Fully network controlled WLAN interworking

Using Wi-Fi/WLAN to offload traffic from mobile networks and use it as an extension to the cellular network to handle the always increasing bandwidth demands of users is becoming more and more interesting from both the operator’s and end user’s point of view.

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Master Researcher, Networks

Master Researcher, Networks

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Some of the reasons for this tendency are the additional spectrum (currently, 85MHz in the 2.4GHz band and close to 500MHz in the 5GHz band), the high data rate offered by WLAN in lower interference conditions and the fact that almost all smartphones and portable devices currently available in the market support Wi-Fi.

Most WLAN deployments today are totally separate from cellular networks, and can be seen as non-integrated from the terminal perspective. Current mobile terminals support a simple WLAN offloading mechanism where all IP traffic is steered to WLAN upon a detection of an accessible WLAN with received signal strength above a certain level. There are several drawbacks of such a WLAN offloading mechanism since no consideration of expected user experience is made, as illustrated in Figure 1. For example, a terminal might be offloaded from a high data rate cellular connection to a low data rate WLAN connection (e.g. loaded WLAN access point).

Figure 1: Drawbacks of current WLAN selection mechanisms

Thus, by facilitating mechanisms whereby terminals get connected to the proper WLAN when needed/available and also distributing the data traffic of the terminal between WLAN and 3GPP, an operator can enhance the user experience in its network. This is illustrated in Figure 2. The black curve shows the performance when WLAN is disabled and only LTE is used, and as expected, when the total load in the network increases, the performance per terminal degrades. The red curve shows the performance if WLAN is enabled and terminals select it whenever they are in WLAN coverage, regardless of the signal quality or load conditions. In low load conditions, this results in most of the terminals getting as low as 50% of the throughput they could have gotten if they have not steered their traffic to WLAN. When a network controlled steering, which considers the radio and load conditions in both LTE and WLAN, is applied (green curve), the performance, especially under low and medium load conditions, is greatly improved.

Figure 2: Enhancing performance via network controlled steering

There is currently quite intense activity in the area of operator-controlled WLAN in several standardization forums. Operator controlled WLAN here refers to a WLAN that is deployed and managed by an operator or its partners.

In 3GPP, WLAN/3GPP interworking features for release 12 are being finalized, aiming to improve operator control w.r.t. how a UE performs access selection and traffic steering between 3GPP and operator-controlled WLANs. A network assisted solution has been agreed upon where the 3GPP RAN provides assistance parameters (such as 3GPP and WLAN signal level thresholds which indicates under which signal conditions the terminal shall steer to 3GPP or WLAN , list of eligible WLANs, etc…) to the terminals, in broadcast and/or dedicated manner, and the terminal is also provided with RAN rules/policies that make use of these assistance parameters. The rules, in conjunction with the assistance parameters, will help the terminal in selecting and steering traffic to the best access network, be it 3GPP or WLAN. Access Network Discovery and Selection Functionality (ANDSF), which has been available since 3GPP release 8 for semi-static access network selection and traffic steering, has also been enhanced for release 12 to make use of the assistance parameters provided from the 3GPP RAN. Figure 3 shows a high level overview of the WLAN interworking feature in 3GPP release 12.

Figure 3: 3GPP release 12 WLAN interworking

In Wi-Fi Alliance (WFA), activities related to certification of WLAN products are undertaken under the umbrella of Hotspot 2.0 specifications. This is, to some extent, driven from the need to make WLAN a viable wireless technology for cellular operators to support high bandwidth offerings in their networks, by offering non-manual seamless authentication and roaming functionalities.

In the network assisted solution being standardized in release 12, the 3GPP RAN just communicates the thresholds for steering to/from WLAN. It does so without an explicit knowledge regarding which terminals are in WLAN coverage and if so, if the WLAN coverage is good or bad. Also, since the decision to steer is made by the terminals, when there is no more traffic to/from a terminal, the 3GPP RAN will not be able to differentiate if the terminal has steered its traffic to WLAN or the traffic has been terminated instead. Thus, the behavior of connected terminals will, in some aspects, be unpredictable. In load balancing situations, we believe that it would be advantageous to know exactly which terminals should be steered to WLAN and when the steering should happen. Such a solution would also be harmonized with the network controlled solution used for intra-3GPP handovers; terminals send measurement reports and receive steering commands from the network based on reported measurements and network load/performance. One realization of a fully network controlled solution is illustrated in Figure 4.

Figure 4: Fully network controlled WLAN interworking

We at Ericsson are currently working on the details of such a fully network controlled solution.

Oumer Teyeb Ericsson Research

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