What you're probably noticing is ARP activity on your network. The new modems such as the Westell 7500 and the Netgear 7550 have a Network List which the modems themselves maintain. To do this they use ARP which is a one to all protocol on a network. It broadcasts to every port BUT the Internet as it scans. It does this to detect devices with static IPs in the same subnet as the rest of your network.
If you want that to stop, log into the modem at »192.168.1.1/
using the username of admin/admin respectively, and then click on "My Connected Network" or "My Network" at the top of the navigation. From here, there will be a button towards the right of the page to disable Network Scanning. This shuts off the network list but it also cuts down on all of that ARP activity when you have no devices connected.