I was a little confused about the communication between mobile phones and base stations, so I searched for information on the Internet and saw a good answer, so I reprinted it and recorded it, respecting the author's achievements, and paying attention to copyright. The link to the original text is as follows:
https://www.zhihu.com/question/29399883
To answer with what I know:
1. The mobile phone has always been listening to the size and clarity of the "sound" from the base station, which is called Measurement. In the idle state (it can be understood as when the mobile phone is not doing anything), the purpose of the measurement is to perform cell reselection (handover is the name of the business in progress, and the reselection only needs the downlink signal, just like listening to the broadcast on the radio, and The handover requires not only the downlink signal, but also the uplink signal of the mobile phone to upload the measurement report so that the network can make a decision). The purpose of reselection is to let the mobile phone select the optimal cell for camping. The principle of reselection is not necessarily the maximum received power, but also related to the interference level, the priority of the cell, and so on. The mobile phone needs to read some parameters related to reselection in the System Informationn Block sent by the base station as the basis for cell reselection, such as when to perform intra-frequency measurement, when to perform inter-frequency and inter-system measurement, After the measurement, how to perform cell reselection according to the result obtained by the measurement so as to camp on the optimal cell and so on. Telecom operators can also control the cell phone to perform better cell reselection by changing these values.
Figure 1
The above figure indicates the simple trigger condition diagram when the mobile phone performs cell reselection in the WCDMA network. The network can control when the mobile phone starts and what kind of cell reselection measurement is started by modifying the value of the ordinate. There are other situations and complicated details in cell reselection, which are omitted here.
2. The time for periodic location update will vary according to different equipment manufacturers and even different regions, most of which are about 30 minutes to an hour. If the mobile phone is shut down in a normal way (such as pressing the power button or shutting down naturally without power), it will initiate a registration process before shutting down. Simply put, it tells the network to shut down, and the network will mark it for registration. If there is a call (paging) for this number, simply reply "The user you dialed is powered off", so as to avoid wasting paging resources. If there is a sudden power failure, such as directly deducting the battery, etc., and the mobile phone does not have time to initiate the registration process, then after the periodic location update time expires and still does not receive the location update message from the mobile phone, the mobile phone is turned off. At this time, if you dial this number, it will also prompt "shut down". Before the periodic location update time expires, the network does not know what the situation of the mobile phone is (such as a sudden power failure or entering a signal dead zone), so when the paging does not get a response from the mobile phone, it will reply "The user you dialed is temporarily Unable to connect". Note: The mobile phone here refers to the mobile phone with a legal SIM/USIM card or a written card.
3. The cellular communication module of the mobile phone in the real shutdown state should be closed, and it will not be in contact with the outside world. The mobile phone generally only has an uplink signal (simply speaking, there is a signal transmission) only during the process of location update, response to paging and business (such as surfing the Internet or making a phone call), and at other times, it silently listens to the downlink signal from the network and then Self-processing and utilization (such as the cell reselection mentioned above, and discontinuous reception of paging messages, etc.), this design can not only save the power of the mobile phone but also reduce the pressure on the network (the uplink signal needs to be processed by the network) . So assuming that the periodic location time is set to 1 hour, the phone will stand by for 8 hours without a call at night, the time required for each update is 3 seconds, and the phone will only transmit a signal for less than half a minute in total (please don’t raise the bar to say I slept on the high-speed train).
for reference only.