datalink layer question

Berhanie · 12-04-2005, 09:31 AM

At home, I access the internet via a cable modem connection. What is the datalink layer protocol (OSI layer 2) used between my home and the ISP's facilities?

fr_laz · 12-05-2005, 06:12 PM

Hi,
PPP as (almost) every single home user...

Berhanie · 12-05-2005, 06:24 PM

Thanks for that, fr_laz. What's confusing to me is my setup, being typical, looks like this:

Code:

                |------------|
My Lan ---------| Cab. Modem |-------------
       (Cat 5)  |------------| (Coax Cable)

So, is it the case that a cable modem accepts Ethernet frames from my lan and
outputs PPP frames? (I thought a cable modem was a layer 1 device.)

fr_laz · 12-07-2005, 09:23 AM

i think you're using pppoe to "talk" with your modem, which means that your linux box is sending ppp frames encapsultaed in ethernet (ppp over ethernet), so the ethernet card of your modem removes the ethernet encapsulation and the ppp frame is sent to your isp...
you can check if your using pppoe in the config files in your /etc/ppp/ dsirectory

Berhanie · 12-07-2005, 09:52 AM

The Cat 5 cable connects my cable modem with my LAN router. It's just a cheap Netgear router. I suppose you're saying that the router automatically outputs PPPoE frames.

Since I can either connect a computer to a router, or a router to another router, my router must be able to handle incoming frames which are either Ethernet or PPPoE.

Am I correct?

Berhanie · 12-07-2005, 10:36 AM

After reading this, I see that a cable modem is more than just a layer 1 device. As such, it would be make more sense for the modem to accomodate the LAN's datalink layer than the other way around. So, I guess my router is outputting ethernet frames which are converted to PPP by the cable modem.