DataLink Protocols | Asynchronous And synchronous

DataLink Protocols

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A protocol in data communication is the set of specifications used to implement one or more layers of the OSI model.

Datalink protocols are sets of specifications used to implement the info datalink layer to this end, they contain rules for line discipline, flow control, and error handling, among others.

Datalink protocols can be divided into two subgroups:-

  • Asynchronous protocols
  • synchronous protocols. 

Asynchronous protocols treat each character during a bitstream independently. synchronous protocols take the entire bitstream and chop it into characters of equal size.

Asynchronous Protocols

Many synchronous datalink protocols have been developed over the last several decades, one of which is these protocols are employed mainly in modems.

Due to its inherent slowness stemming from the required additions of start and stop bits and extended space between frames, asynchronous transmission at this level is being replaced by higher speed synchronous mechanisms.

Data Link Protocols

Asynchronous protocols aren’t complex and are inexpensive to implement. asynchronous transmission a data unit is transmitted with no timing coordination between the sender and receiver.

A receiver doesn’t get to know exactly when a knowledge unit is shipped it only must recognize the beginning and the end of the unit. This is accomplished by using extra bits start and stop bits to frame into the data unit.

There are 5 types of Asynchronous protocols in Datalink protocols, full information about this is given below:-


XMODEM in Datalink protocols, In 1979 ward Christiansen designed a file transfer protocol for telephonic-line communication between PCs. This protocol, now known as XMODEM, may be a half-duplex stop-and-wait ARQ protocol.

The first field is a one-byte start of the header (SHO). The second field is a two-byte header. The first header byte, the sequence number, carries the frame number.

The second harder byte is used to check the validity of the sequence number. The fixed data field holds 128 bytes of data (binary, ASCII, boolean, text, etc.). The last field, CRC, checks for errors within the data field only.

In this protocol, transmission begins with the sending of a NAK frame from the receiver to the sender. Each time the sender sends a frame, it waits for an acknowledgment (ACK) before the next frame can be sent. If instead of a NAK is received, the previously sent frame is sent again.

A frame can also be resent i\if a response is not received by the sender after a specified amount of time. Besides a NAK or an ACK, the sender can receive a cancel single (CAN), which aborts the transmission.


YMODEM is a type of Datalink protocol that is  similar to XMODEM, with the subsequent major differences:

  • The data unit is 1024 bytes.
  • Two CANs are sent to abort a transmission.
  • ITU-T CRC-16 is used for error checking.
  • Multiple files can be sent simultaneously.

ZMODEMis a newer protocol combining features of both XMODEM and YMODEM.


Blocked asynchronous transmission (BLAST) is more powerful than XMODEM. It is a full-duplex with sliding window flow control. It allows the transmission of data and binary files.


KERMIT is another type of datalink protocol, which is designed at Columbia University, is currently the foremost widely used asynchronous protocol.

This file transfer protocol is similar in operation to XMODEM, with the sender waiting for a NAK before it starts transmission. Kermit allows the transmission of control characters as text using two steps.

First, the control character, which is used as text, is transformed into a printable character by adding a fixed number to its ASCII code representation.

Second, the # character is added to the front of the transformed character.

In this way, a control character is used as the text is sent as two characters. When the receiver encounters a # character, it knows that this must be dropped which the next character is a control character. If the sender wants to send a # character, it’ll send two of them.

Synchronous Protocols

Now you are going to learn about synchronous protocols in Datalink protocols, and about their types.

The speed of synchronous transmission makes it the better choice, over the synchronous transmission, For LAN, MAN, and WAN technology. Protocols governing synchronous transmission are often divided into two classes:

  • character-oriented protocols
  • bit-oriented protocols
DataLink Protocols
Character-Oriented Protocols:-

character-oriented protocols (also called byte-oriented protocols) interpret a transmission frame or packet as a succession of characters, each usually composed of one byte (eight bits). All control information is within sort of an existing character encoding system.

Bit-Oriented Protocols:-

Bit-oriented protocols interpret a transfer frame or packet as a succession of individual bits, made meaningful by their placement in the frame and by their juxtaposition with other bits.

Control information during a bit-oriented protocol is often one or multiple bits depending on the information embodied in the pattern.

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