TO DIFFERENT ACCESS TECHNOLOGIES

Technical Field

The present disclosure is generally related to communications networks, and more particularly relates to terminal devices and to the control of which mechanism is used by the terminal device for access network selection and/or traffic steering or routing between networks operating according to different radio access technologies, RATs.

Background

The wireless local-area network (WLAN) technology known as "Wi-Fi" has been standardized by IEEE in the 802.11 series of specifications (i.e., as "IEEE Standard for Information technology— Telecommunications and information exchange between systems. Local and metropolitan area networks— Specific requirements. Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications'). As currently specified, Wi-Fi systems are primarily operated in the 2.4 GHz or 5 GHz bands. The IEEE 802.11 specifications regulate the functions and operations of the Wi-Fi access points or wireless terminals, collectively known as "stations" or "STA," in the IEEE 802.11 , including the physical layer protocols, Medium Access Control (MAC) layer protocols, and other aspects needed to secure compatibility and inter-operability between access points and portable terminals. Because Wi-Fi is generally operated in unlicensed bands, communication over Wi-Fi may be subject to interference sources from any number of both known and unknown devices. Wi-Fi is commonly used as wireless extensions to fixed broadband access, e.g., in domestic environments and in so-called hotspots, like airports, train stations and restaurants. Recently, Wi-Fi has been subject to increased interest from cellular network operators, who are studying the possibility of using Wi-Fi for purposes beyond its conventional role as an extension to fixed broadband access. These operators are responding to the ever-increasing market demands for wireless bandwidth, and are interested in using Wi-Fi technology as an extension of, or alternative to, cellular radio access network technologies. Cellular operators that are currently serving mobile users with, for example, any of the technologies standardized by the 3rd-Generation Partnership Project (3GPP), including the radio-access technologies known as Long-Term Evolution (LTE), Universal Mobile Telecommunications System (UMTS)/Wideband Code-Division Multiple Access (WCDMA), High Speed Packet Access (HSPA) and Global System for Mobile Communications (GSM), see Wi-Fi as a wireless technology that can provide good additional support for users in their regular cellular networks.

Many of today's portable wireless devices (referred to hereinafter as "user equipments", "UEs", or more generally "terminal devices") support Wi-Fi in addition to one or several 3GPP cellular technologies. In many cases, however, these terminal devices essentially behave as two separate devices, from a radio access perspective. The 3GPP-specified radio access network (RAN) and the UE-based modems and protocols that are operating pursuant to the 3GPP specifications are generally unaware of the wireless access Wi-Fi protocols and modems that may be simultaneously operating pursuant to the 802.1 1 specifications. Techniques for coordinated control of these multiple radio-access technologies are needed.

It is currently discussed in 3GPP how 3GPP Radio Access Technologies (RATs) can be integrated and/or interwork with WLAN. The focus of this work is how to perform access network selection and/or traffic steering or routing between 3GPP and WLAN.

Three alternatives for performing access network selection and/or traffic steering or routing between two radio access technologies (RATs), such as a 3GPP RAT and Wi- Fi, are described below. Each of these alternatives are considered to be based on one or more predefined rules, where the rules are predefined in a 3GPP specification.

1. Threshold based approach - This approach is based on conditions and thresholds provided to the terminal device by a first RAT (e.g. a network operating according to a first RAT, such as a 3GPP-specified RAT or WLAN). The conditions and thresholds dictate the situations in which the terminal device should steer traffic from/to a second RAT (e.g. a network operating according to a second RAT, such as another 3GPP- specified RAT or WLAN).

For example, the rule could take the form of Example 1 below where threshold^ threshold2, threshold3 and threshold4 are provided to the terminal device by the 3GPP network (i.e. a network node in the 3GPP network such as an eNodeB, NodeB or radio network controller (RNC)). if (3GPP signal < threshold !) && (WLAN signal > threshold2) {

steerTrafficToWLANO;

} else if (3GPP signal > threshold3) || (WLAN signal < threshold^ {

steerTrafficTo3gpp();

}

Example 1

If the 3GPP signal measured by the terminal device is below thresholdl and the WLAN signal measured by the terminal device is above threshold2, this rule provides that the terminal device should steer traffic to WLAN. Otherwise, if the 3GPP signal is above threshold3 or the WLAN signal is below threshold^ the rule provides that the terminal device shall steer traffic to the 3GPP network.

The term '3GPP signal' herein could mean the signal transmitted by a radio network node belonging to a 3GPP RAT, e.g. a node in a LTE, HSPA, GSM etc. network, and/or it could be the quality of such a signal. The term 'WLAN signal' herein could mean the signal transmitted by a radio network node belonging to WLAN, e.g. an access point (AP) etc., and/or it could be the quality of such a signal. Examples of measurements of 3GPP signals include are reference signal received power (RSRP) and reference signal received quality (RSRQ) in LTE or common pilot channel (CPICH) received signal code power (RSCP) and CPICH Ec/No in HSPA. Examples of measurements of WLAN signals are Received Signal Strength Indicator (RSSI), Received Channel Power Indicator (RCPI), Received Signal to Noise Indicator (RSNI), etc.

2. Traffic steering command based approach - In this approach, a first RAT, e.g. a 3GPP RAT (a network operating according to a 3GPP RAT), controls a terminal device's connection to a second RAT, e.g., a WLAN, based on measurement reporting from the terminal device to the 3GPP network and by the 3GPP network sending traffic steering commands that order the terminal device to steer traffic from/to the second RAT. In short, this approach is based on three messages and some associated procedures that allow the 3GPP network to determine when a terminal device should associate with a WLAN or, more generally, to a network operating according to a second (possibly different) radio access technology (RAT). The procedure is illustrated in Figure 1. The first message, a reporting configuration message (message 1), is sent from the 3GPP network (3GPP radio access network (RAN) 10) to the terminal device 12 and configures the terminal device 12 with a set of criteria (e.g. a set of conditions and/or thresholds) for enabling, detecting, or performing measurements over the second network (WLAN 14).

One possible set of criteria contained in one possible reporting configuration message is as follows:

Received signal strength indicator (RSSI) in WLAN > X

Reference signal received power (RSRP) in 3GPP < Y and/or

· BSS Ioad < Z

Other metrics that can be considered included RCPI, RSNI, RSRP, uplink (UL) and/or downlink (DL) backhaul rate, etc. The terminal device 12 subsequently sends a terminal report, message 2, to the 3GPP network 10, when the criteria given in the first message (message 1) have been fulfilled. The 3GPP network 10 evaluates the content of the terminal report, along with any other reports or information that the network 10 may have available, such as backhaul congestion, delay, subscription information and interference, and determines whether or not to steer the terminal device's traffic to WLAN 14. The third message (message 3), a traffic steering message or command, is an indicator sent from the 3GPP network 10 to the terminal device 12 that the terminal device 12 should steer all or a subset of its traffic to WLAN 14. The traffic steering message may indicate a specific target access point (AP) in the WLAN 14, such as a prioritised AP or WLAN or it could just be a command telling the terminal device 12 to steer its traffic to WLAN 14 and the terminal device 12 and WLAN 14 determine which particular AP should be used. The traffic steering message may also indicate which traffic should be steered to WLAN 14 and/or which traffic should be kept in the 3GPP network 10. 3. Access Network Discovery and Selection Function-based approach - The Access Network Discovery and Selection Function (ANDSF) is an entity defined by 3GPP for providing access discovery information as well as mobility and routing policies to the UE. ANDSF is a new entity added to the 3GPP architecture in Release 8 of 3GPP TS 23.402 (See "Architecture Enhancements for non-3GPP Accesses," 3GPP TS 23.402, v. 1 1.4.0 (Sept. 2012), available at www.3gpp.org). A simplified ANDSF architecture is depicted in Figure 2. As shown in the figure, the ANDSF server 40 is provided that is added to a 3GPP network (that comprises one or more eNodeBs (eNBs) 42 and a gateway (GW) 44) and is only connected to the UE 46, and its main goal is to provide the UE 46 with access network information in a resource efficient and secure manner. The communication between the UE 46 and the ANDSF server 40 is defined as an IP- based S14-interface 48. A Wi-Fi AP 50 is also shown in Figure 2, with the AP 50 being connected to the GW 44. By supplying information about both available 3GPP and non-3GPP access networks to the UE (terminal device) 46, the ANDSF server 40 enables an energy-efficient mechanism of network discovery, where the UE 46 can avoid continuous and energy- consuming background scanning. Furthermore, the ANDSF provides the mobile operators with a tool for the implementation of flexible and efficient UE steering of access mechanisms, where policy control can guide UEs 46 to select one particular radio access network (RAN) over another.

