How WiFi Calling 📞 (VoWiFi) works?
Some time last year, Reliance Jio and AirTel enabled Wifi Calling (Voice over WiFi) for their customers. It was great news for someone like me who always had frequent call drops as I stay in a dense apartment complex. LTE signal is too weak to penetrate so many walls (technically speaking, they reflect and diffract like the phenomena in Young's double-slit experiment).
However, WiFi calling by my carrier Jio behaved interestingly, if I set my router's DNS to Cloudflare or Google Public DNS, it doesn't work. This piqued my interest, I put on my hat and opened my "Mobile Computing" book which I read ages ago — "GSM and LTE networks". And after a little Google search on "VoWiFi call flow" enlightened me with much details (details below).
EDGE, WCDMA, LTE and VoLTE
Cellular networks operate in circuit-switched mode or packet-switched mode. The circuit-switched mode is synchronous and optimized for low latency communication such as phone calls. Packet-switched media is suitable for bursty communications such as mobile internet.
Early mobile networks ☎️ were circuit-switched only (similar to fixed phones). GPRS (2G) introduced packet-switched network access enabling services like MMS, WAP and mobile internet. The phones switched between circuit-switched when taking calls and packet-switched when browsing the internet. This was the reason why you could not talk and browse the internet on GPRS networks.
WCDMA (3G) continued with the same dual-mode approach as well with the enhancement that phones now could use both modes simultaneously allowing calls along with internet access.
LTE 📱 or 4G network is a purely packet-based network. Circuit-switched calls were an expensive legacy, a power-hungry relic of the past (think about 2 antennas) and everyone wanted to get rid of it. However, in some carriers only circuit-switched networks are available for calling, they force your phone to degrade to WCDMA (thus eating more battery as you need 2 different antennas again!). Some maintain for compatibility reasons and to provide a fallback network (2G reaches everywhere!). Reliance Jio famously doesn't.
Thus Voice over LTE (VoLTE) was born. It uses a bunch of IP (internet protocol) telephony network protocols such as SIP and RTMP, which is quite similar to the protocols used in voice and video calling applications (WhatsApp, Viber, Skype etc). Your phone connects to a telephony gateway called IMS (IP Multimedia Services) which is routed to the PSTN (Public Switched Telephony Network) allowing you to make calls. This communication happens directly over the LTE network from your device to a system run by your carrier called Packet Data Gateway (PDG) which bridges you to the IMS and IP (Internet) networks.
Since everything is running on standard IP based communication, it was easy to provide the same set of IMS services over a different IP based access network -- your WiFi. VoWiFi is simply a connection to your carrier's IMS over the public internet.
Your mobile now connects to another server called an Evolved Packet Data Gateway (ePDG) which connects you to the PDG and subsequently the IMS and PSTN. LTE networks are secure and encrypted while public internet is not. Hence, to provide security, the communication to ePDG happens over an IPSec tunnel.
Since ePDG is just an IPSec server, it's IP address is resolved by a DNS query
epdg.epc.mnc<MCC>.mcc<MCC>.pub.3gppnetwork.org where MNC and MCC is the network
and carrier code respectively. You can find those codes by looking up in Android
What do Jio and AirTel do and why doesn't it work on my WiFi
Doing a DNS query for
epdg.epc.mnc861.mcc405.pub.3gppnetwork.org (this is for
Jio Karnataka) or
epdg.epc.mnc45.mcc404.pub.3gppnetwork.org. (this is for
AirTel) over a Public DNS server like Google DNS (220.127.116.11) or Cloudflare
(18.104.22.168) will return a blackhole server. It will not connect. Since my ISP's
DNS was very unreliable, I had set my router to use Cloudflare DNS. VoWiFi didn't
work at all. My phone repeatedly tried to connect to the blackhole server
draining battery forever.
$ dig epdg.epc.mnc861.mcc405.pub.3gppnetwork.org; <<>> DiG 9.10.6 <<>> epdg.epc.mnc861.mcc405.pub.3gppnetwork.org;; global options: +cmd;; Got answer:;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 53335;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1;; OPT PSEUDOSECTION:; EDNS: version: 0, flags:; udp: 1452;; QUESTION SECTION:;epdg.epc.mnc861.mcc405.pub.3gppnetwork.org. IN A;; ANSWER SECTION:epdg.epc.mnc861.mcc405.pub.3gppnetwork.org. 5 IN A 22.214.171.124epdg.epc.mnc861.mcc405.pub.3gppnetwork.org. 5 IN A 126.96.36.199;; Query time: 52 msec;; SERVER: 188.8.131.52#53(184.108.40.206);; WHEN: Fri Jun 12 04:21:18 IST 2020;; MSG SIZE rcvd: 145
However, one day I changed my DNS to use ISP's DNS. VoWiFi connected instantly. Strange. Upon digging I found that the response to the above DNS queries was different. I don't know why.
$ dig @192.168.0.1 epdg.epc.mnc861.mcc405.pub.3gppnetwork.org; <<>> DiG 9.10.6 <<>> @192.168.0.1 epdg.epc.mnc861.mcc405.pub.3gppnetwork.org; (1 server found);; global options: +cmd;; Got answer:;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 61133;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1;; OPT PSEUDOSECTION:; EDNS: version: 0, flags:; udp: 4096;; QUESTION SECTION:;epdg.epc.mnc861.mcc405.pub.3gppnetwork.org. IN A;; ANSWER SECTION:epdg.epc.mnc861.mcc405.pub.3gppnetwork.org. 0 IN A 220.127.116.11epdg.epc.mnc861.mcc405.pub.3gppnetwork.org. 0 IN A 18.104.22.168;; Query time: 35 msec;; SERVER: 192.168.0.1#53(192.168.0.1);; WHEN: Fri Jun 12 04:20:40 IST 2020;; MSG SIZE rcvd: 103
Possibly because these carriers do not want to give away free international roaming, they have set up custom DNS zone files with each of the major ISPs in India. Sadly for me, I now either have an option to drop calls (without VoWiFi) or slow internet (with ISPs DNS).
But thankfully, DNS works in surprising ways as well!
I edited the
hosts file on my router with manual DNS entries for my carrier's ePDG gateways.
It's a hack but it works until my carrier changes their ePDG gateway servers!