It's probably easier (emphasis on the "er") to write a custom firmware for an existing modem.
Someone did a teardown and firmware dump of a couple of Arris modems. The next step would be to try to "jailbreak" the modem by modifying the dumped firmware to e.g. open an interactive serial console on one of the UARTs. The modems are based on a BCM3390 SoC runs Linux so you should be able to get the source code for most of the kernel/userspace (b/c GPL licensing), and work towards replacing the proprietary bits as needed.
If modem manufacturers lock down the SoCs to only run the OE firmware (unlikely IMO), it is possible to obtain new BCM3390 chips from eBay. However, replacing them on existing boards would be a challenge since they are BGA chips. And they're not exactly cheap either.
Re: previous discussion in this thread:
Someone did a teardown and firmware dump of a couple of Arris modems. The next step would be to try to "jailbreak" the modem by modifying the dumped firmware to e.g. open an interactive serial console on one of the UARTs. The modems are based on a BCM3390 SoC runs Linux so you should be able to get the source code for most of the kernel/userspace (b/c GPL licensing), and work towards replacing the proprietary bits as needed.
If modem manufacturers lock down the SoCs to only run the OE firmware (unlikely IMO), it is possible to obtain new BCM3390 chips from eBay. However, replacing them on existing boards would be a challenge since they are BGA chips. And they're not exactly cheap either.
Re: previous discussion in this thread:
- One advantage of having an open source modem would be resistance to vulnerabilities.
- Replacing an ISP-provided modem shouldn't be an issue for most people because the ISP will configure the modem at boot time via standard mechanisms.
