Since last week of Nov 2019, I am having serious issues with Airtel at my home. Somehow 4G signal SNR is very poor and most of the calls just fail on that. Airtel support just mentioned that they are putting a new site in my area in Jan 2020 but fail to explain why suddenly it went so bad. I can imagine that support team staff does not have visibility to network in real-time and likely it would be an issue with the 4G antenna on one or more towers. 2G signal was good but latency was extremely high to connect call plus calls still failed regardless (maybe due to high strain on 2G).
So by the end of December, I just gave up and gave the request for porting to Jio. Yesterday my number started on Jio and around 24 hours later I got access to VoWifi which is quite nice. Both Airtel & Jio are rolling our VoWifi but for Airtel, many users have reported it for being locked on Airtel fixed line only.
What is VoWifi?
Well, it’s just native offloading of voice calls over the fixed-line network and is extremely useful for both network operators as well as end-users. Native IP equipment is cheap and it’s way easy to extend cell coverage at home via Wifi instead of proprietary inbuilding solutions. Plus landline was good for quality but form factor and usability was pain. Now VoWifi kind of merges the fixed-line and cell phones and we get best of both worlds.
I think many people confused BSNL Wings with VoWifi. VoWifi is seamless offloading of calls natively while BSNL Wings offered a separate number, an app to run and a separate billing account to maintain with the operator. BSNL Wings was a bad product idea and really awful execution.
Coming to Jio VoWifi, some misc observations:
It’s working for me on the non-Jio fixed-line connection in Haryana circle. I have IAXN (GEPON FTTH) and Siti broadband (DOCSIS 3.0) at home running in active/backup configuration for high availability.
The device has an IPsec tunnel with 49.44.59.XXX. I see isakmp-nat-keep-alive packets every 20 seconds when I do tcpdump on the router.
On my cell phone (which is an Apple iOS device) I see a tunnel is opened on IPSEC1 port. IPSEC2 & IPSEC3 are there for VoLTE.
IPSEC1 has a link-local IPv6 and an IPv6 from 2405:205:3000::/36 range. The IPv6 on all three IPSEC tunnels is the same as of pdp_ip1 (Cellular data port). I guess that’s probably how they are handing the handovers between VoWifi and VoLTE.
The IPv6 on pdp_ip0 for cellular data is from a different range which again is being announced in the global table as 2409:4051::/36. I wonder why Jio is using publically routable pool for VoLTE and VoWifi. In case of Airtel it a non-routed address space and routes are not announced in the global routing table.
In my test VoWifi to VoLTE was reasonably OK. I tested by shutting off wifi on the cell phone as well as unplugging the cable from wifi AP to test. In both cases call went silent for 1-2 seconds and resumed over VoLTE. It did not come back on VoWifi once Wifi was back live.
Latency from both connections to other endpoint (the router before the IPSEC endpoint) is around 60ms. Either the endpoint is somewhere far off or (more likely) it’s a case of not-so-good routing between Airtel (upstream of my ISP) and Jio.
There’s a quite noticeable improvement in connection time. For IVR based numbers it’s close to 2-3 seconds. Which is a great improvement for someone like me who was on 2G non-VoLTE calls for December!
How to check interfaces and routing table on your phone?
That’s about it. Back to the world of routing. :)