Hi Kevork. I guess it all depends on how much detail we can safely stuff into teenagers' heads in about 200 classes. <br><br>We naturally have to simplify things for them, like when a 4 year old asks "Why is the sky blue?"<br>
<br>:-)<br><br><div class="gmail_quote">On 18 November 2011 15:04, Kevork Krozian <span dir="ltr"><<a href="mailto:kevork@edulists.com.au">kevork@edulists.com.au</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
Hi Friends,<br>
<br>
I shudder when I see answers to questions that are half true.<br>
Example:<br>
<br>
Question 14<br>
<br>
Which statement best describes the purpose of the Open Systems<br>
Interconnections (OSI) physical layer?<br>
<br>
A. to route a packet of data through a network to a remote location<br>
B. to move individual data bits across a shared medium to another<br>
point in the network<br>
C. to ensure that packets of data are assembled and presented to an<br>
application in the correct order<br>
D. to ensure that packets of data received from another point in the<br>
network are free of any errors caused by the medium connecting the two<br>
points<br>
<br>
We agree A ( Network layer ) and C ( Transport layer ) are not the answer.<br>
However both B and D can seem correct.<br>
<br>
The problem for me with B is that there are no ACTUAL bits moving. You<br>
cannot no matter how hard you look notice a 0 or 1 on any medium moving<br>
data. All you will see are distortions that represent these values.<br>
Therefore individual bits are not moving. However, if you take the answer<br>
to mean "representations" of bits are moving then it can be right except<br>
it doesn't just happen on a shared medium eg. switches.<br>
<br>
As for answer D, the Physical layer "can ensure that packets of data<br>
received from another point in the network are free of any errors caused<br>
by the medium connecting the two points"<br>
Here is how.<br>
<br>
Chapter 8 Semester 1 Cisco CCNA<br>
<br>
Code Groups<br>
<br>
Encoding techniques use bit patterns called symbols. The Physical layer<br>
may use a set of encoded symbols - called code groups - to represent<br>
encoded data or control information. A code group is a consecutive<br>
sequence of code bits that are interpreted and mapped as data bit<br>
patterns. For example, code bits 10101 could represent the data bits 0011.<br>
<br>
Code groups are often used as an intermediary encoding technique for<br>
higher speed LAN technologies. This step occurs at the Physical layer<br>
prior to the generation of signals of voltages, light pulses, or radio<br>
frequencies. By transmitting symbols, the error detection capabilities and<br>
timing synchronization between transmitting and receiving devices are<br>
enhanced. These are important considerations in supporting high speed<br>
transmission over the media.<br>
<br>
Although using code groups introduces overhead in the form of extra bits<br>
to transmit, they improve the robustness of a communications link. This is<br>
particularly true for higher speed data transmission.<br>
<br>
Advantages using code groups include:<br>
Reducing bit level error<br>
Limiting the effective energy transmitted into the media<br>
Helping to distinguish data bits from control bits<br>
Better media error detection<br>
<br>
So the physical layer is involved with better error detection and<br>
reduction of bit level errors..<br>
<br>
Therefore, which is the correct answer ?<br>
<br>
<br>
Kevork<br>
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