds (51 Mbit/sec). The problem
has to do with the connection­oriented nature of TCP/IP: TCP sends a very
specific amount of data, known as a ``window'', after which time it expects an
acknowledgment from the other end of the connection. However, this window
size is often very small; the default value for workstations running the SunOS
4.1.4 operating system is only 4 Kbytes.
Fortunately, this problem is well­known in the high­speed networking com­
munity. Networks such as ours are known as ``long fat networks'' (LFN; see RFC
1323). In the case of the SunOS operating system (to which we are constrained
by legacy control software at Keck), we obtained the TCP­LFN package from
Sun Consulting, which purports to support the RFC 1323 extensions. Unfor­

Remote Observing with the Keck Telescope 351
tunately, a number of limitations of SunOS 4.1.4 conspire to prohibit one from
obtaining extremely large window sizes, regardless of the TCP­LFN software.
In our case, the compiled­in kernel limit of 2 Mbytes of Mbuf memory (i.e., IP
packet wrappers) turned out to be the major constraint, limiting our window
size to no more than 1 Mbyte. Indeed, our final tuned network delivered the
expected maximum TCP/IP performance of approximately 15 Mbit/sec (# 1
3
of OC­1). Although perhaps disappointing in a relative sense, this bandwidth
is far in excess of T1 Ethernet speed (1.44 Mbit/sec) and allows an 8 Mbyte
image to be transferred in approximately 5 seconds. As a further comparison,
this bandwidth exceeds by 50% that which is available on the local area Ether­
net network at the Keck Telescope itself. Figure 2 illustrates typical bandwidth
measurements of our network for UDP and TCP, the latter before and after the
network was upgraded to fiber in Hawaii.
While network performance was perhaps not at the level desired, due to
developing infrastructure in Hawaii and idiosyncrasies within the operating sys­
tem, issues of network reliability had far greater impact on our remote observing
operation. The experimental and limited nature of the ACTS program created a
number of di#culties which one would almost certainly not face if using a more
developed and/or commercial satellite system. The impact of the reliability is­
sue is that at least one observer must be sent to Hawaii to use the telescope, in
case of ACTS­related problems.
4. Conclusions
This experiment has explored the data requirements of remote observing with
a modern research telescope and large­format detector arrays. While the maxi­
mum data rates are lower than those required for many other applications (e.g.,
HDTV), the network reliability and data integrity requirements are critical. The
former issue in particular may be the greatest challenge for satellite networks
for this class of application. We have also experimented with the portability of
standard TCP/IP applications to satellite networks, demonstrating the need for
alternative TCP congestion algorithms and minimization of bit error rates.
Reliability issues aside, we have demonstrated that true remote observing
over high­speed networks provides several important advantages over standard
observing paradigms. Technical advantages include more rapid download of data
and the opportunity for alternative communication facilities, such as audio­
and videoconferencing. Scientific benefits include involving more members of
observing teams while decreasing expenses, enhancing real­time data analysis of
observations by persons not subject to altitude­related conditions, and providing
facilities, expertise, and personnel not normally available at the observing site.
Due to the limited scope of the ACTS project, future work from the stand­
point of Keck Observatory will be concerned with establishing a more permanent
remote observing facility via a ground­based network. At least two projects are
under way in this direction: remote observing from the Keck Headquarters in
Waimea, from where up to 75% of observing is now performed every month,
and remote observing from multiple sites on the U.S. mainland using a slower
T1 connection (Conrad et al. 1997, SPIE Proc. 3112). Trial tests of this latter
approach over the Internet have been extremely promising.