The ANDSF supplies three types of information - discovery information, inter-system mobility policies (ISMP) and inter-system routing policies (ISRP). All these are summarized and implemented via ANDSF managed objects (MO), which are communicated to the UE 46 via an over-the-top (OTT) signalling channel.

The discovery information provides the UE with information regarding the availability of different RATs in the UE's vicinity. This helps the UE to discover available (3GPP and non-3GPP) access networks without the burden of continuous background scanning.

Inter-System Mobility Policies (ISMP) are policies which guide the UE to select the most preferable 3GPP or non-3GPP access. The ISMP are used for UEs that access a single access network (3GPP or Wi-Fi) at a time. The ISMP information specifies the behaviour of UEs that can be connected to only one access network at a given time (either 3GPP, WLAN, Worldwide Interoperability for Microwave Access (WiMAX), etc). If the UE, however, supports connection to several access networks at the same time, the operator can use the third type of information, ISRP, to increase the granularity of the RAN selection. In that case, the UEs will be provided with policies that specify how the traffic flows should be distributed over the different RAN. For example, voice might be only allowed to be carried over 3GPP radio access (RA), while Internet video streaming and best-effort traffic can be routed via WLAN.

Thus, the ANDSF approach to access network selection is primarily based on the priority or preference of the RAT that is provided to the UE by the ANDSF server 40.

Summary

It is possible that a terminal device that is capable of supporting both WLAN and cellular systems may use a different mechanism for access network selection and/or traffic steering or routing between WLAN and 3GPP RAT to other terminal devices. This means that in any given situation, some terminal devices may prefer to connect to WLAN for data communication whereas others may select a 3GPP RAT (e.g. LTE). For example the mechanism implemented in a terminal device supporting both WLAN and cellular systems may result in the terminal device connecting to the WLAN even though the signal quality of the cellular system may be better and may therefore better serve the terminal device. This will lead to performance degradation and also underutilization of network resources.

Some terminal devices may implement one or more of the mechanisms described above for access network selection and/or traffic steering or routing between WLAN and 3GPP (for example if required by the 3GPP standards) and/or other mechanisms. However, the behaviour of some terminal devices with respect to the access network selection between WLAN and 3GPP may also or alternatively depend on the implementation of the terminal device (e.g. a manufacturer- or user-specified mechanism).

Existing solutions therefore lead to unpredictable or unspecified behaviour of the terminal device with respect to the access selection between WLAN and 3GPP (from the network's point-of-view). The network operator may therefore not have any or only partial control over the behaviour of the terminal device, and this may lead to severe performance degradation e.g. loss of user and/or system throughput.

The present disclosure provides methods implemented in a network node and a terminal device that enables a mechanism for access network selection and/or traffic steering or routing between a network operating according to a first RAT (e.g. a cellular system such as LTE, HSPA) and a network operating according to a second RAT (e.g. WLAN) based on one of the predefined rules (e.g. one of the mechanisms described above) or based on terminal device autonomous behaviour (i.e. based on the implementation-specific mechanism in the terminal device). More generally, the present disclosure provides methods for enabling the network to control which of a plurality of access network selection and/or traffic steering or routing mechanisms are used by a terminal device. In some embodiments this is ensured by having the network node indicate, by broadcast or dedicated signalling (which can either be implicit or explicit), to the terminal device which access network selection and/or traffic steering or routing mechanism should be applied. Also some embodiments allow for the network node to indicate to the terminal device a time that indicates to the terminal device for how long the terminal device should apply a certain mobility (i.e. access network selection and/or traffic steering or routing) mechanism. Some embodiments provide an approach where the network node can indicate to the terminal device probability values which are used by the terminal device to determine which mobility mechanism should be applied.

In some scenarios a terminal device may receive multiple indications from the network, e.g. from different nodes in the network, so some embodiments provide a mechanism for a terminal device to handle such multiple indications.

Further embodiments describe how a network node can determine a suitable indicator value, and provide that it can be determined based on, for example, subscription information, network state, knowledge about the terminal model, etc. According to a first specific aspect, there is provided a method of operating a network node, the method comprising providing an indication to a terminal device that indicates which of a first mechanism and a second mechanism should be used by the terminal device for access network selection and/or traffic steering or routing between networks operating according to different radio access technologies, RATs.

According to a second specific aspect, there is provided a network node for use in a cellular communications network, the network node comprising processing circuitry and interface circuitry configured to provide an indication to a terminal device that indicates which of a first mechanism and a second mechanism should be used by the terminal device for access network selection and/or traffic steering or routing between networks operating according to different radio access technologies, RATs.

According to a third specific aspect, there is provided a method of operating a terminal device that is capable of communicating with a first network operating according to a first radio access technology, RAT, and a second network operating according to a second RAT, the method comprising receiving an indication from a network that indicates which of a first mechanism and a second mechanism should be used by the terminal device for access network selection and/or traffic steering or routing between the first network and the second network.

According to a fourth specific aspect, there is provided a terminal device for use in a cellular communication network, the terminal device comprising a processing unit; and a transceiver unit that is configured to communicate with a first network operating according to a first radio access technology, RAT, and a second network operating according to a second RAT; wherein the processing unit and the transceiver unit are configured to receive an indication from a network that indicates which of a first mechanism and a second mechanism should be used by the terminal device for access network selection and/or traffic steering or routing between the first network and the second network.

According to a fifth specific aspect, there is provided a computer program product having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable processing unit or computer, the processing unit or computer is caused to perform any of the methods described above. Brief Description of the Drawings

Features, objects and advantages of the presently disclosed techniques will become apparent to those skilled in the art by reading the following detailed description where references will be made to the appended figures in which:

Figure 1 is a diagram illustrating one example of a network interworking feature;

Figure 2 is a block diagram of a simplified ANDSF architecture;

Figure 3 is a diagram illustrating the overall architecture of an LTE network;

Figure 4 illustrates part of an LTE network and a Wi-Fi network; Figure 5 is a block diagram of an exemplary terminal device according to several embodiments;

Figure 6 is a block diagram of an exemplary network node according to several embodiments;

Figure 7 is a flow chart illustrating a method of operating a network node according to an embodiment; and

Figure 8 is a flow chart illustrating a method of operating a terminal device according to an embodiment.

