Thank you sir, i got this message.
pi@ttn-gateway:~ $ sudo tail -f /var/log/syslog
Jan 16 16:11:02 ttn-gateway ttn-gateway[314]: ### [UPSTREAM] ###
Jan 16 16:11:02 ttn-gateway ttn-gateway[314]: # RF packets received by concentrator: 0
Jan 16 16:11:02 ttn-gateway ttn-gateway[314]: # CRC_OK: 0.00%, CRC_FAIL: 0.00%, NO_CRC: 0.00%
Jan 16 16:11:02 ttn-gateway ttn-gateway[314]: # RF packets forwarded: 0 (0 bytes)
Jan 16 16:11:02 ttn-gateway ttn-gateway[314]: # PUSH_DATA datagrams sent: 0 (0 bytes)
Jan 16 16:11:02 ttn-gateway ttn-gateway[314]: # PUSH_DATA acknowledged: 0.00%
Jan 16 16:11:02 ttn-gateway ttn-gateway[314]: ### [DOWNSTREAM] ###
Jan 16 16:11:02 ttn-gateway ttn-gateway[314]: # PULL_DATA sent: 6 (0.00% acknowledged)
Jan 16 16:11:58 ttn-gateway kernel: [ 131.272705] Under-voltage detected! (0x00050005)
Jan 16 16:12:02 ttn-gateway kernel: [ 135.432711] Voltage normalised (0x00000000)