More answers from our webinar titled “WiFi Calling – Enabling a Seamless Customer Experience”

Aricent recently hosted a webinar titled “WiFi Calling - Enabling a Seamless Customer Experience”. In this webinar our experts discussed the various advantages and benefits of WiFi calling and how it can be implemented to provide a seamless customer experience.

Please Click here to view the archived version of the webinar.

Given the huge number of questions asked during the webinar, we were not able to answer every one of them live, so we've addressed them below.

1. How does EAP SIM for WiFi work with this infrastructure?

EAP-SIM or EAP-AKA is used for mutual authentication between the UE and the EPDG as a part of the IKEv2-based IPSEC establishment. In addition, should the WiFi be a public one, it is also possible that the WiFi connection may have been set up using EAP-SIM or EAP-AKA with 802.1X to begin with – but this is not necessary, and in fact the WiFi calling infrastructure does not need to know whether this is the case.

2. Will the IP address of UE change during seamless handover? Please elaborate on interoperability between WLAN and LTE

IP address of the UE only exists if the type of connection is WiFi or LTE, so for GSM/UMTS ⇌ LTE and GSM/UMTS ⇌ WiFi handovers this question is not applicable. For LTE ⇌ WiFi handovers, the IP address will typically not change during the handover, as there is a ‘Handover Indication’ flag in the Create Session Request GTP message for S5, S2a & S2b interfaces requesting IP address preservation. Even if this mechanism is not used, the SIP/IMS messaging allows for a change in IP address in an on-going call with some more extra messaging.

3. What are the enterprise fire wall considerations?

It is essential that an IPSEC tunnel be set up between the UE and the EPDG, and considering the prevalence of NAT, the EPDG supports tunneling of IPSEC over UDP and other NAT traversal techniques (including STUN & TURN.) So WiFi calling from inside enterprise WiFi networks should work unless explicitly blocked by firewall policy.

4. Would like to know if it's possible to setup a WiFi calling only MNO with the ability to roam on other networks.

It certainly is, and this can be the basis for a ‘WiFi First’ business. In such a case, the MNO in question obtains an MCC & MNC from the national regulator – but does not require any spectrum license. It requires to install a HSS (and can issue SIM cards), and also needs to install EPDG + PGW + PCRF + CSCF + TAS + SCC-AS + ATCF + ATGW functions (can be in a single box), and also a ‘PSTN Gateway’. It can then sign roaming agreements with other networks.

5. How do carriers guarantee a QoS over WiFi calling?

Due to the regulatory restrictions on the unlicensed spectrum, it isn’t really possible for a carrier to avoid interference on the wireless leg of any WiFi call, and so it cannot provide absolute guarantee of QoS. This is in spite of the QoS mechanisms available in the WiFi MAC layer which can be triggered by DiffServ markings on voice packets. If the call path traverses the open Internet, it is generally on best-effort basis, and so it is similarly not feasible to guarantee QoS, this is the case when the call is taking place over the EPDG (S2b). However, if S2a (SaMOG) is used along with a managed backhaul network to connect the TWAN and PGW, QoS can be managed on the backhaul leg at least. We would like to re-iterate, however, that if the Internet connection is high quality, a good QoE will be obtained even in absence of guaranteed QoS.

6. With VoWiFi, if, when & under what circumstances will WiFi be only network needed inside of buildings? What are the constraints? How WiFi and small cells work in the market place. What is the impact of VoWiFi on companies who provide DAS solutions

If every phone supports WiFi calling, and if every phone supports 5 GHz WiFi, and if the WiFi network in the building allows every phone lawfully inside it to connect through it, then WiFi is all that will be needed inside the building. Current problem is that a majority of phones only support WiFi on 2.4 GHz (which is crowded), and that WiFi roaming in public areas is a bit too ad-hoc (though Hotspot 2.0 deployments may solve this eventually) so in public indoor spaces a full solution needs small cells on the licensed spectrum – and/or DAS to complete the indoor coverage. If the building is a home, however, these conditions are roughly satisfied (2.4 GHz inside homes isn’t nearly as crowded).

