High-Level Data Link Control (HDLC) generally provides flexibility to simply support all options that are possible in various data transfer modes and configurations. To provide flexibility, HDLC basically uses and explains three different types of frames. Type of frame is basically determined by control field of frame. Each type of frame generally serves as an envelope for transmission of various types of messages. These three different classes of frames used in HDLC are given below.
1. I-frame : I-frame stands for Information frames. This frame is generally used for transporting user data from network layer. These frames actually carry actual data or information of upper layer and some control information. This frame carries data along with both send sequence number and an acknowledgment number. It can also be used to piggyback acknowledgement information in case of ABM (Asynchronous Balanced Mode). The first bit of this frame of control field is 0.
2. S-frame : S-frame stands for Supervisory frames. These frames are basically required and essential for error control and flow control. They also provide control information. It contains or includes only an Acknowledgment number. First two bit of this frame of control field is 10. S-frame does not have any information fields. This frame contains send and receive sequence numbers.
- Receive Ready (RR)
- Receive Not Ready (RNR)
- Reject on frame N(R) (REJ)
- Selective reject on N(R) (SREJ), etc.
3. U-frame : U-frame stands for Unnumbered frames. These frames are also required in various functions like link setup and disconnections. These frames basically support control purposes and are not sequenced. First, two-bit of this frame of control field is 11. Some U-frame contains an information field depending on the type. These frames are also used for different miscellaneous purposes along with link management. U-frame is required for managing link itself. This frame does not include any type of acknowledgment information i.e. in turn it includes or is contained in sequence number. These frames are generally reserved for system management.
- Set Normal Response Mode (SNRM)
- Set Asynchronous Response Mode (SARM)
- Set Asynchronous Balanced Mode (SABM)
- Unnumbered Polling (UP)
- Unnumbered Acknowledge (UA), etc.
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