Most cable modems have an LED labelled Cable or Data or D/S, which indicates when data arrives from the cable network. Some users get worried that this LED flashes even when their PC is doing nothing or is switched off. If they are running a firewall, they are puzzled that the firewall does not register any traffic. There is no need for concern: some essential data (DHCP and ARP protocols) is broadcast into the cable network and arrives at every cable modem whether your PC needs it or not. You need not worry that someone is trying to hack into your PC just because the data light is flashing.

If the rate of flashing goes up when your PC is online (compared to when your PC is switched off or disconnected), but you do not know of any reason why your PC should be generating network traffic, then that might be a source of concern.