Email Transmission Rates

For some time we’ve been confused by e-mail data rates and cost, as it applies to boat-based communications. Recently, Mike Parker, a new Sundeer owner and old-time glider guru, brought us up to speed on this subject. Here’s what we learned.

There are eight bits of data in one byte. One byte is more or less equal to a single character (i.e. one byte – or eight bits – equals a letter, number, space, punctuation mark, etc.).

When you hear about a 2400 baud connection over Iridium or Mini-M sat com, this is essentially equivalent to 2400 bits per second of data. Since there are eight bytes in a character, divide the data rate-let’s say 2400 baud-by eight to see how many characters you can send per second. 2400/8 = 300 characters per second.

When you hear about 9600 baud over Globalstar sat phones, you divide 9600 by eight to get 1200 characters per second.

The average word contains about five characters, so this means the slower sat phone connections can send text at roughly 50 words/second or 3000 words/minute-which is why text-only messages don’t take much connect time.

One of our main concerns is with sending photos and Excel spreadsheet files. Most of the larger web photos published on run about 18,000 to 24,000 bytes of data-this would be a heavily compressed JPG image. To see how long it would take to transmit an 18K byte photo, we first have to convert from bytes to bits by multiplying 18,000 x 8 to get 144,000 bits. Then we divide this by the data rate or 144,000/2400 bits per second to give us 60 seconds of time for the data transmission.

To this we have to add a bit of “overhead” at the beginning and end of each transmission for connection and disconnect.

Bottom line – we can send an average photo like SetSail uses for about a minute of transmission time on the slower services.

Posted by Steve Dashew  (February 13, 2005)

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