Method, apparatus and computer program

Abstract

A first terrestrial network function (TNF) in a core network receives from a second TNF a request for user equipment (UE) reachability information, determines the UE reachability information based on

Description

A communication network can be seen as a facility that enables communications between two or more communication devices, or provides communication devices access to a data network. A mobile or wireless communication network is one example of a communication network. A communication device may be provided with a service by an application server. Such communication networks operate in according with standards such as those provided by 3GPP (Third Generation Partnership Project) or ETSI (European Telecommunications Standards Institute). Examples of standards are the so-called 5G (5th Generation) standards provided by 3GPP. SUMMARY According to an aspect there is provided an apparatus for a core network, the apparatus comprising means for providing a first terrestrial network function configured for: receiving, from a second terrestrial network function of the core network, a request for user equipment reachability information for a user equipment; based on the receiving of the request, determining the user equipment reachability information for the user equipment based at least on location information indicative of a location of the user equipment and information about one or more satellites of the non-terrestrial network; and sending, to the second terrestrial network function, the user equipment reachability information for the user equipment. The user equipment reachability information may comprise information indicating, for each of the one or more satellites: a first time window during which a respective satellite is reachable by the core network; and a second time window during which the user equipment is reachable by the respective satellite. Determining the user equipment reachability information for the user equipment may comprise assessing the future location of the user equipment based on its last known location and on an assessment of the user equipment’s trajectory. The means may be for providing a first terrestrial network function further configured for: obtaining, from a unified data management function or access and mobility management function serving the user equipment, the information indicating the location of the user equipment. The information associated with each of the satellite may comprise information indicating one or more of: the satellite’s constellation identifiers; the satellite’s location; the satellite’s velocity; the satellite’s ephemeris; or the satellite’s non-terrestrial network function identifiers. Determining the user equipment reachability information for the user equipment may be further based on: information indicating a trajectory of the user equipment; and / or information indicating a velocity of the user equipment. The means may be for providing the first terrestrial network function further configured for registering, at a network repository function, a profile for the first terrestrial network function, the profile comprising information indicating one or more of: non-terrestrial network function identifiers supported by the first terrestrial network function; satellite operator identifiers supported by the first terrestrial network function; or public land mobile network identifiers supported by the first terrestrial network function. The request for user equipment reachability information may comprise information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. The means may be for providing a first terrestrial network function further configured for: determining, based on locally configured information and / or information obtained from at least one of a unified data management function, an access and mobility management function, or another network function, information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. Determining the user equipment reachability information based on the request may comprise determining the user equipment reachability information based on the information indicating one or more of: the identity of the user equipment; the location of the user equipment; the expected travel route of the user equipment; the identity of the one or more satellites; the identity of the operator of the one or more satellites; and a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites. The user equipment reachability information may further comprise information indicating a probability or confidence level of the user equipment being reachable. The user equipment may be expected to be reachable via one or more of the following modes: a first mode in which the user equipment is expected to be reachable via the satellite that previously received a registration request from the user equipment; a second mode in which the user equipment is expected to become reachable via a first satellite that has or will have a coverage area in which the user equipment is located and has or will have a connection to the terrestrial network; or a third mode in which the user equipment is expected to be reachable via a second satellite that has a connection to the terrestrial network and a third satellite that has a coverage area in which the user equipment is located and has an inter-satellite connection with the second satellite. The request for user equipment reachability information may comprise information indicating one or more of the modes to be used. According to an aspect there is provided an apparatus comprising means for providing a second terrestrial network function configured for: sending, to a first terrestrial network function, a request for user equipment reachability information for a user equipment; receiving, from the first terrestrial network function, the user equipment reachability information for the user equipment; and determining, based on the received user equipment reachability information for the user equipment, how and when to send at least one of: messages, paging, or user plane packets addressed to the user equipment. The user equipment reachability information for the user equipment may comprise information indicating, for each of the one or more satellites: a first time window during which a respective satellite is reachable by the core network; and a second time window during which the user equipment is reachable by the respective satellite. Determining how and when to send the at least one of: messages, paging or user plane packets addressed to the user equipment may comprise: determining at least one satellite of the one or more satellites to send the at least one of: messages, paging, or user plane packets addressed to the user equipment to; and determining when to send the messages, paging or user plane packets addressed to the user equipment to the determined at least one satellite. Determining the at least one satellite of the one or more satellites may comprise determining at least one network function provided by the at least one satellite to send the at least one of: messages, paging, or user plane packets addressed to the user equipment to. The means may be for providing a second terrestrial network function further configured for: sending, to the determined at least one satellite, the messages, paging or user plane packets addressed to the user equipment during the first time window. The request for user equipment reachability information may comprise information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. The means may be for providing a second terrestrial network function further configured for: determining, based on locally configured information and / or information obtained from at least one of a unified data management function, an access and mobility management function, or another network function, information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. Determining the user equipment reachability information based on the request may comprise determining the user equipment reachability information based on the information indicating one or more of: the identity of the user equipment; the location of the user equipment; the expected travel route of the user equipment; the identity of the one or more satellites; the identity of the operator of the one or more satellites; and a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites. The user equipment reachability information may further comprise information indicating a probability or confidence level of the user equipment being reachable. The user equipment may be expected to be reachable via one or more of the following modes: a first mode in which the user equipment is expected to be reachable via the satellite that previously received a registration request from the user equipment; a second mode in which the user equipment is expected to become reachable via a first satellite that has or will have a coverage area in which the user equipment is located and has or will have a connection to the terrestrial network; or a third mode in which the user equipment is expected to be reachable via a second satellite that has a connection to the terrestrial network and a third satellite that has a coverage area in which the user equipment is located and has an inter-satellite connection with the second satellite. The request for user equipment reachability information may comprise information indicating one or more of the modes to be used. According to an aspect, there is provided an apparatus for a core network, the apparatus being for providing a first terrestrial network function, the apparatus comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the first terrestrial network function at least to: receive, from a second terrestrial network function of the core network, a request for user equipment reachability information for a user equipment; based on the receiving of the request, determine the user equipment reachability information for the user equipment based at least on location information indicative of a location of the user equipment and information about one or more satellites of the non-terrestrial network; and send, to the second terrestrial network function, the user equipment reachability information for the user equipment. The user equipment reachability information may comprise information indicating, for each of the one or more satellites: a first time window during which a respective satellite is reachable by the core network; and a second time window during which the user equipment is reachable by the respective satellite. The at least one processor may be configured to cause the apparatus to assess the future location of the user equipment based on its last known location and on an assessment of the user equipment’s trajectory. The at least one processor may be configured to cause the first terrestrial network function to: obtain, from a unified data management function or access and mobility management function serving the user equipment, the information indicating the location of the user equipment. The