[Year 12 SofDev] SD 2011 exam - MC Q14

Sat Nov 19 09:39:56 EST 2011

Hi Mark,

I continue to be troubled by this question. Evidently the writers of the
question expect there be no role for the physical layer in error
detection/control/correction.

Here are some sources that describe extensive error handling capability at
the physical layer:

http://www.oohoo.org/Book/MindShare-PCIe-System/0321156307_ch11lev1sec7.html

http://my.safaribooksonline.com/book/hardware/0321156307/physical-layer-logi
c/ch11lev1sec7

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4784785 

This covers fundamental behavior of a 802.11g network which already has
embedded error correction at the physical layer, under unicast and broadcast
modes

Error detection and handling appears at a number of layers in the OSI. When
the physical layer reports an error either "caused by the medium" or during
transmission over the medium, the data link layer then organizes to attempt
to deliver/dispatch again. 

So as with the question on the worm example - it depends what you mean by
the word "infect" -  we now need to ask what we mean with "ensure" in:

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 

Without the physical layer reporting problems that insurance lapses. The
fact both layers ( physical and data link ) are involved in this does not
remove the role of the physical layer. 

Take Care

Kevork Krozian

Edulists Creator Administrator

www.edulists.com.au

tel: 0419 356 034

From: sofdev-bounces at edulists.com.au [mailto:sofdev-bounces at edulists.com.au]
On Behalf Of Mark KELLY
Sent: Friday, 18 November 2011 3:26 PM
To: Year 12 Software Development Teachers' Mailing List
Subject: Re: [Year 12 SofDev] SD 2011 exam - MC Q

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
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