Detailed Description

In the discussion that follows, specific details of particular embodiments of the present teaching are set forth for purposes of explanation and not limitation. It will be appreciated by those skilled in the art that other embodiments may be employed apart from these specific details. Furthermore, in some instances detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or in several nodes. Some or all of the functions described may be implemented using hardware circuitry, such as analog and/or discrete logic gates interconnected to perform a specialized function, application-specific integrated circuits (ASICs), programmable logic arrays (PLAs), digital signal processors (DSPs), reduced instruction set processors, field programmable gate arrays (FPGAs), state machines capable of performing such functions, etc. Likewise, some or all of the functions may be implemented using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Where nodes that communicate using the air interface are described, it will be appreciated that those nodes also have suitable radio communications circuitry. Moreover, the technology can additionally be considered to be embodied entirely within any form of computer-readable memory, including non- transitory embodiments such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein. Hardware implementations of the present teachings may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analog) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and/or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions.

In terms of computer implementation, a computer is generally understood to comprise one or more processors or one or more controllers, and the terms computer, processor, and controller may be employed interchangeably. When provided by a computer, processor, or controller, the functions may be provided by a single dedicated computer or processor or controller, by a single shared computer or processor or controller, or by a plurality of individual computers or processors or controllers, some of which may be shared or distributed. Moreover, the term "processor" or "controller" also refers to other hardware capable of performing such functions and/or executing software, such as the example hardware recited above.

The discussion that follows frequently refers to "terminal devices", although other generally equivalent terms such as "mobile devices", "communication devices", "mobile stations" and particularly "UEs" - which is a 3GPP term for end user wireless devices - may also be used. It should be appreciated, however, that the techniques and apparatus described herein are not limited to 3GPP UEs (i.e. UEs or terminal devices that are capable of operating according to one or more 3GPP standardised technologies), but are more generally applicable to end user wireless devices (e.g., portable cellular telephones, smartphones, wireless-enabled tablet computers, target devices, device-to-device (D2D) UEs, machine-type UEs or UEs capable of machine- to-machine (M2M) communication, laptop embedded equipment (LEE), laptop mounted equipment (LME), Universal Serial Bus (USB) dongles, etc.) that are useable in cellular systems and that are capable of communicating with a radio access network (RAN) using one or multiple carriers or cells (e.g. known as a carrier aggregation (CA) mode in LTE). It should also be noted that the current disclosure relates to end user terminal devices that support one or more wide-area cellular technologies, such as any of the wide-area radio access standards maintained by 3GPP, and a wireless local area network (WLAN) technology, such as one or more of the IEEE 802.1 1 standards. End user devices are referred to in Wi-Fi documents as "stations," or "STA" - it should be appreciated that the term "UE" or "terminal device" as used herein should be understood to refer to a STA, and vice-versa, unless the context clearly indicates otherwise. It should also be noted that the current disclosure also relates to end user wireless devices that support both a wide-area cellular technology, such as any of the wide-area radio access standards maintained by 3GPP, and a non-3GPP standardized RAT.

The term 'radio network node' or 'network node' is used herein and should be understood as referring to any type of network node serving a UE and/or that is connected to another network node or network element or any radio node from where a UE receives a signal or message. Examples of a network node include NodeB, base station (BS), multi-standard radio (MSR) radio node such as MSR BS, eNodeB, network controller, radio network controller (RNC), base station controller (BSC), relay, donor node controlling relay, base transceiver station (BTS), access point (AP), wireless router, transmission points, transmission nodes, remote radio unit (RRU), remote radio head (RRH), nodes in distributed antenna system (DAS), core network node, mobility management entity (MME), ANDSF server etc.

In addition to the above, it will be appreciated that the term "base station" (and thus also "network node") comprises in a general sense any node transmitting radio signals in the downlink (DL) to a terminal device and/or receiving radio signals in the uplink (UL) from the terminal device. Some example base stations are eNodeB, eNB, Node B, macro-/micro-/pico-/femto-cell radio base station, home eNodeB (also known as a femtocell base station), relay, repeater, sensor, transmitting-only radio nodes or receiving-only radio nodes. A base station may operate or at least perform measurements in one or more frequencies, carrier frequencies or frequency bands and may itself be capable of carrier aggregation. It may also be a single-radio access technology (RAT), multi-RAT, or multi-standard node, e.g., using the same or different base band modules for different RATs. The signalling between the terminal devices and the network nodes (e.g. a base station or another node in the RAN or core network) described below is either via direct links or logical links (e.g. via higher layer protocols and/or via one or more other network nodes). For example, signalling from a coordinating node may pass another network node, e.g., a radio node.

It will be appreciated that, as used herein, the 3GPP RATs include any one or more of the technologies standardised by the 3rd-Generation Partnership Project (3GPP), including the radio-access technologies known as Long-Term Evolution (LTE), Universal Mobile Telecommunications System (UMTS)/Wideband Code-Division Multiple Access (WCDMA), High Speed Packet Access (HSPA), Code Division Multiple Access 2000 (CDMA2000), Global System for Mobile Communications (GSM), GSM Enhanced Data rates for GSM Evolution (EDGE) Radio Access Network (GERAN). A "WLAN", as used herein, may comprise of any type of wireless LAN such as Wi-Fi, WiMAX or compliant to any IEEE 802.11 standard etc.

Overall E-UTRAN architecture - An exemplary Evolved UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network (E-UTRAN) architecture is shown in Figure 3. The E-UTRAN architecture 210 consists of base stations 220, 230, 240 called enhanced NodeBs (eNBs or eNodeBs), which provide the E-UTRA user plane and control plane protocol terminations towards the User Equipment (UE). The eNBs 220, 230, 240 are interconnected with each other by means of the X2 interface 250, 252, 254. The eNBs 220, 230, 240 are also connected by means of the S1 interface 260, 262, 264, 266 to the EPC 270 (Evolved Packet Core), more specifically to the MME 280, 290 (Mobility Management Entity), by means of the S1-MME interface, and to the Serving Gateway 280, 290 (S-GW) by means of the S1-U interface. The S1 interface supports many-to-many relations between MMEs / S-GWs and eNBs.

The eNB 220, 230, 240 hosts functionalities such as Radio Resource Management (RRM), radio bearer control, admission control, header compression of user plane data towards the UE, and routing of user plane data towards the serving gateway. The MME 280, 290 is the control node that processes the signalling between the UE and the core network 270. The main functions of the MME 280, 290 are related to connection management and bearer management, which are handled via Non Access Stratum (NAS) protocols. The S-GW 280, 290 is the anchor point for UE mobility, and also includes other functionalities such as temporary downlink data buffering while the UE is being paged, packet routing and forwarding the right eNB 220, 230, 240, gathering of information for charging and lawful interception. The PDN (Packet Data Network) Gateway (P-GW - not shown in Figure 3) is the node responsible for UE IP address allocation, as well as Quality of Service (QoS) enforcement. The 3GPP document "Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall Description; Stage 2," 3GPP TS 36.300, v. 1 1.3.0 (Sept. 2012), available at www.3gpp.org, and the references therein provide details of the functionalities of the different nodes shown in Figure 3.

Figure 4 illustrates a network where the LTE radio access parts (eNBs) 320, 322 and a Wi-Fi wireless access point 310 are both connected to the same P-GW 340. In the case of the LTE radio access parts, the eNBs 320, 322 are connected to the P-GW 340 via an S-GW 330. A UE 300 is shown that is capable of being served both from the Wi-Fi Access Point 310 and the LTE eNBs 320, 322. Arrows 350 and 352 illustrate the uplink (UL) and downlink (DL) transmissions between the UE 300 and the Wi-Fi AP 310 respectively and arrows 360 and 362 illustrate the uplink (UL) and downlink (DL) transmissions between the UE 300 and the eNBs respectively. Figure 4 illustrates one possible way of connecting a Wi-Fi access network 302 to the same core network as the 3GPP-specified access network 304. The gateways (Trusted Wireless Access Gateway, TWAG, evolved Packet Data Gateway, ePDG, etc) between Wi-Fi AP and P- GW are omitted for simplicity. It should be noted that the presently disclosed techniques are not restricted to scenarios where the Wi-Fi access network 302 is connected in this way; the techniques can be applied to scenarios where the networks are more or completely separate.

