This week Tessares exhibited at MWC’21 in Barcelona. We went there for the purpose of demonstrating Wi-Fi Cellular Convergence using our Seamless Handover solution we successfully tested in our labs earlier this year.

During the past year, worldwide cellular data transmission has grown by 38% but operator revenues are down 2%. Operators are deploying 5G to increase capacity but it will be difficult for them to increase their prices to cover this cost.¹

Many mobile operators are also fixed network operators with millions of subscribers using Wi-Fi at home and at public hotspots. Initiatives like OpenRoam make it easier for subscribers to access these Wi-Fi hotspots. Most end users do not know or care about which network they use, especially if they have unlimited data plans. It makes sense for operators to use the least cost network.

Multipath TCP (MPTCP) is an extension of TCP which allows one TCP connection to be conveyed over several paths, even over different physical networks.

The 0-RTT Transport Converter allows client devices to use TCP extensions like MPTCP that are not supported by web servers or applications. This allows MPTCP to use multiple access networks without making changes to applications or servers. The MPTCP connections are terminated by the 0-RTT proxy on a Hybrid Access Gateway located between the server and the client devices where the networks converge.

On creating the connection, the client immediately sends a 0-RTT Convert message to the specified IP address and port of the 0-RTT converter. To minimize the latency, 0-RTT Convert protocol uses TCP Fast Open to exchange the address and port of the remote server during the connection’s handshake. 

Demonstration

At MWC21 in Barcelona this week we have been demonstrating a solution that we developed in our lab earlier this year. To accomplish this we ported the off-tree implementation of MPTCP to Linux kernel 4.9 used by the Xiaomi Pocophone F1 running Android 9.

Fig. 1 – Experimental setup schema

For the first scenario we measured handover performance for a Twitch client viewing video over TCP.  The smartphone was connected to a public LTE and a private Wi-Fi network at the same time.  The 0-RTT Converter is hosted in the public cloud and is configured to prioritise Wi-Fi.

When Wi-Fi is available, all of the traffic goes over Wi-Fi but when Wi-Fi is disconnected the connection seamlessly transfers to LTE.  Of course when Wi-Fi is available again, the connection seamlessly transfers back to Wi-Fi.  This minimises the use of the more costly LTE network without negatively impacting the user’s experience.

Fig. 2 – When Wi-Fi is available, it is prioritised but the user experience is not negatively impacted.

In the second test scenario, the xDSL (which is limited to 10Mbps for the purpose of the test) is not sufficient to carry the high volume of Twitch traffic. The data that cannot be carried over the Wi-Fi network is carried over the cellular network. This enables the video to be streamed at a higher resolution and without any interruptions. We also disconnect the Wi-Fi (at 28 seconds) and the session just continues over cellular. In this way we have shown that when Wi-Fi is available, the Wi-Fi network can carry the traffic overflowing to LTE only as necessary, reducing LTE usage while providing a level of user experience that would not have been possible on Wi-Fi alone.

Fig. 3 – When Wi-Fi cannot support full resolution video streaming, LTE can be used to boost the bandwidth.

Standards Based Road Map

This protocol has been adopted by the Broadband Forum for hybrid access and the 3GPP for Wi-Fi Cellular Convergence by introducing a new feature named ATSSS. ATSSS stands for Steering (network selection), Switching (changing from one network to another) and Splitting (using two networks at the same time).  ATSSS allows for the creation of policies that determine how multiple networks are used. The 3GPP selected the 0-RTT protocol to convert (proxy) TCP to MPTCP for ATSSS. 

Cellular network operators can deploy 0-RTT Converter servers in their backbone where their networks converge. In the long run, these deployments will migrate to 3GPP ATSSS in the 5G core but the Tessares overlay ATSSS solution allows operators to achieve Wi-Fi Cellular convergence today without upgrading to a 5G core.

Enterprises with campuses with private 4G/5G and Wi-Fi can take advantage of 0-RTT Converters as part of the service provider’s Multi-Access Edge Computing platform.

Conclusion

We have demonstrated an end to end, standards based solution which allows a smartphone to seamlessly move between networks for true Wi-Fi Cellular Convergence without negatively impacting the customer experience. Cellular network operators with access to fixed network assets can take advantage of this solution without having to wait for an upgrade to a 5G core and with 4G also being supported.

Learn more about how we modified a Smartphone to use MPTCP and 0-RTT

This work is supported by NGI POINTER (NGI Program for Open Internet Renovation) which has received funding from the European Commission, as part of the Horizon 2020 Research and Innovation Programme, under Grant Agreement Nº871528.

^{1 https://tefficient.com/category/analysis/mobile-data-usage-analysis/}