Title of Invention: Downlink Data Packet Transmission Method and Device Abstract: A method and a device for transmitting downlink data packets are provided. The method includes: receiving a paging message transmitted by a network side device; and in a case that a data transmission indication indicating transmission exists in the paging message, receiving the downlink data packets transmitted by the network side device. Detailed Description: CROSS-REFERENCE TO RELATED APPLICATION This application is a U.S. national phase application of a PCT Application No. PCT/CN2017/109835 filed on Nov. 8, 2017, which claims a priority of the Chinese patent application No. 201611005126.3 filed in China on Nov. 11, 2016, a disclosure of which is incorporated herein by reference in its entirety. TECHNICAL FIELD The present disclosure relates to the field of communication technology, and in particular, to a method for transmitting downlink data packets, and a device for transmitting downlink data packets. BACKGROUND With a continuous development of wireless technology, a fifth generation (5G) mobile communication technology has become a development direction of a next generation mobile communication technology. According to current 5G requirements defined by the International Telecommunication Union (ITU), in a future 5G era, a connection density will reach 1 million/Km2, and an end-to-end minimum delay will reach 1 ms. This puts extremely high demands on a standardization of mobile communication techniques. Meanwhile, the 5G will support all vertical industries in future, and new technologies will incubate many new business forms. In a requirement definition of a 3rd Generation Partnership Project (3GPP), application scenarios having been determined include following scenarios: enhanced Mobile Broadband (eMBB); massive Machine-Type-Communications (mMTC); ultra Reliable and Low Latency communications (uRLLC); wherein, the mMTC scenario is mainly optimized for an Internet of Things (IoT) scenario, such as smart water meters, smart electricity meters, wearable devices, etc. One of main features of this scenario is: a large number of terminals and connections, relatively small data transmission requirements and data concurrency. On the other hand, the uRLLC scene is also an important scene in 5G communication technology, such as a Vehicle to Everything scenario, virtual reality (VR)/augmented reality (AR) applications, telemedicine and the like. In this scenario, an end-to-end user plane latency and an end-to-end signaling plane latency are extremely low. However, in a related system, a method for transmitting downlink data packets to a terminal generally includes steps as follow: a network side device transmits a paging message to a terminal; after the terminal receives the paging message, the terminal initiates a random access request message; the network side device responds to the random access request message, and returns a random access response message, and then transmits an initial Radio Resource Control (RRC) message according to data transmission parameters configured in the random access response message; and finally according to the initial RRC message from the network side device, a procedure for establishing a RRC connection is initiated; and after the RRC connection is established, the downlink data packets are transmitted to the terminal. SUMMARY Embodiments of the present disclosure provide a method for transmitting downlink data packets, and a device for transmitting downlink data packets, to solve a problem that a delay is overlarge when transmitting the downlink data packets. In a first aspect, a method for transmitting downlink data packets is provided in the present disclosure. The method includes: receiving a paging message transmitted by a network side device; and in a case that a data transmission indication indicating transmission exists in the paging message, receiving the downlink data packets transmitted by the network side device. In a second aspect, a method for transmitting downlink data packets is provided in the present disclosure. The method includes: transmitting a paging message to a terminal; in a case that a data transmission indication indicating transmission exists in the paging message, transmitting the downlink data packets to the terminal. In a third aspect, a method for transmitting downlink data packets is provided in the present disclosure. The method includes: transmitting a paging message to a network side device, wherein the paging message is used to indicate whether to transmit the downlink data packets to a terminal or not; transmitting the downlink data packets to the network side device, so that the network side device transmits the downlink data packets to the terminal according to the paging message. In a fourth aspect, a terminal is provided in the present disclosure and includes: a paging message reception module configured to receive a paging message transmitted by a network side device; and a first data packet reception module configured to: in a case that a data transmission indication indicating transmission exists in the paging message, receive downlink data packets transmitted by the network side device. In a fifth aspect, a network side device is provided in the present disclosure and includes: a first paging message transmission module, configured to transmit a paging message to a terminal; and a first data packet transmission module configured to: in a case that a data transmission indication indicating transmission exists in the paging message, transmit downlink data packets to the terminal. In a sixth aspect, a core network device is provided in the present disclosure and includes a second paging-message transmission module, configured to transmit a paging message to a network side device, wherein the paging message is used to indicate whether to transmit downlink data packets to a terminal or not; and a third data-packet transmission module configured to transmit the downlink data packets to the network side device, so that the network side device transmits the downlink data packets to the terminal according to the paging message. In a seventh aspect, a terminal is provided in the present disclosure and includes: at least one processor, a storage, at least one network interface, and a user interface; and a bus system, wherein the at least one processor, the storage, and the at least one network interface and the user interface are coupled together by the bus system, the at least one processor