Wi-Fi offloading helps network operators cope with rapidly increasing data traffic from mobile devices, writes Adrian Schumacher from Rohde & Schwarz.
Smart devices make it convenient to access the Internet from virtually anywhere because the cellular network can deliver content over broadband links such as HSPA+ and LTE at speeds similar to those in the home or office. Many mobile users are streaming music and video or sending e-mails with large attachments, the accumulated data traffic can easily approach the capacity of the cellular network. There have already been many instances in which required capacity has exceeded available capacity, resulting in websites that are slow to load and video that is almost impossible to view.
The percentages of web browsing, e-mail, music and video over mobile links is set to explode in the next few years.
Although there are some true “on-the-go” uses of data-intensive applications, most mobile subscribers are stationary when they watch a video, listen to music or browse websites. Places of highest traffic density – hotspots – are found indoors in urban areas where users tend to be stationary and do not necessarily have to depend on the cellular network because other broadband technologies are available.
Smart offloading and the architecture
In order to cope with increasing data traffic areas, mobile network operators (MNOs) can add more cells or install more advanced base stations. Both strategies require a great deal of planning and are expensive. These costs can be minimized, however, since all smartphones and tablets have integrated Wi-Fi. Thanks to the relatively high data rates and the availability at home and public places, people tend to use Wi-Fi often. In fact, about 70% of global smartphone-originated traffic already goes over Wi-Fi, according to a recent study.
Because the subscribers consuming high data volumes usually have an unlimited data plan (or a “flat-rate plan”) MNOs can derive significant benefit from passive offloading.
WLAN has the benefits of small cells such as home use and hotspots. It does not interfere with the frequency bands of macro-cells. The 5GHz band has a bandwidth of 500-800 MHz available for WLAN. Therefore, MNOs perceive WLAN as a financially attractive complement to their cellular network. As a result, LTE networks are being deployed for area coverage with WLAN networks for capacity in high traffic areas.Smart offloading is the controlled data traffic offload from the cellular network to Wi-Fi (and in the reverse direction). It should happen seamlessly so the user does not need to search for an appropriate network and enter user credentials such as a password. Ideally, the MNO can control the data offload dynamically and selectively based on the location, time of day, network loading, user’s subscription and other factors. For the actual offloading, all the required technologies are available and will be discussed in the following sections. Figure 1: an example of an offload scenario. As the smartphone moves from a home or office through the city, it automatically hands over to Wi-Fi hotspots where available.3GPP architectureAs a result of studies carried out during the development of the system architecture evolution (SAE), support for non-3GPP access systems such as WiMAXTM and WLAN was added and interfaces for interworking between these systems were defined. Figure 2: Trusted non-3GPP access to the EPC Trusted and untrusted connection requests are treated differently for non-3GPP access. Figure 2 shows the main network entities for the trusted access, e.g. if the WLAN network uses secure transmissions. The untrusted access is routed from the Packet Data Network Gateway (PDN GW) over the S2b interface to an evolved packet data gateway (ePDG) that acts as a filter and firewall, then over the SWn interface to the WLAN. In that case, it is required that a device connected over WLAN sets up an IPsec tunnel to the ePDG.IEEE 802.11u and Wi-Fi Hotspot 2.0Connecting to a WLAN access point (AP) generally requires the user and device to select the desired AP and provide appropriate credentials for authentication. The IEEE 802.11 standardization group has released an amendment (IEEE 802.11u) in 2011 that includes media access control (MAC) enhancements that automate and speed up the process of joining a network. It basically extends the beacon that APs broadcast, and adds protocols such as the access network query protocol (ANQP).The Wi-Fi Alliance defines a set of functions from the IEEE 802.11 WLAN standards and its amendments that are guaranteed to work on Wi-Fi Certified™ products. In 2012, the Alliance also released the Wi-Fi Hotspot 2.0 specification, which defines the required capabilities for APs, mobile devices, and the operators.Several protocols and functions are required for seamless data traffic offload. These range from the possibility of controlling the actual offload to an authenticated secure link to the IP flow mobility for session continuity.Access network discovery and selection functionMobile phones today typically have