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OSI "DATA LINK" AND "TRANSPORT" LAYER 12 years 4 months ago #3911

  • babarali
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Hi ,

I have a question related to OSI layers (Data Link & Transport). question is i have read in book that

1. data link layer provide supports for connection oriented and connection less services and also provides for sequencing and flow control.

2. also Transport layer provide support for connection oriented and connection less tranmission and also perform 2 functions "Flow Control by sliding window and relaibilty provide by sequencing numbers"

my question is how data link provide sequencing and flow control ?
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Re: OSI "DATA LINK" AND "TRANSPORT" LAYER 12 years 4 months ago #3917

  • Chris
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Good question! I personally haven't ever come across a similar description for the datalink layer. Flow control can be provided in some sence e.g when collisions occur or when two workstations are beggining a data transfer.... but sequencing?

My opinion is that you've got a book with errors as they usually occur. The author must have got these mixed up and as a result, they ended up in the datalink description aswell;

A quick search on google will provide more than enough evidence that this is a classical error, unless anyone else can prove elsewise :wink:

Chris Partsenidis.
Founder & Editor-in-Chief
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Re: OSI "DATA LINK" AND "TRANSPORT" LAYER 12 years 4 months ago #3925

  • tfs
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I have.

This was actually a confusing issue when dealing with the CCNA test, in that the question would be something like:

What layer handles reliability and flow control.

Both 2 and 4 do. But there would be something that was subtly different about the question that would be the tip off. It was very confusing.

Here is an article that mentions it:

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Re: OSI "DATA LINK" AND "TRANSPORT" LAYER 12 years 4 months ago #3974

  • IFTY
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babar ali

i think the book u got is not correct and the second chris also couldnt explain correctly
according to my knowledge transport layer is responsible for error recovery and flow control.
so dont think that data link layer provides flow control......

i think first u have to understand that what actually flow control is?ok let me explain
it is the process of controlling the rate at which a computer sends data.flow control prevents unnecessary congestion by attempting to send data at a rate that the network can currently ccomudate.a sender of data might be sending the data fast then the receiver can receive the data so the receiver discards the data.also,the sender might be sending the data faster then the intermediate switching devices(switches,routers)can forward the data,also causing discards.the receiving computer can have insufficient buffer space to receive the next incoming frame or possibly the CPU is too busy to process the incoming frame.flow control attempts to reduce
unnecessary discarding of data.with flow control,the sender can be slowed enough that the original PDU can be forwarded to the receiving computerand the receiving computer can process the PDU.
flow control protocols dont prevent the lose of data as a result of congestion,these protocols simply reduce the amount of lost data.with flow control,the sender is artificially slowed or throttled so that it sends data less quickly then it could without flow control.

there are three methods of implementing flow control
2)congestion avoidance

buffering simply means that the computers reserve enough space that bursts of incoming data can be held until attempt is made to actually slow the transmission rate of the sender data.

congestion avoidance:
the computer receiving the data notices that its buffers are filling and it sends a packet toward the sender,signaling the sender to stop transmitting.having processed
the data the receiver send a packet to the sender to start again transmitting data.

a window is the maximum amount of data that sender can send without getting an acknowledgment.if no acknowledgment received by the time the window is filled,the sender must wait for acknowledgment.
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Re: OSI "DATA LINK" AND "TRANSPORT" LAYER 12 years 4 months ago #3979

  • sahirh
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IFTY, Chris did explain correctly.. think about it.. ethernet at the data link layer does provide error correction and recovery.. thats why its CSMA/CD -- Carrier sense multiple access with collision detection.. when frames collide, ethernet has the back-off algorithm to attempt to recover from the error.

As far as flow control goes, it also has a rudimentary form of flow control -- the MTU.. or Max Transmission Unit.. which for ethernet is 1500 bytes.

You are right TCP has a much more advanced flow control system.. the sliding window as its called.. which provides dynamic flow control.

As far as sequencing is concerned.. I don't understand how the link layer can provide it.. but then again I've forgotten what method ethernet uses to ensure frames dont come out of sync...
Sahir Hidayatullah. Staff - Associate Editor & Security Advisor
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Re: OSI "DATA LINK" AND "TRANSPORT" LAYER 12 years 4 months ago #3990

  • IFTY
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data link layer provides error detection not correction :D
transport layer performs error correction :D
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