performs, by invoking programs or instructions stored in the storage, the method according to the first aspect. In an eighth aspect, a network side device is provided in the present disclosure and includes: at least one processor, a storage, at least one network interface, and a user interface; and a bus system, wherein the at least one processor, the storage, and the at least one network interface and the user interface are coupled together by the bus system, the at least one processor performs, by invoking programs or instructions stored in the storage, the method according to the second aspect. In a ninth aspect, a core network device is provided in the present disclosure and includes: at least one processor, a storage, at least one network interface, and a user interface; and a bus system, wherein the at least one processor, the storage, and the at least one network interface and the user interface are coupled together by the bus system, the at least one processor performs, by invoking programs or instructions stored in the storage, the method according to the third aspect. In a tenth aspect, a nonvolatile storage medium is provided in the present disclosure, and includes instructions stored on the nonvolatile storage medium, wherein when the instructions are executed by a processor, the processor performs the method according to the first aspect. In an eleventh aspect, a nonvolatile storage medium is provided in the present disclosure, and includes instructions stored on the nonvolatile storage medium, wherein when the instructions are executed by a processor, the processor performs the method according to the second aspect. In a twelfth aspect, a nonvolatile storage medium is provided in the present disclosure, and includes instructions stored on the nonvolatile storage medium, wherein when the instructions are executed by a processor, the processor performs the method according to the third aspect. In the embodiments of the present disclosure, the paging message transmitted by the network side device is received. In a case that the data transmission indication indicating the transmission exists in the paging message, the downlink data packets transmitted by the network side device are received. In this way, after the paging message is received, the downlink data packets may be directly transmitted, so that a case in the related art that the downlink data packets may not be transmitted until a RRC connection is established after the paging message is received may not be needed, thereby reducing the delay of transmitting the downlink data packets. BRIEF DESCRIPTION OF THE DRAWINGS In order to illustrate technical solutions of some embodiments of the present disclosure more clearly, drawings used in description of some embodiments of the present disclosure will be described hereinafter briefly. Obviously, the following drawings merely relate to some embodiments of the present disclosure, and based on these drawings, a person skilled in the art may obtain other drawings without paying any creative effort. FIG. 1 is a flowchart of a method for transmitting downlink data packets according to some embodiments of the present disclosure; FIG. 2 is a flowchart of a method for transmitting downlink data packets according to some embodiments of the present disclosure; FIG. 3 is a schematic diagram of an example of the method for transmitting downlink data packets according to some embodiments of the present disclosure; FIG. 4 is a schematic diagram of an example of the method for transmitting downlink data packets according to some embodiments of the present disclosure; FIG. 5 is a flowchart of a method for transmitting downlink data packets according to some embodiments of the present disclosure; FIG. 6 is a flowchart of a method for transmitting downlink data packets according to some embodiments of the present disclosure; FIG. 7 is a flowchart of a method for transmitting downlink data packets according to some embodiments of the present disclosure; FIG. 8 is a flowchart of a method for transmitting downlink data packets according to some embodiments of the present disclosure; FIG. 9 is a structural schematic diagram of a terminal according to some embodiments of the present disclosure; FIG. 10 is a structural schematic diagram of a terminal according to some embodiments of the present disclosure; FIG. 11 is a structural schematic diagram of a terminal according to some embodiments of the present disclosure; FIG. 12 is a structural schematic diagram of a terminal according to some embodiments of the present disclosure; FIG. 13 is a structural schematic diagram of a terminal according to some embodiments of the present disclosure; FIG. 14 is a structural schematic diagram of a terminal according to some embodiments of the present disclosure; FIG. 15 is a structural schematic diagram of a network side device according to some embodiments of the present disclosure; FIG. 16 is a structural schematic diagram of a network side device according to some embodiments of the present disclosure; FIG. 17 is a structural schematic diagram of a network side device according to some embodiments of the present disclosure; FIG. 18 is a structural schematic diagram of a network side device according to some embodiments of the present disclosure; FIG. 19 is a structural schematic diagram of a network side device according to some embodiments of the present disclosure; FIG. 20 is a structural schematic diagram of a network side device according to some embodiments of the present disclosure; FIG. 21 is a structural schematic diagram of a core network device according to some embodiments of the present disclosure; FIG. 22 is a structural schematic diagram of a core network device according to some embodiments of the present disclosure; FIG. 23 is a structural schematic diagram of a core network device according to some embodiments of the present disclosure; FIG. 24 is a structural schematic diagram of a terminal according to some embodiments of the present disclosure; FIG. 25 is a structural schematic diagram of a terminal according to some embodiments of the present disclosure; FIG. 26 is a structural schematic diagram of a network side device according to some embodiments of the present disclosure; and FIG. 27 is a structural schematic diagram of a core network device according to some embodiments of the present disclosure. DETAILED DESCRIPTION Technical solutions of some embodiments of the present disclosure will be described hereinafter in a clear and