What is the difference between an ADSL operating in modem mode versus bridge mode?

What is the difference between an ADSL operating in modem mode versus bridge mode? The scenario is that we have a router supporting ADSL, and a second router with more advanced options. Two routers are connected so that first can synchronize with the ADSL and pass the public IP the the second router.

Now, what is the difference when setting the ADSL router into the modem mode. And what additional option can we have when using the bridge mode (if any)?

tojazenn

Posted 2015-04-27T21:11:06.450

Reputation: 1

Can you please edit your question to provide the exact make and model of the ADSL modem you are using? – JakeGould – 2015-04-27T21:49:11.137

There is no need for a model version. I just would like to know what is the difference. What are the pros for one or the other. – tojazenn – 2015-04-28T07:36:34.907

Please read my answer for a better understanding. Some makes and models use the terms “bridge mode” and “modem mode” interchangeably. And some have distinct differences. But there is no generic “better.” – JakeGould – 2015-04-28T19:22:45.130

Answers

Unclear what the make and model of the ADSL modem you are using is, but in most cases, bridge mode and modem mode are 100% the same.

That said, if this is a modem that—for example—has a USB connector in addition to an Ethernet port, then modem mode would only work on the USB connector while bridge mode would be something that would work on a networking level via the Ethernet port.

JakeGould

Posted 2015-04-27T21:11:06.450

Reputation: 38 217

I suspect that you mean by modem mode, the 'normal' mode in which you have to program the modem with password and username of you ISP provider.

If that is the case, that's also the difference: In modem mode, the connection with the ISP is established and monitored by the modem itself. Eg., many ISPs use PPPoE or a variant to establish the real connection. This done by the modem in modem mode, while in bridge mode, it's the computer which does all the 'high level' stuff. The PC sends the username and password to the ISP - the modem then just acts as a bridge.

In bridge mode, the modem pretty much only syncs with the phone line signals, and decodes/encodes the packets. The PC takes care of the rest.

This has the advantage that it is easy to change modems - the user data are only present in the PC.

jcoppens

Posted 2015-04-27T21:11:06.450

Reputation: 629

I thought that modems are for sync-ing only. And that they are passing the username/password provided from the end device (or a router). Hence, the PPPoA information is stored on a computer (router). – tojazenn – 2015-04-28T07:41:26.760

Well, if you buy an ADSL modem, what you get is in fact (modem+controller), where the controller can do many things, such as manage the PPP protocol, NAT translation, firewall, MAC filtering, etc. Or the controller can be configured to act as a bridge, where all the above functions are taken over by the computer. The specs of bridgeing are in RFC1483. – jcoppens – 2015-04-28T14:48:33.690