In a typical communications network, also referred to as a wireless communications system, wireless communications network, cellular network or cellular system, a user Equipment (UE), communicate via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A user equipment is a device by which a subscriber may access services offered by an operators network and services outside operator's network to which the operators radio access network and core network provide access, e.g. access to the Internet. The user equipment may be any device, mobile or stationary, enabled to communicate over a radio channel in the communications network, for instance but not limited to e.g. mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop or Personal Computer (PC). The user equipment may be portable, pocket storable, handheld, computer comprised, or vehicle mounted user equipments, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
User equipments are enabled to communicate wirelessly in the communications network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the cellular network.
The cellular network covers a geographical area which is divided into cell areas. Each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. evolved Node B (eNB), eNodeB, NodeB, B node, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
The communications network may apply to one or more radio access technologies such as for example Long Term Evolution (LTE), LTE Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile Communications (GSM), or any other Third Generation Partnership Project (3GPP) radio access technology, or other radio access technologies such as Wireless Local Area Network (WLAN).
In for example LTE, user equipments expect a new network to support all the services from a legacy network. To meet these needs, Inter-technology mobility is an important feature. In LTE, voice service over LTE is Internet Protocol Multimedia Subsystem (IMS)-based Voice Over Internet Protocol (VoIP). LTE is a packet data network and VoIP is used for supporting voice on packet networks.
Inter-technology mobility is also important for introduction of new services. Inter-technology mobility, enables that a new service may be rolled out network-wide even though the wireless broadband access technology that best and most efficiently supports it has only been deployed in the highest traffic areas. Inter-technology mobility provides a bridge between the old and new access networks enabling seamless service continuity for the user over a wide area.
Inter-technology mobility may simplify rollout of a new LTE where voice services is moved to VoIP over IMS in conjunction with the deployment of an LTE access network by using inter-technology mobility together with a functionality called Single Radio Voice Call Continuity (SRVCC). SRVCC is an LTE functionality that allows a VoIP/IMS call in the LTE packet domain to be moved to a legacy circuit domain, e.g. GSM/UMTS or CDMA. UMTS is short for Universal Mobile Telecommunications System.
When a user equipment with an ongoing IMS voice call in LTE looses its LTE coverage, provided the 2G/3G, i.e. Circuit Switched (CS) network, does not support VoIP, the user does SRVCC to 2G/3G and continues the voice call in the CS network through a Mobile Switching Centre Server (MSC). The MSC is a 3G core network element which controls the network switching subsystem elements. When the user equipment gets back into LTE coverage, the operator may want for different reasons to move the user equipment back to LTE. That procedure is called return SRVCC (rSRVCC). Another use case for rSRVCC may also be that the user equipment was camping in 2G/3G and started a CS voice call in 2G/3G through the MSC. After some time the user equipment gets into LTE coverage, upon which the rSRVCC is triggered.
A handover of an ongoing voice call from LTE to a 3G or 2G network, or a handover of an ongoing voice call from 2G/3G to LTE is done by using a mechanism called a dedicated bearer. In general, a bearer may be a logical channel that carries some information. A bearer may also be referred to as a radio resource. One Evolved Packet System (EPS) bearer is established when the user equipment 101 connects to the Packet Data Network (PDN) and remains connected throughout the lifetime of the connection. It is also called a default bearer. A default bearer provides always-on Internet Protocol (IP) connectivity to the network. Any additional EPS bearer is called a dedicated bearer. Dedicated bearers contexts are established when a service in the network requests a prioritising of IP packets belonging to a specific media stream between two IP addresses and TCP/UDP ports. A dedicated bearer is a bearer that carries traffic for IP flows that have been identified as requiring a specific packet forwarding treatment. A dedicated bearer is request by a user equipment to transmit data with a particular Quality of Service (QoS). TCP is short for Transmission Control Protocol and UDP is short for User Datagram Protocol.
When doing handover between different radio technologies, the bearer resource demands may differ in the source and in the target systems. One example is rSRVCC. When doing rSRVCC HO from a 2G/3G CS system to a VoIP based LTE system the user equipment may have been allocated a number of Packet Switched (PS) bearers and also a Circuit Switched (CS) bearer in the source system. After HandOver (HO) to LTE, the CS bearer is not available to the user equipment. It needs to be replaced by a new PS bearer to carry the voice media. If there are no resources available in the target system, the new voice specific PS bearer may not be allocated once the user equipment has been handed over to the target system. In this case the voice call will be interrupted. Then it would have been better to interrupt the rSRVCC and let the user equipment remain in 2G/3G. This is due to that rSRVCC may be triggered from optimizations rather than from loss of radio coverage. The rSRVCC procedure does not allow check for resources in the target system for non-existing bearers, only already allocated bearers may be checked.
The existing method for rSRVCC may be performed in different ways depending on if the source system is Dual Transfer Mode (DTM) based or non-DTM. In other words it is depending on if the PS service and the CS service may be done in parallel or only one at the time. Also this procedure is not really specified completely yet in 3GPP. DTM is a protocol based on the GSM standard that allows simultaneous transfer of CS voice and PS data over the same radio channel.
The rSRVCC procedure for the non-DTM case is shown here, but the pre-allocation is also applicable to the DTM based procedure.
In the non-DTM case, the existing rSRVCC procedure is comprises the following steps, which steps may be performed in any suitable order:
Step 1
The source radio access network node triggers that handover to LTE is needed and sends a signal about this to the MSC. The source radio access network node may be for example an eNB or a Radio Network Controller (RNC) or a Base Station Controller (BSC).
Step 2
The MSC sends a CS to PS handover request to the target MME.
Step 3
The target MME request, from the source SGSN, the existing Packet data protocol (PDP) contexts, i.e. bearers.
Step 4
The MME sends a request to the eNB to allocate the bearers.
Step 5
The eNB transmits a reply to the request to the MME. The request comprises indications about result of the allocation to the MME.
Step 6
The MME sends a CS to PS acknowledgement to the MSC.
Step 7
The MSC sends a handover command to the source radio access network node, e.g. an eNB or a RNC. The handover command triggers the user equipment to move to LTE.
Step 8
When the user equipment appears in LTE, a new Guaranteed Bit Rate (GBR) bearer that may be used for voice is allocated and the voice call continues.
A disadvantage of this is that the network utilization is not optimized, and that the user equipments experience may be exposed to service interruptions.