This is about modifying my SMC-D3GNV which is a Cable Modem, a Wireless Router and a VOIP telephone adapter. While most people have their home networks configured nicely, when it comes to the cable modem Comcast often ships devices like this which unfortunately cannot be reconfigured easily if at all. Most people complain about very low quality of the wireless portion of this device which also applies for other makes like Technicolor, Cisco, etc. Unfortunately Comcast customized firmware has restricted the options that users can do to make changes to such devices by using the usual web interface. To be honest, some Comcast customers were able to make this device to work as a plain modem (Bridged Mode) thus disabling the Wi-Fi portion and the router just by calling Comcast customer service. Unfortunately that solution is not permanent and if you reset the device, you will be on Comcast CSR mercy again. Here in a few steps I will show you how to permanently disable the Wi-Fi section of this modem/router. This is reversible process and you may put everything back the way it was but I sincerely doubt that anyone would do that unless you must return the equipment back to Comcast.
Step 1: Disassembling the Comcast Gateway SMC-D3GNV
For this procedure you will need a Torque driver T9 for the screws removal shown on the picture. There are 4 screws on the bottom and 2 screws on the right side cover that need to be removed before you are going to pry the left cover.
Step 2: RF Module Removal
Once you remove the left cover you will locate the RF module in the upper right corner on the main board. You'll find three antenna cables with miniature connectors attached to the module. Remove the connectors and two screws that hold the module to the main board. Be aware that the screws have washers under the board and you must use needle-nose pliers to hold them under the board to be able to unscrew them. Once the screws and the cables are removed, remove the whole RF module. You may also remove the antennas attached to the cables, they are stuck with double-side sticky tape and easy to remove. Also you may leave the antennas the way they are since they don't have any function at this time. You may keep all the removed parts for later reassembly if needed.
Step 3: Reassembly of the Modem/Router
Put the covers back and screw down the screws without the RF module and use the device as a regular modem/router. The device won't even notice that you removed the RF module and it will report on its own web page that your Wi-Fi network is still active. This in case if you know how to log in (default address: 10.0.0.1, default username: "admin", default password: "password".). The main benefit is that there won't be any RF signal generated to already overused 2.4 GHz band. The signal that this device used to create was strong enough to interfere with other routers you may have especially that this type of device occupies 2 different channels at the same time overlapping 8 channels in the 2.4 GHz band hence using much more bandwidth than regular routers do. Mine was using both Ch.3 and Ch.7 which occupied almost the whole available band since one standard channel actually overlaps four channels (2+2 adjacent channels). It also used to create additional Xfinity Wi-Fi network to serve external unknown Comcast passing by customers unless you have disabled it. For this unauthorized bandwidth sharing for the Xfinity network, Comcast was sued by some customers.
Removal of the RF module also reduces the unnecessary heat dissipation and will keep the temperature of the device lower. For your Wi-Fi planning I strongly suggest you to use some spectrum analyzing software in order to choose free of interference channels instead of letting the device to do it for you. Comments and questions are welcome.
Step 4: Apendix: Network Configuration
This is about configuration of the LAN network following the modification on this device. You would usually connect a second or even a third Wi-Fi router to your LAN in order to get strong enough signal to cover your residence. You may connect all Wi-Fi routers in a daisy chain or to serve separate LAN networks. Look at the images to see which combination better works for you. You may still use all ports of all routers to provide Internet but the computers may or may not be on the same LAN with the rest of the computers (connected directly or wirelessly) depending on the combination you choose.
If you need all of your wireless devices to be on the same network due to media sharing, you need to use the daisy chain configuration. That means only one router may act as a DHCP server to assign IP addresses to all network users (doesn't matter which one) while the other router(s) must have DHCP server disabled. Also the home IP address of the routers themselves should be in the same subnet , e.g. 192.168.0.1 for the first router, 192.168.0.2 for the second, 192.168.0.3 for the third router etc. In this case you will not use the WAN port on any router since the second and the third router (if needed) actually will not be routing anything, they will act as access points in this case. The inter connection will be on the LAN side only in order to preserve the same subnet.
The other combination is when you don't care about sharing files and other media among the users so you will leave the routers to do the routing and each one's DHCP server will have their users assigned IP addresses in their different subnets with IP addresses as shown on the pictures. In this case the WAN ports are used normally as an Internet connection from the previous source, whether it is the main router or the second router's LAN port.