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. At receiver’ end, data link layer picks up signals from hardware and assembles them into frames.
A frame has the following parts:
Frame Header: It contains the source and the destination addresses of the frame and the control bytes.
Payload field: It contains the message to be delivered.
Trailer: It contains the error detection and error correction bits. It is also called Frame Check Sequence (FCS).
Flag: Two flag at the two ends mark the beginning and the end of the frame.
A frame header contains the destination address, the source address and three control fields kind, seq, and ack serving the following purposes:
Kind: This field states whether the frame is a data frame or it is used for control functions like error and flow control or link management etc.
seq: This contains the sequence number of the frame for rearrangement of out – of – sequence frames and sending acknowledgements by the receiver.
ack: This contains the acknowledgement number of some frame, particularly when piggybacking is used.
Frames can be of two types, fixed sized frames and variable sized frames.
Fixed-sized Frames: The size of the frame is fixed and so the frame length acts as delimiter of the frame. It does not require delimiting bits to identify the start and end of the frame. Example : ATM cells.
Variable – Sized Frames: 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.