OSI model layer of a DSL modem

[stu17323]

I know the actual modem works at layer one, modulating and demodulating data. But a modem with an ethernet port obviously communicates over at least layer two. After googling around a bit almost everyone agrees DSL modems work at layer 2, I would agree with this if the device connected to it used the same public IP address given to the modem. Although I would argue that if the modem assigns a new private IP address to the device it works at layer 3.

What do you think?

 

[bill001g]

The problem you have is your are confusing term. It the device is a DSL "MODEM" then it does not use IP address so it is purely a layer 2 device....well sorta. If it does anything with IP addresses it is a "ROUTER" which makes it a layer 3 device.

DSL is a really bad example to try to discuss OSI layers. Now you still have a physical layer on the DSL but the layers above that are based on ATM which does not follow the standard OSI concept. It actually has a bunch of layers that make up what would be considered layer2.

ATM used to be very popular years ago but you now seldom see it used in anything except DSL.

 

[stu17323]

The problem you have is your are confusing term. It the device is a DSL "MODEM" then it does not use IP address so it is purely a layer 2 device....well sorta. If it does anything with IP addresses it is a "ROUTER" which makes it a layer 3 device.

DSL is a really bad example to try to discuss OSI layers. Now you still have a physical layer on the DSL but the layers above that are based on ATM which does not follow the standard OSI concept. It actually has a bunch of layers that make up what would be considered layer2.

ATM used to be very popular years ago but you now seldom see it used in anything except DSL.

Thanks for replying.

Yeah I was trying to discuss the part of a normal 'DSL modem' after the actual modem part. (The part the deals with the communication with the device connected to it). Not DSL.

So your saying these normal modems with an ethernet port communicate with the connected device over layer 3? This makes perfect sense to me for a router that gives the device a private IP address via DHCP then the default gateway as the modem's IP address.

Although what I do not understand is how a modem can allow the connected device to use the public IP address assigned by the ISP. (Which I understand some do?) How does the computer know to send all internet traffic to the MAC address of the modem without IP?

 

[bill001g]

Again there is not the concept of mac address on DSL/ATM. It uses a ATM identifier to send the data between the ISP device and the modem.

You seldom see a actual DSL modem without a router. Any time you even say "IP address" it immediately becomes a layer 3 device.

Now let say you have a real modem without router capability and want to plug a PC directly into it. To avoid a massive confusing explanation of the PPPoE and PPPoA I will just call it PPP. What the PC does is build a special "tunnel" using PPP between itself and the ISP equipment on the far end. Since it is point to point it does not need any actual layer 2 addresses, it knows that anything it sends into the PPP connection goes to the other end. Now even though technically it is traveling over the ethernet cable it is running PPP...PPPoE to be more specific.

So in the process of setting up the PPP connection the PC gets a IP address from the ISP. It also gets a default gateway ip. It maps this default gateway IP to the tunnel directly. It pretty much says anything going to the default gateway (the ISP router) just send it on the PPP link and it will get there. So when you are using PPP it does not need any mac addresses to function.

Now when you have a modem/router rather than just a simple modem the router is doing all the PPP stuff behind the covers. It get the public IP address. The LAN side of the router is your standard ethernet where you are getting a private ip.

 

[stu17323]

Thank you so much you have completely answered my question! I now know what I need to read.