complete manner in conjunction with the drawings of the embodiments. Obviously, the described embodiments merely relate to a part of, rather than all of, the embodiments of the present disclosure. Based on these embodiments of the present disclosure, all other embodiments obtained by a person skilled in the art without paying any creative effort fall within the scope of the present disclosure. In view of a problem that a transmission delay is significant when transmitting downlink data packets in the related art, some embodiments of the present disclosure provide a method for transmitting downlink data packets and a device for transmitted downlink data packets. Referring to FIG. 1, FIG. 1 is a flowchart of a method for transmitting downlink data packets according to some embodiments of the present disclosure. The method illustrated in FIG. 1 includes following steps 101-102. Step 101: receiving a paging message transmitted by a network side device. The paging message may be a paging message transmitted to the terminal after the network side device receives downlink data packets to be transmitted the terminal. Further, the network side device may further receive a paging message transmitted by a core network (CN) device; and the paging message indicates that the core network receives the downlink data packets to be transmitted to the terminal and is to transmit the downlink data packets to the network side device. Further, the step 101 may also include: receiving a paging message transmitted by the network side device before an air-interface signaling connection is established. An air-interface signaling may be an end-to-end data transmission signaling protocol. For example, a related air-interface signaling includes a Radio Resource Control (RRC) signaling. Step 102: in a case that a data transmission indication indicating transmission exists in the paging message, receiving the downlink data packets transmitted by the network side device. In the embodiments of the present disclosure, the paging message may indicate the foregoing data transmission indication in an explicit or implicit manner, and the data transmission indication may include two states: data transmission or no data transmission. For example, in a case that the indicating is performed in the explicit manner, the paging message indicates whether data transmission exists or not through specific information; in a case that the indicating is performed in the implicit manner, the paging message may indicate whether data transmission exists or not by means of existence or non-existence of the specific information. For example, in a case that the specific information exists, the data transmission exists, and vice versa. The data transmission indication indicating the transmission may be understood as a case in which the network side device transmits the downlink data packets to the terminal without state transition of the terminal, or as a case in which the network side device transmits all or part of the downlink data packets to the terminal without the state transition of the terminal, and a remaining part of the downlink data packets may be transmitted after the state transition of the terminal, and the embodiments of the present disclosure are not limited thereto. The data transmission indication indicating the non-transmission may be understood as a case in which the network side device needs to transmit the downlink data packets after the terminal performs the state transition, that is, the terminal needs to perform the state transition before the downlink data packets are received by the terminal. The state transition herein may refer to a change in a connected state of a RRC connection. For example, the state transition may be a transition from a state in which a RRC connection is not-established to a state in which the RRC connection is established. It should be noted that, in the embodiments of the present disclosure, a RRC_connected state may be a complete RRC_connected state or a fully active state. After the terminal obtains the paging message, the terminal may identify whether the data transmission indication indicating the transmission exists in the paging message or not. In a case that the data transmission indication is identified as indicating the transmission, receiving of the downlink data packets transmitted by the network side device may be performed. It should be noted that the receiving of the downlink data packets transmitted by the network side device may be understood as: an action of receiving the downlink data packets is performed, and the downlink data packets may be successfully received in the action, or the downlink data packets may not be successfully received in the action. That is, in step 102, the downlink data packets may be successfully received, or may not successfully be received. Following may be implemented by the foregoing steps: the terminal receives the paging message, and the data transmission indication indicates that the downlink data packets transmitted by the network side device may be directly received, and the state transition is not required, so that a delay of transmitting the downlink data packets may be reduced. Moreover, power consumptions of the terminal and the network side device may be reduced, and overheads of network signaling may be reduced. For example, under a condition that the downlink data packets of the terminal are only data packets of small sizes, the data packets may be directly transmitted without the state transition, such as in cases of notifying a network error and needing a re-attach procedure, or notifying a system information update procedure, or just for receiving application program configuration information of a mMTC terminal, etc. Subscription information of the terminal indicates that data is allowed to be received without the state transition, or a quality of service (QoS) feature of the downlink data packets allows data to be transmitted without the state transition. The data may be transmitted directly without the state transition, so as to reduce the delay of transmitting downlink data packets, and also reduce the power consumption of the terminal and reduce the overhead of the network signaling. In the embodiments of the present disclosure, the foregoing method may be applied to any terminal having a communication function, such as: a mobile phone, a tablet personal computer, a laptop computer, and a personal digital assistant (PDA), a Mobile Internet Device (MID) or Wearable Device. It