HDLC

• High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO).

• Most important data link control protocol.

From where it started???

• Synchronous Data Link Control (SDLC) by IBM

• Advanced Data Communication Control Procedure (ADCCP) by ANSI

Basic Characteristics– Primary station • Responsible for controlling the operation of the

link. • Handles error recovery• Frames issued by the primary station are called

commands.

– Secondary station• Operates under the control of the primary station. • Frames issued by a secondary station are called

responses.• The primary station maintains a separate logical

link with each secondary station.

– Combined station• Combine the features of both primary and

secondary station.

Two Link Configurations

Primary

Secondary Secondary

Commands

Responses

Combined Combined

commands/Responses

Unbalanced Mode

Balanced mode

Three Data Transfer Modes– Normal Response Mode (NRM)

• Used with an unbalanced configuration.• Primary may initiate data transfer to secondary.• But secondary may only transmit data in response to

a command from a primary.• NRM is used on multidrop lines.

– Asynchronous Balanced Mode (ABM) • Used with balanced configuration.• Either combined station may initiate transmission

without receiving permission from the other combined station.

• Widely used, efficient used to connect from one device from one another.

– Asynchronous Response Mode (ARM) • Used with an unbalanced configuration.• The secondary may initiate transmission without

permission of the primary.

Frame Structure of HDLC

• FLAG FIELD:-Frame contains unique pattern at both ends.-No assurance of occurrence of same bit pattern somewhere inside the frame.-Bit Stuffing is used.

Original Pattern:

111111111111011111101111110After bit-stuffing:

1111101111101101111101011111010• ADDRESS FIELD:

-Identify the secondary station that transmitted or is to received the frame.

• CONTROL FIELD:The first one or two bits are used to identify the type of frame. Defines three types of frames:-

-INFORMATION FRAME(I-FRAME)-Carry data to be transmitted for the user.-Flow and error control data, using the ARQ

mechanism, are piggybacked on an I-Frame.-SUPERVISORY FRAME(S-FRAME)

-Used for flow and error control whenever piggybacking is impossible-UNNNUMBERED FRAME(U-FRAME)

-Used for link management, and can also be used to transfer user data.

-They exchange session management and control information between connected devices.

• All of the control field contain poll/final (P/F) bit.

• It is called Poll when set by the primary station to obtain a response from a secondary station.

• Final when set by the secondary station to indicate a response or the end of transmission. In all other cases, the bit is clear.

There are four different supervisory frames:• Receive Ready (RR)

Bit Value = 00Indicate that the sender is ready to receive more data.

• Receive Not Ready (RNR)Bit value = 01 Acknowledge some packets and request no more be sent

until further notice.• Reject (REJ)

Bit value = 10 Requests immediate retransmission starting with N(R).

• Selective Reject (SREJ)Bit value = 11Requests retransmission of only the frame N(R).

• FRAME CHECK SEQUENCE FIELD:-Error detecting code calculated from the remaining bits of the frame and exclusive of flags.

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