Wi-Fi is not as ubiquitous as cellular but finding Wi-Fi is like finding a coffee shop. Though there are initiatives to make it more ubiquitous as it has potential to cater largest share of internet data flowing over wireless access networks.

Before going into the main theme of cellular (note, though i want to generalize here for cellular but i am actually specific to LTE/EPC network while being talking for cellular) and Wi-Fi convergence, let’s go through some of the points from the cisco VNI report for 2015-2020.

Global mobile data traffic grew 74 percent in 2015. Global mobile data traffic reached 3.7 exabytes per month at the end of 2015, up from 2.1 exabytes per month at the end of 2014.

Mobile offload exceeded cellular traffic for the first time in 2015. Fifty-one percent of total mobile data traffic was offloaded onto the fixed network through Wi-Fi or femtocell in 2015. In total, 3.9 exabytes of mobile data traffic were offloaded onto the fixed network each month.

And the forecast …..

Mobile data traffic will reach the following milestones within the next 5 years:

●   Monthly global mobile data traffic will be 30.6 exabytes by 2020.

●   The number of mobile-connected devices per capita will reach 1.5 by 2020.

●   The average global mobile connection speed will surpass 3 Mbps by 2017.

●   The total number of smartphones (including phablets) will be nearly 50 percent of global devices and connections by 2020.

●   Because of increased usage on smartphones, smartphones will cross four-fifths of mobile data traffic by 2020.

●   Monthly mobile tablet traffic will surpass 2.0 exabytes per month by 2020.

●   4G connections will have the highest share (40.5 percent) of total mobile connections by 2020.

●   4G traffic will be more than half of the total mobile traffic by 2016.

●   More traffic was offloaded from cellular networks (on to Wi-Fi) than remained on cellular networks in 2015.

●   Three-fourths (75 percent) of the world’s mobile data traffic will be video by 2020.

3GPP is continuously evolving its Wi-Fi integration architecture, more attentively since 2008 with rel-8 along with LTE and recent advancement in rel-12 and in rel-13, where it going to be more like RAN scenario as the radio specific standards will incorporate Wi-Fi network related parameters. 3GPP has created the dichotomy of Wi-Fi access as trusted and not-trusted and its solution evolution has been carried around this dichotomy so far. Though Wi-Fi has developed to the advancement to have it as secure access and there are mechanism, like HOTSPOT 2.0, defined for making Wi-Fi as trusted network, But Wi-Fi is more like a mushroom network and in perception is always untrusted.

Assuming Wi-Fi ‘untrusted’ is also the need for providing ubiquitous solution as that makes the solution universal to accommodate any kind of Wi-Fi access, that’s where the ePDG has become synonymous to Wi-Fi offload solution. ePDG provide IPsec tunnel with the end device over the underlying Wi-Fi access network and authenticate devices (with AAA server) and provide access to EPC core. But Rel 11 onward 3GPP is more inclined to refine it trusted access solution that is SaMOG/TWAG based with the assumption of the secure radio access due to HOTSPOT 2.0 compliance Wi-Fi APs.

All the advancements for the need of Wi-Fi offload, like HOTSPOT 2.0, ANDSF, RAN specific incorporation's in rel-12 & rel-13 etc., are of necessity but could not be presumed as of being 'must'. The transformation is sequel not a leap forward. We have to think about a universal approach cater to all, at least for few more years - the ecosystem need to emerge in time.

What's the crux.....

3GPP solution for Wi-Fi integration with cellular has been under the grip of what I call a inter-RAT cellular paradigm like hand off from one RAT to other RAT. Which is not feasible at all between these two technology as they are of different nature and from different origin. we always think to integrate the two non-isotopic networks with complete hand offs i.e. if a device move from one to other it should completely be with the one, more like 3GPP inter-RAT hand-offs paradigm. But those two networks are of their own sovereignty and completely unrelated and with such paradigm the required control or seamless transfer is not feasible at all. Therefore 3GPP solutions find only point of convergence, at EPC/PGW (rel-8 onwards), as for the shake of creating a IPCAN session and retain the IP address and so the mobility. Solution introduced interfaces with AAA server for network authentication and for retaining the context, QoS, etc.

The solution are seemingly more inclined to IETF kind of approaches for access connectivity, i.e. connect, authenticate, solicit and access with mobility through anchor router like HA or here PGW. These integration approaches are superimposed with conceptual perception of hand off which is deliberately made feasible but not there by design.

