Traffic offloading procedures

The typical mobile traffic offloading scenario consists of six major steps: offloading initiation, context collection, offloading decision, network associ- ation, data transmission, and offloading termination.

1. Offloading Initiation – The offloading procedure can be initiated by the network side (network-driven offloading), or by the mobile system 15 _{Wi-Fi Direct, WiFi Alliance:}

http://www.wi-fi.org/discover-and-learn/ wi-fi-direct

(user-driven offloading). Network-driven offloading can be triggered by dedicated signaling protocols such as router advertisement [76], enabling operators to dynamically manage and balance the traffic load. User-driven offloading is often triggered by applications that need to access the Internet for content, which is based on the demand of the user.

The network-driven offloading introduces overhead in terms of extra signaling and potential energy cost, but it can offer timely and op- timized offloading guidance based on the comprehensive knowledge from the network side, such as network structure and condition. On the other hand, user-driven offloading avoids the extra signaling cost but lacks network context, making it less efficient for users at high moving speed.

In the initiation phase, it is important to keep the existing data flows uninterrupted to guarantee consistent service experience. For data flows that are sensitive to connectivity interruption, one good practice is to keep using the existing channel until the ongoing flows complete and then offload new flows to the new channel. For flows that can tolerate connectivity interruption, we could apply the delay-transfer scheme [5] to postpone the data transmission for a delay tolerance threshold and then offload the ongoing flows to the new channel once the connectivity is established.

2. Context Collection – The context information is essential for mobile traffic offloading as input to make the offloading decision. Users can obtain context information either from network operators or from their surrounding access environment. The key information includes the user location, potential offloading targets, condition of access net- work and connection details such as extended service set identifica- tion (ESSID) or MAC addresses, signal-to-noise ratio of WiFi access points, and wireless fingerprint information [77].

The collected context information will be sent to either remote con- trolling servers or local management components. For the remote option that utilizes cloud support, a dedicated signaling channel is required such as cellular data connection. Therefore the proposals relying on remote support are limited by the channel condition, es- pecially when such a channel is congested. This also affects the scal- ability due to the dependence on a centralized entity. Compared to the remote approach, the local solution does not depend on external entities. However, by relying solely on local resources, context infor-

mation can be incomplete or less accurate compared to the remote option.

3. Offloading Decision – The decision process involves computation ac- cording to the pre-defined algorithms and operation logic, and deliver- ing control messages to mobile devices to carry out traffic offloading. By taking the context information as input, an offloading decision can be made either at the network side or using local resources on the mobile device.

By offloading the computation to the network side, we can improve ef- ficiency in terms of latency by using the powerful hardware. However, this approach depends on the infrastructure support and requires net- work connectivity. On the other hand, local decision is more flexible and robust to network conditions, but at the cost of local resources such as energy. The local operation also suffers from the limitation that there is limited external knowledge available for improving the accuracy of offloading decisions.

4. Network Association – Based on the offloading decision, mobile de- vices need to perform network association to enable traffic offloading. The association process includes access/peer discovery via pre-defined configuration protocols such as DHCP [78] and DNS [79, 80] to es- tablish connectivity to the target offloading networks.

When users are moving at high speed, the connectivity period for offloading is often short. This demands an efficient association sup- ported by optimized protocols to avoid excessive association cost, which can decrease the time for data transmission.

5. Data Transmission – As the key part of mobile traffic offloading, data transmission determines how much data can be offloaded from the congested mobile networks in order to improve the overall service quality. Depending on the type of traffic such as real-time streaming, delay-tolerant traffic and web surfing, the offloading design can utilize the characteristic of traffic for optimization.

In the short period of offloading, which is typical for mobile users, the bitrate and condition of the wireless access can affect the off- loading efficiency. At the same time, the offloading design also needs to consider the hardware limitation on existing mobile devices such as restricted size and output of wireless antenna.

6. Offloading Termination – A successful offloading session should be terminated by closing the temporary offloading connection and smoothly

switching to other available networks. The prior research on handover mechanisms has illustrated how to seamlessly migrate from one access network to another [55], [63]–[68], [81]–[84].

To achieve efficient and smooth termination, guidance can be ob- tained from the network side or from the local heuristic prediction to spot potential connectivity [85, 86, 87]. The termination process is one of the key factors affecting the adoption of mobile traffic off- loading.