should be noted that a specific type of the terminal is not limited in the present disclosure. In addition, in the embodiments of the present disclosure, the network side device may be a base station, and the base station may be a macro station, such as a Long Term Evolution (LTE) evolved Node B (eNB), a 5G NR NB, or the like; or a small station, such as a low power node (LPN), a pico base station, a femto base station, etc., or may be an access point (AP). The network side device may also be a network node including a central unit and a plurality of transmission-reception points (TRP) managed and controlled by the central unit. In addition, one or more cells are managed by one base station (for example: cells having different frequency points or formed through sector splitting). It should be noted that a specific type of the network side device is not limited in the present disclosure. In the method for transmitting downlink data packets of the embodiments of the present disclosure, the paging message transmitted by the network side device is received; in a case that the data transmission indication indicating the transmission exists in the paging message, downlink data packets transmitted by the network side device are received. In this way, under a condition that the paging message is received, the downlink data packets may be directly transmitted, so that a case in the related art that after the paging message is received, the downlink data packets may not be transmitted before a RRC connection is established may not be needed, thereby reducing a delay for transmitting the downlink data packets. Referring to FIG. 2, FIG. 2 is a flowchart of a method for transmitting downlink data packets according to some embodiments of the present disclosure. The method illustrated in FIG. 2 includes following steps 201-203. Step 201: receiving a paging message transmitted by a network side device. The paging message may be a paging message transmitted to a terminal after the network side device receives downlink data packets to be transmitted to the terminal. Further, the network side device may receive a paging message transmitted by the core network device, the paging message indicates that the core network device receives the downlink data packets of the terminal and is to transmit the downlink data packets to the network side device. Further, step 201 may include receiving a paging message transmitted by the network side device before an air-interface signaling connection is established. Step 202: generating a Message 1 (Msg1) of a random access procedure according to the paging message, and transmitting, by using the Message 1 of the random access procedure, reply information corresponding to the data transmission indication to the network side device, wherein the network side device transmits the downlink data packets according to the reply information. In this step, after the terminal receives the paging message, the terminal may generate the Message 1 of the random access procedure according to the paging message, and transmit the reply information corresponding to the data transmission indication by using the Message 1. The reply information may indicate that the data transmission indication is agreed or disagreed, that is, the reply information may be whether an operation corresponding to the data transmission indication is performed or not. For example, in a case that the data transmission indication indicates the transmission, the reply information may mean that the downlink data packets transmitted by the network side device may be received or the downlink data packets transmitted by the network side device may not be received. The above-mentioned reply information may be used to ensure that the network side device negotiates with the terminal when the network side device is to transmit the downlink data packets to the terminal, in order to avoid wasting resources overhead in a case that the network side device directly transmits the downlink data packets to the terminal but the terminal cannot receive the downlink data packets. In addition, the above Message 1 may be a random access request message. The above Message 1 may indicate the above reply information in an explicit or implicit manner. For example, the reply information may include an identifier for indicating whether to receive the downlink data packets transmitted by the network side device or not, wherein the receiving herein indicates a state is not transitioned, and not receiving indicates a state is transitioned. For example, in a case that the Msg1 includes a 1-bit information, then ‘1’ may indicate that the state transition is performed, that is, in a case that the data transmission indication indicates the transmission, that is, ‘1’ indicates disagreement of state-transition, and in a case that the data transmission indication indicates the non-transmission, then ‘1’ indicates agreement of state-transition. ‘0’ means that the state transition is not performed. That is, in a case that the data transmission indication indicates the transmission, that is, 0 indicates agreement of the state-transition, and in a case that the above data transmission indication indicates the non-transmission, ‘0’ indicates disagreement of the state-transition. Optionally, the reply information may include a random access preamble sequence for indicating whether to receive the downlink data packets transmitted by the network side device or not. That is, a specific preamble group is used to identify whether to perform the state transition. The network side device receives the random access preamble sequence and may determine whether the terminal may receive the downlink data packets transmitted by the network side device or not. For example, the network side device and the terminal pre-negotiate which random access preamble sequence indicates that the terminal is to receive the downlink data packets transmitted by the network side device, and which random access preamble sequence indicates that the terminal is not to receive the downlink data packets transmitted by the network side device. In the embodiments, whether the downlink data packets transmitted by the network side device are to be received or not is indicated by the random access preamble sequences, the content of the Message 1 needs not to be added, so that transmission overhead may be reduced. Optionally, the random access preamble sequences are provided with an identifier bit used to indicate whether the downlink data packets