multi-mode chipsets that support multiple radio access technologies such as GSM and WCDMA. It won’t be long before many phones also have LTE, WLAN and perhaps TD-SCDMA. The access network discovery and selection function (ANDSF) provides the user equipment (UE) with policies on intersystem mobility and routing, and assists in access network discovery. It offers a way for MNOs to dynamically control and define preferences — that is, how, where, when, and for what purpose a device can use a certain radio access technology. It can be used for both inter-technology and intra-technology access network selection but it should not influence the network selection and reselection procedures as already specified in 3GPP.ANDSF was first introduced with 3GPP Rel. 8 (TS 24.312). The ANDSF server is an entity in the Evolved Packet Core (EPC) that communicates with the client UE over the S14 interface (TS 23.402) which is realized above IP level. By sending an open mobile alliance (OMA) device management (DM) message, the UE gets the policies. These are described with managed objects (MOs) and are defined in XML.Generic Advertisement Service and the Access Network Query ProtocolIf a Wi-Fi device has found an AP and is allowed to connect, it is still not certain that this particular AP is authentic. In order to discover additional details, a mechanism called generic advertisement service (GAS) was introduced. This allows a device to query information from the AP. Figure 3: IEEE 802.11u/Hotspot 2.0 Access Network Query ProtocolIn Figure 3, an AP that is compliant with Wi-Fi Hotspot 2.0 begins the interaction by sending beacons that are extended with more information. Using the access network query protocol (ANQP), a device sends a “GAS initial request” that inquires which roaming consortium it belongs to, which network authentication to use, a NAI realm list, or even the location and WAN metrics. The device will receive the “GAS initial response” with these information elements and can then decide whether or not to connect. If the AP is already very busy, has a slow connection to the Internet, or belongs to a provider that charges excessive rates, a device may keep looking for another AP. When it decides to connect, it already knows how to authenticate.AuthenticationWi-Fi networks are considered secure if they use the appropriate authentication method. The IEEE 802.1X standard describes such a method where the authenticator checks the supplicant’s credentials with an authentication server (RADIUS). It uses the extensible authentication protocol (EAP). If a device automatically discovers a WLAN hotspot belonging to the subscriber’s MNO, it needs to authenticate automatically. For the user it is only seamless if he/she does not need to enter any username and password. For that purpose there are EAP methods that make use of the available SIM/USIM in the device. The MNO’s hotspot can verify the UE’s identity with the MNO’s Home Subscriber Server (HSS), and in that case EAP-SIM, EAP-AKA or EAP-AKA’ can be used. Devices without SIM/USIM will need to use either EAP-TLS with a certificate or EAP-TTLS with MSCHAPv2 if a username and password can be supplied. Flow MobilityTo support a seamless handover, some sort of IP flow mobility must be implemented. Client-based mobility is one option. This requires that the device runs a special stack that can handle changes in the connection. The Dual Stack for Mobile IPv6 (DSMIPv6, IETF RFC 5555) is an example that enables session continuity for IPv4 and IPv6 packets. On the other hand, there is network based mobility that does not require changes in the devices. Proxy Mobile IPv6 (PMIPv6, IETF RFC 5213) is an example of that. For full flow mobility across all interfaces, however, additional extensions are required. On the 3GPP side, protocols such as local IP access (LIPA) allow the direct routing of traffic between devices in the same cell, and selected IP traffic offload (SIPTO) for routing the Internet traffic directly. These protocols are standardized in 3GPP TS 23.829. A newer protocol developed for Rel.10 and onward is IP flow mobility (IFOM, 3GPP TS 23.261). The rapid increase of mobile data traffic demands an infrastructure that can deliver huge amounts of data less expensively than current cellular technologies. WLAN is a widely used and accepted technology. With the help of enhancements such as IEEE 802.11u and Wi-Fi Hotspot 2.0 as well as non-3GPP access standardized by 3GPP, MNOs can use these technologies to complement each other. In fact, many mechanisms and protocols already exist and operators such as Swisscom, KDDI and MetroPCS have been using Wi-Fi offloading for quite some time. But the variety of options must be reduced to one or a few options for a wide-spread and interoperable success. Future network upgrades and smartphone generations will continuously enhance the experience. At the same time, new extensive and complex tests need to be performed to ensure the dynamic provisioning and correct interpretation of policies.
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