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Framing in Data Link Layer
In the physical layer, data transmission involves synchronised transmission of bits from the source to the destination. The data link layer packs these bits into frames.
Data-link layer takes the packets from the Network Layer and encapsulates them into frames. If the frame size becomes too large, then the packet may be divided into small sized frames. Smaller sized frames makes flow control and error control more efficient.
Then, it sends each frame bit-by-bit on the hardware. At receiver's end, data link layer picks up signals from hardware and assembles them into frames.
Parts of a Frame
A frame has the following parts
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Frame Header It contains the source and the destination addresses of the frame.
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Payload field It contains the message to be delivered.
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Trailer It contains the error detection and error correction bits.
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Flag It marks the beginning and end of the frame.
Types of Framing
Framing can be of two types, fixed sized framing and variable sized framing.
Fixed-sized Framing
Here the size of the frame is fixed and so the frame length acts as delimiter of the frame. Consequently, it does not require additional boundary bits to identify the start and end of the frame.
Example ATM cells.
Variable Sized Framing
Here, the size of each frame to be transmitted may be different. So additional mechanisms are kept to mark the end of one frame and the beginning of the next frame.
It is used in local area networks.
Two ways to define frame delimiters in variable sized framing are
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Length Field Here, a length field is used that determines the size of the frame. It is used in Ethernet (IEEE 802.3).
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End Delimiter Here, a pattern is used as a delimiter to determine the size of frame. It is used in Token Rings. If the pattern occurs in the message, then two approaches are used to avoid the situation
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Byte Stuffing A byte is stuffed in the message to differentiate from the delimiter. This is also called character-oriented framing.
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Bit Stuffing A pattern of bits of arbitrary length is stuffed in the message to differentiate from the delimiter. This is also called bit oriented framing.
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