transmitted by the network side device are to be received or not. That is, the identifier bit in the random access preamble sequence may be an identifier bit used to indicate whether the downlink data packets transmitted by the network side device are to be received or not, so that the network side device may directly determine, by identifying the identifier bit, whether the terminal receives the downlink data packets transmitted by the network side device or not, so as to quickly determine, through the identifier bit, whether the terminal may receive the downlink data packets transmitted by the network side device or not, so as to improve a transmission efficiency of downlink data packets. Optionally, after receiving the paging message transmitted by the network side device, before generating the Message 1 of the random access procedure according to the paging message and transmitting the reply information corresponding to the data transmission indication to the network side device by using the Message 1 of the random access procedure, the method further includes: determining, according to parameters of the terminal, whether to receive the downlink data packets transmitted by the network side device or not; wherein, the reply information is used to indicate whether to receive the downlink data packets transmitted by the network side device or not. The parameters of the terminal may be parameters in the terminal itself, for example, UE (User Equipment) capability information or energy consumption information of the terminal. The parameters of the terminal may also be measurement information of the terminal. In the embodiments, the network side device may transmit the downlink data packets according to the parameters of the terminal determined by the terminal, so as to avoid a case in which the network side device directly transmits the downlink data packets to the terminal, but the terminal cannot receive the downlink data packets due to the parameters of the terminal, and an effect of saving resources is achieved. Optionally, the step of determining whether to receive the downlink data packets transmitted by the network side device according to the parameters of the terminal includes: determining, according to the UE capability information of the terminal, whether to receive the downlink data packets transmitted by the network side device or not; or determining, according to the energy consumption information of the terminal, whether to receive the downlink data packets transmitted by the network side device or not, or determining, according to the measurement information of the terminal, whether to receive the downlink data packets transmitted by the network side device or not. The UE capability information may be capability information indicating whether the terminal or the User Equipment itself supports receiving the downlink data packets without transitioning states of the terminal. In a case that the UE capability information indicates the capability of receiving the data packets without transitioning states of the terminal, the terminal may receive the downlink data packets transmitted by the network side device and notify the network side device by using the above reply information. Otherwise, the terminal notifies, by using the above reply information, the network side device of incapability of receiving the downlink data packets. The foregoing capability information may enable the network side device to transmit the downlink data packets according to UE capability information of the terminal determined by the terminal, so as to avoid a case that the network side device transmits the downlink data packets to the terminal directly but the capability of the terminal cannot enable the terminal to receive the downlink data packets, thereby causing wasting resource overheads, and to achieve an effect of saving resource overheads. The foregoing energy consumption information may be that the terminal detects the energy consumption information of the terminal itself. In a case that the energy consumption information indicates that the terminal may receive the data packets without transitioning states of the terminal, the downlink data packets transmitted by the network side device may be received, and the network side device is notified through the above reply information. Otherwise, the terminal notifies, through the above reply message, the network side device of the incapability of receiving the downlink data packets by the terminal. The foregoing energy consumption information may enable that, in a case that the downlink data packets are to be transmitted without transitioning the states of the terminal, the downlink data packets must be transmitted under a condition that the power consumption of the terminal supports receiving the data packets, so as to avoid a case that the network side device transmits the downlink data packets to the terminal directly but the power consumption of the terminal cannot enable the terminal to receive the downlink data packets, thereby causing wasting resource overheads, and to achieve the effect of saving resource overheads. The foregoing measurement information may be information obtained by the terminal through measurement. For example, the terminal performs measurement on a network to obtain measurement information indicating a condition of the network. In a case that the measurement information indicates that the network condition is good, the terminal may determine that data packets may be received without transitioning the states of the terminal, and the downlink data packets transmitted by the network side device may be received, and the network side device is notified by using the reply information. Otherwise, the network side device is notified that the downlink data packets cannot be received by the terminal. The foregoing measurement information may enable the network side device to transmit downlink data packets when a current condition of a network is good, so as to avoid a case that the network side device directly transmits the downlink data packets to the terminal but the condition of the network cannot enable the terminal to receive the downlink data packets, thereby causing wasting resource overheads, and to achieve the effect of saving resource overheads. It should be noted that the reply information is not limited to be used to make a reply when the data transmission indication indicates the transmission, and may also be used to make a reply when the data transmission indication