Hardware Implementations - Several of the techniques and methods described below may be implemented using radio circuitry and electronic data processing circuitry provided in a terminal device. Figure 5 illustrates features of an example terminal device 400 according to several embodiments. Terminal device 400, which may be a UE configured for operation with an LTE network (E-UTRAN) and that also supports Wi-Fi, for example, comprises a transceiver unit 420 for communicating with one or more base stations (eNBs) as well as a processing circuit 410 for processing the signals transmitted and received by the transceiver unit 420. Transceiver unit 420 includes a transmitter 425 coupled to one or more transmit antennas 428 and receiver 430 coupled to one or more receiver antennas 433. The same antenna(s) 428 and 433 may be used for both transmission and reception. Receiver 430 and transmitter 425 use known radio processing and signal processing components and techniques, typically according to a particular telecommunications standard such as the 3GPP standards for LTE. Note also that transmitter unit 420 may comprise separate radio and/or baseband circuitry for each of two or more different types of radio access network, such as radio/baseband circuitry adapted for E-UTRAN access and separate radio/baseband circuitry adapted for Wi-Fi access. The same applies to the antennas - while in some cases one or more antennas may be used for accessing multiple types of networks, in other cases one or more antennas may be specifically adapted to a particular radio access network or networks. Because the various details and engineering tradeoffs associated with the design and implementation of such circuitry are well known and are unnecessary to a full understanding of the techniques described herein, additional details are not shown here.

Processing circuit 410 comprises one or more processors 440 coupled to one or more memory devices 450 that make up a data storage memory 455 and a program storage memory 460. Processor 440, identified as CPU 440 in Figure 5, may be a microprocessor, microcontroller, or digital signal processor, in some embodiments. More generally, processing circuit 410 may comprise a processor/firmware combination, or specialized digital hardware, or a combination thereof. Memory 450 may comprise one or several types of memory such as read-only memory (ROM), random-access memory, cache memory, flash memory devices, optical storage devices, etc. Because terminal device 400 supports multiple radio access technologies, processing circuit 410 may include separate processing resources dedicated to one or several radio access technologies, in some embodiments. Again, because the various details and engineering tradeoffs associated with the design of baseband processing circuitry for mobile devices are well known and are unnecessary to a full understanding of the techniques described herein, additional details are not shown here.

Typical functions of the processing circuit 410 include modulation and coding of transmitted signals and the demodulation and decoding of received signals. In several embodiments, processing circuit 410 is adapted, using suitable program code stored in program storage memory 460, for example, to carry out any of the embodiments described below. Of course, it will be appreciated that not all of the steps of these embodiments are necessarily performed in a single microprocessor or even in a single module.

Similarly, several of the techniques and processes described herein can be implemented in a network node, such as an eNodeB or other node in a 3GPP network. Figure 6 is a schematic illustration of a node 500 in which a method embodying any of the presently described network-based techniques can be implemented. A computer program for controlling the node 500 to carry out a method according to any of the relevant embodiments is stored in a program storage 530, which comprises one or several memory devices. Data used during the performance of a method according to the embodiments is stored in a data storage 520, which also comprises one or more memory devices. During performance of a method embodying the present techniques, program steps are fetched from the program storage 530 and executed by a Central Processing Unit (CPU) 510, retrieving data as required from the data storage 520. Output information resulting from performance of a method embodying the presently- described techniques can be stored back in the data storage 520, or sent to an Input/Output (I/O) interface 540, which includes a network interface for sending and receiving data to and from other network nodes and which may also include a radio transceiver for communicating with one or more terminal devices.

Accordingly, in various embodiments, processing circuits, such as the CPU 510 in Figure 6, are configured to carry out one or more of the techniques described in detail below. Likewise, other embodiments include radio network controllers including one or more such processing circuits. In some cases, these processing circuits are configured with appropriate program code, stored in one or more suitable memory devices, to implement one or more of the techniques described herein. Of course, it will be appreciated that not all of the steps of these techniques are necessarily performed in a single microprocessor or even in a single module.

Network controlled selection of mobility procedures

As noted above, a terminal device that is capable of supporting both WLAN and cellular networks (or more generally two or more networks operating according to different RATs) may be capable of using one or more of a plurality of different mechanisms for deciding which network to access and/or which network to steer or route traffic to. Some mechanisms are defined in the standards (also known as specifications) relating to the RAT (e.g. the 3GPP standards or specifications relating to LTE, HSPA, GSM, etc.) or are otherwise known to both the network and the terminal device, whereas other mechanisms may be specific to the way in which a particular terminal device is implemented (e.g. the mechanism could be defined by the terminal device manufacturer or be user-configurable, etc.). These terminal device-specific mechanisms are not defined in a standard or specification relating any of the RATs the terminal device is capable of supporting. It will be appreciated that in some cases the terminal device-specific mechanisms may employ rules or conditions that are similar to those used in the threshold-based mechanism or traffic steering command-based mechanism described above, except that in this case, the rules or conditions are not known to the network (or at least not defined in a standard for the RAT that the network is operating according to).

The threshold-based mechanism will be described in 3GPP TS 36.331 : "3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification" and 3GPP TS 36.304: "3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode". The ANDSF based access mechanism is described in 3GPP TS 24.312 v12.4.0 "3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Access Network Discovery and Selection Function (ANDSF) Management Object (MO)".

In view of these different mechanisms, and the possibility that a given terminal device can be capable of implementing more than one type of mechanism, for example a mechanism defined in the standards and a terminal device implementation-specific (i.e. non-standardised) mechanism, the behaviour of some terminal devices with respect to access network selection/traffic steering between WLAN and 3GPP may be difficult for the network(s) to predict or control, which can lead to severe performance degradation in the network, e.g. manifesting as a loss of user and/or system throughput.

The present disclosure provides methods implemented in a network node and a terminal device that enables network control over which mechanism of a plurality of mechanisms should be used by the terminal device for access network selection and/or traffic steering or routing between networks operating according to different RATs, such as a first network operating according to a first RAT (e.g. a cellular system such as LTE, HSPA) and a second network operating according to a second RAT (e.g. WLAN).

In particular embodiments, the present disclosure provides for network control of whether the terminal device uses a first mechanism, such as a network-dependent mechanism (i.e. a mechanism that is dependent on the network sending some information, parameters, thresholds, messages and/or control signalling from the network to the terminal device) or a mechanism specified in a standard relating to one of the RATs, or a second mechanism, such as a terminal device implementation- specific mechanism for access network selection and/or traffic steering or routing. The network-dependent mechanisms and mechanisms specified in a standard include any of the threshold-based mechanism, the traffic steering command-based mechanism and/or the ANDSF-based access mechanism described above. The term mobility between first and second RATs or first and second networks is interchangeably used herein with access network selection or selection between first and second RATs or first and second networks. They are considered to have the same meaning in that the selected RAT/network is responsible for serving, some or all, of the traffic of the terminal device. The network-dependent mechanisms, which are also referred to as mechanisms that are based on predefined rule (i.e. predefined in a standard), with the rules being known to the network and the terminal devices, are in some places herein termed as a "first method" or "first mechanism" of access network selection/traffic steering between the first RAT and the second RAT. In general, according to a predefined rule a terminal device can be required to select the RAT for access selection based on a comparison of one or more radio (signal) measurements on first and/or second RATs with corresponding thresholds, and/or based on an indicated priority or preference associated with the first and second RATs. Terminal devices may also be configured with one or more parameters related to the predefined rules e.g. thresholds for comparing measurement results of a first RAT and/or second RAT. Examples of predefined rules are provided in the threshold-based mechanism, the traffic steering command-based mechanism and the ANDSF-based access mechanism described above. However, it will be appreciated that the embodiments are not limited to these examples.

