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Explain Operating System Interface OSI Layer 2 – Data Link Layer

About Data Link Layer – OSI Model

– At layer 2 of the OSI model, we have the data link layer.
– The basic function of this layer is to provide a means for transferring the data between the elements of the network that is both functional and procedural.
– It’s another major function is the detection and correction of errors that might take place in the preceding layer i.e., the physical layer.
– Originally this intended for the following two media:
Point – to – point and
Point – to – multipoint

– These are characteristics of WAM (wide area media).
– A standard that provides a high speed LAN over the existing cables is the ITU – T and it consists of a complete data link layer that is capable of both flow control and error correction.
– It achieves by the means of a ‘selective repeat sliding window protocol’.
– The bits are taken from the physical layer and arranged in to frames (i.e., the logical sequences) by LAN and WAN services.
– However, it is not necessary that all the bits should fit in the frames.
– So, we can sum up the functions of data link layer as follows:
a) Framing
b) Flow control
c) Access control
d) Physical addressing
e) Error control
f) MAC or media access control

– Data transfer in the physical link is achieved through the data link layer only.
– This transfer however, might be reliable or un-reliable depending on how the frames are received and accepted by the data link protocols.
– There are other data link protocols that might not have any type of check-sum for checking up on the errors in transmission.
– Then what happens in such cases?
– The services such as flow control, re-transmission, error checking and acknowledgements are provided by the higher level protocols.
– The response to the requests from successive layers in data link layer is achieved through the semantics of OSI architecture.
– Data link layer provides two modes of communication namely:
a) The connection oriented communication and
b) The connection-less communication

Sub-Layers of Data Link Layer

There are two sub layers in this data link layer:
1. Logical link control sub – layer:
– This one is the uppermost sub – layer.
– Multiplex protocols are implemented over the OSI layer 2 and sometimes provide acknowledgement, flow control and the error notification.
– The data link is addressed and controlled by this sub layer only.
– Further, the mechanisms to be used for controlling the exchange of the data between the sender and receiver machines and for addressing of the stations.

2. Media access control sub – layer:
– This is the sub – layer that has the authority of determining for whom to grant the access based up on certain conditions.
– This layer has a more frame like structure and consists of all the MAC addresses.
– Media access control is available in two forms namely – Distributed and Centralized.
– Frame synchronization i.e., the process of determining the beginning and the end of the data packets is another major purpose of this layer.
– It offers 4 means of synchronization as mentioned below:
a) The time based approach: In this approach, a significant amount of time is put between the frames. Here the existing gaps may get lost or new ones might be added because of some external factors.

b) Character counting: the number of the characters is noted in the header of the frame. But this method is prone to be disturbed if any fault gets in the way. It finds difficulty in keeping pace with the synchronization.

c) Byte stuffing: in this approach a special byte sequence precedes the frame and is succeeded by some other sequence. Thus the start and end marks can be recognized by the receiver.

d) Bit stuffing: this is somewhat similar to the byte stuffing approach but makes use of flags instead of using special byte sequences. This approach is quite easy for maintaining the synchronization.

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