Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 35)

Stamper, D .A., 1989: Business D ata C omm unications, 2 Edition, Benjamin/C um m ings Publishing C o. nd 10
Ltd., R edwood, C A , U .S .A .
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The W ide Area N etwork is sim ilar in nature to a LAN but is designed to connect geographically
distant locations. W ithin m any countries national or regional governm ents operate W AN’s for
internal use (e.g., transfer of m eteorological data from observing stations to the central forecast
office). If these can be accessed to transfer data over long distances, significant operating cost
m ay be saved, albeit at the expense of slower data transfer rates.
More sophisticated m eans of transferring data from rem ote locations are through radio frequency,
cellular telephone and satellite transm issions. An exam ple of the latter m ethod is the United
States NASA aerosol optical property network AERONET. In this case, a global network of
instrum ents obtains m easurem ents of aerosol optical properties that are transm itted once per
hour via satellite to the Goddard Space Flight Center (G SFC ) for analyses. This m ethod, however,
is lim ited by the am ount of data that can be transm itted through existing m eteorological satellites.
W hatever the m eans of com m unication selected, Stam per (1989) provides an excellent set of 10
criteria on which to base the decision. Each criterion should be considered, even though it m ay
not be significant in the final selection process.
Cost: this includes the price of the m edium selected, the installation of the
necessary equipm ent (e.g., cable), software and hardware requirem ents
(e.g., drivers and com puter cards) specific to the m edium and the ancillary
cost of expansion, if and when needed.
Speed (capacity): this is broken into response tim e (the tim e required for each individual
transaction) and aggregate data rate (the am ount of inform ation transm itted
per unit tim e). An exam ple of such is m odem com m unication with a data
logger every hour to download m ean values of clim ate variables. The
response tim e is the tim e it takes the m odem s to connect, while the
aggregate tim e is the tim e it takes to download the data. In this case, the
m ore com plex the m odem s the greater the response tim e in determ ining
speed and com pression type, while the aggregate data rate m ay be of little
im portance because the am ount of data is only several thousand bytes.
Conversely, when transferring Mb of data, the aggregate data rate becom es
the m ost im portant factor.
Availability: Is the m edium available when there is a need to utilize it? For exam ple, if
using com m on carrier telephone lines, does one get a ‘busy’ signal at the
tim es data is to be transferred, or is the telephone system so busy that lines
are unavailable (e.g., during special holidays).
Expandability: Can the system be enlarged for increased dem and? This can be an
increase either in the num ber of stations using the com m unication system or
in the am ount of data being transm itted through the system . An exam ple of
the latter would be the upgrading of telephone m odem s to higher baud rates
to handle increased am ounts of data transfer over the sam e tim e period
(increased aggregate data rate).
Errors: All m eans of data transm ission are subject to signal distortion, which can
produce errors in the data. To reduce this problem , data com m unication
environm ents transm it redundant data to detect if such errors have occurred.
The m ore com plex the m ethod used for detecting such errors, the slower the
data throughput, but the higher the probability that the data will be error free.
The num ber of copies of the data and how long each copy is m aintained
should in part be correlated to the frequency of data transm ission errors. In
turn this will dictate part of the overall cost of the system .
Security: The ease of access by outsiders increases the threat of breaches in
security. This can vary from som eone accidentally interrupting a data
transfer to vandals physically or electronically destroying equipm ent and