Satellite
E-mail: High Speed, Low Cost
Comparison of transfer protocols via satellite
The standard e-mail transfer protocols like POP3 and SMTP require many dialogue steps for each single message and also increase the data volume by 33 % using the Base64 encoding. This makes their use very expensive over satellite links with a high propagation delay and a high price per minute. Optimized software can significantly reduce the cost and make even large attachments affordable: High speed means low cost.
How POP3 and SMTP work - step by step
POP3 (Post Office Protocol 3) and SMTP (Standard Mail Transfer Protocol) are most commonly used in local networks and in the Internet to receive and transmit e-mails. For connecting an Internet service provider (ISP) over an analogue modem, an ISDN adapter, a DSL line or cable modem, PPP (point-to-point protocol) is used as an envelope around the TCP/IP packets. Typical e-mail programs like Microsoft Outlook, Netscape Messenger and most others go through the following steps when retrieving and sending one mail during a connection (see table):
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This means that 23 steps are required to send one mail and receive one. Every single step requires waiting for an answer from the other end. This is no problem at all within a local-area network or even via a terrestrial line since the roundtrip delay is only a few milliseconds.
But over a geostationary Inmarsat satellite the roundtrip delay is typically more than one second. (Note that this time is significantly longer than one might expect from the height of the satellite orbit; it "should" be around 0.25 s for 36,000 km, but much additional time is eaten up by the internal processing and error correction!) Even if the DNS server is really fast and the mail server load is low, this means that this simple mail exchange will take at least 23 seconds. Unfortunately, that's not all:
- Attachments are Base64-encoded, packing 6 data bits into 8 transfer bits (33 % more data!)
- Modems need about 15...20 s for negotiation until they connect (not for ISDN/M4/F77)
- The data packets need to go from the ISP to the mail server if it is not located at the ISP
The sum of all these times easily reaches 45...60 seconds - for sending one short mail and receiving one, resulting in a typical cost of 2 US$ or more using a conventional e-mail program.
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Optimized protocol: 23 steps shrinked to 4
For Inmarsat connections as used by ships or land mobiles, Vizada (former name: France Telecom Mobile Satellite Communications) and Shamrock Software have developed a highly efficient e-mail software called SkyFile. The transfer protocol needs significantly less steps than PPP, POP3 and SMTP, resulting in much faster and also cheaper connections.
Even
more, instead of waiting until all mails are sent before retrieving mails,
SkyFile receives and sends mails at one time - full duplex. The transfer
itself uses a large transmission window, i.e. several KB of data are sent
without waiting for an acknowledgement. It also uses pipelining which allows
the transfer of the next file to start before the previous one is
acknowledged. This avoids unwanted breaks in the data stream.
Transparent data and online compression
While many modems do have a compression feature, it works very inefficiently if data is encoded with the Base64 algorithm used by conventional SMTP/POP3 transfers, since the original character string repetitions are no longer visible in the encoded data stream. In contrary, SkyFile does not encode attachments but transfers them transparently as binary data, saving not only 33 % volume overhead Base64 but also gaining a much better compression. The terrestrial gateway in the land earth station (LES) then encodes the transparent data into an SMTP-compatible Base64 data stream before sending it to the Internet, and vice versa.
The software also does not rely on the more or less efficient MNP5 or V.42bis compression algorithms used in modems, but uses its own ZIP-like compression which also works for ISDN connections.
Another feature is that the transfer of a large file has not to be repeated from its very beginning if the connection breaks for some reason. The transfer will continue seamlessly at the point where it stopped as soon as the user dials again.
The following form calculates the typical connection time for SkyFile in comparison to conventional e-mail programs like Outlook when receiving one file. If you change the file type or size, the result will follow your data automatically, taking into account how much the file can be compressed. The connection time includes modem negotiation and log-in duration.
Spam and virus filtering
Compression and duplex transfer is one thing, avoiding unwanted e-mails another one. Spam and viruses make up more than half of all Internet e-mails today, but many filtering solutions leave it to the client to get rid of it. This includes Bayesian filters which have to be trained by each user by marking good and bad e-mails so that the filter learns for whom which e-mails are spam and which are ham.
While this may be acceptable for terrestrial users with a flat-rate Internet access, satellite users demand an efficient filtering at the gateway in the land earth station -- before unwanted e-mails are transferred to them over an expensive satellite link. Due to the high transfer cost, they even accept more rigid filter settings. Please see our spam article for some techniques used in the SkyFile satellite e-mail gateway.
© 06/2007 Shamrock Software GmbH
