Device offline/online every 30 mins

Hello!
In the events report i get device offline/online events every half an hour. could you please help me understand the reason for this behaviour and how can I solve it?
i assume online/offline alarms are generated by traccar server when the device connects/disconnects.

2018-12-31 17:29:39	deviceOffline
2018-12-31 17:29:57	deviceOnline
2018-12-31 17:59:26	deviceOffline
2018-12-31 17:59:59	deviceOnline
2018-12-31 18:29:13	deviceOffline
2018-12-31 18:29:25	deviceOnline

Corresponding logs from tracker-server.log are:

2017-12-31 17:28:45  INFO: [71497B32] id: 8170870218, time: 2017-12-31 17:28:43, lat:  lat: xx.xxxxx, lon: yy.yyyyy, speed: 0.0, course: 0.0
2017-12-31 17:29:05 DEBUG: [71497B32: 5013 < 213.233.85.132] HEX: 2481708702181629033112174425749606026093953e000000ffffbbff000000000000000000e20156cc2baee3
2017-12-31 17:29:05  WARN: Incorrect string value: '\xC8\x99i, B...' for column 'address' at row 1 - SQLException (... < QueryBuilder:477 < DataManager:342 < DefaultDat
aHandler:27 < ...)
2017-12-31 17:29:05  INFO: [71497B32] id: 8170870218, time: 2017-12-31 17:29:03,  lat: xx.xxxxx, lon: yy.yyyyy, speed: 0.0, course: 0.0
2017-12-31 17:29:07 DEBUG: [71497B32: 5013 < 213.233.85.132] HEX: 2a48512c383137303837303231382c56312c3136323930352c412c343432352e373439362c4e2c30323630392e333935332c45
2c3030302e32302c3030302c3331313231372c46464646424246462c3232362c30312c32323232302c313131383223
2017-12-31 17:29:07  WARN: Incorrect string value: '\xC8\x99i, B...' for column 'address' at row 1 - SQLException (... < QueryBuilder:477 < DataManager:342 < DefaultDat
aHandler:27 < ...)
2017-12-31 17:29:07  INFO: [71497B32] id: 8170870218, time: 2017-12-31 17:29:05, lat: xx.xxxxx, lon: yy.yyyyy, speed: 0.2, course: 0.0
2017-12-31 17:29:13  INFO: [71497B32] disconnected
2017-12-31 17:29:13  INFO: [927F9C09] connected
2017-12-31 17:29:24 DEBUG: [927F9C09: 5013 < 213.233.85.132] HEX: 2481708702181629233112174425749706026093956e000000ffffbbff000000000000000000e20156cc2baee4
2017-12-31 17:29:25  WARN: Incorrect string value: '\xC8\x99i, B...' for column 'address' at row 1 - SQLException (... < QueryBuilder:477 < DataManager:342 < DefaultDat
aHandler:27 < ...)
2017-12-31 17:29:25  INFO: [927F9C09] id: 8170870218, time: 2017-12-31 17:29:23,  lat: xx.xxxxx, lon: yy.yyyyy, speed: 0.0, course: 0.0
2017-12-31 17:29:44 DEBUG: [927F9C09: 5013 < 213.233.85.132] HEX: 2481708702181629433112174425751606026093945e000000ffffbbff000000000000000000e20156cc2baee5
2017-12-31 17:29:44  WARN: Incorrect string value: '\xC8\x99ti, ...' for column 'address' at row 1 - SQLException (... < QueryBuilder:477 < DataManager:342 < DefaultDat
aHandler:27 < ...)

Device is chinese OBD gps tracker:

CXZT

XM_GT08B_SW_7.0 2017/05/19
ID:8170870218
IP:ip_address 5013
BAT:6
APN:live.vodafone.com
GPS:A-10-4
GSM:28
ICCID:8940011708682484146F

You have a problem with database character encoding. It has been discussed before. Please use search.

thank you Anton!
however, i checked the db url and it seems fine:

<entry key='database.url'>jdbc:mysql://localhost:3306/traccar?useSSL=false&amp;allowMultiQueries=true&amp;autoReconnect=true&amp;useUnicode=yes&amp;characterEncoding=UTF-8&amp;sessionVariables=sql_mode=''</entry>

what would be the relation between character encoding and periodic device connect/disconnect status changes? i suspect this is a different issue.
could the device "voluntary" disconnect from server?
what is the difference between deviceOffline which seems to be linked to a disconnect message and "status unknown" which is declared by server after several minutes of inactivity?
the other device i am using does not disconnect/connect every 30 mins.

Device can disconnect from the server. You can see it in the log.

For difference between offline and unknown search forum. It has been answered before more than once.