indicates the non-transmission. For example, in a case that the data transmission indication indicates the non-transmission and state transition is needed, the user terminal may reply to the network side device that the state transition is needed, or reply that the state transition is not needed and the downlink data packets may be transmitted directly. It should be noted that, in the embodiments of the present disclosure, the step 202 is optional, that is, the step 202 may also be not performed. For example, after the network side device transmits the paging message, the network side device may directly transmit the downlink data packets through resource locations agreed between the terminal and the network side device in advance, or may directly broadcast the downlink data packets. A manner in which the downlink packets are transmitted is not limited in the embodiments of the present disclosure. Step 203: receiving the downlink data packets transmitted by the network side device in a case that the data transmission indication indicating the transmission exists in the paging message. When the terminal recognizes that the data transmission indication indicates the transmission, the terminal may receive the downlink data packets transmitted by the network side device. It should be noted that the receiving of the downlink data packets transmitted by the network side device may be understood as: performing an operation of receiving the downlink data packets, and the downlink data packets may be successfully received, or may not be successfully received after the operation is performed. That is, in step 202, the downlink data packets may be successfully received, or may not be successfully received. Optionally, in a case that a random access preamble exists in the paging message, the data transmission indication indicating the transmission exists in the paging message; or the paging message includes a data transmission indication, and the data transmission indication is used to indicate to transmission or non-transmission. In the embodiments, by adding the random access preamble, such as a random access preamble sequence, to the paging message, that is, as long as the random access preamble exists in the paging message, the transmission is indicated in the paging message, i.e., the downlink data packets are transmitted without transitioning the states of the terminal. Otherwise, in a case that no random access preamble exists, the non-transmission is indicated in the paging message, that is, no downlink data packet is transmitted without transitioning the states. A cost of the paging message may be reduced by the random access preamble to save network resources. Therefore, in a case that the non-transmission is indicated in the paging message, additional information is not needed to be added to the paging message. In the embodiments, the data transmission indication may be added to the paging message, and indication contents of the data transmission indication may be the transmission or the non-transmission. For example, the transmission or the non-transmission may be indicated by using 1 bit, such as ‘1’ is used to indicate the transmission, ‘0’ is used to indicate the non-transmission. Optionally, the paging message may further carry an air-interface identifier. The air-interface identifier may be used to decode the downlink data packets transmitted by the network side device, or the air-interface identifier may be used to decode a message through which the network side device transmits the downlink data packets. The air-interface identifier may be allocated to the terminal by the network side device. The air-interface identifier may be used to decode the downlink data packets by the terminal effectively or to decode the message including the downlink data packets by the terminal, and may prevent other terminals from acquiring the downlink data packets transmitted by the network side device, so as to improve security of transmitting the downlink data packets. Optionally, the step of receiving the downlink data packets transmitted by the network side device includes: receiving a ‘Message 2’ (Msg2) of a random access procedure transmitted by the network side device. The Message 2 includes a downlink scheduling indication (a DL grant) and the downlink data packets; decoding the Message 2 according to the air-interface identifier, and acquiring the downlink scheduling indication; determining time-frequency domain locations indicated by the downlink scheduling indication; receiving, at the time-frequency domain locations, the downlink data packets transmitted by the network side device. In the embodiments, the Message 2 includes the downlink scheduling indication and the downlink data packets, so that the terminal may receive, at the time-frequency domain locations indicated by the downlink scheduling indication, the downlink data packets transmitted by the network side device, and thus there is no need to additionally add a scheduling message, thereby reducing an overhead of system signaling. It should be noted that, in the embodiments, in a case that the step 202 is not performed, a same technical effect may be implemented. For example, in the embodiments of the present disclosure, the transmitting the Message 1 of the random access procedure to the network side device does not include the foregoing step of transmitting the reply information. Of course, in some scenarios, it is also possible that the terminal does not transmit the Message 1 of the random access procedure to the network side device. Optionally, the downlink scheduling indication is used to indicate multiple resources, and the network side device repeatedly transmits downlink data packets at time-frequency domain locations corresponding to the multiple resources; or the downlink scheduling indication is used to indicate multiple resources, and the network side device transmits multiple Redundancy Versions (RVs) of the same downlink data packet at time-frequency domain locations corresponding to the multiple resources; or the downlink scheduling indication is used to indicate a resource, and the network side device transmits the downlink data packets at a time-frequency domain location corresponding to the resource. In the embodiments, the downlink scheduling indication (the DL grant) may be configured to allocate multiple resources, and the network side device repeatedly transmits the downlink data packets at the time-frequency domain locations corresponding to the multiple