What is claimed is:1. A transmission method, comprising:receiving, from a second network node via a E1 interface, a first indication indicating that activity of a terminal device is inactive, wherein the E1 interface is an interface between a first network node and the second network node; andindicating to the second network node via the E1 interface that the terminal device is entering an inactive state, wherein the first network node is a control plane node of a third network node, the second network node is a user plane node of the third network node, and the third network node includes at least one of a packet data convergence protocol layer function, a service data adaptation protocol layer function, or a radio resource control layer function, wherein:the first network node comprises a radio resource control (RRC) protocol layer and a packet data convergence protocol (PDCP) protocol layer, and wherein the first network node processes a signaling radio bearer (SRB) by using the PDCP protocol layer.2. The method according to claim 1, wherein the method further comprises:receiving a second indication from the second network node, wherein the second indication indicates that the second network node receives downlink data of the terminal device from a core network.3. The method according to claim 2, wherein initiates, in response to the second indication, a radio access network paging within a radio access network notification range to notify the terminal device.4. The method according to claim 1, wherein:the second network node comprises a service data adaptation protocol (SDAP) protocol layer and a second PDCP protocol layer, and wherein the second network node processes a data radio bearer (DRB) by using the second PDCP protocol layer.5. The method according to claim 1, wherein the method further comprises:sending an inactivity timer of the terminal device to the second network node.6. The method according to claim 1, wherein the first network node is a control plane node of a centralized unit (CU-CP), the second network node is a user plane node of the centralized unit (CU-UP), and the third network node is the centralized unit (CU).7. An apparatus comprising:at least one processor, and a memory storing instructions for execution by the at least one processor;wherein, when executed, the instructions cause the apparatus to perform operations comprising:receiving, from a second network node via a E1 interface, a first indication indicating that activity of a terminal device is inactive, wherein the E1 interface is an interface between the apparatus and the second network node; andindicating to the second network node via the E1 interface that the terminal device is entering an inactive state, wherein the apparatus is a control plane node of a third network node, the second network node is a user plane node of the third network node, and the third network node includes at least one of a packet data convergence protocol layer function, a service data adaptation protocol layer function, or a radio resource control layer function, wherein:the apparatus comprises a radio resource control (RRC) protocol layer and a packet data convergence protocol (PDCP) protocol layer, and wherein the apparatus processes a signaling radio bearer (SRB) by using the PDCP protocol layer.8. The apparatus according to claim 7, wherein, when executed, the instructions cause the apparatus to perform operations comprising:receiving, a second indication from the second network node, wherein the indication indicates that the second network node receives downlink data of the terminal device from a core network.9. The apparatus according to claim 8, wherein, when executed, the instructions cause the apparatus to perform operations comprising:initiating, in response to the second indication, a radio access network paging within a radio access network notification range to notify the terminal device.10. The apparatus according to claim 7, wherein:the second network node comprises a service data adaptation protocol (SDAP) protocol layer and a second PDCP protocol layer, and wherein the second network node processes a data radio bearer (DRB) by using the second PDCP protocol layer.11. The apparatus according to claim 7, wherein, when executed, the instructions cause the apparatus to perform operations comprising:sending an inactivity timer of the terminal device to the second network node.12. The apparatus according to claim 7, wherein the apparatus is a control plane node of a centralized unit (CU-CP), the second network node is a user plane node of the centralized unit (CU-UP), and the third network node is the centralized unit (CU).13. An apparatus, comprising:at least one processor, and a memory storing instructions for execution by the at least one processor;wherein, when executed, the instructions cause the apparatus to perform operations comprising:sending, to a first network node via a E1 interface, a first indication indicating that activity of a terminal device is inactive, wherein the E1 interface is an interface between the first network node and the apparatus; andreceiving, from the first network node via the E1 interface, indication that the terminal device is entering an inactive state, wherein the first network node is a control plane node of a third network node, the apparatus is a user plane node of the third network node, and the third network node includes at least one of a packet data convergence protocol layer function, a service data adaptation protocol layer function, or a radio resource control layer function, wherein:the apparatus comprises a service data adaptation protocol (SDAP) protocol layer and a second packet data convergence protocol (PDCP) protocol layer, and wherein the apparatus processes a data radio bearer (DRB) by using the second PDCP protocol layer.14. The apparatus according to claim 13, wherein, when executed, the instructions cause the apparatus to perform operations comprising:sending, a second indication to the first network node, wherein the indication indicates that the apparatus receives downlink data of the terminal device from a core network.15. The apparatus according to claim 14, wherein the second indication triggers a radio access network paging within a radio access network notification range to notify the terminal device.16. The apparatus according to claim 13, wherein:the first network node comprises a radio resource control (RRC) protocol layer and a first packet data convergence protocol (PDCP) protocol layer, and wherein the first network node processes a signaling radio bearer (SRB) by using the first PDCP protocol layer.17. The apparatus according to claim 13, wherein, when executed, the instructions cause the apparatus to perform operations comprising:receiving an inactivity timer of the terminal device from the first network node.18. The apparatus according to claim 13, wherein the first network node is a control plane node of a centralized unit (CU-CP), the apparatus is a user plane node of the centralized unit (CU-UP), and the third network node is the centralized unit (CU).