Why do they call a ADSL modem a modem ?

[simonpolly]

Modem = modulate/demodulate i-e digital to analogue and analogue to digital.

 

[kevindgas]

surely that is what it does
analogue phone line to a digital computer? but i'm most likely talking bllx!!

edit: and yes this is what we use!

 

[Synapse]

Exactly, because it's still modulating and demodulating analogue signals.

The main difference is that ADSL uses a much higher frequency range and a greater number of channels to increase throughput, however these are still analogue signals, albeit much more complex than traditional voice frequency modems.

What wouldn't make sense is something like an ISDN modem, since that technically isn't a modem but a terminal adapter since signals on an ISDN line are digital.

 

[TicklyT]

Years ago, around about the time Pontius learnt to fly, digital communication over phone lines was carried out using a modulator to convert the 1s and 0s into different tones. The tones, carried over the phone network, were fed into a demodulator, and converted back into 1s and 0s. communication back the other way needed another modulator, phone line and demodulator.

When techniques for both sending and receiving over the same line were devised, modulators and demodulators were built into the same box - a combined Modulator - demodulator.

Things have moved on a lot since, but local loop DSL signals are still essentially transmitted by modulating analogue carrier signals superimposed on local lines originally designed for analogue speech trasmission, so the name has stuck.

Why do we call the white box at the heart of a domestic heating system a boiler, when the last thing we want it to do is boil?

 

[punchy]

Modem = modulate/demodulate i-e digital to analogue and analogue to digital.

Surely the answer is misinformation! An ADSL 'Brouter' which is exactlly what you have at home is a device that is a cross between a Bridge and a Router. It does do packet switching although is purely a bridge between your network and the DSLAM in the exchange. The packet switching is in your home network (bridge) the routing is to the DSLAM (your ISPs device in the exchange). A proper router would be programmable in all areas as oppose to just IP add and usernames with a few other functions (a proper router would have a console port at least). Try doing the CCNA course, its a 2 year course at night school if you want to learn or 6 weeks crash if you just want to pass the test, I would recommend the 2 years at night school.

ADSL (Asymmetric Digital Subscribers Loop) is Openreach's preffered method of delivery bigger downstream than upsteam.

SDSL (symmetrical Digital Subscribers Loop) was very expensive for large networks who wanted to use BT as an SP and had equal upstream and downstream. This was all but ceased 3 years ago unless it has been resurrected, I am sure they kept existing customers and did not take on new business.

Broadband (Typically used by NTL now Virgin) One pipe up the street co axial then split to each house all the way down the street. This goes back to a cabinet which is served by Fibre optic i.e. no need for an exchange. This is the reason they call it Fibre broadband but you do have a contention ratio as you share the co axial pipe back to the cabinet as oppose to ADSL where you have your own pipe to the exchange before you start sharing bandwidth at the DSLAM.

The simple answer is BT decided to call it a modem as that is what people were used to. The companies who made them called them routers once they got into the ethernet connection and away from USB as they were at first. See netacad at Cisco for more information.

 

[Monkeh]

Comodembridgerouterdecs.

 

[plugwash]

Modem = modulate/demodulate i-e digital to analogue and analogue to digital.

Which is exactly what a ADSLmodem is. Digital signals from the PC, modulated signals on the phone pair. The modulation scheme being designed to work arround the fact that phone pairs have pretty horrible characteristics at those frequencies and to adapt to varying line conditions.

A typical broadband router is a two port router with NAT capability, typically the NAT capability can be disabled and real IPs used on the LAN side if the ISP offers this (better ones generally do, sometimes at an extra charge). It may have a built in ADSL modem or it may have a WAN side ethernet port for connecting to a seperate DSL or cable modem. Some of them even had USB ports for use with the BT issue ADSL modem. Typically there is also a switch and sometimes also a wireless access point on the LAN side.

 

[plugwash]

A proper router would be programmable in all areas as oppose to just IP add and usernames with a few other functions (a proper router would have a console port at least).

Gimping the configuration interface does not make the device not a router.

BTW they mostly do have pads for a console port, they just don't bother with the level shifters and external connector

 

[Paul_C]

Surely the answer is misinformation! An ADSL 'Brouter' which is exactlly what you have at home is a device that is a cross between a Bridge and a Router. It does do packet switching although is purely a bridge between your network and the DSLAM in the exchange. The packet switching is in your home network (bridge) the routing is to the DSLAM (your ISPs device in the exchange).

You still need the modem to communicate with the DSLAM at the exchange, since the data has to be modulated onto the carrier to send and then demodulated at the other end.

So whatever other routing, bridging, NAT, or other functions the box in question performs, if it is linked over DSL, then it incorporates a modem.