7. Is WiFi a threat or boost for Telco Operators engaged on LTE? Why we need WiFi calling while telcos are working on VoLTE?

WiFi itself may be considered some kind of threat to the LTE mobile data business, but WiFi calling is actually a boost because it lets the carrier reap some revenue out of users who have chosen to use WiFi coverage instead of LTE. The reason why a user chooses WiFi instead of LTE can be because WiFi is cheaper or free (as in homes), or it may be because the LTE coverage quality where he is located is inadequate – in either case the carrier has already lost the user for mobile data. With WiFi calling the carrier can pull the user back a little with the promise of continued connectivity to the telephone network.

8. Does WiFi cover 9-1-1 calling?

WiFi calling does offer 911/112 emergency calls – but if the call is coming in over S2b (EPDG), the location of the caller can in general only be localized to the ‘911 address on file’ recorded with the home carrier. It may sometimes be possible to do better, but paradoxically because of the ability of WiFi calling to work from behind layers of NAT, it may become impossible to locate the origin on the call in real time based on the connection itself. Maybe the right way to deal with this is:

  • Have the dialer use cellular connection for emergency calls if any cellular signal is usable (over-riding user preferences)
  • If no cellular signal is usable, have the phones (which are smartphones after all) to report whatever it can see in its radio environment – which cellular cells are visible, what WiFi networks are visible, GPS/GLONASS readings – and use Google Maps style geolocation techniques to deduce the position.

9. What roles does ANDSF play? You have clients making the network decision vs a server?

ANDSF is orthogonal to WiFi calling. ANDSF is a feature that is supposed to assist (or maybe even force) selection of WiFi networks. WiFi calling does not depend on how the WiFi network was chosen.

10. How is Hotspot 2.0 and Docsis 3.1 improving the WiFi calling capability and experience and is this what is behind the Cablevision Freewheel service that has recently entered the market.

Hotspot 2.0 helps phones with SIM/USIM cards gain WiFi access in public hotspots, and to that extent it extends the reach of WiFi calling.

11. Is this not something like integrating WiFi and celluar network together and making it transparent to end user?

Yes, this is exactly what it is – but only for calling and texting.

12. Who do you think will benefit the most due to WiFi calling: fixed operators or mobile operators - and why?

The main beneficiary will be the mobile operators – they will be able to stem some revenue loss (to OTT voice), compensate for inadequate indoor coverage and alleviate radio congestion. The fixed broadband operator benefits very little as the data volume of the WiFi calling traffic is very small.

13. What revenue can be generated for telcos by adopting WiFi calling?

Mainly it can stem loss of voice revenue to ‘over-the-top’ competition. Please refer answer 7

14. What charging or billing requirements will telcos need to address WiFi Calling?

MS billing & charging will be sufficient to tackle WiFi calling – be it ‘contract’ (post-paid) or prepaid.

15. This 'hot' handover between current CDMA and Generic WiFi is exactly what Republic Wireless has engineered. Maybe the industry should be talking to them about licensing their solution.

They are surely on to a good thing – though they aren’t pioneers of ‘hot’ handover (that was UMA several years ago). We are not aware of the technical details of their implementation. What we have discussed in the webinar is how to improve on UMA for GSM & UMTS – and how to integrate CDMA into the mix too.

16. Is the "calling back" actually done at layer 3?

It is done above layer 3.

17. If you have different private WiFi networks, how is the handover handled? In this case all these private WiFi networks (hotspots) need to be integrated together to take up the WiFi calls and also recognize the phone devices under consideration.

As long as the phone can connect to all the hotspots in question automatically, handover will still occur – the phone detects the change and dials the STI URI again to reconnect. There is no need to integrate the WiFi networks.

18. Since we're relying upon IMS access, is this VoLTE?

It reuses a lot of VoLTE infrastructure if it happens to be available, but as it doesn’t use LTE radio access, it isn’t VoLTE as such.

