After my initial post on this topic yesterday, I kept thinking about about the potential implications of eventually also providing LTE IMS voice and Internet service over One NZ Starlink in New Zealand ranging from satellite density over data rates to emergency calls. Follow me into the world of wild speculation…
Why Only SMS so Far?
As One NZ says on their web page, there are not enough Starlink Direct-To-Device capable satellites in orbit yet to provide continuous service. From my point of view it thus makes sense to only offer SMS text messaging at this time. This is a non-real time service and although it is perhaps a bit inconvenient to wait for a message to be sent for a few minutes or perhaps to even re-send it manually once you notice that coverage is indicated in the status bar, it still gets the job done. In other words, it’s a service that can work with intermittent coverage.
Voice calls are another thing entirely. So even assuming (rather than knowing) that voice calls would already work now while a satellite is overhead, having the connection drop every few minutes is a no go for such a service. The same goes for applications that use the Internet. Non-voice and non-real time services mostly use TCP, a protocol which does not work well at all when connectivity is lost for minutes at a time. Even on higher layers, apps are expecting continuous connectivity to work properly. If connectivity intermittently fails, apps often show error messages to the user rather than re-trying on their own in the background every now and then.
So it makes sense to limit connectivity to SMS for now until a reasonable service continuity can be assured with an increasing number of satellites. It would be interesting to know when the number of satellites will be high enough to ensure this for New Zealand, which is fortunately completely within the main Starlink coverage zone (see screenshot above, taken from here).
Data Rates and Speech Codec
Next, let’s think a bit about datarates for a voice call. As per the information I linked to in my previous post, Starlink probably uses a 5 MHz channel in the 1800 MHz band for LTE services in New Zealand. As a single satellite probably has many spot beams and there will probably be several D2D satellites covering New Zealand at some point, I guess capacity will be the least problem. I assume the main limitation is the non-directional antenna and transmit power on the smartphone side. Hence, the lower the data rate in the uplink direction, the narrower the uplink channel can be configured and the more conservative the modulation and coding scheme selected can be. For LTE IMS mobile to mobile calls, the EVS speech codec is typically used today that can switch dynamically between 5.9 and 24.4 kbps. Here’s a SIP SDP excerpt of a smartphone for this codec:
a=rtpmap:112 EVS/16000
a=fmtp:112 br=5.9-24.4;bw=nb-swb;cmr=1;max-red=0
So assuming some logic is put into the IMS network to distinguish ground based calls from satellite calls, it might be possible to limit satellite based calls to a low data rate, thus making the connection more stable. Or the system adapts the bandwidth and hence speech quality based on how well the uplink works during the connection. Or adaptation is not necessary at all and the maximum bandwidth will always be reached when a satellite is in view. All pure speculation, but mechanisms are in place already should flexibility be required.
12 Miles and Beyond?
One NZ also points out that their satellite based service will work within 12 miles around New Zealand. That’s the territorial water around a country. In theory, service would work much further out, it’s an artificial boundary. Which makes me wonder: Why isn’t Starlink allowed or doesn’t want to offer LTE voice and D2D Internet service out in the open sea around New Zealand? This is not a rhetorical question, does any one know if there is any kind of international law prohibiting Starlink to eventually offer Direct-To-Device LTE service on the oceans, up to 12 miles to a coast?
Emergency Calls For All?
All right and one more topic for today, emergency calls: For terrestrial telecommunication, national regulators typically require network operators to connect emergency calls no matter if a user has a network subscription or not. It’s free of charge. But how about emergency calls over satellite, will the same apply here as well? Or, in other words, let’s say I’m a One NZ prepaid subscriber and have not signed up for One NZ Starlink LTE service, can I make an emergency call when in dire need? Or I’m a roamer and not a subscriber of One NZ at all, and my home network operator has no roaming agreement with One NZ, will I still be allowed to do an emergency call over One NZ Starlink? I wonder what the local regulator will decide on this matter. I’m sure there are quite different opinions on the topic from the different parties involved.
So far so good, let’s hope some answers will come in the near future 🙂