information associated with each of the satellite may comprise information indicating one or more of: the satellite’s constellation identifiers; the satellite’s location; the satellite’s velocity; the satellite’s ephemeris; or the satellite’s non-terrestrial network function identifiers. The at least one processor may be configured to cause the first terrestrial network function to determine the user equipment reachability information for the user equipment further based on: information indicating a trajectory of the user equipment; and / or information indicating a velocity of the user equipment. The at least one processor may be configured to cause the apparatus to register, at a network repository function, a profile for the first terrestrial network function, the profile comprising information indicating one or more of: non-terrestrial network function identifiers supported by first terrestrial network function; satellite operator identifiers supported by the first terrestrial network function; or public land mobile network identifiers supported by the first terrestrial network function. The request for user equipment reachability information may comprise information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. The at least one processor may be configured to cause the apparatus to: determine, based on locally configured information and / or information obtained from at least one of a unified data management function, an access and mobility management function, or another network function, information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. The at least one processor may be configured to cause the apparatus to determine the user equipment reachability information based on the information indicating one or more of: the identity of the user equipment; the location of the user equipment; the expected travel route of the user equipment; the identity of the one or more satellites; the identity of the operator of the one or more satellites; and a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites. The user equipment reachability information may further comprise information indicating a probability or confidence level of the user equipment being reachable. The user equipment may be expected to be reachable via one or more of the following modes: a first mode in which the user equipment is expected to be reachable via the satellite that previously received a registration request from the user equipment; a second mode in which the user equipment is expected to become reachable via a first satellite that has or will have a coverage area in which the user equipment is located and has or will have a connection to the terrestrial network; or a third mode in which the user equipment is expected to be reachable via a second satellite that has a connection to the terrestrial network and a third satellite that has a coverage area in which the user equipment is located and has an inter-satellite connection with the second satellite. The request for user equipment reachability information may comprise information indicating one or more of the modes to be used. According to an aspect, there is provided an apparatus for second terrestrial network function, the apparatus comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the second terrestrial network function at least to: send, to a first terrestrial network function, a request for user equipment reachability information for a user equipment; receive, from the first terrestrial network function, the user equipment reachability information for the user equipment; and determine, based on the received user equipment reachability information for the user equipment, how and when to send at least one of: messages, paging, or user plane packets addressed to the user equipment. The user equipment reachability information for the user equipment may comprise information indicating, for each of the one or more satellites: a first time window during which a respective satellite is reachable by the core network; and a second time window during which the user equipment is reachable by the respective satellite. The at least one processor may be configured to cause the second terrestrial network function to: determine at least one satellite of the one or more satellites to send the at least one of: messages, paging, or user plane packets addressed to the user equipment to; and determine when to send the messages, paging or user plane packets addressed to the user equipment to the determined at least one satellite. The at least one processor may be configured to cause the second terrestrial network function to determine at least one network function provided by the at least one satellite to send the at least one of: messages, paging, or user plane packets addressed to the user equipment to. The at least one processor may be configured to cause the second terrestrial network function to: send, to the determined at least one satellite, the messages, paging or user plane packets addressed to the user equipment during the first time window. The request for user equipment reachability information may comprise information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. The at least one processor may be configured to cause the apparatus to: determine, based on locally configured information and / or information obtained from at least one of a unified data management function, an access and mobility management function, or another network function, information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. The at least one processor may be configured to cause the apparatus to determine the user equipment reachability information based on the information indicating one or more of: the identity of the user equipment; the location of the user equipment; the expected travel route of the user equipment; the identity of the one or more satellites; the identity of the operator of the one or more satellites; and a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites. The user equipment reachability information may further comprise information indicating a probability or confidence level of the user equipment being reachable. The user equipment may be expected to be reachable via one or more of the following modes: a first mode in which the user equipment is expected to be reachable via the satellite that previously received a registration request from the user equipment; a second mode in which the user equipment is expected to become reachable via a first satellite that has or will have a coverage area in which the user equipment is located and has or will have a connection to the terrestrial network; or a third mode in which the user equipment is expected to be reachable via a second satellite that has a connection to the terrestrial network and a third satellite that has a coverage area in which the user equipment is located and has an inter-satellite connection with the second satellite. The request for user equipment reachability information may comprise information indicating one or more of the modes to be used. According to an aspect there is provided a method performed by a first terrestrial network function, the method: receiving, from a second terrestrial network function of the core network, a request for user equipment reachability information for a user equipment; based on the receiving of the request, determining the user equipment reachability information for the user equipment based at least on location information indicative of a location of the user equipment and information about one or more satellites of the non-terrestrial network; and sending, to the second terrestrial network function, the user equipment reachability information for the user equipment. The user equipment reachability information may comprise information indicating, for each of the one or more satellites: a first time window during which a respective satellite is reachable by the core network; and a second time window during which the user equipment is reachable by the respective satellite. Determining the user equipment reachability information for the user equipment may comprise assessing the future location of the user equipment based on its last known location and on an assessment of the user equipment’s trajectory. The method may comprise: obtaining, from a unified data management function or access and mobility management function serving the user equipment, the information indicating the location of the user equipment. The information associated with each of the satellite may comprise information indicating one or more of: the satellite’s constellation identifiers; the satellite’s location; the satellite’s velocity; the satellite’s ephemeris; or the satellite’s non-terrestrial network function identifiers. Determining the user equipment reachability information for the user equipment may be further based on: information indicating a trajectory of the user equipment; and / or information indicating a velocity of the user equipment. The method may comprise registering, at a network repository function, a profile for the first terrestrial network function, the profile comprising information indicating one or more of: nonterrestrial network function identifiers supported by first terrestrial network function; satellite operator identifiers supported by the first terrestrial network function; or public land mobile network identifiers supported by the first terrestrial network function. The request for user equipment reachability information may comprise information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. The method may comprise: determining, based on locally configured information and / or information obtained from at least one of a unified data management function, an access and mobility management function, or another network function, information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. Determining the user equipment reachability information based on the request may comprise determining the user equipment reachability information based on the information indicating one or more of: the identity of the user equipment; the location of the user equipment; the expected travel route of the user equipment; the identity of the one or more satellites; the identity of the operator of the one or more satellites; and a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites. The user equipment reachability information may further comprise information indicating a probability or confidence level of the user equipment being reachable. The user equipment may be expected to be reachable via one or more of the following modes: a first mode in which the user equipment is expected to be reachable via the satellite that previously received a registration request from the user equipment; a second mode in which the user equipment is expected to become reachable via a first satellite that has or will have a coverage area in which the user equipment is located and has or will have a connection to the terrestrial network; or a third mode in which the user equipment is expected to be reachable via a