Iridium Bandwidth and Internet Download Speeds

There is a lot of misinformation about the actual bandwidth (ie internet download speed) of a normal Iridium internet data connection (excluding Iridium Pilot)

Actual Iridium Bandwidth from Iridium 9505a / 9555 / 9522 phone to satellite

Actually the theoretical channel bandwidth is 3,466.67 bps. Error correction, etc generally reduces this to around 2,400 bps usable download speed

The answer is that Iridium has just one speed from the phone (ISU) up to the satellite and this is a nominal 2,400 bit/s (or baud) connection. No matter how much you tinker of fiddle with the modem settings or compression systems, still the native speed between you and the satellite is a fixed 2,400 baud connection.

Of course this also assumes that you have full and strong signal strength (which in turn normally requires you to have a dedicated external Iridium antenna).

But when I connect the computer says "Connected at 9,600" (or sometimes 19,200)

The speed which flashes up when windows connects to the internet is the speed reported back by the modem driver. In the case of the Iridium it actually tends to report something like the speed that the cable between the phone and the computer is running at, and it never reports the actual download speed of the phone to the satellite

Interpreting the Iridium Internet Connection speed messages

The how and why (V.110):

The faster speed of 9,600 is achieved by using the standard AT command sequence:

This is standard across all modern modems and basically switches the modem into an ISDN emulation mode running at (an emulated) 9,600 baud (called V.110). The connection is then made instantly and all data is transfered digitally from end to end (no conversion to analogue).

Clearly this is the most optimal way to use a digital telephone connection!

If you see the "Connected at 19,200" message then this indicates that you are connecting like an old style analogue mode (ie it does that warble and spends some time synchronising speeds with the far end).

The implications of this are that it normally takes around 20 seconds of chargeable airtime to connect to the Iridium ISP and the connection is usually not quite optimal and more vulnerable to interference and disconnects. Essentially the Iridium is converting the signal into analogue squeeks and whistles, transmitting them over a digital network then the other end is converting these modem noises back to digital again (that's what a modem does!). Clearly it's less optimal to use a digital network to use analogue tones and whistles to transmit what started out as digital data in the first place!

However, if you see the "Connected at 9,600" message then curiously this means that you are connected via a faster ISDN digital connection. The implications of this are that there is no warble while each end handshakes and instead you are directly connected to the Iridium Direct Internet gateway in just 10 seconds!

Conclusion: "Connected at 9,600" is good. Curiously, "Connected at 19,200" means your Iridium internet connection is actually slower..

But Iridium advertises a 10kbit/s internet service from Iridium Direct Internet 2.0 (DI2) and Apollo Emulator

Did Apollo Emulator compress data?

Actually as far as we can tell the Apollo Emulator product was mainly useful for connection "spoofing". This means that if the real internet connection dropped, then the Apollo Emulator would pretend that it was still there and prevent your email application from showing an error message. The product doesn't appear to offer any additional compression over and above normal PPP compression.

The problem we saw with the spoofing is that you only get around 60 seconds before your email program gets bored and disconnects anyway, which isn't very much time to fix the original problem and redial - hence it wasn't a terribly useful product and is now discontinued at Iridium

When the computer connects to the internet it uses a protocol called PPP. This protocol includes a compression component and assuming your data is compressible then you have the possibility to transmit data faster than the native Iridium channel rate would imply.

For example if hypothetically your data compressed at a rate of 4:1 then you might manage to squeeze 4x 2.4Kbit = 9.6Kbit down the Iridium. Hence the quoted 10Kbit/s rates that they claim to offer.

So the actual effective bandwidth across Iridium will be influenced by how "compressible" your data is. If you are transmitting a lot of very easily compressed text data then your actual transfer speeds can be faster then the nominal 2.4Kbit/s of the Iridium internet connection.

Note: In fact the compression rates achieved by the fairly bruteforce compressor in the normal Iridium PPP connection are less than can be achieved by a smarter compression product. Hence the reason that MailASail offers optional compression software with our teleport-mail email service. The Teleport-Mail system uses quite a number of ways to increase the effective internet download speeds over Iridium and one of the incremental improvements we found we could make was to pre-compress the data. Overall using Teleport-Mail will likely give you around a 10x speedup compared with a normal POP email account

How important is the Iridium signal strength on internet download speeds?

Incredibly this is probably the single most important factor which influences Iridium download speeds!

Consider that the Iridium handset has 5 bars of signal strength (depends how you count them, but lets count 5 as full strength). Roughly speaking your effective download speeds vary with signal strength:

  • 5 bars Iridium signal => Full speed (obviously)
  • 4 bars Iridium signal => 50% speed
  • 3 bars Iridium signal => 10% speed (gulp)
  • 2 bars and below usually either fail to connect or will transfer data only very slowly

Clearly it's very important to have the best signal strength possible for best Iridium download speed!

Why does the speed vary so much with signal strength?

In fact the speed does not vary with signal strength. As we said above, the Iridium runs a roughly fixed speed 2,400 baud connection speed all the time. However, as the signal strength drops then the data being transmitted is likely to get garbled and corrupted.

Consider that the computer transmits data in "chunks" in roughly 5 second chunks over Iridium. If there is any corruption during that (roughly) 5 second transmission then the whole data chunk is corrupted and useless. The computer must then re-order that chunk of data and retransmit it... Clearly as the probability of each chunk getting corrupted goes up then the computer starts to spend all of it's time retransmitting stuff again and again until it gets through.

Conclusion - so what are the real Iridium download speeds

The nominal Iridium bandwidth, is a little less than 20KB/minute. So keep your transfers of sensible size given that download speed.

Effective speeds will vary depending on:

  • Signal strength (use a proper external antenna if possible)
  • Compression (text might transfer faster than a picture)
  • Compression technology (use a compression system optimised for Iridium, especially use email compression)
  • Optimal dialup settings (make sure that you are connecting using the digital settings and getting the "you are connected at 9,600" message)