[Year 12 IT Apps] Modems and Routers thread..

Tue Oct 16 09:18:03 EST 2007

Hi Kent,

 A multiplexor at the exchange ( DSLAM) is simply an aggregation device for multiple DSL input lines from customers.
Were you thinking of some other multiplexor  ?

Regards

Kevork

>>> "Kent Beveridge" <kbeveridge at stbc.vic.edu.au> 15/10/2007 4:36 pm >>>
Scuse my ignorance, but where would a multiplexor fit here, or wouldnt it? Yep, I know its not the question, but now I just got curious!

This conversation thread is actually an interesting one given all our different perspectives/backgrounds. 

Kent Beveridge,
I.T. co-ordinator
St. Brigids Catholic Sec. College
Horsham
email.. kbeveridge at stbc.vic.edu.au 

|<3|\|7  b3\/3r1D93 ?  ;-)

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From: itapps-bounces at edulists.com.au on behalf of Kevork Krozian
Sent: Mon 10/15/2007 3:09 PM
To: Year 12 IT Applications Teachers' Mailing List
Subject: Re: [Year 12 IT Apps] VITTA ITA exam 2 - my ideas of errata

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

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