second satellite that has a connection to the terrestrial network and a third satellite that has a coverage area in which the user equipment is located and has an inter-satellite connection with the second satellite. The request for user equipment reachability information may comprise information indicating one or more of the modes to be used. According to an aspect there is provided a method performed by a second terrestrial network function, the method comprising: sending, to a first terrestrial network function, a request for user equipment reachability information for a user equipment; receiving, from the first terrestrial network function, the user equipment reachability information for the user equipment; and determining, based on the received user equipment reachability information for the user equipment, how and when to send at least one of: messages, paging, or user plane packets addressed to the user equipment. The user equipment reachability information for the user equipment may comprise information indicating, for each of the one or more satellites: a first time window during which a respective satellite is reachable by the core network; and a second time window during which the user equipment is reachable by the respective satellite. Determining how and when to send the at least one of: messages, paging or user plane packets addressed to the user equipment may comprise: determining at least one satellite of the one or more satellites to send the at least one of: messages, paging, or user plane packets addressed to the user equipment to; and determining when to send the messages, paging or user plane packets addressed to the user equipment to the determined at least one satellite. Determining the at least one satellite of the one or more satellites may comprise determining at least one network function provided by the at least one satellite to send the at least one of: messages, paging, or user plane packets addressed to the user equipment to. The method may comprise: sending, to the determined at least one satellite, the messages, paging or user plane packets addressed to the user equipment during the first time window. The request for user equipment reachability information may comprise information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. The method may comprise: determining, based on locally configured information and / or information obtained from at least one of a unified data management function, an access and mobility management function, or another network function, information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. Determining the user equipment reachability information based on the request may comprise determining the user equipment reachability information based on the information indicating one or more of: the identity of the user equipment; the location of the user equipment; the expected travel route of the user equipment; the identity of the one or more satellites; the identity of the operator of the one or more satellites; and a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites. The user equipment reachability information may further comprise information indicating a probability or confidence level of the user equipment being reachable. The user equipment may be expected to be reachable via one or more of the following modes: a first mode in which the user equipment is expected to be reachable via the satellite that previously received a registration request from the user equipment; a second mode in which the user equipment is expected to become reachable via a first satellite that has or will have a coverage area in which the user equipment is located and has or will have a connection to the terrestrial network; or a third mode in which the user equipment is expected to be reachable via a second satellite that has a connection to the terrestrial network and a third satellite that has a coverage area in which the user equipment is located and has an inter-satellite connection with the second satellite. The request for user equipment reachability information may comprise information indicating one or more of the modes to be used. According to an aspect, there is provided a computer readable medium comprising instructions which, when executed by an apparatus for a core network, the apparatus providing a first terrestrial network function, cause the first terrestrial network function to perform at least the following: receiving, from a second terrestrial network function of the core network, a request for user equipment reachability information for a user equipment; based on the receiving of the request, determining the user equipment reachability information for the user equipment based at least on location information indicative of a location of the user equipment and information about one or more satellites of the non-terrestrial network; and sending, to the second terrestrial network function, the user equipment reachability information for the user equipment. The user equipment reachability information may comprise information indicating, for each of the one or more satellites: a first time window during which a respective satellite is reachable by the core network; and a second time window during which the user equipment is reachable by the respective satellite. Determining the user equipment reachability information for the user equipment may comprise assessing the future location of the user equipment based on its last known location and on an assessment of the user equipment’s trajectory. The instructions, when executed by the apparatus, may cause the first terrestrial network function to further perform: obtaining, from a unified data management function or access and mobility management function serving the user equipment, the information indicating the location of the user equipment. The information associated with each of the satellite may comprise information indicating one or more of: the satellite’s constellation identifiers; the satellite’s location; the satellite’s velocity; the satellite’s ephemeris; or the satellite’s non-terrestrial network function identifiers. Determining the user equipment reachability information for the user equipment may be further based on: information indicating a trajectory of the user equipment; and / or information indicating a velocity of the user equipment. The instructions, when executed by the apparatus, may cause the first terrestrial network function to further perform registering, at a network repository function, a profile for the first terrestrial network function, the profile comprising information indicating one or more of: nonterrestrial network function identifiers supported by first terrestrial network function; satellite operator identifiers supported by the first terrestrial network function; or public land mobile network identifiers supported by the first terrestrial network function. The request for user equipment reachability information may comprise information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. The instructions, when executed by the apparatus, may cause the first terrestrial network function to further perform: determining, based on locally configured information and / or information obtained from at least one of a unified data management function, an access and mobility management function, or another network function, information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. Determining the user equipment reachability information based on the request may comprise determining the user equipment reachability information based on the information indicating one or more of: the identity of the user equipment; the location of the user equipment; the expected travel route of the user equipment; the identity of the one or more satellites; the identity of the operator of the one or more satellites; and a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites. The user equipment reachability information may further comprise information indicating a probability or confidence level of the user equipment being reachable. The user equipment may be expected to be reachable via one or more of the following modes: a first mode in which the user equipment is expected to be reachable via the satellite that previously received a registration request from the user equipment; a second mode in which the user equipment is expected to become reachable via a first satellite that has or will have a coverage area in which the user equipment is located and has or will have a connection to the terrestrial network; or a third mode in which the user equipment is expected to be reachable via a second satellite that has a connection to the terrestrial network and a third satellite that has a coverage area in which the user equipment is located and has an inter-satellite connection with the second satellite. The request for user equipment reachability information may comprise information indicating one or more of the modes to be used. According to an aspect, there is provided a computer readable medium comprising instructions which, when executed by an apparatus providing a second terrestrial network function, cause the second terrestrial network function to perform at least the following: sending, to a first terrestrial network function, a request for user equipment reachability information for a user equipment; receiving, from the first terrestrial network function, the user equipment reachability information for the user equipment; and determining, based on the received user equipment reachability information for the user equipment, how and when to send at least one of: messages, paging, or user plane packets addressed to the user equipment. The user equipment reachability information for the user equipment may comprise information indicating, for each of the one or more satellites: a first time window during which a respective satellite is reachable by the core network; and a second time window during which the user equipment is reachable by the respective satellite. Determining how and when to send the at least one of: messages, paging or user plane packets addressed to the user equipment may comprise: determining at least one satellite of the one or more satellites to send the at least one of: messages, paging, or user plane packets addressed to the user equipment to; and determining when to send the messages, paging or user plane packets addressed to the user equipment to the determined at least one satellite. Determining the at least one satellite of the one or more satellites may comprise determining at least one network function provided by the at least one satellite to send the at least one of: messages, paging, or user plane packets addressed to the user equipment to. The instructions, when executed by the apparatus, may cause the second terrestrial network function to further perform: sending, to the determined at least one satellite, the messages, paging or user plane packets addressed to the user equipment during the first time window. The request for user equipment reachability information may comprise