There are several things you should know about iridium. Here are a few facts.
1. it takes about 20 secconds to bring up a ppp connection with iridium
2. the line turn around latency varies with the number of satellites in the link and can be as long as 3 seconds. This means that when you do ping over an iridium connection the ping could take 2-3 seconds. This is very important (see below). The closer you are to AZ and the fewer the satellites in the link the faster the line turn around.
3. Currently only one gateway is used for all civilian communication.
4. the nominal raw rate for iridium is 2400 baud which translates to about 18Kbytes per minute. It actually runs between 12 and 15Kbytes per minute for an average connection.
5. the iridium connection is duplex which means that you can send and receive concurrently over the link.
So what does this all mean for a user wanting to do e-mail over Iridium using standard internet
1. Most e-mail clients do not support compression. This means that after you bloat your e-mail by converting it to mime format (mime increases a binary attachment size by as much as 40%) you transfer it over the link with 0 compression. On average text files compress by 50% which means that you could save 2x in your air time bill if you simply compressed the data before it is transmitted. If you compress multiple files into one block then your compression factors go up and your corresponding transmission
costs go down. So... adding compression is a good idea.
2. Direct internet, provided for free by iridium, does compression. So... by adding direct internet to the equation you gain in performance.. However, there are something you should know about direct internet.
a. it only does run length encoding compression. Since it doesnt know the amount of data the compression cant be as efficient as that done by an application to application block compression scheme.
b. It doesnt work on vista
c. The dialup connection to the internet needs to be brought up manually. Once the connection is up it takes direct internet a while to negotiate with the compression server before the system is in place. This means that instead of looking at 20 second connection times, you must wait about 45 seconds before you start transmitting data. If you start transmitting data before the compression agent has finished initializing itself then the data connection runs without compression.
d. once you are finished with your session you must bring the connection down manually.
because of (c) there is no practical way to configure the auto dialers in e-mail clients such as outlook express to dial, send/receive, disconnect. when using Direct internet.
d. SSL mail connections (for both sending and receiving) cant be compressed. By definitiion encrypted data is randomized and cant be compressed. This means that if your e-mail service uses SSL you gain absolutely nothing by using Direct Internet. Actually... your performance is works since compressing a compressed stream results in a larger amount of data to be transfered.
e. Direct internet does no mid file restart, large attachment handling, header pruning, pipelining, or spam filtering. See below.
3. latency is a killer. As mentioned above 2-3 seconds for line turn arounds is typical over iridium. Looking at the SMTP protocol which is used by common mail clients such as thunderbird and outlook express you will note that 5 line turn arounds are required to setup the transfer and one line turn around is required for each email address. So... if you are sending an e-mail to 5 recepients then at works (5 + 5)*3 ==> 30 connections of air time are lost
before you send a single
characeter of your email over the link.
Receiving mail is also bad and latency is still a problem. After you do the pop authentication (which takes 5 line turn arounds) you have one line turn around for the directory listing and then one line turn around for the request of every file. Once the files are all downloaded then you have one line turn around for every file requesting the delete.
4. No mid file restart.
The killer here is that since the POP deletes come after all the files have been received, if you have a connection drop in the middle of your transmission
you must start all over again! Sending mail is not as bad since the client gets an ACK from the server for every mail sent so you dont have to resend mails that have already been sent. If, however, you get a drop in the middle of an e-mail you are sending then you must resend that e-mail from the start.
5. no duplex operation.
Thunderbird and outlook express as well as most e-mail clients in the market run in simplex mode. They send mail first, then receive. This means that you dont have full use of all the available bandwidth. a 40% in time savings can be had if you were to send/reveive mail at the same time.
6. Standard e-mail clients/servers do not do header pruning. Take a look at a standard internet mail header. They can be huge. 5 Kbytes is not uncommon. Standard mail clients do nothing about reducing the size of the mail headers. Chopping the mail headers so that the contain the minimum amount of info (date, from, to, subject) can tremendously reduce the amount of data that is transfered over the link.
7. No intelligent attachment control... What would happen if someone were to send you a 1Mb picture as an attachment while you were out at sea? Your e-mail would be stuffed. Sure... you could configure you e-mail client to only list the headers of e-mail but this is a tremendously inefficient use of bandwidth and its expensive in air time.
8. No intelligent spam controll. Over 80% of all internet mail is spam. On average in a 24 hour period our service filters out 30 spam e-mails per active user on the system. Some users (i.e. me) can receive as much as several hundred spam e-mails in a 24 hour period if the system does not support intelligent spam control. There are many services that include this but you want to be certain that the system you use is friendly to satellite
So... why is XGate (and other application to application compression systems) worth using? Because they address all of the deficiences mentioned amove.
XGate does the following.
1. It does block compression for maximum compressibility of the data trasnmitted.
2. it does mid file restart... after a disconnect XGate will start the transfer of data exactly where it left off.
3. I does protocol overhead removal
removing all the dead time in standard internet clients by pipelining the ACKS. XGate has a total of 3 line turn arounds total for a connection independent of the number of files being sent/received.
4. duplex operation for maximum bandwidth utilization
5. intelligent and efficient large mail management
6. great spam controll offering several levels of server side customizable services.
7. convinient link control. With the push of a single
button XGate will dial the connection, bring up the internet connection, send/receive, and disconnect.
So... if you are doing more than a single occassional text e-mail over iridium then XGate should be considered. XGate will save you considerable $$ in air time and reduce the frustration of doing e-mail over a slow expensive and contanquerous link.
XGate runs natively under windows, Mac OSX, and Debian Linux
. All features are supported on all platforms.
One last thing... receiving SMS on iridium is free. up to 160 characters can be received for free on the phone. Sending SMS, however, is not free. it costs about $0.50 (varies with your plan) to send up to 160 character SMS from your phone. Using XGate to send e-mail is much more cost effective than using SMS for your messaging.
Luis Soltero, Ph.D., MCS