[Year 12 SofDev] SD 2011 exam - MC Q

[Mark KELLY]

Hi Kevork.  I guess it all depends on how much detail we can safely stuff
into teenagers' heads in about 200 classes.

We naturally have to simplify things for them, like when a 4 year old asks
"Why is the sky blue?"

:-)

On 18 November 2011 15:04, Kevork Krozian <kevork at edulists.com.au> wrote:

> Hi Friends,
>
>  I shudder when I see answers to questions that are half true.
> Example:
>
> Question 14
>
>  Which statement best describes the purpose of the Open Systems
> Interconnections (OSI) physical layer?
>
>  A.      to route a packet of data through a network to a remote location
>  B.      to move individual data bits across a shared medium to another
> point in the network
>  C.      to ensure that packets of data are assembled and presented to an
> application in the correct order
>  D.      to ensure that packets of data received from another point in the
> network are free of any errors caused by the medium connecting the two
> points
>
> We agree A ( Network layer ) and C ( Transport layer ) are not the answer.
> However both B and D can seem correct.
>
> The problem for me with B is that there are no ACTUAL bits moving. You
> cannot no matter how hard you look notice a 0 or 1 on any medium moving
> data. All you will see are distortions that represent these values.
> Therefore individual bits are not moving. However, if you take the answer
> to mean "representations" of bits are moving then it can be right except
> it doesn't just happen on a shared medium eg. switches.
>
> As for answer D, the Physical layer "can ensure that packets of data
> received from another point in the network are free of any errors caused
> by the medium connecting the two points"
> Here is how.
>
> Chapter 8 Semester 1 Cisco CCNA
>
> Code Groups
>
> Encoding techniques use bit patterns called symbols. The Physical layer
> may use a set of encoded symbols - called code groups - to represent
> encoded data or control information. A code group is a consecutive
> sequence of code bits that are interpreted and mapped as data bit
> patterns. For example, code bits 10101 could represent the data bits 0011.
>
> Code groups are often used as an intermediary encoding technique for
> higher speed LAN technologies. This step occurs at the Physical layer
> prior to the generation of signals of voltages, light pulses, or radio
> frequencies. By transmitting symbols, the error detection capabilities and
> timing synchronization between transmitting and receiving devices are
> enhanced. These are important considerations in supporting high speed
> transmission over the media.
>
> Although using code groups introduces overhead in the form of extra bits
> to transmit, they improve the robustness of a communications link. This is
> particularly true for higher speed data transmission.
>
> Advantages using code groups include:
> Reducing bit level error
> Limiting the effective energy transmitted into the media
> Helping to distinguish data bits from control bits
> Better media error detection
>
> So the physical layer is involved with better error detection and
> reduction of bit level errors..
>
> Therefore, which is the correct answer ?
>
>
> Kevork
> _______________________________________________
> http://www.edulists.com.au - FAQ, Subscribe, Unsubscribe
> IT Software Development Mailing List kindly supported by
> http://www.vcaa.vic.edu.au - Victorian Curriculum and Assessment
> Authority and
> http://www.vcaa.vic.edu.au/vce/studies/infotech/softwaredevel3-4.html
> http://www.vitta.org.au  - VITTA Victorian Information Technology
> Teachers Association Inc
>



-- 
Mark Kelly
Manager of ICT, Reporting, IT Learning Area
McKinnon Secondary College
McKinnon Rd McKinnon 3204, Victoria, Australia
Direct line / Voicemail: +613 8520 9085, Fax +613 9578 9253
kel at mckinnonsc.vic.edu.au
VCE IT Lecture Notes: http://vceit.com
Moderator: IT Applications Edulist <http://www.edulists.com.au/>

Want a good time? Call 0112358. Ask for Mr Fibonacci.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.edulists.com.au/pipermail/sofdev/attachments/20111118/86a28149/attachment.html