Electronics are not built to constantly be running at all times, and power cycling (a fancy way of saying unplugging the power cord) helps with most issues. Like other electronics, one of the biggest reasons you may benefit from rebooting your modem is because when you leave your modem on for too long, it will tend to stop performing at its best after a while. Rebooting your modem can fix Internet connection problems, improve slow moving connections and resolve wireless issues, which also affect your TV service that is being transmitted over an internet connection. By unplugging the power cord from the back of the modem you are giving it the breather it needs.
Another reason is that when your modem is on all day, every day, it is slowly desynchronizing from the Internet Service Provider (ISP). This desynchronization causes the packets you are sending and receiving to become corrupt, requiring them to be resent, and thus, lowering your speed.
Try unplugging your modem on a monthly basis to stay proactive. This will not cause any harm to your electronics as it is simply a way of rebooting the system. If your problems continue please call your local NU-Telecom office and a representative will lend a helping hand.
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