CCNA newbie question about OSI model

JimF · Guest

From what I have read, error detection happens at the Application Level.
Error-Handling is done at the Transport level. Error correction happens at
the Data Link layer (Logical Link Control). The way I read this is that the
app says "error", the Transport layer says "did it get here?" and the Data
link layer says "send it again". Is this the right way to think of it? My
source is
http://www.geocities.com/SiliconValley/Monitor/3131/ne/osimodel.html Any
help would be appreciated. JimF

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magic · Guest

Error handling depends on the protocol used on each of the OSI layers.
Generally, the transport layer, network layer and datalink layer may be
involved in error checking.
Application layer is the last place where data errors can be found and the
action can be taken- it is the most usefull when using connectionless
protocol in transport layer (UDP). In the case of using TCP, the transport
layer is responsible for data inegrity and error handling.

Regerds,
magic

André Franke · Guest

Imagine an internetwork as the communication between the CEOs of two
companies:
City A
City B
Comp X
Comp Y
7 Appl CEO______________________________________CEO 7 Appl
6 Prese Chief Secretary______________________Chief Secretary 6 Prese
5 Sessi Secretary_______________________________Secretary 5 Sessi
4 Trans Fed-Ex Driver_______________________Fed-Ex Driver 4 Trans
3 Netw Fed Ex Internal Distribution____Fed Ex Internal Distribution 3
Netw
2 Datal Fed Ex Internal Truck____________Fed Ex Internal Truck 2 Datal
1 Physi Highway_________________________________Higway 1 Physi

Now if CEO X [7] wants to send CEO Y a letter, he lets his Chief Secretary
[6] type it (encoding) and take care that it will be sent. The 1st level
secretary [5] hands the letter to the Fed Ex Driver, telling him that her
collegue at Comp Y sits - let's say in Room 223 (that would represent a
port number) at Comp Y's site but she doesn't know the correct address.
Now the Driver puts the letter into a box (Segment) inside his truck [4]
labeled Comp Y and adds a sticky "Room 223". Once the letter arrives at
the Fed Ex Internal Distribution [3](Router), the whole box with the
letter is placed in a larger box (Packet) and since here the correct
address is known or can be easily found out, the address is written on the
larger box and the number of the responsible City BFed Ex Distribution as
well as a route map. The Driver of the big Fed Ex Internal Truck [2] puts
that larger box into his big fat truck (Frame) and drives along the
Highway [1] to the City B Fed Ex Internal Distribution, where he takes the
large box out. He found his destination, because of the route map. If his
truck had an accident and everything burnt to ashes noone would have been
informed (At least in network belongings that is so. This is just error
checking. The only consequence of a detected error would be that the
letter never reaches the destination.)
But let's assume it was ok.
He now removes the number and the map, leaving only the address of comany
Y on the box.
From the City B Fed Ex Internal Distribution someone is having a look if
the larger box is damaged. If so he throws it into the bin and again noone
knows. (Again just error detection)
But let's assume it was ok.
The Fed Ex Driver in City B gets the smaller box and checks again, if tha
smaller box is ok. If it's not, he sends a note back to his collegue in
City A telling him something went wrong. Now it begins to sound like a
fairy tale, because City A's Driver has a magic copy of the small box and
the letter. So he sends it again. (This time it is error recovery).
But let's assume it was ok.
He takes out the letter and brings it to room 223 (remember the port
number) where the secretary thanks him and runs to the Chief secretary of
Comp Y. The Chief Secretary again reads out the letter, so the CEO Y
doesn't have to break his golf training. If the words of the letter don't
make any sense, or if it is unreadable the CEO will of course take action
and ask the CEO X what that is supposed to be. And what those CEOs do
after that is what we call error correction. You may also say that this is
just another story.
Summary :
Datalink Layer = Error Detection (if error = silently discard)
Network Layer = Error Detection (if error = silently discard)
Transport Layer = Error Detection and Recovery (if error = request
retransmission)
All these check only for damaged encapsulations while the Transport layer
doesn't look inside either but checks if a checksum matches the delivered
one, so it seems to be ok. There is still a chance that there are errors,
but none caused by transportation. And any kind of remaining errors are
thing of the application layer.

regards
André