Computer Communication And Networking slides (HDLC) B.E Electronic Engineering By Raveena Kumari(14ES28)

2. Definition
• HDLC ( high-level data link control)
• HDLC Is a group of protocols and rules for
transmitting data between network points
(called nodes).
• Developed by ISO.

3. description
• It works of layer 2, (data link) that encapsulate
layer 3 frame
• Data is organized in unit called frame and sent
across a network to the destination that verify
its successful arrival.
• It also manage the flow at which data is sent
• HDLC use synchronous transmission.

4. DATA LINK CONFIGURATION
• UNBALANCED: consist of one primary and
one(point to point) or more(multipoint)
secondary stations and support both full
duplex and half duplex transmission.
• BALANCED: consist of two combined station
supports both full duplex and half duplex
communication

5. Data transfer modes/Response modes
• NORMAL RESPOND MODE (NRM): Use unbalanced configuration.
The primary may initiate data transfer to secondary, but the
secondary may only transmit data in response to the command
from the primary.
• ASYNCHRONOUS RESPOND MODE(ARM): Use with an unbalanced
mode. Secondary may initiate transmission without permission of
primary.
• ASYNCHRONOUS BALANCE MODE(ABM): Used with balanced
configuration. Either combined station may initiate the data
without any permission of other combined data.(can only send
information not command and respond to command)
•

6. STATIONS TYPES
• PRIMARY: responsibility for controlling the operation of the
link,
• handles error recovery
• Frame issued by the primary called command
• SECONDARY: operate under the control of primary
• Frame issued by the secondary called response
• Primary station maintain the logical link with secondary.
•
• COMBINED: Each station can act as both primary and
secondary

7. FRAMING MODE
• There are three different classes of frames used in HDLC
• (1) I-frame (information frames): which carry actual
information. Such frame can also send ACK.
•
• (2) S-frame ( Supervisory frames): used when sender and
receiver want to sent ACK only, contain no information. It
can also be used for error and flow control.
•
• (3) U-frame (Unnumbered frames): use to link setup,
responsible to maintain terms and argument between the
sender and receiver.

8. FRAME STRUCTUR
• I-FRAME:
• U-FRAME:
• S-FRAME:
•

9. FLAG FIELD
• Indicate start and ending frame (closing and
opening flag)
• It indicate the start and ending of frame with a
unique pattern 011111110 which could be
found inside the frame
• To destruct the inner structure a method is
used called bit stuffing
• BIT STUFFING: The sender will insert 0 after
occurrence of 5

10. ADDRESS FIELD
• The address field identify the secondary station
that transmit or receive the frame
• Can be of 1 byte or multiple byte
1 is used to indicate the end of byte
0 is sued to indicate the continuity of byte
ADDRESS of 1 byte 1
1st byte 0 2nd byte 0 3rd byte 1

11. INFORMATION FIELD
• Present only in I-frame and some in U-
frame(management information does nit
contain DATA)
• MANAGEMENT INFORMATION: to maintain
the link, response mode, terms and arguments
between sender and receiver.

12. FCS FIELD
• The frame check sequence (FCS) is an error
detecting code calculated from remaining bit
of frame.
• Exclusive of flag the normal code is 16 bit

13. CONTROL FIELD
• The control field is different in different frame
as shown below:
• I-FRAME:
• S-FRAME:
• U-FRAME:
0 SEQUENCING P/F ACKNOWLEDGMENT
1 0 SUPERVISORY FUNCTION BITS P/F ACKNOWLEDGMENT
1 1 UNNUMBERED FUNCTION P/F UNNUMBERED FUNCTION

14. • POLL/FINAL (P/F): tells the direction of frame
• Poll => primary to secondary
• Final => secondary to primary
• Supervisory function: 2 bit code
• 00 = The Receive-ready
• 01 = Receive-not-ready
• 10 = REJ initiates
• 11 = selective reject
• Unnumbered function bit: usually of 32 bits sends large
number of information.
• Sequencing : the informative data that is to be send