Ocean Navigator: Newsletter #2


Sailing Away and Leaving It All Behind (Or Taking It With You)


Offshore data connectivity has been somewhat limited in the recreational marine world. Without the room to mount large satellite antenna domes, yachts have been left out of the high-bandwidth world of web surfing at sea. We've been able to get by with lower bandwidth options that allowed us to connect to the Internet to send a few emails or even look at a web page or two.

The first breakthrough in the world of offshore data was Inmarsat's Mini-M service which debuted about five years ago. Mini-M uses a small dome about 9" wide. Inside the dome is a tracking antenna that stays locked on a satellite even as the boat pitches and rolls. Using spot-beam technology, the satellites focus their energy on the most frequently travelled parts of the world and cover the northern hemisphere almost entirely, and some of the southern hemisphere as well. In addition to allowing you to make and receive phone calls, Mini-M also allows you to connect your computer to the Internet (or your office network) at 2400 bits per second (bps). You're charged by the minute, with rates usually around $2.50 per minute.

For the uninitiated - a brief description of bits per second or bps: Each time you press a key on your keyboard, your computer translates that keypress into a particular letter or punctuation mark. Each of these letters/marks takes up exactly one byte of the computer's memory. Computer memory is typically measures in megabytes (MB) - roughly one million bytes. A computer with 128 MB of memory would be able to remember about 128 million keystrokes at a time.

In order to send any of these letters or bytes to another computer, they first need to be broken down into smaller chunks called bits. There are eight bits inside each byte (and yes, there are also nibbles - but we won't go there just yet).

So, when a vendor advertises a speed (or bandwidth) of 2400 bps, or bits per second. We can divide this by eight (8 bits per each byte) and calculate that we can send roughly 800 bytes in one second. There's generally about a 20% overhead that needs to be accounted for. This is taken up by the computers "talking" to each other making sure that everything is getting through without errors. So 800 bytes per second, less 20% is about 640 bytes per second. So far, I've typed a little over 3,000 keystrokes (bytes) - so this email would take about 5 seconds to send (3,000 divided by 640) over a 2400 bps connection. Not bad.

However, the web is a different story -- Ocean Navigator's home page with all of the associated graphics is 141,792 bytes. Divide this by 640 bytes per second and we'll find that it would take 222 seconds or about 4 minutes to transfer the page and its graphics. At $2.50 per minute, this comes out to about $10 to download a web page.

Reality would show that it would take even longer. Most web pages are actually composed of several separate files -- one for each picture or drawing on the screen. In Ocean Navigator's case, there are 43 separate files that make up the home page. Each file is requested one at a time, so this amounts to 43 separate requests from your computer to the Internet. There's a lot of "waiting" involved here -- The dialog between your computer and the web site goes something like this:

Your computer: "Dear Ocean Navigator Web Server, please send me the home page and let me know what other files I'll need to request."

ON Web Server: "OK. Here's the home page and a list of files."

Your computer: "OK. Received the home page intact. Thank you."

ON Web Server: "You're welcome. Have a nice day."

Your computer: "Dear Ocean Navigator Web Server, please send me the banner image for the top of the web page."

ON Web Server: "OK. Here's the file you asked for."

Your computer: "OK. Received the file intact. Thank you."

ON Web Server: "You're welcome. Have a nice day."

. . . and so on for each of the 43 files involved. You can see that there would be a lot of time spent waiting for a response -- especially during busy times. All this waiting would be over and above the 30 minutes we calculated above. It quickly becomes apparent why most yachts can't afford to offer web surfing at sea.

Let's not pick on Ocean Navigator exclusively -- the New York Times home page is even bigger. For a landline connection, these sizes are not unreasonably large -- they can be downloaded in less than a minute over a 56 Kbps modem (roughly 56,000 bits per second - more or less).

However, there are several things you can do to enable you to get critical information off the web at sea. One easy and very effective tip is to turn off the display of all of the images and pictures on web pages. In Internet Explorer, you'll find this option under Tools - Internet Options - Advanced - Multimedia - Show Pictures. Just uncheck the box, and then take another look at Ocean Navigator's home page. You'll see all the text, but the images will be gone. This would bring the size of the page down from 141,792 bytes to only 39,948 bytes. This could be sent in one minute.

You can bookmark the pages you need most. If they're filled with text as opposed to graphics, they'll likely download pretty quick.

Real world tests have typically shown a rate of about 10,000 bytes per minute or a little over 150 bytes per second -- significantly slower than our calculated rates for a 2400 bps connection. The slowdown is caused by latency (the time it takes for a signal to go from your boat up to the satellite and back down to the ground), error correction techniques, and network congestion - including Internet response times.

Don't fret though - the world of communications is improving rapidly. Globalstar now offers 9600 bps connections, and SeaTel recently announced a system called WaveCall that bundles three of these connections together for a combined rate of 28,800 bps.

KVH's TracNet will offer 400,000 bps, and Inmarsat announced a marine data service at 64,000 bps - which can be bundled in groups of 2 for a total of 128 bps. As you might imagine, this high-speed options do cost a bit more than their low speed counterparts.

Happy surfing.

- Dan Piltch
dpiltch@MarineComputer.com


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