Framing in Data Communication

Framing is the process of dividing bit stream of information into logical groups of equal size. Framing is done both with asynchronous communication and synchronous communication. In asynchronous communication, the sender is responsible for breaking up the continues bit stream generated into frames and send these frame on the communication medium after regular gaps in time. In synchronous communication, the receiver is responsible for dividing the continues bit stream from communication channel into logical frames. The efficiency of communication has a great impact of the properties on ht e frames.
Generally, three main types of frames are discussed in this topic. These are :
Information Frames
Supervisory Frames or Control Frames
Unnumbered Frames
Information Frames :
Information frames for Information frames generally used to transfer user data and control information related to user dat. These types of frames act like an envelope for user data. The flag field is used for identifying beginning and end of the frame. It also serves as a synchronous pattern for the receiver. The address field of the frame contains the address of the destination computer on the network. Address field can be of one byte or of multiple bytes depending upon the network. Larger networks need longer addresses then smaller networks. The control field is the third field in HDLC frame and it contains error control and follow control information. The control field is dependent upon the frame type. If the first bit of the control field is zero than frame will be an information, if the first bit of control field is 1 and the second bit 0 then the frame is a supervisory.

Supervisory or Control Frames :
Supervisory or Control Frames are used for carrying control information. The control information id generally concerned with the flow of data from a particular sub network and information related to common errors occurring in a network. In case of synchronous communication, they are no gaps in data so special bit patterns are used to mark the beginning and ending of a logical frame. Supervisory frames are generally used for acknowledgement, flow control and error control is a network. A S-Frame does not have an information field then also they carry message to the receiving stations.These message are sent by using a particular type of S-Frame and the contest of the transmission. A two-bit code or inside each S-Frame determines its type.

Unnumbered Frames :
Unnumbered Frames are used to transfer section management and control information between connected devices. Unnumbered Frames do contain an information field but they do not contain user data. The main information carried by these frames is in the control field. Unnumbered Frames carry a to bit prefix before and three bit suffix. After the P/F bit together these two segments can be used to create 32 different types of Unnumbered Frames.

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