Learning by Doing: CISCO Certified Network ... - SCN Research

layer adds LLC, CRC, and MAC information. The LLC is just instructions on how to get
from layer 1 to layer 3. MAC information is the hexadecimal, 48-bit, physical address of
the source and destination. The CRC is an error-checking mechanism for the data link
layer. It essentially works like this: Now that the “frame” is nearly completed the overall
number of bits is divided by a unique prime number (a number divisible only by one and
itself…17 and 31 are most common). With all the overhead of the headers and footers
our individual frames may be 6808 bytes in size by now. So the CRC divides 6808 by 17
(I picked which one our network is using arbitrarily)..and we get 400 with a remainder of
8. The 17 is attached along with the remainder of 8. When this frame gets to Casey the
division will take place again. If the same remainder is attained then Casey will assume
everything came over ok. Also, since all of our Ethernet, Token Ring, Frame Relay,
ATM, etc. is found on the data link layer that information also is added (before the CRC
stuff). Finally the entire frame is passed to the physical layer where it is converted from
hex into decimal and transmitted over the network. On Casey’s computer the information
is received, checked and re-assembled. In our case 6 chunks of information that are 6808
bytes are received (40,848). If we follow our same compression ratio of 5:3 then we
would expect the 40,848 to be un-compressed to over 68,000 bytes. However, since all
of the headers and footers are removed after being de-compressed our original message
will be back to its original size of 50,000 bytes. This is why, when you download
something from the Internet, a 100,000 byte download counts up to about 130,000 bytes
before being “finished” but then is only 100,000 bytes when you look at it. Aha!
Mysteries of the Internet Revealed! Even better than Geraldo and the Capone’s Vault.
Step-By-Step Instructions:
Ok…so those are the definitions/encapsulations that they asked you to know. Let’s take
a few seconds to re-write them in our own words.
Layer CISCO definition Your definition
Application
Presentation
identifies and establishes the
availability of intended
communication partners,
synchronizes cooperating
applications, and establishes
agreement on procedures for
error recovery and control of
data integrity. “browsers”
translates multiple data
representation formats by
using a common data
representation format.
“concerned with data
structures and negotiation data
transfer syntax” “encoding,
representation of data, ASCII”
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