[Year 12 IT Apps] VITTA ITA exam 2 - my ideas of errata

Tue Oct 16 10:55:02 EST 2007

Hi Mark,

 Yes it is a complex area.
The fact you can hear a dial up modem is not what modulation/demodulation is limited to as much as we have become used to it.
The human hearing range is only incidental to the frequency of transmission. The fact transmission with mod/demod can't be heard in DSL/Cable modems does not change the point about use of waveforms/frequencies to model digital data. We just can't hear these frequencies. 

You are right. We have become accustomed to our senses dictating what the technology is doing just like a router then a modem to connect. In reality, a modem is just an interface that is now absorbed into the routers.

Now, I hope that hasn't caused more confusion ........

Back to work - exams are looming and the kids have been dragging their feet . 
Back in my days ......

Keep well
Kevork

>>> Mark Kelly <kel at mckinnonsc.vic.edu.au> 16/10/2007 8:17 am >>>
Thanks, Kevork.  It is a complex topic once you start delving into it. 
I suppose what I meant about modulation/demodulation is that cable and 
DSL modems don't convert digital data to and from _sound_ as the dial-up 
modems do.  Or at least I hope they don't  :-)

Wiki has an informative page on DSL - http://en.wikipedia.org/wiki/Dsl 

Kevork Krozian wrote:
> Hi Mark and Russell,
> 
>  I would add a few more cents here to the discussion. 
> 
> The primary role of a router is " path determination and switching"
> meaning it decides which interface an incoming packet has to be sent out
> and how to transfer 
> - switch -  the packet to that outbound interface. Various modules
> within routers ( eg. ADSL module, WAN modules ) can be used to provide
> connectivity to a WAN without a modem. Also, routers can provide
> security functions through firewall services, that is not their primary
> role.
> So, a router connects LANs to either other LANs or WANs - it is a
> connectivity device for different networks. Compare with a switch which
> is a connectivity device for devices WITHIN a network.
> 
> Broadband modems ( DSL and cable modems ) use complex waveforms to
> carry digital data ( http://en.wikipedia.org/wiki/Modem ) therefore
> still modulate/demodulate and therefore are analog in nature. The term
> broadband refers to the ability to carry multiple signals ( voice, data,
> TV ) as opposed to baseband which carries one type of signal. 
> A DSLAM ( Digital Subscriber Line Access Multiplexer ) is found at the
> exchange which allows aggregation of incoming ADSL signals to connect to
> the digital network (internet ) backbone. So, despite the publicity an
> ADSL digital service is only digital up to the DSLAM after which it is
> analog up to the customer premises. That is still better than dial up no
> doubt.
> 
> Cable modems act like a bridge between a customer's LAN and the coax
> cable network of the provider though they can provide higher layer
> functions such as routing and other tasks
> http://en.wikipedia.org/wiki/Cable_modem . The point is there is no
> sign of fibre optic media here. 
> 
> As in past years, to all readers,  Cisco Networking Academy ( a VET
> subject ) is the way to go if you wish to formally learn more about this
> fascinating area of IT.
> 
> Best Wishes
> 
> Kevork
> 
> 
>>>> Russell Edwards <edwards.russell.t at edumail.vic.gov.au> 15/10/2007
> 2:22 pm >>>
> Hi Mark, I agree with most of what you have said, just a couple of  
> minor points
> 
> On 15/10/2007, at 1:36 PM, Mark Kelly wrote:
>> Question 1
>> Which of the following enables several LANS to connect over a WAN?
>> Suggested answer: D. Router.
>>
>> A router protects a LAN from the outside world, but its role is not 
> 
>> principally to enable communications.  I suspect the best answer  
>> would be a modem.
> 
> The primary role of the router is indeed to enable communications  
> between two or more networks. It may include a firewall of some sort  
> (=protection from the outside world) but need not.   A modem is not  
> necessary as you may have a data link that doesn't require modulation/
> 
> demodulation-- though you'd be hard pressed to find one that could  
> offer distances that classify as "wide area"!
> 
>> Question 3
>> Which type of backup uses two sets of backup media and backs up the 
> 
>> most recent changes on the second media?
>> Suggested answer: B. Differential.
>>
>> This can*t be right.,  The question is very odd.  Two sets of  
>> backup media? From http://www.backuptool.com/incremental% 
>> 20backup.htm...
> 
> I suppose what they're getting at is that incremental backups might  
> use more than twos set of media. But in fact, the terms are about  
> _what_ you backup, _when_ -- not _where_ you put the data (i.e.
> media).
> 
>> A modem is, for most people today, a digital device for ADSL or  
>> cable internet that has no modulation or demodulation happening!   
>> The question should have referred to analogue modems if it wanted  
>> this answer.
> 
> No, ADSL and cable are still analog modems. ADSL goes over copper  
> line just as dialup, simply using a much higher (analog) bandwidth  
> than plain dialup and avoiding the voice band. Cable modems (afaik)  
> use an optic fibre data link so again this is a modem, as a carrier  
> wave (in this case a light wave) is modulated to transport data.   
> Even wireless, 3G or satellite internet will rely on a pair of modems 
> 
> (with an electromagnetic wave as a carrier) as part of the link, even 
> 
> though the term isn't usually used in reference to them.
> 
> cheers
> 
> Russell Edwards
> Whittlesea Secondary College
> 

-- 
Mark Kelly
Manager - Information Systems
McKinnon Secondary College
McKinnon Rd McKinnon 3204, Victoria, Australia
Direct line / Voicemail: 8520 9085
School Phone +613 8520 9000
School Fax +613 95789253
kel AT mckinnonsc.vic.edu.au

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