HDLC is a bit-oriented synchronous data link layer protocol that can provide the error-free transmission of data to specific destinations and control the speed at which information is sent. It supports both connection-oriented and connectionless services. It is a set of protocols that represents rules for sending data between network points. Data in HDLC is arranged into units known as frames and is transmitted across networks to specified destinations.
HDLC frames are sent over synchronous links or asynchronous links, which do not indicate the beginning and end of frames. This is completed using a frame delimiter or flag, which includes a specific sequence of bits that are not visible within a frame.
There are three types of HDLC frames which are as follows −
Information Frames (IF)
Supervisory Frames (SF)
Un-numbered Frames (UF)
Given below is the HDLC Frame Format
Flag Address Control User Information FCS Flag
Flag Address Control FCS Flag
Flag Address Control Management Information FCS Flag
I-Frames can transfer user data and control data relating to user data. S-frames can send control data, and frequently, data link layer architecture and error controls. U-frames are constrained for framework management.
Every frame on the connection must begin and end with a flag sequence Field (F). Stations linked to the data link must always listen for a flag sequence is an octet treating like 01111110. Flags are continually transmitted onto the connection among frames to maintain the connection active.
It can identify the primary or secondary stations’ participation in the frame connection or response.
HDLC uses the control field (C) to determine how to control the transmission procedure. The field contains the commands, responses and sequence numbers used to provide the link’s data stream accountability.
There are three control field formats are as follows −
Information transfer formatThe frame can send the user data among two devices.
Supervisory FormatThe control field implements control functions including acknowledgment of frames, request for re-transmission, and requests for definite suspension of frames being transmitted.
Unnumbered FormatThis control field format can also be used for the control objective. It can execute link initialization, link disconnection, and some link control services.
The control field’s fifth-bit position is referred to as the poll/final bit, or p/f bit. It can be recognized when it is transmitted to 1. If it is set, more, it is ignored. The secondary station responds to the p bit by sending a data or status frame to the primary station with the p/f bit set to f = 1.
This field is not frequently in an HDLC frame. It is only actual, when the data transfer format is being utilized in the control field.
FCS is a HDLCs error detection field. This field contains a 16 bit or 32-bit cyclic redundancy check bit.