resources, so that a successful rate of receiving the downlink data packets by the terminal may be improved. The DL grant may be configured to allocate multiple resources, and the network side device transmits multiple Redundancy Versions (RVs) of the same downlink data packet at the time-frequency domain locations corresponding to the multiple resources, wherein the multiple Redundancy Versions (RVs) may be the same or different redundancy versions of the same data packet, in which way, the successful rate of receiving downlink data packets by the terminal may be increased. In the embodiments, the downlink scheduling indication may also be configured to allocate a resource, and the network side device transmits the downlink data packets at the time-frequency domain location corresponding to the resource; the case may be realized when the time-frequency domain resources are not enough. That is, the network side device may reasonably schedule resources to transmit downlink data packets by using the foregoing downlink scheduling indication according to the condition of the network, so as to achieve an effect that the network resource is reasonably utilized. Optionally, the Message 2 further includes at least one of: an uplink scheduling indication of a Message 3 (Msg3) of the random access procedure, an uplink timing used for a uplink synchronization, or a signaling used for the uplink power control. In the embodiments, the foregoing Message 2 may be used to transmit to the terminal an uplink scheduling indication (a UL grant) of the Message 3 of the random access procedure, an uplink timing (TA) used for the uplink synchronization, and the signaling (P) used for the uplink power control, so that in a case that the terminal needs to perform the transmission of the Message 3, and/or in a case that the uplink synchronization needs to be performed, or when the uplink power control needs to be performed, no additional signaling is needed so as to save the signaling overheads. Optionally, the receiving the downlink data packets transmitted by the network side device includes: successfully receiving the downlink data packets transmitted by the network side device; or failing to receive the downlink data packets transmitted by the network side device. In the embodiments, it possibly exist that at the time of receiving the downlink data packets by the terminal, there may be a case in which the reception succeeds or the reception fails. Optionally, after the receiving the downlink data packets transmitted by the network side device, the method further includes: in a case that the downlink data packets transmitted by the network side device are successfully received, stopping receiving the downlink data packets and/or establishing an air-interface signaling connection; receiving a downlink scheduling indication re-allocated by the network side device in a case that the downlink data packets transmitted by the network side device are not successfully received, and receiving, according to the downlink scheduling indication, the downlink data packets re-transmitted by the network side device; or in a case that the downlink data packets transmitted by the network side device are not successfully received, transmitting the Message 3 of the random access procedure to the network side device, wherein the Message 3 is used to request to enter a connected state by the terminal. In the embodiments, in a case that the downlink data packets transmitted by the network side device are successfully received, the terminal may stop receiving the downlink data packets and/or establishing the air-interface signaling connection, wherein it is also possible that no air-interface signaling is established herein because the downlink data packets have been successfully received. In addition, in the embodiments, in a case that the reception is not successful, the downlink scheduling indication re-allocated by the network side device may be received so as to receive the downlink data packets again and so as to increase the successful rate of receiving the downlink data packets. Optionally, under a condition that the reception is failed, the Message 3 may be transmitted to enter the connected state, such as a RRC connected state. After entering the connected state, the terminal may receive the downlink data packets so as to avoid a possibility that the terminal cannot receive the downlink data packets. Here, the re-reception under a situation that the downlink packets are not successfully received, and the request to enter the connected state may be performed based on a contention-based random access. In a case that the downlink data packets are not successfully received, the network side device may be notified by means of a NACK message or in other manners, which is not limited in the embodiments of the present disclosure. Optionally, the paging message may further carry the random access preamble index, and the Message 1 in the random access procedure carries a random access preamble corresponding to the random access preamble index. In the embodiments, a non-contention-based random access procedure may be applied. The random access preamble index may be an index of random access preambles in a preamble group B, for example, a preamble index (group-B). Of course, the preamble group B may also be replaced by a preamble group A or another preamble group, which is not limited thereto in the present disclosure. The Message 1 may also carry random access preambles corresponding to the random access preamble index, so that the network side device receives the Message 1 transmitted by the terminal, and further transmits the downlink data packets. In the embodiments, since the downlink data packets are transmitted between the network side device and the terminal in a non-contention manner, the successful rate of transmitting the downlink data packets may be improved. It should be noted that, in the embodiments, in the case that the step 202 is not performed, the same technical effect is achievable, that is, the Message 1 of the random access procedure is transmitted by the terminal to the network side device, and does not include the reply information. In addition, in the embodiments, in a case that the terminal fails to receive the downlink data packets, the state transition of the terminal may not be performed. Optionally, the data transmission indication is an indication indicating whether the network side device needs to transmit downlink data packets or not and being determined