That's where term 'Wi-Fi offload' does not bring the fidelity, as there is no feasibility by design. The most buzzing term in these solutions is ‘seamlessness’ and also the theme of differentiation and innovation for the solutions mustering up for Wi-Fi offload.

What is of interest to us...

Wi-Fi is not there for hand off or to say in popular term offload. It could be leveraged as associated network. An associated data path to LTE network by converging at RAN or IP-RAN through a Local PGW and keeping the common EPC core. This convergence is possible if S1AP will be terminated at this Local PGW and MME is kept transparent to converged Local network. EPC/PGW while creating the bearer, based on IPCAN session (contains the policy), will provide the information, based on policy, which will help Local PGW to distinguish and forward the data flow.

Wi-Fi and cellular could be converged at radio access level and Wi-Fi can be used an associated data channel for the mobile access. This feasibility is well accepted now and companies like Qualcomm have more obsessed approches. Qualcomm is pursuing the aggregation at radio link level with a point of convergence at PDCP layer (https://www.qualcomm.com/invention/research/projects/lte-advanced/lte-wi-fi-interworking). 

On the convergence of two at RAN, the small cell forum is also putting its attention through research and industry surveys. Its recent whitepaper, feb 2016, titled "Industry perspectives, trusted WLAN architectures and deployment considerations for integrated Small-Cell Wi-Fi (ISW) networks" SCF states at section 2.0 Integrated small cell Wi-Fi (ISW) networks.....

"other interesting alternatives are possible, namely integration in the SC-APs (i.e. RAN-based integration) and/or in SC-gateways (i.e. GW-based integration).

Here, the Integration function resides at the edge, possibly in an integrated ISW-AP. RAN-based integration of licensed/unlicensed access is now being addressed by 3GPP release 13, including approaches for RAN based integration of Wi-Fi and LTE.

Finally, architectures that integrate Wi-Fi and SCs at the gateway level are possible. For example, the SC-GW (i.e. H(e)NB-GW) as well as Wi-Fi GW (i.e. ePDG and/or TWAG/TWAP) may be realized together, along with associated integration functions. At the time of writing, these architectures are still in consideration and development."

So that's the future.....

This could be done at network level, where you need not to go all the way to PGW at EPC but having a local PGW at RAN/IP-RAN network. The advantage it will bring is the common EPC for both cellular and Wi-Fi networks i.e. no need for the transfer of PDN connectivity context like an inter-RAT scenarios. Only thing is that the solution is not for any kind of Wi-Fi device but it is for cellular device equipped with Wi-Fi as we need cellular for all the service control level functionality.

Service delivery is well controlled through the 3GPP PCC architecture that define PCRF, PCEF (PGW) and Application server (AS) level interaction and coordination also referred to as IPCAN session. In EPC we need bearer to carry the traffic for specific service. This bearer traffic is delivered to RAN to reach to end devices over air interfaces or radio channels. EPC provide all the necessary information as QoS parameter for specific bearer to RAN for required radio channel capacity. The interface between EPC and RAN is S1 (S1-AP for control path and S1-U for data path).

Instead of having direct interface between eNB and EPC if we keep a Local PGW node (L-PGW) to interface with EPC and provide convergence of Wi-Fi network at this L-PGW . A replica of this PCC architecture can be implemented at this L-PGW level which will decide to deliver the service at Wi-Fi of Cellular radio. This will definitely need the modification on RAN and EPC interfaces like s1AP and NAS etc. as L-PGW has to coordinate with MME. 

There is nothing to scratch to get go, the provisions of such nature framework and already there with 3GPP, like for LIMONET (http://www.3gpp.org/DynaReport/WiVsSpec--500028.htm). We can leverage this for convergence of the two networks at local network level with a common core.

The Standards for LIPA specific work at 3GPP (23.859) provide the data path connectivity compressed till local network, although the signaling for that remain intact like a normal PDN connectivity. This could be taken in principle for the convergence of two networks at local network level.

We believe strongly that Service provider can create yielding business case around a associated Wi-Fi network. The convergence at RAN and at network layer level would be a pure software solution with existing infrastructure and can be a ready to move solution. Other specific approaches will be requiring necessary ecosystem around for their success.

We are actively seeking the support and sponsors to extend our POC work. We need support from service provider and OEMs for widen-out our POC development. Please feel free to contact us for detail and discussion.

Note : We are strongly pursuing on our believe on next gen network with a central theme of "Homogeneous connectivity through heterogeneous networks" the solution for Wi-Fi convergence to cellular is inclined to this theme. We believe technology like MEC is going to give a master boost to our center theme.