Kippzonen BSRN Scientific Solar Monitoring System Instrukcja Użytkownika Pobierz pdf (Strona 35)

Stamper, D.A., 1989: Business Data Communications, 2 Edition, Benjamin/Cummings Publishing Co.

Ltd., Redwood, CA, U.S.A.

The Wide Area Network is similar in nature to a LAN but is designed to connect geographically

distant locations. Within many countries national or regional governments operate WAN’s for

internal use (e.g., transfer of meteorological data from observing stations to the central forecast

office). If these can be accessed to transfer data over long distances, significant operating cost

may 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 transmissions. An example of the latter method is the United

States NASA aerosol optical property network AERONET. In this case, a global network of

instruments obtains measurements of aerosol optical properties that are transmitted once per

hour via satellite to the Goddard Space Flight Center (GSFC) for analyses. This method, however,

is limited by the amount of data that can be transmitted through existing meteorological satellites.

Whatever the means of communication selected, Stamper (1989) provides an excellent set of

criteria on which to base the decision. Each criterion should be considered, even though it may

not be significant in the final selection process.

Cost: this includes the price of the medium selected, the installation of the

necessary equipment (e.g., cable), software and hardware requirements

(e.g., drivers and computer cards) specific to the medium and the ancillary

cost of expansion, if and when needed.

Speed (capacity): this is broken into response time (the time required for each individual

transaction) and aggregate data rate (the amount of information transmitted

per unit time). An example of such is modem communication with a data

logger every hour to download mean values of climate variables. The

response time is the time it takes the modems to connect, while the

aggregate time is the time it takes to download the data. In this case, the

more complex the modems the greater the response time in determining

speed and compression type, while the aggregate data rate may be of little

importance because the amount of data is only several thousand bytes.

Conversely, when transferring Mb of data, the aggregate data rate becomes

the most important factor.

Availability: Is the medium available when there is a need to utilize it? For example, if

using common carrier telephone lines, does one get a ‘busy’ signal at the

times 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 demand? This can be an

increase either in the number of stations using the communication system or

in the amount of data being transmitted through the system. An example of

the latter would be the upgrading of telephone modems to higher baud rates

to handle increased amounts of data transfer over the same time period

(increased aggregate data rate).

Errors: All means of data transmission are subject to signal distortion, which can

produce errors in the data. To reduce this problem, data communication

environments transmit redundant data to detect if such errors have occurred.

The more complex the method used for detecting such errors, the slower the

data throughput, but the higher the probability that the data will be error free.

The number of copies of the data and how long each copy is maintained

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 someone accidentally interrupting a data

transfer to vandals physically or electronically destroying equipment and

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