A frame is a unit of communication in the data link layer. 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 reassembles them into frames.
A frame is composed of four types of fields, namely kind, seq, 𝑎𝑐𝑘 and info. The first three fields contain control information about the frame and collectively form the frame header. Besides, the frame header contains the destination and the source addresses. The info field contains the data frame that may be either of fixed length or of variable length. The frame may have a trailer, also called frame check sequence (FCS). It has flags at both ends to mark its beginning and end.
The position of header in a frame is shown in the following diagram:
A frame header contains the destination address, the source address and three control fields: kind, seq, ack and 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.