information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. The instructions, when executed by the apparatus, may cause the second terrestrial network function to further perform: determining, based on locally configured information and / or information obtained from at least one of a unified data management function, an access and mobility management function, or another network function, information indicating one or more of: an identity of the user equipment; a location of the user equipment; an expected travel route of the user equipment; an identity of the one or more satellites; an identity of the operator of the one or more satellites; a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; or a storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available. Determining the user equipment reachability information based on the request may comprise determining the user equipment reachability information based on the information indicating one or more of: the identity of the user equipment; the location of the user equipment; the expected travel route of the user equipment; the identity of the one or more satellites; the identity of the operator of the one or more satellites; and a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites. The user equipment reachability information may further comprise information indicating a probability or confidence level of the user equipment being reachable. The user equipment may be expected to be reachable via one or more of the following modes: a first mode in which the user equipment is expected to be reachable via the satellite that previously received a registration request from the user equipment; a second mode in which the user equipment is expected to become reachable via a first satellite that has or will have a coverage area in which the user equipment is located and has or will have a connection to the terrestrial network; or a third mode in which the user equipment is expected to be reachable via a second satellite that has a connection to the terrestrial network and a third satellite that has a coverage area in which the user equipment is located and has an inter-satellite connection with the second satellite. The request for user equipment reachability information may comprise information indicating one or more of the modes to be used. According to an aspect, there is provided a non-transitory computer readable medium comprising program instructions that, when executed by an apparatus, cause the apparatus to perform at least the method according to any of the preceding aspects. In the above, many different embodiments have been described. It should be appreciated that further embodiments may be provided by the combination of any two or more of the embodiments described above. DESCRIPTION OF FIGURES Embodiments will now be described, by way of example only, with reference to the accompanying Figures in which: Figure 1 shows a representation of a 5th generation communication system; Figure 2 shows a representation of an apparatus for the communication system of Figure 1 according to some example embodiments; Figure 3 shows a representation of an apparatus according to some example embodiments; Figure 4 shows methods according to some examples; and Figure 5 shows examples of communication systems. DETAILED DESCRIPTION A non-exhaustive list of some abbreviations used throughout the present disclosure is provided below for reference: 5GS = 5G system AMF = Access and Mobility Management Function NAS = Non-Access Stratum NF = Network Function NTNF = Non-Terrestrial Network Function SAT = Satellite TNF = Terrestrial Network Function UE = User Equipment In the following certain embodiments are explained with reference to communication devices capable of communication with a communication system comprising a non-terrestrial network (NTN) which includes one or more satellites, where a satellite of the one or more satellites includes one or more access nodes of an access network and one or more network functions of a core network of the communication system. Before explaining in detail the exemplifying embodiments of the methods and apparatuses of the present disclosure, a 5th generation communication system (5GS), an access network and a core network (5GC) thereof, and communication devices are briefly explained with reference to Figures 1,2 and 3. Figure 1 shows a schematic representation of a 5G communication system (5GS). The 5GS may comprise a user equipment (UE), an access network such as a 5G radio access network (5G-RAN) or next generation radio access network (NG-RAN), a 5G core network (5GC), and one or more application functions. An application function may be deployed in the 5GS as trusted application function or may be deployed or host on one or more application servers of the data network. Such application functions are untrusted application functions. The 5GS connects the UE to a data network the access network and the 5GC (e.g., a UPF of the 5GC). The 5G-RAN may comprise one or more radio access nodes, such as gNodeB (gNB). A gNB may include one or more gNodeB (GNB) distributed units connected to one or more gNodeB (GNB) centralized units. The 5GC may comprise the following network functions: Network Slice Selection Function (NSSF); Network Exposure Function; Network Repository Function (NRF); Policy Control Function (PCF); Unified Data Management (UDM); Application Function (AF); Authentication Server Function (AUSF); an Access and Mobility Management Function (AMF); and Session Management Function (SMF), and a user plane function (UPF). Figure 1 also shows the various logical interfaces (N1, N2 etc.) that may be implemented between the various elements of the system - for example the N1 interface between the AMF and communication device or terminal; the N2 interface between the AMF and a RAN node; the N3 interface between the RAN node and UPF; the N4 interface between the UPF and SMF; the N6 interface between the UPF and the Data Network (DN); the N8 interface between the AMF and the UDM; the N10 interface between the SMF and UDM; the N11 interface between the SMF and AMF; the N12 interface between the AMF and AUSF; the N13 interface between the AUSF and UDM; the N29 interface between the NEF and SMF; and the N33 interface between the NEF and AF. Figure 2 illustrates an example of a control apparatus 200 for controlling a function of the access network (e.g., a 5G-RAN or the NG-RAN illustrated in Figure 1) illustrated on Figure 1. The control apparatus 200 may comprise at least one random access memory (RAM) 211a, at least on read only memory (ROM) 211b, at least one processor 212, 213 and a network interface 214. The at least one processor 212, 213 may be coupled to the RAM 211a and the ROM 211b. The at least one processor 212, 213 may be configured to execute an appropriate software code 215. Execution of software code 215 may for example may cause the apparatus to perform operations for controlling a function of the access network. The software code 215 may be stored in the ROM 211b. The control apparatus 200 may be interconnected with another control apparatus 200 for controlling another function of the 5G-RAN or the NG-RAN. In some embodiments, each function of the 5G-RAN or the NG-RANis deployed or hosted on a control apparatus 200. In alternative embodiments, two or more functions of the 5G-RAN or the NG-RAN may share a control apparatus. Figure 3 illustrates an example of a communication device 300, such as the UE illustrated on Figure 1. The communication device 300 may be provided by any device capable of sending and receiving radio signals. Non-limiting examples of a communication device 300 comprise a user equipment, a mobile station (MS) or mobile device such as a mobile phone or what is known as a ’smart phone’, a computer provided with a wireless interface card or other wireless interface facility (e.g., USB dongle), a personal data assistant (PDA) or a tablet provided with wireless communication capabilities, a machine-type communications (MTC) device, an Internet of things (loT) type communication device or any combinations of these or the like. The communication device 300 may comprise a transceiver for transmitting and / or receiving, for example, wireless signals carrying communications, for example radio signals. The communications may be one or more of voice, electronic mail (email), text messages, multimedia data, machine data and so on. The communication device 300 may receive wireless signals (e.g., radio signals) over an air or radio interface 307 via appropriate apparatus for receiving and may transmit wireless signals via appropriate apparatus for transmitting radio signals. In Figure 3 transceiver is designated schematically by block 306. The transceiver 306 may comprise, for example, a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the mobile device and may comprise one or more antenna elements. The antenna arrangement may be a multi-input multi output (MIMO) antenna. The communication device 300 may be provided with at least one processor 301, at least one memory ROM 302a, at least one RAM 302b and other possible components 303 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with access networks (e.g., the 5G-RAN or NG-RAN illustrated in Figure 1) and other communication devices. The at least one processor 301 is coupled to the RAM 302b and the ROM 302a. The at least one processor 301 may be configured to execute an appropriate software code 308. The software code 308 may for example allow to perform one or more operations of the communication device. The software code 308 may be stored in the ROM 302a. The processor, the ROM, and the RAM, the transceiver and other circuitry of the communication device (e.g., a modem) can be provided on a circuit board, in chipsets, or in a system on chip. The circuit board, chipsets or system on chip is denoted by reference 304. The communication device 300 may optionally have a user interface such as key pad 305, touch sensitive screen or pad, combinations thereof or the like. Optionally one or more of a display, a speaker and a microphone may be provided depending on the type of communication device. Non-terrestrial networks (NTNs) are being integrated into 5G communication systems. NonTerrestrial Networks may be defined as networks, or segments of networks, using an airborne or space-borne vehicle (herein referred to as a satellite) to embark a transmission equipment relay node or access node (e.g., base station). As used herein, an access node deployed on a satellite is referred to as a satellite-based access node that provides at least some of the functionality of an access node, such as a gNB. In some examples a NTN may include one or more satellite constellations that are operated by the same constellation operator or different satellite operators. Each satellite constellation is a group of artificial satellites working together as a system. The satellites of a given satellite constellation may be operated by the same satellite operator or different satellite operators. Reference is made to Figures 