For instance, another example of a predefined rule is that the terminal device selects between the first and the second RATs based on the following scheme: If (3GPP signal < threshold-!) && (WLAN signal > threshold2)

steerTrafficToWLAN i.e. select second RAT (e.g. WLAN)

Else

steerTrafficTo3GPP i.e. select first RAT (e.g. LTE) The terminal device implementation-specific mechanisms, which are also referred to herein as 'terminal autonomous mechanisms', 'terminal device implementation specific', 'implementation up to terminal', and 'mechanisms that are not specified in a (or any) standard relating to the (or any) RAT' are also for simplicity termed herein as a "second method" or a "second mechanism" of access network selection and/or traffic steering or routing between the first RAT and the second RAT. The autonomous mechanism can be based on the implementation of the terminal device, which can, for example be determined by the terminal device manufacturer, the operating system provider, and/or the user of the terminal device, and is not specified in a standard relating to any of the RATs. Thus, the terminal device implementation-specific mechanism can be based on hard coded programs, software installed on the terminal device, application programs or managed autonomously by the operating system, etc. The terminal device implementation-specific mechanisms can also be considered to be 'network-independent', in the sense that the network (operating according to the first or second RAT) has no control or influence over the access network selection and/or traffic steering or routing by the terminal device.

Figure 7 illustrates a method of operating a network node to provide network control over the mechanism used by the terminal device for access network selection and/or traffic steering or routing between networks operating according to different RATs.

In step 101 , the network node determines which of a plurality of mechanisms for access network selection and/or traffic steering or routing should be used by a specific terminal device, a specific group of terminal devices or all terminal devices under the control or influence of the network node (for example where the network node is a base station, this could be all terminal devices being served by the base station). More particularly, the network node determines which of a first mechanism (as defined above) and a second mechanism (also as defined above) should be used by one or more terminal devices for access network selection and/or traffic steering or routing. The network node can determine which of the access network selection/traffic steering mechanisms to use in a number of different ways, which are described in more detail below.

Then, in step 103, the network node provides an indication to the terminal device(s) which indicates to the terminal device(s) which mechanism should be used. The indication can take any one of a number of different forms, which are also described in more detail below.

Figure 8 illustrates a corresponding method of operating a terminal device. In step 1 11 , the terminal device receives an indication from a network (e.g. a network that the terminal device is currently connected to or associated with) that indicates which of a plurality of mechanisms should be used by the terminal device for access network selection and/or traffic steering or routing between networks operating according to different RATs. More particularly, the indication indicates which of a first mechanism (as defined above) and a second mechanism (also as defined above) should be used by the terminal device for access network selection and/or traffic steering or routing. As noted above, the indication can take any one of a number of different forms, which are described in more detail below.

In step 1 13, the terminal device uses the indicated mechanism to perform access network selection and/or traffic steering or routing.

In one possible embodiment, the indication sent in step 103 and received in step 11 1 indicates which mechanism shall be applied by the terminal device. In another possible embodiment, the indication can be interpreted by the terminal device as a suggestion to the terminal device, but the decision of which mechanism to apply is ultimately taken by the terminal device. For example, the terminal device may not be capable of applying the indicated mechanism and in that case the terminal device may be permitted to apply a different mechanism to the one indicated. In a simple embodiment, the indication sent by the network node in step 103 to the terminal device can comprise one bit of information indicating whether the terminal device should use a first mechanism or a second mechanism for access network selection and/or traffic steering or routing between the first and second RATs. In another embodiment, the indication may provide additional information such as which one of a number of predefined rules or which combination of predefined rules should be used by the terminal device for access network selection and/or traffic steering or routing. As a result, in some embodiments the indication is also interchangeably termed herein as 'RAT selection information' or 'RAT access selection information' or 'RAT selection indicator'. These aspects are further elaborated below.

The network node may determine which of the plurality of mechanisms for the terminal device(s) to use in step 101 based on one or more criteria. For example if a high data rate with guaranteed quality of service is required by the terminal device then the network node may determine that a first mechanism (i.e. one requiring control or influence from the network) should be used over a mechanism that is terminal device implementation specific. This will improve the chances that the terminal device maintains a connection with the strongest RAT in terms of signal quality and also quality of service. Another criterion for selecting between first and second mechanisms can be based on the type of subscription of the terminal device or the user of the terminal device. For example the network node may determine that the second mechanism should be used if the subscription is a lower-priced subscription. This is elaborated in more detail below. As noted above, it may be possible for a terminal device to be capable of operating according to multiple first mechanisms, in which case the indication provided in step 103 could indicate whether the terminal device should apply a terminal device autonomous mechanism for mobility, or apply one (or a specific one) of the multiple mechanisms based on predefined rules. For example, the terminal device may be able to implement one access stratum mechanism such as the threshold based mechanism or the traffic steering command based mechanism described above, and one mechanism defined in higher layers such as the ANDSF based access selection. It may also depend upon the terminal device is capable of using one or a plurality of predefined rules. The terminal device can inform the network via signalling whether it is capable of using one or a plurality of predefined rules for the purposes of access network selection and/or traffic steering or routing. If the indication indicates that the terminal device should apply a terminal device autonomous mobility mechanism, then the terminal device would do so, and if the indication indicates that a mechanism based on predefined rules should be applied then the terminal device may apply one of the available mechanisms or a combination of them.

Means and contents of indication - The indication, which is also referred to herein as 'RAT selection information', provided by the network node to the terminal device in step 103 may be an explicit indication from the network node to the terminal device (i.e. it can comprise an explicit signal sent from the network node to the terminal device). This explicit indication could, for example, be a one-bit flag which takes one value (e.g. 0) if the terminal device is to apply a terminal device autonomous mechanism for mobility between the first and second RATs, and the other value (e.g. 1) if the terminal device should apply a standardised mechanism for mobility.

In some embodiments, the indication sent to the terminal device in step 103 may further comprise one or more pieces of the following additional information to further enhance the access selection between the two RATs: - An indication of a specific predefined rule or network-dependent/standardised mechanism to be used by the terminal device in case more than one mechanism is present in the standards and the terminal device supports (or it is possible that the terminal device supports) more than one mechanism; - An indication of the time duration that a certain access network selection and/or traffic steering or routing mechanism should be used (this is also referred to as a 'mechanism occupancy time duration');

- An indication of a probability value for using a certain access network selection and/or traffic steering or routing mechanism.

In some embodiments, the indication of time duration ('time duration parameter') may comprise at least two non-overlapping and successive timing values: T1 and T2. For example it may be predefined that the terminal device shall use a first mechanism and a second mechanism over time duration T1 and T2 respectively. Exemplary, non- limiting, values of T1 and T2 are 10 seconds and 5 seconds respectively. The terminal device may either use T1 and T2 for applying first and second mechanisms once or it may use them periodically i.e. use the first and second mechanisms over T1 and T2 respectively and repeat periodically. In some embodiments, a starting reference time of T1 may be predefined or configured by the network node. For example, first time T1 may start at a certain reference time (e.g. from system frame number (SFN) = 0) or some specified time period At (e.g. 50 ms) after receiving the RAT selection information.

