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On 15/04/2012 11:54, Gordon Henderson wrote:
This I know - knew anyway - amazing how much you forget 8 years out of the game :)On Sun, 15 Apr 2012, Julian Hall wrote:On 14/04/2012 21:36, Philip Hudson wrote:It's wrong for all broadband not just *DSL, in fact even more so for cable connections. However, inasmuch as Modulator/Demodulator converts digital to analogue and back again, that was the job of the original equipment as it converted a digital input into sound that could be carried over the phone line, and reversed the process the other end. Given that AFAIK it's digital all the way now with fibre in use and even *DSL over copper wire doesn't employ an analogue signal anymore, I agree the term 'modem' is inaccurate. However, in terms of it being 'the device which sends and receives information to/from the computer over a remote network' the general public (and let's be fair technicians too) have got used to that bit of kit being called a modem and so rightly or wrongly it has passed into the lexicon of IT terminology as a modem.Can't believe nobody's confirmed for Tony that he's spot on about the term "modem" being wrong for *DSL. It's pure marketing-speak, a holdover from pre-broadband days, something the punters understood and was close enough in function. Less sure of my ground now: I think it's correct to call it a codec, which we usually think of as a software library these days, but can be hardware. Gordon would know.It is correct to call it a modem because that's what it is. An analogue signal is MODulated by the digital signal and DEModulated at the other end. In-between the analogue signal can be corrupted, lost, found, buried in soft peat and recycled as firelighters. It's a miracle it works at all sometimes.Actually, its probably more correct to call it analogue signals - as there are several carriers in ADSL.Even with Cable - they are still modems. Cable Internet uses a system called DOCSIS
- and it's an analogue carrier signal modulated with the digital signal over co-ax cable - unlike Co-Ax Ethernet which is baseband (You put 5V on the cable to indicate a "1" and 0V to indicate a "0"). You can use different carrier frequencies to carry different information - so the way you get Internet and TV at the same time is to use different carrier frequencies and filters to split the signals out. DOCSIS uses a carrier of (IIRC) 10MHz - it can get away with much higher carrier frequencies than ADSL as it's co-ax cable and thus not prone to radiating the signal out, nor accepting interferance in.If your home broadband is supplied on copper then the digital signals are carried over it modulated on-top of an analogue carrier signal. In ADSL, the carrier is between 32KHz and about 1.2MHz - the lower frequencies are used for the upstream and the higher frequencies for the downstream. The bands are divided into "buckets". Part of the negotiation phase involves testing each frequency to see which work and which don't - and various factors affect this - line length, local conditions, etc. Some technologies extend the upstream frequeneices into the downstream ones to give you more upstream at the expense of less downstream speed (Annexe M in this country)So just because we can't hear it, doesn't mean its not via a modem. The principles are the same, the carrier frequencies have been shifted from the audio range to radio range.And when it's fibre... don't think it's always "pure" digital either. Analogue modulation techniques are being used with Fibre to increase its capacity. If you want to know more, lookup QAM but my head starts to spin at that point.Gordon
Fair enough.. I sit corrected :) Julian -- The Mailing List for the Devon & Cornwall LUG http://mailman.dclug.org.uk/listinfo/list FAQ: http://www.dcglug.org.uk/listfaq