5a and 5b, which show two examples of communication systems comprising a non-terrestrial network (NTN) where some network functions of a core network of the communication system are deployed at satellites of the NTN and other network functions of the core network are deployed at (e.g., hosted on) an apparatus (e.g., a computing system) located on the ground. A network function deployed at a satellite is referred to herein as a terrestrial network function and a network function deployed at an apparatus located on the ground is referred to as a terrestrial network function. It should be understood that in some examples different network functions other than those shown in Figures 5a and 5b may be deployed at satellites of the NTN. In the example communication system shown in Figure 5a, NTNFs are deployed at a satellite 500 (e.g., a satellite of a satellite constellation that is in low-Earth orbit or mid-Earth orbit) of the NTN. Each respective NTNF is capable of communicating with one or more TNFs of the core network 502 provided by terrestrial-based apparatus(es) via a feeder link 504 between the satellite 500 and a ground station connected to the core network 502. As shown in the example of Figure 5b, the satellite may also provide some core network functionality. For example, the satellite may comprise an AMF, UPF, SMF etc. Figure 5b shows examples of different logical interfaces between various TNFs in the core network 502 and NTNFs at the satellite 500. It should be understood that while lines depicting the logical interfaces are shown, communications between the various TNFs and NTNFs (e.g. between the UDM of the core network 502 and the AMF of the satellite 502) pass through the feeder link 504 which provides the transport interface for control plane messages between TNFs and NTNFs and for NAS messages between a UE and the TNFs. In some examples the core network 502 may comprise a first TNF, which may be a new network function (which may be referred to herein as a “UE Reachability Estimator” or URE as shown in Figures 5a and 5b) or an existing network function (such as a NEF) that includes the functionality of the first TNF disclosed herein). The functionality of the first TNF will be discussed in more detail below. In some examples, a core network may comprise a terrestrial portion and a non-terrestrial portion. The terrestrial portion of the core network (referred to herein as a terrestrial core network) may comprise a computing system hosting various network functions (referred to herein as terrestrial network functions). The non-terrestrial portion of the core network may include network functions deployed on satellites of a satellite constellation (referred to as nonterrestrial network functions herein). NTNs provide coverage to a wide area by providing communication devices connectivity over the regions that may be expensive or difficult to cover with terrestrial networks (e.g., rural areas, vessels in oceans, airplanes). NTNs may enhance coverage by offering wide-area coverage and ensuring service availability, continuity, and scalability, and may represent a coverage extension for the terrestrial network. NTN systems may therefore be an effective solution to complement terrestrial networks in providing services over uncovered or underserved geographical areas. In some examples a NTN may comprise one or more satellites that have network functions deployed thereon (e.g., the software or computer program code for one or more network functions is stored in a satellite) for direct communication with devices on the ground. Such NTNs may be termed as having “regenerative” satellite architectures. A NTN that is part of a communication system may establish a communication link (herein referred to as a “feeder link”) between a satellite of the NTN and a ground station connected to a core network of the communication system. The feeder link may be used to transmit data (e.g., user plane packets) and control plane signalling (e.g., NAS messages) between the satellite and the core network for various purposes, such as control of the satellite for communications with a user equipment, synchronization, and transfer of data to the ground station for forwarding to a user plane function of the core network deployed at the satellite. The feeder link may therefore be an important aspect for the operation of the satellite, especially in scenarios where the satellite includes NFs of the core network and the satellite is part of a Non-Terrestrial Network (NTN). However, there may be times when the feeder link between the satellite and the ground station connected to ground is absent or interrupted. For example, when a satellite is flying over an ocean (or other remote geographical area), there may be no ground station that is within communication range of the satellite to establish a feeder link with. During these times of absence of a feeder link, the Network Functions of a core network that is connected to the ground station (which may be referred to herein as terrestrial network functions or TNFs) may not have continuous connectivity with the NFs of the core network deployed at the satellites (which may be referred to herein as NTN network functions or NTNFs). The absence of a feeder link or interruption to the feeder link may cause problems, such as problems in connectivity and delivery of services to UEs served by satellite of the NTN. If the TNF is unaware of uncertain as to the time and method by which to establish communication with a user equipment via one or more satellites, there may be problems that arise. For example, there may be an unnecessary delay in providing services to the UE (e.g., if the UE is reachable before the TNF is aware), or there may be inefficient use of network resources (e.g., if TNF attempts to send messages to the UE but the feeder link to the satellite is not available). Some examples of the present disclosure may address some of these problems. Reference is made to Figure 4, which shows methods according to some examples. At 400, a method comprises receiving, from a second terrestrial network function of the core network, a request for user equipment reachability information for a user equipment. At 402 the method comprises, based on the receiving of the request, determining the user equipment reachability information for the user equipment based at least on location information indicative of a location of the user equipment and information about one or more satellites of the non-terrestrial network. At 404, the method comprises sending, to the second terrestrial network function, the user equipment reachability information for the user equipment. In some examples steps 400-404 may be performed by a first terrestrial network function. At 406, a method comprises sending, to a first terrestrial network function, a request for user equipment reachability information for a user equipment. At 408 the method comprises receiving, from the first terrestrial network function, the user equipment reachability information for the user equipment. At 410 the method comprises determining, based on the received user equipment reachability information for the user equipment, how and when to send at least one of: messages, paging, or user plane packets addressed to the user equipment. In some examples steps 406-410 may be performed by a second terrestrial network function. As explained previously, in some examples the core network 502 may comprise a first TNF, which may be a new network function (which may be referred to herein as a “UE Reachability Estimator” or URE as shown in Figures 5a and 5b) or an existing network function (such as a NEF) that includes the functionality of the first TNF disclosed herein). The first TNF may be configured to determine UE reachability information fora UE (e.g., the UE shown in Figure 5a or 5b). The UE reachability information for a UE may comprise information indicating a first time window (which may be referred to herein as a feeder link availability window) at which a satellite capable of serving the UE is reachable by the core network 502 via the feeder link 504. The UE reachability information for the UE may comprise information indicating a second time window (which may be referred to herein as a UE-SAT link availability window) in which the UE is reachable by the satellite 500. In some examples a second TNF (e.g., a second NF of the core network 502) may request, from the first TNF, the UE reachability information for a UE, and in response the request, the first TNF may provide the UE reachability information for the UE. In some embodiments, the second TNF may use the UE reachability information for the UE o set the second TNF’s local timer for a NAS procedure, to determine when and / or how to send NAS messages to the UE (e.g., to deliver a mobile terminated service), and / or to determine when and / or how to page the UE. The second TNF may be comprised in the terrestrial portion of a core network 502, of a public land mobile network (PLMN). In some examples the first TNF may determine the UE reachability information for a UE based on information about a location of the UE (generally referred to herein as UE location information) (and in some examples information about a trajectory of the UE and / or a velocity of the UE) and information associated with one or more satellites of the NTN. The information associated with the one or more satellites of the NTN may comprise, for each respective satellite, information indicating one or more of: the location of the respective satellite (generally referred to as a satellite’s location), a velocity of the respective satellite and / or ephemeris the respective satellite. For example, the first TNF may determine the first time window and / or the second time window based on the information associated with the UE location and the information associated with the one or more satellites. In some examples the UE reachability information for a UE may comprise information indicating a time that the UE is expected to be reachable by the satellite 500 and a time duration for which the UE is expected to be reachable by the satellite 500. In some examples the UE may be reachable by more than one satellite. In such cases, the UE reachability information for the UE may comprise, for each satellite via which the UE is reachable, information indicating a time that the UE is expected to be reachable by the satellite and time duration for which the UE is expected to be reachable by satellite. In some examples the information indicating the time that the UE is expected to be reachable may comprise information indicating an earliest time that the UE is expected to be reachable by a satellite (e.g, the satellite 500). In some examples the UE reachability information may comprise a “reachability schedule” (RS). The RS may comprise information indicating the time(s) and duration(s) that the UE is expected to be reachable by a satellite comprising the second TNF. The RS may further comprise information indicating the satellite (and optionally it’s NTNFs) through which the UE