In some embodiments, time duration T1 may further comprise two or more smaller sub- durations over which particular mechanisms (e.g. particular predefined rules) are to be used by the terminal device. That is T1 = Tn+ ... +T _{1 N} (where N is an integer that is greater than 1). For example the terminal device may use a first standardised mechanism (e.g. the threshold-based mechanism) and a second standardised mechanism (e.g. the ANDSF based access selection mechanism) over time durations Tii and T ₁₂ respectively.

In embodiments where a probability parameter or value is indicated to the terminal device in step 103, the probability parameter may comprise at least two probability values: P1 and P2. The terminal device can, based on the received probability parameters, determine which of a first mechanism and a second mechanism should be applied and used by the terminal device. For example the network node may signal to the terminal device two probability parameters P1 and P2, where the sum of P1 and P2 is 1 (i.e. P1 + P2 = 1). Alternatively, where the sum of P1 and P2 is 1 , the network node may only need to signal one of these probability values, since the terminal device could calculate P2 using P1 , and vice versa.

The terminal device would then use these probability values to determine which mobility mechanism should be used. For example, the terminal device can apply a particular one of the mobility mechanisms with probability P2 (e.g. a terminal device autonomous mechanism for mobility (i.e. a second mechanism), and with probability P1 apply a first mechanism based on a predefined rule specified in a standard. If there are more than two possible mobility mechanisms the network could indicate more probability parameters, i.e. one per mobility mechanism. The probability parameter P1 associated with the first mechanism sent to the terminal device by the network in step 103 may further comprise of two or more sub-values over which particular predefined rules or standardised mechanisms are to be used by the terminal device. That is P1 = Pn+... +P _{1 N} (where N is an integer that is greater than 1), and P1 + P2 = 1. For example the terminal device may use predefined rule 1 , rule 2 and rule N with probability Pn, P ₁₂ and P _{1 N} respectively.

In this probability-based approach, the terminal device can be configured to generate a random number between 0 and 1 and if the random value is below e.g. P1 , then the terminal device would apply the mobility mechanism associated with P1 , otherwise if the random number is above P1 the terminal device would apply the other mobility mechanism.

Furthermore, in case more than two mobility mechanisms are considered, the network node may only send probabilities for a subset of these mobility mechanisms, and for the mobility mechanisms that the network node has not provided probabilities it could be left to the terminal device to select or determine the probabilities, as long as the mobility mechanism(s) for which a probability has been provided is selected with the provided probability. For example, if three mobility mechanisms are considered, the network node may only send P2, but not P1 or P3. The terminal device can then apply mobility mechanism 2 with probability P2, but it could be left to the terminal device to decide what values P1 and P3 take, and hence also what with which probabilities mobility mechanism 1 and 3 are to be selected.

In contrast to the above embodiments, the indication provided by the network node to the terminal device in step 103 may be an implicit indication. For example, an implicit indication of the mechanism to use for access network selection and/or traffic steering or routing by be provided by signalling to the terminal device one or more parameters (e.g. threshold values, conditions, ANDSF policies) or commands (e.g. a traffic steering command) relating to a specific mechanism.

For example, if the network node sends to the terminal device one or more parameters which relate to a mobility mechanism based on predefined rules specified in a standard then the terminal device can be configured to consider that the network node has indicated that the terminal device should apply that mobility mechanism. In this embodiment, the terminal device can also be configured to consider that the network node has indicated that the terminal device should apply a terminal device autonomous mobility mechanism if the terminal device does not receive parameters for a particular standardised mobility mechanism from the network node. The indication provided in step 103 by the network node may be broadcasted to all terminal devices and/or sent with dedicated signalling to one or more specific terminal devices.

In the event that a network node both broadcasts an indication and sends an indication with dedicated signalling, a terminal device can be configured to use one of the indications if it receives both the broadcasted indication and an indication over dedicated signalling. For example the terminal device can be configured to give higher priority to the indication received over dedicated signalling. This embodiment allows the network node to broadcast one indication which is applicable to a majority of the terminal devices, and allows the network to specify different behaviour for some particular terminal devices using dedicated signalling.

Nodes that provide the indication - The indication or RAT selection information may be provided by a network node in a Radio Access Network (RAN) e.g. an eNode B in LTE or an RNC in UTRA frequency division duplexing (FDD). A benefit of providing the indication from a network node in the RAN is that the RAN would likely have a better understanding of the current load situation in the RAN (and hence the need to offload traffic to WLAN) than higher parts of the network. So, if from the RAN's point of view there is a need to perform mobility to WLAN, the network node in the RAN can indicate to the terminal device that the terminal device should apply an appropriate mobility mechanism (e.g. one based on predefined rules in a standard) that may lead to the terminal device performing mobility to WLAN. Furthermore, the RAN (or a node in the RAN) may be able to affect or impact a mechanism based on predefined rules (such as the threshold-based mechanism or traffic steering command-based mechanism) to ensure the desired mobility result, and hence the RAN could take this into consideration when deciding content or information to be included in the indication.

Alternatively, the indication or RAT selection information may be provided to the terminal device from another network node, e.g. a core network node such as an ANDSF server or mobility management entity (MME). In case the ANDSF server provides this indication, then it may be provided as part of an ANDSF policy e.g. an ANDSF managed object (MO). A benefit of sending the indication from a core network node is that the core network node may have more information about the subscription of the terminal device which may be relevant when deciding which type of mechanism the terminal device should apply.

Handling of multiple indications - As noted above it is possible for a terminal device to receive multiple indications, e.g. one through broadcast signalling and one through dedicated signalling, or one indication from the network node in the RAN and one indication from an ANDSF server. Different methods for handling multiple indications are possible, and the terminal device can be configured to use a rule to decide which one of the received indications should be followed when selecting the mechanism for access network selection and/or traffic steering or routing between the first and the second RATs.

One possible method is that the terminal device gives higher priority to one of the multiple indications and the terminal device applies the indication with the highest priority. For example, the terminal device may give higher priority to an indication received in dedicated signalling than broadcast signalling. As another example, the terminal device may give higher priority to an indication received from the network node in the RAN than to an indication from an ANDSF server, which means that if the network node in the RAN indicates that the terminal device should follow a standardised mechanism for mobility the terminal device would do so even if the ANDSF policy indicates that a terminal device autonomous mechanism for mobility should be used. In another example, it is possible that the terminal device decides the priority order of the multiple indications based on whether each indication was provided explicitly or implicitly. In particular the terminal device may give higher priority to an explicit indication than to an implicit indication. In another embodiment, a terminal device can be configured to use AND-logic and/or OR-logic when determining which mobility mechanism the terminal device should apply if multiple indications are received. In one example implementation of this embodiment, the terminal device may apply AND-logic to decide whether to use an autonomous mobility mechanism. In particular, the terminal device may only apply an autonomous mobility mechanism if it is indicated to do so by both a network node in the RAN and the ANDSF server. This implementation would have benefit if there is no coordination between the entities which can provide the indications (e.g. RAN node and ANDSF server) and the terminal device should then only apply an autonomous mobility mechanism if both the RAN and the ANDSF deem this suitable. Similar behaviour could be achieved alternatively with OR-logic.

