HDLC is a protocol that defines rules for transmitting data between network points. It organizes data into frames and sends them to specified destinations, while managing transmission pace. HDLC supports three station types, three data transfer modes, and three frame types. It is commonly used at the OSI model's layer 2 and transmits synchronous data between serial links using frames with fields for flags, addresses, control data, and cyclic redundancy checks.

1. .
Present By
Showkot Usman

2. High-Level Data Link Control (HDLC)
• A high-level data link control defines rules for transmitting data between
network points.
• Data in an HDLC is organized into units called frames and is sent across
networks to specified destinations. HDLC also manages the pace at which
data is transmitted. HDLC is commonly used in the open systems
interconnection (OSI) model's layer 2.

3.  Defines three types of stations
 Primary
 Secondary and
 Combined
Defines three types of data transfer mode : -
1. Normal Response mode
2 . Asynchronous Response mode
3. Asynchronous Balanced mode

4. HDLC
The three modes of data transfer operations are
1.Normal Response Mode (NRM)
- Secondary station can send ONLY when the primary station instruct it
to do so
2. Asynchronous Response Mode (ARM)
- More independent secondary station.
- Can send data or control information without explicit permission to do
so (note that it is still can not send commands)

5. 3. Asynchronous Balanced Mode (ABM)
 Mainly used in point-to-point links, for communication between
combined stations.
 Either stations can send data, control information and commands

6. Three types of frames
Unnumbered
 information
 Supervisor

7. HDLC (High-level Data Link Control) is a group of protocols documented in ISO 3309
for transmitting synchronous data between serial links (Point-to-Point nodes).

8. The HDLC frame consists of Flag, Address, Control, Data, and CRC fields as shown. The bit length of
each field is given below

9. Any Questions ……???

10. Thank You