is expected to be reachable during the indicated time(s) and duration(s). The RS may further comprise information indicating the latest time the satellite has reached the terrestrial network before covering the UE. As used herein, “covering” the UE may refer to the UE being within the coverage area of the satellite such that a UE to satellite link is available. Examples of RS are provided below in Table 1 and Table 2. It should be understood that these examples are provided by way of example only, and that in some examples different RSs may be implemented. First Time Window (Feeder link availability window) Second Time Window (UE-SAT link availability window) SAT ID Start End Start End SAT000 01 12:00 GMT 16:00 GMT 18:00 GMT 19:00 GMT SAT000 02 11:20 GMT 13:40 GMT 19:50 GMT 20:05 GMT Tab e 1 - example RS structure with non-overlapping windows for a given satellite First Time Window (Feeder link availability window) Second Time Window (UE-SAT link availability window) SAT ID Start End Start End SAT000 01 12:00 GMT 16:00 GMT 11:00 GMT 13:00 GMT SAT000 02 11:20 GMT 13:40 GMT 12:50 GMT 14:05 GMT Table 2 - example RS structure with overlapping windows for a given satellite In some examples, such as that shown in Table 1 and Table 2, there may be more than one satellite through which the UE is reachable by a TNF. In some examples, such as that shown in Table 1, the time window in which the UE is reachable via a first satellite may not overlap with the time window in which the UE is reachable via a second satellite. In other examples, such as that shown in Table 2, the time window in which the UE is reachable via a first satellite may overlap with the time window in which the UE is reachable via a second satellite. In some examples the UE may be considered reachable via a given satellite when the second time window (UE-SAT link availability window) is available during or after the first time window (Feeder link availability window). In some examples, during the first time window, messages addressed to the UE may be sent via the feeder link from the second TNF to the satellite. The satellite may store those messages until the start of the second time window, during which the satellite may send the stored messages to the UE via the UE-SAT link. For example, with reference to Table 1, the second TNF may send messages addressed to the UE to SAT00001 via the feeder link during the first time window, which is between 12:00 and 16:00 GMT. SAT0001 may store those messages (providing that there is storage capacity available) until the start of the second time window, and then send the stored messages to the UE during the second time window, which is between 18:00 and 19:00 GMT. In some examples the first time window and second time window may overlap, such as shown in Table 2 above. In such examples the second TNF may send messages addressed to the UE to the satellite (e.g. SAT0002) which may then send the messages to the UE without storing the messages. For example, the second TNF may send messages addressed to the UE to SAT0002 via the feeder link at 13:10 GMT (during the first time window), and as the UE-SAT link is available at this time (the second time window is between 12:50-14:05 GMT) the satellite may send the messages to the UE without storing them. In some examples, the UE may be stationary, and therefore the RS may be relatively accurate. However, in some examples, the UE may be mobile and the RS may be more uncertain due to mobility of the UE (e.g., due to the UE moving). In some examples the first TNF may determine the UE reachability information for a UE based on information associated with the UE’s mobility (e.g. information about a trajectory of the UE and / or a velocity of the UE). For example, the first TNF may determine the UE reachability information based on information about one or more of: the UE’s trajectory, the UE’s velocity, and / or a UE’s flight pathor flight plan, and any other information regarding the mobility of the UE. In some examples the UE reachability information for a UE may include information indicating a probability or confidence level of the UE being reachable via the satellite. For example, the RS may comprise information indicating a probability or confidence level of the UE being reachable by a satellite. In some examples the UE may be reachable via different “reachability modes”, which define how the UE is reachable. In some examples the UE may be reachable by one or more of the following modes: • Same SAT mode (SSM); • Any SAT mode (ASM); or • Fastest reach mode (FRM). In some examples the modes may be configured by the network operator. In some examples, the request for UE reachability information for the UE sent from the second TNF to the first TNF may comprise information indicating one or more of the reachability modes to be used. The first TNF may then provide the UE reachability information for the UE accordingly. In some examples (for example when no mode is indicated in the request, or when the indicated mode is not supported by the network operator), the first TNF may be configured to assume a default configuration or default mode (which may be set by the network operator) to be used when providing the UE reachability information. In some examples, in SSM it may be assumed that the satellite that receives a registration request for the UE to register with the ground-based network is the satellite to reply to the UE, and thus that the UE is reachable via that satellite. For example, when the UE registers with the ground based network (e.g., a portion of the core network located on the ground) via a first satellite (e.g. SAT00001), the first TNF may determine that the UE is reachable via the first satellite when operating in SSM. In some examples, in ASM it may be assumed that the UE may be reached by the first satellite that can cover the UE and that this satellite will have a feeder link to the ground based network before the satellite covers the UE. In some examples, in FRM it may be assumed that satellites can utilize inter-satellite links to exchange information for delivering a service to the UE. For example, the ground station connected to the ground based network (e.g., portion of the core network located on the ground) may have a feeder link established to a first satellite (e.g. SAT00001), which may establish an inter-satellite link with a second satellite (e.g. SAT00002) covering the UE (e.g., the UE being within the coverage area of the second satellite such that a UE to second satellite link is available). In this way, a service provided by a TNF may be routed to the UE via the first and second satellites, which may be faster than waiting for the first and / or second satellites to be both reachable by the ground station via a feeder link and covering the UE. In some examples, the request for UE reachability information for a UE sent from the second TNF to the first TNF may include an identifier of the UE that identifies the UE (generally referred to as a UE identifier) and reachability mode(s) to be used. The request may further comprise one or more of the following: • UE location information (e.g., tracking area information of the UE, UE co-ordinates etc.); • Expected travel route information for the UE; • Identifier of a satellite, identifier of a satellite constellation, an identifier of an operator of the satellite (e.g., the satellite operator identifier (ID)) and / or and identifier of an operator of the satellite constellation (e.g., satellite constellation operator identifier (ID)) • Types of each NTNF (e.g., NTNF type(s)) or identifiers of each NTNF (e.g., NTNF identifiers (ID(s)) deployed on the satellite; • A storage quota per NTNF, which may indicate how much storage a NTNF has already been utilized or is left available. In some examples the satellite may store messages (e.g. SMS, NIDD etc.) to be sent to the UE as the satellite moves, until the UE is within coverage of the satellite. The UE reachability information may take into account whether a given satellite has storage quote available to process and store messages addressed to a target UE until such a time that the messages can be delivered. In some examples, instead of any of the abovementioned information that may be comprised in the request, the first TNF may instead obtain the information based on locally configured information and / or information that the first TNF obtains from a UDM, AMF and / or another NF. For example, if an identifier of a satellite, an identifier of a satellite constellation, an identifier of an operator of a satellite and / or an identifier of an operator of satellite constellation operator is not provided in the request then the first TNF may assume a default satellite and / or satellite constellation (based on locally configured information) when determining the UE reachability information for a UE. As a further example, when information about a location of the UE (generally referred to as UE location information) is not available to the first TNF, the first TNF may obtain the UE location information. In some examples the first TNF may obtain the UE location information from a UDM or AMF serving the UE. In some examples the UDM may store information including the identifier of a satellite constellation (generally referred to as satellite constellation ID) or an identifier of a satellite operator (generally referred to as a satellite operator ID) if the serving AMF of the UE has registered this information at the UDM. The UDM may provide this information when the UDM receives a request for UE location information for a UE from the first TNF. In some examples the UDM may store information indicating the last known tracking area identifier or registration area (e.g., a set of tracking area identifiers) for a UE. The UDM may provide this information when the UDM receives a request for UE location information from the first TNF. In some examples the first TNF may obtain the UE location information from the AMF with which the UE is registered, or an AMF in a set of AMFs with which the UE is registered. In some examples, the AMF (or an AMF in the set of AMFs) may be a NTNF AMF. In some examples the first TNF may use a UE’s location information (e.g., last known registration area of the UE) to determine or estimate the UE’s location based on the UE location information. In some examples the first TNF may determine the satellite and / or the satellite constellation that is currently covering that area based on the UE location information. For example, the first TNF may determine, based on the satellite constellation ID or satellite operator ID, the UE’s location information, the satellite ephemeris information, the first time window and / or the second time window. In some examples the first TNF may register its NF profile at an NRF. The profile of the first TNF may comprise information indicating one or more of: the satellite constellation IDs of satellite constellations supported by the first TNF (e.g., satellite constellations the first TNF is capable of providing services to); the satellite operator