Basis and criteria for selection of mobility procedures - In taking a decision about which mechanism a terminal device should use for access network selection and/or traffic steering or routing in step 101 , a network node can consider different types of information. A few examples are explained below.

As noted above, in some embodiments the network node can consider information related to the subscription for the terminal device (or user of the terminal device) when deciding which mobility mechanism the terminal device should apply. For example a terminal device that has a higher priced subscription may expect the network operator to provide the best possible user experience for this subscriber. In that case the network node may indicate a 'first mechanism' should be applied as this could give the network, and hence the network operator, more control over the mobility of the terminal device and hence can ensure that the terminal device is performing mobility such that the best user experience is achieved. On the other hand, the network operator may consider that for a terminal device with a lower price subscription it is not as critical to control the mobility mechanism, and hence for such a terminal device, the network node could indicate that a terminal device autonomous mobility mechanism (i.e. second mechanism) can be used.

In some embodiments, the network node may additionally have information about the terminal device and possibly information about the autonomous behaviour of the terminal device available when making the decision in step 101 about which mechanism should be used. The network node can evaluate what mobility decision may be made by the terminal device autonomous mechanism, and if the network node determines that the terminal device autonomous mechanism is suitable then the network node could indicate to the terminal device that the autonomous behaviour should be applied. However, the autonomous behaviour of another terminal device may not be suitable and hence the network node has the possibility of preventing that terminal device from applying its autonomous mechanism for mobility. The network could obtain the information about the terminal device by knowing the particular model and/or version of the terminal device. For example, the network node may know that the terminal device is of a certain brand and/or of a certain model, and based on this information the network node can determine from a database or previous experience of that type of terminal device what the expected autonomous behaviour is.

Consider, for example, that the network node has information that a certain terminal device is of brand X and model Y which could potentially have a certain autonomous behaviour. The network node could then determine if the autonomous behaviour is suitable in the network or whether behaviour consistent with a mechanism defined in a standard is more appropriate. For example the network node can use historical data related to the terminal device performance (e.g. user throughput) to determine which method leads to better performance. Based on such determination the network node could indicate whether the terminal device should apply the autonomous behaviour for access selection between the two RATs.

The network node may also consider the current state of the network when determining whether the terminal device should apply an autonomous behaviour or apply a predefined (i.e. standardised) mobility mechanism. For example, in a scenario where the network load is low it may not be critical which mechanism the terminal device uses and it can be left to terminal device implementation which behaviour should be applied, i.e. autonomous behaviour. However, in a high load scenario it may be critical that a certain terminal device does not take an autonomous mobility decision as this could lead to, for example, an uneven load distribution between the possible access networks. The network load can be expressed in terms of one or more of: mean transmit power, mean throughput or bit rate, mean utilization of physical channels (e.g. resource block usage etc). As an example the load can be considered low if mean throughput is below 30%; otherwise it can be considered moderate or high if it is above 70%. Hence the network node could in this scenario indicate to the terminal device that the terminal device should apply a first mechanism which could possibly be, to some extent, controlled by the network node, and the network node could ensure that a certain action is taken by the terminal device to improve the overall system performance. Thus, there is provided methods for enabling a network to control which of a plurality of access network selection and/or traffic steering or routing mechanisms are used by a terminal device. The proposed methods ensure better mobility performance of terminal devices between WLAN and cellular systems, and provides that user and system performance is improved for a terminal device supporting and using both WLAN and cellular systems. The network resources are more efficiently used since the proposed methods ensure that the terminal device is served by the most suitable of the available RATs. Embodiments of the proposed methods also enable a network operator to select the best possible method for access network selection and/or traffic steering or routing between WLAN and cellular systems in accordance with the requirements and subscription of the subscriber.

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the teaching of the present application. For example, it will be readily appreciated that although the above embodiments are described with reference to parts of a 3GPP network, embodiments will also be applicable to like networks, such as a successor of any current 3GPP network, having like functional components. Therefore, in particular, the terms 3GPP and associated or related terms used in the above description and in the enclosed drawings now or in the future are to be interpreted accordingly. Examples of several embodiments have been described in detail above, with reference to the attached illustrations of specific embodiments. Because it is not possible, of course, to describe every conceivable combination of components or techniques, those skilled in the art will appreciate that the present techniques can be implemented in other ways than those specifically set forth herein, without departing from essential characteristics of the teaching of this application. The present embodiments are thus to be considered in all respects as illustrative and not restrictive. Example embodiments of the present invention are illustrated below. Example Embodiments

1. A method of operating a network node, the method comprising:

providing an indication to a terminal device that indicates which of a first mechanism and a second mechanism should be used by the terminal device for access network selection and/or traffic steering or routing between networks operating according to different radio access technologies, RATs.

2. The method of embodiment 1 , wherein the first mechanism is a network-dependent mechanism that requires at least one parameter or command to be signalled to the terminal device from the network.

3. The method of embodiment 1 or 2, wherein the first mechanism comprises a mechanism based on one or more predefined rules. 4. The method of embodiment 3, wherein the one or more predefined rules are known to the network node and the terminal device.

5. The method of any of embodiments 1-4, wherein the first mechanism comprises a mechanism that is defined in a standard relating to one of the RATs.

6. The method of embodiment 5, wherein the standard is a 3GPP standard.

7. The method of any of embodiments 1-6, wherein the second mechanism is a terminal device implementation-specific mechanism. 8. The method of embodiment 7, wherein the terminal device implementation-specific mechanism comprises a mechanism based on one or more rules.

9. The method of embodiment 8, wherein the one or more rules are not defined in a standard relating to any of the RATs.

10. The method of any of embodiments 1-9, wherein the first mechanism comprises a threshold-based mechanism in which the network node signals conditions and thresholds to a terminal device that are used by the terminal device to determine which of a plurality of networks operating according to different RATs to access and/or steer traffic to.

1 1. The method of any of embodiments 1-10, wherein the first mechanism comprises a traffic steering command-based mechanism in which the network node sends a command to the terminal device indicating which of a plurality of networks operating according to different RATs the terminal device is to access and/or steer traffic to.

12. The method of any of embodiments 1-1 1 , wherein the first mechanism comprises an Access Network Discovery and Selection Function, ANDSF, based access selection mechanism in which an ANDSF policy is provided to the terminal device that indicates a priority or preference of the RAT to be used by the terminal device.

13. The method of embodiment 1 , wherein the first mechanism comprises one of:

(i) selection between a first network operating according to a first RAT and a second network operating according to a second RAT by the terminal device based on the comparison of at least one first signal measurement with a first threshold and at least one second signal measurement with a second threshold, and wherein the first and second signal measurements are performed by the terminal device on signals from the first and second networks respectively;

(ii) selection between the first network and the second network by the terminal device based on an explicit command sent from the network node, which command is sent in response to receiving a measurement report from the terminal device relating to the first network and the second network; and (iii) selection between the first network and the second network by the terminal device based on information sent from the network node, which information specifies a priority or preference between the first RAT and the second RAT. 14. The method of any of embodiments 1-13, wherein the network node is a network node in a network operating according to a cellular RAT.

15. The method of embodiment 14, wherein the network node is a base station, NodeB, eNodeB, radio network controller, RNC, base station controller, BSC, a core network node or an Access Network Discovery and Selection Function, ANDSF, server.

16. The method of any of embodiments 1-13, wherein the network node is a network node in a network operating according to a wireless local area network, WLAN, RAT.

