Explaining the functions of cable gateway with wirelessAP device

Question

Theoretical question. Let's say I have a device that is described as a standard home cable (DOCSIS) gateway with wirelessAP to which I connect a number of end devices.

The end devices are directly connected with this device via wireless.

Can we say that this device:

1. acts as a router and separates the home network of mine from the Internet?

2. acts as if it has a switch between this router and my end devices?

Link to manual: http://setuprouter.com/router/technicolor/tc7200-u/manual-1685.pdf

Answer 1

Can we say that this device:
1. acts as a router and separates the home network of mine from the Internet?

Yes, but it's more than a router. There doesn't appear to be a way to make it do plain IP routing between two subnets. So it's more properly referred to as an NAPT gateway (NAPT = Network Address and Port Translation, which is what people mean 99% of the time they write NAT).

2. acts as if it has a switch between this router and my end devices?

Yes. In this kind of device, the LAN and WLAN ports are bridged (switched) together. You can think of your box like this:

                               +--------+---[2.4GHz Wi-Fi AP]
+-------+   +--------------+   |        +---[5GHz Wi-Fi AP]
| modem +---+ NAPT gateway +---+ switch +---LAN1
+-------+   +--------------+   |        +---LAN2
                               |        +---LAN3
                               +--------+---LAN4

While we're being so precise, note that a Wi-Fi AP is much more like an Ethernet hub (repeater) than a switch, because it does not allow full-duplex operation. So it's as if all of your 2.4GHz devices are on one "hub", and all your 5GHz devices are on another "hub", and those two hubs are connected to the same switch as your LAN ports.

Also note that in many home gateway designs, it's slightly more complex than this, but in a mostly transparent way. For example, the Wi-Fi AP radios may not be connected to the same physical switch chip as the LAN ports, but the LAN switch and the AP radios are bridged (switched) together via software switching in the main SOC (CPU) that runs the whole gateway.

Answer 2

The device is functionally identical to having a regular wireless router (which is a type of AP) with it's upstream port connected to a regular DOCSIS modem (and in fact, most such combined Modem/AP devices are in fact such a combination, but using traces on a circuit board instead of a physical Ethernet cable).

From a network topology perspective, it is both:

1. A NAT router (all internal clients appear externally as one client with whatever your external IP address is), just like almost all consumer access points (in contrast to many corporate AP's, which are typically run as 802.3 to 802.11 bridges, not routers).

2. A layer 3 gateway (because it's converting from IEE 802.11 and/or IEEE 802.3 signaling and framing to ITU-T J.112/J.122/J.222 signaling and framing), just like any modem that provides an ethernet connection that you connect your computer to.