IDs of satellites supported by the first TNF (e.g., satellites the first TNF is capable of providing services to); and the identifiers of the PLMNs (generally referred to a PLMN IDs) supported by the first TNF (e.g., PLMNs the first TNF is capable of providing services to). In this way, the second TNF may discover the first TNF and request the UE reachability information accordingly. Having received the user equipment reachability information for a user equipment, the second TNF may determine, based on the received user equipment reachability information, when and / or how to send messages (e.g. NAS messages), page the UE, and / or send user plane packets addressed to the user equipment. For example, the second TNF may determine which satellite is to send the messages, paging messages, and / or user plane packets to the user equipment. This may for example be based on which satellite will be able to provide the messages, paging messages and / or the user plane packets to the user equipment most quickly. In another example, the determined satellite may be the first satellite that provides the required NTNF type - e.g., if the second TNF requires a particular NTNF in the satellite to provide the messages, paging messages for the UE, and / or user plane packets to the user equipment, then the determined satellite would be a satellite that provides the required NTNF. As yet another example, if the storage capacity required for storing the messages, paging messages and / or user plane packets to the user equipment exceeds the available storage capacity at a first satellite, then the second TNF would not select the first satellite for sending the messages, paging messages, and / or user plane packets to the user equipment. As another example, the second TNF may adjust or assign a value of a timer (e.g., a timer relating to a NAS procedure) at the second TNF based on the user equipment reachability information for the user equipment. The second TNF, having determined the satellite to send the messages, paging messages, and / or user plane packets to the user equipment, may then determine when to send the messages, the paging messages, and / or user plane packets to the user equipment. This may for example comprise determining the first time window associated with the determined satellite. The second TNF may then send, to the determined satellite, the messages, the paging messages, and / or user plane packets to the user equipment during the first time window. Thus in some examples a second TNF may obtain, from a first TNF, UE reachability information. The first TNF may be a new NF or an existing NF adapted to perform the previously described functions. The first TNF may determine the UE reachability information based at least on UE location information and information associated with one or more satellites. The second TNF may then determine, based on the received user equipment reachability information, when and / or how to send at least one of: messages, paging, or user plane packets addressed to the user equipment. The second TNF may then send the at least one of: messages, paging, or user plane packets addressed to the user equipment to the determined satellite when the feeder link to that satellite is available. In this way, the second TNF is enabled to reach to reach a UE to send messages, paging messages, and / or user plane date to the UE. Some examples may further minimize delays in providing messages, paging messages, and / or user plane packets to the UE via satellite. In some examples there is provided an apparatus for a core network, the apparatus comprising means for providing a first terrestrial network function configured for: receiving, from a second terrestrial network function of the core network, a request for user equipment reachability information for a user equipment; based on the receiving of the request, determining the user equipment reachability information for the user equipment based at least on location information indicative of a location of the user equipment and information about one or more satellites of the non-terrestrial network; and sending, to the second terrestrial network function, the user equipment reachability information for the user equipment. In some examples there is provided an apparatus for a core network, the apparatus being for providing a first terrestrial network function, the apparatus comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the first terrestrial network function at least to: receive, from a second terrestrial network function of the core network, a request for user equipment reachability information for a user equipment; based on the receiving of the request, determine the user equipment reachability information for the user equipment based at least on location information indicative of a location of the user equipment and information about one or more satellites of the non-terrestrial network; and send, to the second terrestrial network function, the user equipment reachability information for the user equipment. In some examples there is provided an apparatus comprising means for providing a second terrestrial network function configured for: sending, to a first terrestrial network function, a request for user equipment reachability information for a user equipment; receiving, from the first terrestrial network function, the user equipment reachability information for the user equipment; and determining, based on the received user equipment reachability information for the user equipment, how and when to send at least one of: messages, paging, or user plane packets addressed to the user equipment. In some examples there is provided an apparatus for second terrestrial network function, the apparatus comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the second terrestrial network function at least to: send, to a first terrestrial network function, a request for user equipment reachability information for a user equipment; receive, from the first terrestrial network function, the user equipment reachability information for the user equipment; and determine, based on the received user equipment reachability information for the user equipment, how and when to send at least one of: messages, paging, or user plane packets addressed to the user equipment. It is understood that references in the above to various network functions (e.g., to an AMF, an SMF, TNF etc.) may comprise apparatus that perform at least some of the functionality associated with those network functions. Further, an apparatus comprising a network function may comprise a virtual network function instance of that network function. It should be understood that the apparatuses may comprise or be coupled to other units or modules etc., such as radio parts or radio heads, used in or for transmission and / or reception. Although the apparatuses have been described as one entity, different modules and memory may be implemented in one or more physical or logical entities. It is noted that whilst some embodiments have been described in relation to 5G networks, similar principles can be applied in relation to other networks and communication systems. Therefore, although certain embodiments were described above by way of example with reference to certain example architectures for wireless networks, technologies and standards, embodiments may be applied to any other suitable forms of communication systems than those illustrated and described herein. It is also noted herein that while the above describes example embodiments, there are several variations and modifications which may be made to the disclosed solution without departing from the scope of the present invention. As used herein, “at least one of the following: ” and “at least one of ” and similar wording, where the list of two or more elements are joined by “and” or “or”, mean at least any one of the elements, or at least any two or more of the elements, or at least all the elements. In general, the various embodiments may be implemented in hardware or special purpose circuitry, software, logic or any combination thereof. Some aspects of the disclosure may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the disclosure is not limited thereto. While various aspects of the disclosure may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof. As used in this application, the term “circuitry” may refer to one or more or all of the following: (a) hardware-only circuits (such as implementations in only analog and / or digital circuits) and (b) combinations of hardware circuits and software, such as (as applicable): (i) a combination of analog and / or digital hardware circuit(s) with software / firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.” This definition of circuitry applies to all uses of this term “means” in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and / or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device. The embodiments of this disclosure may be implemented by computer software executable by a data processor of the mobile device, such as in the processor entity, or by hardware, or by a combination of software and hardware. Computer software or program, also called program product, including software routines, applets and / or macros, may be stored in any apparatus-readable data storage medium and they comprise program instructions to perform particular tasks. A computer program product may comprise one or more computerexecutable components which, when the program is run, are configured to carry out embodiments. The one or more computer-executable components may be at least one software code or portions of it. Further in this regard it should be noted that any blocks of the logic flow as in the Figures may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD. The physical media is a non-transitory media. The term “non-transitory,” as used herein, is a limitation of the medium itself (i.e., tangible, not a signal) as opposed to a limitation on data storage persistency (e.g., RAM vs. ROM). The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may comprise one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASIC), FPGA, gate level circuits and processors based on multi core processor architecture, as non-limiting examples. Embodiments of the disclosure may be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate. The scope of protection sought for various embodiments of the disclosure is set out by the independent claims. The embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the disclosure. 5 The foregoing description has provided byway of non-limiting examples a full and informative description of the exemplary embodiment of this disclosure. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the 10 appended claims. However, all such and similar modifications of the teachings of this disclosure will still fall within the scope of this invention as defined in the appended claims. Indeed, there is a further embodiment comprising a combination of one or more embodiments with any of the other embodiments previously discussed.