by the network side device according to service feature information of the downlink data packets; or the data transmission indication is an indication indicating whether the network side device needs to transmit downlink data packets or not and being determined by the network side device according to the service feature information of the downlink data packets and network side information acquired by the network side device; or the data transmission indication is a data transmission indication transmitted by the core network and received by the network side device; or the data transmission indication is an indication obtained by the network side device through modifying a data transmission indication transmitted by the core network. In the embodiments, the foregoing data transmission indications may be determined by the network side device or the core network, and may be determined according to the service feature information of the downlink data packets or according to the service feature information and the network side information. For example, in a case that the service feature information indicates that the downlink data packets are allowed to be transmitted without a need for the state transition, the data transmission indication may be determined to be a data transmission indication indicating transmission of data packets, and in a case that the service feature information indicates that the downlink data packets are allowed to be transmitted with the need for the state transition, the data transmission indication is a data transmission indication indicating non-transmission of data packets; or in a case that the network side information indicates that the downlink data packets are currently allowed to be transmitted without the need for the state transition, the data transmission indication may be determined to be a data transmission indication indicating transmission of the data packets; and in a case that the network side information indicates that the downlink data packets are allowed to be transmitted with the need for the state transition, and the data transmission indication may be determined to be a data transmission indication indicating non-transmission of data packets. In the embodiments, since the data transmission indication is determined according to the service feature information, or according to the service feature information and the network side information, it may be prevented that a waste of transmission resources is caused by transmitting, to the terminal without the state transition of the terminal, data being only allowed to be transmitted with the need for the state transition, thereby saving the transmission resources. Moreover, since the foregoing data transmission indication may be determined according to the network side information, an effect of optimizing the network environment may be achieved. For example, when the network is congested, the downlink data packets may be transmitted without the the state transition of the terminal, thereby reducing network congestion. Optionally, the network side information includes at least one of a network side load status or operator configuration data. The network side load status may include a load condition of the network side device and/or a load condition of the core network, and the operator configuration data includes whether the operator is configured to support data transmission without a state transition of a terminal. In the embodiments, since the foregoing data transmission indication is determined based on at least one of the network side load status or the operator configuration data, it may be ensured that transmitting downlink data packets without the state transition may more easily be supported by the network side so as to improve the successful rate of transmitting downlink data packets. Optionally, the foregoing service feature information includes at least one of a service type, service cache information, a service quality requirement of a service, a service priority, or subscription information of the terminal. The service type may include at least one of a smart water meter service, a Vehicle to Everything service, or an eMBB service. The service cache information may be a quantity of a user uplink data cache, for example, a specific cache quantity (such as XX bit); the foregoing service quality requirement of a service may be a QoS parameter (which may also be called a user data QoS feature), such as a data packet loss rate, a maximum allowable delay, a forwarding priority, an allocation reservation priority (ARP), bit rate rules (such as a maximum allowed bit rate, etc.). In the embodiments, the data transmission indication may be determined according to the foregoing service feature information, so that the downlink data packets transmitted without the state transition are downlink data packets transmitted without the need for the state transition and under a support of the service feature information, so as to improve the successful rate of transmitting the downlink data packets. For example, in a case that the delay required for the downlink data packets is low, or the quantity of the user uplink data cache is low, it may be determined that data transmission without the state transition is supported, that is, the data transmission indication is determined to indicate the transmission. In a case that the required delay is high and the quantity of the user uplink data cache is high, it may be determined that data transmission without the state transition is not supported, that is, the data transmission indication is determined to indicate the non-transmission. A determination rule for determining based on the service feature information whether data transmission without the state transition is supported or not is not enumerated herein one by one. Optionally, the service cache information is represented by a cache quantity or by a cache level; the service quality requirement is represented by a service quality parameter feature, or is represented by numbers used for numbering groups of service quality parameter features, or is represented by a packet loss rate and the maximum allowable delay. The service cache information may be represented by a cache quantity (such as N bit) or a cache level (high, medium, low, or level numbers corresponding to amounts of data to be transmitted (for example, 1 to 100 bits are numbered as group 1, and a group number 1 is used at this time); and the
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