17. The method of embodiment 16, wherein the network node is an access point or a wireless router.

18. The method of any of embodiments 1-17, wherein the RATs comprise at least one cellular RAT, such as Long-Term Evolution, LTE, Universal Mobile

Telecommunications System, UMTS, Wideband Code-Division Multiple Access, WCDMA, High Speed Packet Access, HSPA, Code Division Multiple Access 2000, CDMA2000, Global System for Mobile Communications, GSM, GSM Enhanced Data rates for GSM Evolution, EDGE, Radio Access Network, GERAN, and at least one wireless local area network, WLAN.

19. The method of any of embodiments 1-18, wherein the indication is provided using broadcast or dedicated signalling. 20. The method of any of embodiments 1-19, wherein the first mechanism is a network-dependent mechanism that requires at least one parameter or command to be signalled to the terminal device from the network, and the indication comprises the at least one parameter or command. 21. The method of any of embodiments 1-20, wherein the indication further comprises timing duration parameters, T1 , T2, that indicate respective time durations during which the terminal device is to use the first mechanism and the second mechanism for access network selection and/or traffic steering or routing.

22. The method of any of embodiments 1-21 , wherein the indication further comprises probability parameters, P1 , P2, that indicate respective probabilities for the terminal device to use the first mechanism and the second mechanism for access network selection and/or traffic steering or routing, wherein P1 + P2 = 1.

23. The method of any of embodiments 1-22, wherein the method further comprises the step of:

determining which of the first mechanism and the second mechanism should be used by the terminal device for access network selection and/or traffic steering or routing between networks operating according to the different radio access technologies, RATs.

24. The method of embodiment 23, wherein the step of determining comprises using any one or more of:

(i) information relating to the subscription level of the terminal device;

(ii) information on the model and/or version of the terminal device;

(iii) information on the historical performance of the terminal device; and

(iv) information on the load in one or more of the networks operating according to the different RATs.

25. A computer program product having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable processing unit or computer, the processing unit or computer is caused to perform the method of any of embodiments 1-24.

26. A network node for use in a cellular communications network, the network node comprising:

processing circuitry and interface circuitry configured to provide an indication to a terminal device that indicates which of a first mechanism and a second mechanism should be used by the terminal device for access network selection and/or traffic steering or routing between networks operating according to different radio access technologies, RATs.

Further embodiments of the network node are also contemplated in which the processing circuitry and interface circuitry is configured to perform the method according to any of embodiments 2-24 described above.

27. A method of operating a terminal device that is capable of communicating with a first network operating according to a first radio access technology, RAT, and a second network operating according to a second RAT, the method comprising:

receiving an indication from a network that indicates which of a first mechanism and a second mechanism should be used by the terminal device for access network selection and/or traffic steering or routing between the first network and the second network.

28. The method of embodiment 27, wherein the first mechanism is a network- dependent mechanism that requires at least one parameter or command to be signalled to the terminal device from the network. 29. The method of embodiment 27 or 28, wherein the first mechanism comprises a mechanism based on one or more predefined rules.

30. The method of embodiment 29, wherein the one or more predefined rules are known to the network and the terminal device.

31. The method of any of embodiments 27-30, wherein the first mechanism comprises a mechanism that is defined in a standard relating to one of the RATs.

32. The method of embodiment 31 , wherein the standard is a 3GPP standard.

33. The method of any of embodiments 27-32, wherein the second mechanism is a terminal device implementation-specific mechanism.

34. The method of embodiment 33, wherein the terminal device implementation- specific mechanism comprises a mechanism based on one or more rules. 35. The method of embodiment 34, wherein the one or more rules are not defined in a standard relating to any of the RATs. 36. The method of any of embodiments 27-35, wherein the first mechanism comprises a threshold-based mechanism in which the network signals conditions and thresholds to a terminal device that are used by the terminal device to determine which of a plurality of networks operating according to different RATs to access and/or steer traffic to.

37. The method of any of embodiments 27-36, wherein the first mechanism comprises a traffic steering command-based mechanism in which the network sends a command to the terminal device indicating which of a plurality of networks operating according to different RATs the terminal device is to access and/or steer traffic to.

38. The method of any of embodiments 27-37, wherein the first mechanism comprises an Access Network Discovery and Selection Function, ANDSF, based access selection mechanism in which an ANDSF policy is provided to the terminal device that indicates a priority or preference of the RAT to be used by the terminal device.

39. The method of embodiment 27, wherein the first mechanism comprises one of:

(i) selection between a first network operating according to a first RAT and a second network operating according to a second RAT by the terminal device based on the comparison of at least one first signal measurement with a first threshold and at least one second signal measurement with a second threshold, and wherein the first and second signal measurements are performed by the terminal device on signals from the first and second networks respectively;

(ii) selection between the first network and the second network by the terminal device based on an explicit command sent from one of the first network and the second network, which command is sent in response to receiving a measurement report from the terminal device relating to the first network and the second network; and

(iii) selection between the first network and the second network by the terminal device based on information sent from one of the first network and the second network, which information specifies a priority or preference between the first RAT and the second RAT. 40. The method of any of embodiments 27-39, wherein the RATs comprise at least one cellular RAT, such as Long-Term Evolution, LTE, Universal Mobile Telecommunications System, UMTS, Wideband Code-Division Multiple Access, WCDMA, High Speed Packet Access, HSPA, Code Division Multiple Access 2000, CDMA2000, Global System for Mobile Communications, GSM, GSM Enhanced Data rates for GSM Evolution, EDGE, Radio Access Network, GERAN, and at least one wireless local area network, WLAN. 41. The method of any of embodiments 27-40, wherein the indication is received over broadcast or dedicated signalling.

42. The method of any of embodiments 27-41 , wherein the first mechanism is a network-dependent mechanism that requires at least one parameter or command to be signalled to the terminal device from the network, and the indication comprises the at least one parameter or command.

43. The method of any of embodiments 27-42, wherein the indication further comprises timing duration parameters, T1 , T2, that indicate respective time durations during which the terminal device is to use the first mechanism and the second mechanism for access network selection and/or traffic steering or routing.

44. The method of any of embodiments 27-43, wherein the indication further comprises probability parameters, P1 , P2, that indicate respective probabilities for the terminal device to use the first mechanism and the second mechanism for access network selection and/or traffic steering or routing, wherein P1 + P2 = 1.

45. The method of any of embodiments 27-44, wherein the method further comprises the step of:

performing access network selection and/or traffic steering or routing between networks operating according to the different RATs using the mechanism indicated in the received indication.

46. The method of any of embodiments 27-45, wherein the method further comprises the step of: receiving a second indication from a or the network that indicates which of the first mechanism and the second mechanism should be used by the terminal device for access network selection and/or traffic steering or routing between the first network and the second network; and

determining which of the indication and the second indication to use to determine which of the first mechanism and the second mechanism should be used by the terminal device.

47. The method of embodiment 46, wherein the one of the indication and the second indication is received over broadcast signalling and the other one of the indication and the second indication is received over dedicated signalling; and wherein the step of determining comprises using the one of the indication or the second indication received over dedicated signalling. 48. A computer program product having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable processing unit or computer, the processing unit or computer is caused to perform the method of any of embodiments 27-47. 49. A terminal device for use in a cellular communication network, the terminal device comprising:

a processing unit; and

a transceiver unit that is configured to communicate with a first network operating according to a first radio access technology, RAT, and a second network operating according to a second RAT;

wherein the processing unit and the transceiver unit are configured to receive an indication from a network that indicates which of a first mechanism and a second mechanism should be used by the terminal device for access network selection and/or traffic steering or routing between the first network and the second network.

Further embodiments of the terminal device are contemplated corresponding to method embodiments 27-48 above.