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Binary Synchronous Communication (BISYNC)

Last Updated : 03 May, 2023
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Binary Synchronous Communication (BISYNC) is basically a character or byte-oriented form of communication which means that the groups of bits or bytes are the important elements of transmission rather than a stream of bits. BISYNC was established or originated by IBM in 1960’s. It generally includes characters and procedures for simply controlling the establishment or development of a valid connection and transmission of data. It is a half-duplex link protocol that has replaced the Synchronous transmit-receive (STR) protocol usually used with second-generation computers. It is also known as Basic Mode Protocol that is required for transmission of bit-oriented data basically known as transparent mode. It is also being replaced largely by much more efficient protocol of IBM i.e. Synchronous Data Link Control (SDLC) that is usually under SAN (Systems Network Architecture). BSC or BISYNC also used to describe various types of data packets as given below :

  • ENQ (Enquiry)
  • ACK (Acknowledge)
  • NAK (Negative Acknowledge)
  • EOT (End of Transmission)

Types of Frames : There are basically two types of BSC or BISYNC frames as shown below : 1. Control Frame : These frames are basically required for exchange of information or data among devices to develop or obtain initial connection, provide flow and error control, and also disconnect the devices when the session is completely over. This frame does not contain any header.  
2. Data Frame : These frames are basically required to carry user data. It is also used to show the direction of data transfer. BISYNC allows for two types of block messages i.e. Single-block and multiple-block message. The only difference in both of these blocks is that data in single-block messages would be terminated with ETB (End of Transmission Block) character rather than an ETX (End of Text) character. Single Block Data Frame :  
Multiple Block Data Frame :  
Control Characters for BISYNC : Some of the Standard Control Protocols that are used in BISYNC frames are given below :

Character ASCII Code Function
ACK 0 DLE 0 Good even frame received or ready to receive
ACK 1 DLE 1 Good odd frame received
DLE DLE  Data Transparency Marker
EOT EOT Sender Terminating
ETX ETX End of Text in message
ETB ETB End of Transmission block: ACK required
SOH SOH Header Information Begins
STX STX Text Begins
SYN SYN Alert receiver to the incoming frame


  1. Reliability: BISYNC is a reliable protocol for data transfer because it uses synchronous communication. This means that the sender and receiver are synchronized, and data is transferred in a continuous stream, reducing the possibility of errors.
  2. Flexibility: BISYNC can be used with different types of data transmission lines, including leased lines and dial-up lines. This makes it a flexible protocol that can be used in various applications.
  3. Error Detection: BISYNC includes error detection mechanisms that help to identify and correct transmission errors. This is important in ensuring the accuracy of data transfer.
  4. Flow Control: BISYNC also includes flow control mechanisms that regulate the flow of data between the sender and receiver. This ensures that the receiver can process the data without getting overwhelmed by a flood of data.


  1. Complexity: BISYNC is a relatively complex protocol, which can make it difficult to implement and maintain. This complexity can also result in longer processing times, which can impact performance.
  2. Limited Speed: BISYNC was developed in the 1960s when data transfer speeds were much slower than today. As a result, it is not ideal for high-speed data transfer applications.
  3. Lack of Compatibility: BISYNC is not compatible with other communication protocols, which can limit its usefulness in some applications. This can result in the need for additional hardware and software to support data transfer between different systems.
  4. Cost: Implementing BISYNC can be expensive due to the need for specialized hardware and software. This can make it less attractive for smaller organizations or those with limited budgets.

Overall, BISYNC is a reliable and flexible communication protocol that can be used in a variety of applications. However, its complexity and limited speed may make it less attractive in certain situations, and its lack of compatibility with other protocols can limit its usefulness.

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