19. How does QoS work in WiFi?

QoS for WiFi has been discussed in detail in answer 5

20. Why TWAN needs Security GW if the WiFi access is trusted? What about ePDG that the WiFi access is untrusted?

The security gateway is necessary to secure the communication between the TWAN (Access Point & Controller if any) at the deployment site and the carrier’s central office. This is obviously important if this traffic passes over the Internet, but it is also important if the traffic passes over some 3rd party managed network. While the carrier and the WiFi network may trust each other, they do not necessarily trust the 3rd party managed network operator with the content of data flowing between them. Note that the EPDG protects the traffic between the phone and the carrier, while the security gateway shown for the TWAN only protects the traffic between the WiFi network and the carrier. When TWAN is used, the WiFi calling traffic doesn’t need to flow through the EPDG if the TWAN supports the new SCM or MCM modes – but if the older TSCM is the only mode supported then the phone cannot figure out that it is in fact using a TWAN and will thus connect through the EPDG (i.e. set up an IPSEC tunnel) anyway.

21. In your opinion, with 1.3 million hotspots, how will MSO's (cable operators) integrate with this process as interface secured operators in the future? And how will the QOS be for the consumers?

Initially, no integration is required – if the end-user can get Internet access over the hotspot, he can do WiFi calling. Subsequently, if the MSO-s choose to become ‘trusted WiFi’ partners for the carriers, the end-users will see a small improvement in battery use for their WiFi calls.

22. How is the UE configured for WiFi ⇌ LTE handover from a radio perspective? I was under the impression Sprint was already supporting WiFi calling? Yes/No?

Sprint does support WiFi calling – but does not support WiFi ⇌ Cellular handovers as yet.

23. What role does 802.11ai have in WiFi calling/roaming? Is that required and when will that be available?

IEEE 802.11ai is about fast initial link set up (under 100 ms), and is a very good thing. WiFi calling and all other WiFi applications will benefit from it – but it isn’t required as such. The infrastructure at the carrier end is entirely transparent to whether it is in use.

24. How do you identify trusted and untrusted WiFi access network?

When using 802.1x/EAP authentication for WiFi, if the phone uses EAP-AKA’, the WiFi network can indicate over the EAP transaction that trusted WiFi access (Single Connection Mode or Multiple Connection Mode) to the phone’s home carrier is available. If the phone supports any of these modes, it can proceed to set up a trusted connection.

25. In slide 17 why TWAN needs Security GW while ePDG doesn't need where the WiFi access is untrusted?

The EPDG is itself a security gateway (like a VPN server). Also see answer 20 above.

26. What is the expected impact of WiFi calling on Enterprise solution? Especially, if I already have WiFi APs in place.

WiFi calling will be possible through enterprise WiFi networks unless the enterprise implements a policy that blocks them.

27. Do you expect the VoWifi traffic to eclipse the VoLTE in the future?

Cisco certainly thinks so (please see their VNI report dated February 2015)

28. Where do we need Security GW and IPsec when we do WiFi Calling?

WiFi calling traffic traverses the Internet, where the privacy of the traffic cannot be assured. IPSEC between the phone and the EPDG or WiFi Network and Security Gateway assures that privacy. See answer 20.

29. How will the WiFi calling deliver the location to the PSAP within 300 meters?

An IP address delivered is not location information. Please refer answer 8

30. What network speed will be required to use WiFi? Will DSL work?

2 Mbps or more should be good enough – and DSL will certainly work if the rates in both directions are at least this number.

31. Will WiFi calling be free, or will minutes still be billed to the service provider?

It is up to the service provider – they will have the ability to bill the minutes, but whether they choose to do so is up to them. They may choose, for instance, to bill these minutes at a lower rate to incentivize the use of WiFi, or zero-rate such access altogether.

32. Are you finding that WiFi calling can actually help relieve network strain for carriers and free up band-width for them? This could be a selling feature?

Yes, WiFi calling can go a long way in reducing the strain on carrier networks.

33. What about security and privacy of my calls in WiFi network or set of WIFI networks?

They are secured – as secure as they are when you call through a cellular radio connection.

34. What are the economics of the WiFi calling solution? What is the relative cost of a voice minute of use over licensed mobile compared to the various WiFi calling scenarios?

A WiFi call does not using the radio spectrum licensed to the carrier – and allows the carrier to accommodate another call at the same time over that radio spectrum.


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