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dsl modem - protocols used?

Posted on 2003-10-28
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Last Modified: 2010-04-11
I've been wondering about the way the dsl modems work in terms of protocols.
Since, I could not find much of documentation on this:
What protocol is used for the connection between the pc and the modem?
 
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Question by:sinexec
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by:qwaletee
ID: 9637446
The PC and modem typically connect over Ethernet. Some modems that offer a USB option connect via USB, with the PC thinking that at the other end of the USB cable is an external Ethernet adapter.
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by:qwaletee
ID: 9637451
(That's technicaly 10Base-T -- 10 mbit Ethernet over twisted pair wiring.  It could use 100Base-T, but since all DSL modems are slower than 10 anyway, they don't bother putting in 100-capable hardware for the PC connection.  All current Ethernet cards for PCs are 10/100 autosensing, so that is not a problem.)
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by:sinexec
ID: 9637590
May be I did not word my question clearly.
Ethernet is a layer 2 protocol, but what protocol is used for the communication between pc and modem over Ethernet.
It does not seem to be IP, since the modem's ip-address does not appear to matter - the communication still takes place. Or I am wrong?
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by:JConchie
ID: 9637852
Ethernet is not a protocol, it is a set of standards:

"802.3 - Hardware standards for Ethernet cards and cables
802.5 - Hardware standards for Token Ring cards and cables
802.2 - The new message format for data on any LAN
The 802.3 standard further refined the electrical connection to the Ethernet. It was immediately adopted by all the hardware vendors. Today all cards and other devices conform to this standard"

You are either using TCP/IP  or netbeui (which is a broadcast protocol) if you are using windows machines, appletalk if macs or spx/ipx if novell.  

Given your comment on the modems ip address, I would bet that you are using netbeui.
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qwaletee earned 85 total points
ID: 9637978
There are lots of different setups.  PPPoE (guess which protocol!) will work differently from static.  I have seen static setups where the DSL modem was an IP router -- and others, where it was not.  USB will obvisouly use the USB driver.

With the exception of PPPoE, the PC really does not know anything except for IP/ARP addressing.  If the modem has an IP on teh smae subnet, then the PC will use it as a gateway.  if it does not, then the mdem is acting as a bridge, and the PC is still just putting out IP.ARP packets, with the bridge picking up the gateway-bound packets and forwarding them on via the ATM DSLAM connection.
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by:sinexec
ID: 9638034
Thanks. I've had PPP over Ethernet in my mind, but I wasnt sure...
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