What is happening in ADSL modem(In bridge mode)?

Question

I am using broadband connection via BSNL and I get service in a twisted pair. ADSL technology is used for broadband connectivity. ADSL modem normally have two modes of operation (Bridge mode and PPPoE mode). I would like to know what is happening in the modem when it functions as Bridge mode. I would like to substitute it with a circuit or a PCB module. Any help would be great.

I have a ADSL input and I need ethernet output. Thanks for any help.

Solution 1

I have a ADSL input and I need ethernet output.

Then you need an ADSL modem.

I would like to substitute it with a circuit or a PCB module.

The ADSL modem is actually a rather complex telephony device that has a microcontroller and firmware. It has to negotiate the line parameters before the connection is established (aka training). You can only replace this functionality with another ADSL modem.

I would like to know what is happening in the modem when it functions as Bridge mode.

The unit that you're calling an "ADSL modem" probably has both modem and router functionality contained within it.
By "router" I do not mean "wireless". (What people commonly refer to as a "router" is really a "wireless router" which has router, wireless access point and Ethernet switch functionality.)
By router functionality I am referring to services that include NAT, DHCP, DNS and firewall.

Bridge mode is for disabling the router functionality within the unit so that you can use just the ADSL modem functionality. You are by-passing or going over the router functionality. You still have to properly configure the modem to connect to your ISP with the proper ATM VPI/VCI settings. You would also still need to attach a router to protect your LAN.

See ADSL modem+router in bridge mode tutorial.

Solution 2

Networking is a complex topic. And the terminology often isn't intelligent.

What you have are devices with multiple functions.. routing is a function, modem is a function, PPP is a function.

A network bridge and a network switch are functionally the same thing. Some say a bridge often has 2 ports and the network mediums differ, whereas a switch tends to have multiple ports and the network mediums are the same. The logic of what it does is the same though. So if you look up switching, that's the function.

But also, they can get the name of router, or switch, even when they technically might not really need to do any routing or switching. For example, aside from juniper and cisco, who sell routers that do a lot of routing, the items sold as routers by others, are technically routers, but only just about. They have 2 interfaces and on one of the interfaces is a network switch.

I recommend the book 'computer networking first step' by wendell odom. He has some ugly analogies but you can skip the analogies and he has some good stuff.

A switch is receiving "packets" and redirecting them.

But you could then probably simplify the device a lot if e.g. it has only one computer connected to it then there's more functionality there than it needs.

Many of these devices, particularly older ones, you could typically telnet to them and get a console up, 'cos they not only run linux. So a Router might have a whole routing table in there but really since it only has 2 interfaces it could probably get pretty far (i'm not sure how far but perhaps pretty far), by simply saying if a packet comes in one side then send it out the other side. (unless it's destined for local). It perhaps doesn't technically need a whole routing table to do that. Maybe similar for "bridging" mode. If going from scratch from an electronics background you might be able to make it simpler particularly if it only has one computer connected to it.

Also, I suggest you look into a PCI DSL modem.. That is a bridging dsl modem. As an electronics person, that may interest you. And when they say bridging that also means it isn't doing any PPP(logging in), so the PPP is then done on the computer from windows. As is it when a modem is put in bridge mode.. or it can be done by a downstream device.. e.g. by a "router" plugged into the modem where the router supports ppp.

Comments

[Manu M.]

Thank you. On more time I need to confirm this. Using an ADSL modem router in bridge mode means we are using it only as a ADSL modem. Router part is doing the PPPoE and other stuffs. That means ADSL Modem simply converts incoming ADSL singal to ethernet format only. Right? Is there any single chip or SOC available for this (ADSL to Ethernet conversion) Thanks a lot.

[Manu M.]

Thank you for your answer. I got lot of information regarding this topic. I will try to read that book too.

[barlop]

@ManuM. Router and PPPoE are two different functions. But the box they call a router might sometimes do PPP too. When in bridge mode it won't do PPP. I don't know much about the ADSL to Ethernet aspect.

[barlop]

-1 It is absolutely wrong to refer to NAT and DHCP and DNS and Firewall as "router functionality", and I think you know that very well. I notice you missed out PPPoE from the list and then he started wondering whether PPPoE is part of the routing like the other parts. He should know that NAT and DHCP and DNS are not part of routing and neither is PPP. Your answer is wrong and clearly misled him, understandably so.

[sawdust]

Your understanding seems correct. Availability of ADSL chips is questionable, and usually requires quantity purchase and NDA. If you did get a chip, there's still a lot of external supporting circuitry and firmware required. You're underestimating the sophistication/complexity of ADSL. I have first-hand experience developing xDSL firmware; I'm not guessing.

[sawdust]

@barlop -- For simplicity I divided the the functionality of the typical ADSL "modem" into modem versus router halves, as typified by the common label "modem+router". So what name do you use to collectively refer to these services commonly found in SOHO (wireless) routers and ADSL "modems"?

[barlop]

@sawdust Well can call them "modem routers" to signify that it is a box that includes those two functions. But the issue of your error isn't so much what to call it since you're writing an explanation and your explanation was wrong.

[sawdust]

@barlop -- One more time: what name do you use to collectively refer to services (NAT, DHCP, DNS, firewall) commonly found in SOHO (wireless) routers and ADSL "modems"?

[barlop]

Let us continue this discussion in chat.