Link Layer's role in TCP

Posted on 2011-05-04
Last Modified: 2012-05-11
Hello Experts,

Can we rely on link layer flow & error control for reliable data transmission without using TCP? Kindly explain elaborately.

Many thanks.
Question by:mshankarak
    LVL 6

    Assisted Solution

    Transport Layer – helps to solve problems during data transmission and reception over the network.  When incorrect transmission occurs, this layer initiates the retransmission of data.
    Network Layer – handles addressing of messages and translation of logical addresses into physical addresses.  It defines the path to be followed by transmitted data.  This layer also manages network traffic problems such as congestion of data.
    Data Link Layer – organizes data from the Network Layer in the source computer into a logical structure known as a frame.  It then ensures error-free transmission of frames to the Data Link Layer in the destination computer.  Contains the Logical Link Control and Media Access Control sub layers.
    LLC – the upper of the two sublayers.  It manages data-link communication by defining multiple Service Access Points (SAPs).  Defined by standard 802.2.
    SAPs – are connection points that facilitate communication between each of the seven layers in the OSI protocol stack.  Computers use these SAPs to transfer information from the LLC sublayer to the upper OSI layers.
    MAC – This sublayer handles data transfer from the OSI Physical layer to the physical medium.  It is responsible for delivering error-free data between two computers in a network.  Defined by standards 802.3, 802.4, 802.5, and 802.12.

    So, your answer is No.  To summarize:
    The Transport Layer Protocols add error-handling information to data to ensure reliable transmission.
    The Network Layer Protocols provide addressing and routing of data.
    The Data Link Layer Protocols add error-checking information to data and handle retransmission requests.

    TCP is a transport Protocol.
    IP is a network protocol.
    LVL 6

    Assisted Solution

    As an example, look at the differences between TCP and UDP.  With UDP, you are not guaranteed that every packet will arrive, so by the time the file makes it to its destination and back up the OSI model to the Application layer, you are not sure that you got every piece of the file you were expected.  That does not mean that there was a error during the transmission, it just means that some packets were dropped by whatever router/device along its path.  Hence, you have two flavors, TCP and UDP.  

    Author Comment

    Though I have got most of what you wanted to express, could you kindly also state what kind of errors / problems could be faced if TCP wasn't used ?

    Many thanks
    LVL 6

    Accepted Solution

    In the example I gave above between TCP and UDP.  That is the difference.  TCP packets are gauranteed to arrive, be recombined in the proper order and delivered to the application layer or their is a transmission error.  UDP does not guarantee packet delivery like that.

    For instance, if you are copying a file, you want to make sure that you get every piece of the file for reassembly at the destination in the correct order, so you would use TCP.

    If you are playing a game like on xbox, the arrival of every packet is not required for it to function, hence UDP.  For instance, in an xbox first person shooter, each xbox would send information to the other about where players are located or the directino they are facing, but sometimes the screen is jumpy because UDP is being used and some of the packets are being dropped or recieved out of order with that information.  So the xbox just uses the current information it has, because it was autocorrecting for the lack of having all the data or recieving it out of sequence.

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