I received an email from Comcast indicating my modem needed to be replaced.
“We've noticed you have an older modem. It's time to update your equipment so that you can enjoy faster Internet speeds now available in your area.”
This has happened before and it wasn’t necessary to replace my modem. My experience may help other customers faced with this question. Comcast’s specifications on my currently owned modem Arris TM822 indicates download speed up to 214 Mbps. This figure is lower than the speed Arris claims on its own website—up to 343. A cable provider can limit the download speed according to provided plans but my actual speed with Comcast Blast as measured by Speedtest today is 281 Mbps on my iPad and 256 on my Windows laptop more than Comcast says the Arris should handle. This is wireless, not wired and I’ve seen no problems with my connection.
Before you start replacing equipment, check your speed with Speedtest and adjust WiFi adapter settings if necessary. Don’t automatically believe Comcast’s warning about your modem.