Claims

1. An apparatus for a core network, the apparatus comprising means for providing a first terrestrial network function configured for:receiving, from a second terrestrial network function of the core network, a request for user equipment reachability information for a user equipment;based on the receiving of the request, determining the user equipment reachability information for the user equipment based at least on location information indicative of a location of the user equipment and information about one or more satellites of the nonterrestrial network; andsending, to the second terrestrial network function, the user equipment reachability information for the user equipment.

2. The apparatus of claim 1, wherein the user equipment reachability information for the user equipment comprises information indicating, for each of the one or more satellites:a first time window during which a respective satellite is reachable by the core network; anda second time window during which the user equipment is reachable by the respective satellite.

3. The apparatus of claim 1 or 2, wherein determining the user equipment reachability information for the user equipment comprises assessing the future location of the user equipment based on its last known location and on an assessment of the user equipment’s trajectory4. The apparatus of claim 3, wherein the means is for providing a first network function further configured for:obtaining, from a unified data management function or access and mobility management function serving the user equipment, the information indicating the location of the user equipment.

5. The apparatus of claim 3 or 4, wherein the information associated with each of the satellite comprises information indicating one or more of:the satellite’s constellation identifiersthe satellite’s location;the satellite’s velocity;the satellite’s ephemeris; orthe satellite’s non-terrestrial network function identifiers.

6. The apparatus of any preceding claim, wherein determining the user equipment reachability information for the user equipment is further based on:information indicating a trajectory of the user equipment; and / or information indicating a velocity of the user equipment.

7. The apparatus of any preceding claim, wherein the means is for providing the first network function further configured for registering, at a network repository function, a profile for the first network function, the profile comprising information indicating one or more of:satellite constellation identifiers of satellite constellations supported by first terrestrial network function;satellite operator identifiers of satellites supported by the first network function; or public land mobile network identifiers supported by the first network function.

8. An apparatus comprising means for providing a second terrestrial network function configured for:sending, to a first terrestrial network function, a request for user equipment reachability information for a user equipment;receiving, from the first terrestrial network function, the user equipment reachability information for the user equipment;determining, based on the received user equipment reachability information for the user equipment, how and when to send messages, page the UE, send user plane packets addressed to the user equipment, and / or adjust a NAS timer of the second terrestrial network function.

9. The apparatus of claim 8, wherein the user equipment reachability information for the user equipment comprises information indicating, for each of the one or more satellites:a first time window during which a respective satellite is reachable by the core network; anda second time window during which the user equipment is reachable by the respective satellite.

10. The apparatus of claim 8 or 9, wherein determining how and when to send the at least one of: messages, paging or user plane packets addressed to the user equipment comprises:determining at least one satellite of the one or more satellites to send the at least one of: send messages, to page the user equipment, or send user plane packets addressed to the user equipment to; anddetermining when to send the messages, page the user equipment, or send user plane packets addressed to the user equipment to the determined at least one satellite.

11. The apparatus of claim 10, wherein determining the at least one satellite of the one or more satellites comprises determining at least one network function provided by the at least one satellite to send the at least one of: messages, paging, or user plane packets addressed to the user equipment to.

12. The apparatus of claim 10 or 11 when dependent on claim 9, wherein the means is further for:sending, to the determined at least one satellite, the messages, paging or user plane packets addressed to the user equipment during the first time window.

13. The apparatus of any preceding claim, wherein the request for user equipment reachability information comprises information indicating one or more of:an identity of the user equipment;a location of the user equipment;an expected travel route of the user equipment;an identity of the one or more satellites;an identity of the operator of the one or more satellites;a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; ora storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available.

14. The apparatus of any of claims 1 to 12, wherein the means is for:determining, based on locally configured information and / or information obtained from at least one of a unified data management function, an access and mobility management function, or another network function, information indicating one or more of:an identity of the user equipment;a location of the user equipment;an expected travel route of the user equipment;an identity of the one or more satellites;an identity of the operator of the one or more satellites;a type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites; ora storage quota indicating how much storage a non-terrestrial network function deployed on the one or more satellites has left available.

15. The apparatus of claim 13 or 14 when dependent on any of claimsl to 7, wherein determining the user equipment reachability information based on the request comprises determining the user equipment reachability information based on the information indicating one or more of:the identity of the user equipment;the location of the user equipment;the expected travel route of the user equipment;the identity of the one or more satellites;the identity of the operator of the one or more satellites; anda type and / or identity of one or more non-terrestrial network functions deployed on the one or more satellites.

16. The apparatus of any preceding claim, wherein the user equipment reachability information further comprises information indicating a probability or confidence level of the user equipment being reachable.

17. The apparatus of any preceding claim, wherein the user equipment is expected to be reachable via one or more of the following modes:a first mode in which the user equipment is expected to be reachable via the satellite that previously received a registration request from the user equipment;a second mode in which the user equipment is expected to become reachable via a first satellite that has or will have a coverage area in which the user equipment is located and has or will have a connection to the terrestrial network; ora third mode in which the user equipment is expected to be reachable via a second satellite that has a connection to the terrestrial network and a third satellite that has a coverage area in which the user equipment is located and has an inter-satellite connection with the second satellite.

18. The apparatus of claim 17, wherein the request for user equipment reachability information comprises information indicating one or more of the modes to be used.

19. An apparatus for a core network, the apparatus being for providing a first terrestrial network function, the apparatus comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the first terrestrial network function at least to:receive, from a second terrestrial network function of the core network, a request for user equipment reachability information for a user equipment;based on the receiving of the request, determine the user equipment reachability information for the user equipment based at least on location information indicative of a location of the user equipment and information about one or more satellites of the nonterrestrial network; andsend, to the second terrestrial network function, the user equipment reachability information for the user equipment.

20. An apparatus for second terrestrial network function, the apparatus comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the second terrestrial network function at least to:send, to a first terrestrial network function, a request for user equipment reachability information for a user equipment;receive, from the first terrestrial network function, the user equipment reachability information for the user equipment; anddetermine, based on the received user equipment reachability information for the user equipment, how and when to send at least one of: messages, paging, or user plane packets addressed to the user equipment.

21. A method performed by a first terrestrial network function, the method:receiving, from a second terrestrial network function of the core network, a request for user equipment reachability information for a user equipment;based on the receiving of the request, determining the user equipment reachability information for the user equipment based at least on location information indicative of a location of the user equipment and information about one or more satellites of the nonterrestrial network; andsending, to the second terrestrial network function, the user equipment reachability information for the user equipment.

22. A method performed by a second terrestrial network function, the method comprising: sending, to a first terrestrial network function, a request for user equipment reachability information for a user equipment;receiving, from the first terrestrial network function, the user equipment reachability information for the user equipment; anddetermining, based on the received user equipment reachability information for the user equipment, how and when to send at least one of: messages, paging, or user plane5 packets addressed to the user equipment.42

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