SABM Set Asynchronous Balanced Mode

SABM (Set Asynchronous Balanced Mode) is a control command used in data communication protocols, particularly in the context of the X.25 protocol suite. It is primarily employed in establishing and terminating logical connections between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) in packet-switched networks.

SABM is part of the Link Access Procedure, Balanced (LAPB) protocol, which is a data link layer protocol used for reliable transmission of data over a variety of network media, including X.25 networks. LAPB ensures that data is transmitted reliably between DTE and DCE by providing flow control, error detection, and retransmission capabilities.

When a DTE wishes to establish a connection with a DCE, it sends an SABM frame to the DCE. The SABM frame contains specific control information, including the address of the DTE and DCE, as well as the command code indicating that the DTE wants to enter Asynchronous Balanced Mode. The Asynchronous Balanced Mode is a state of operation where the DTE and DCE can exchange data frames in a balanced manner, with acknowledgments and retransmissions as needed.

Upon receiving an SABM frame, the DCE acknowledges the request by sending a response frame called an Unnumbered Acknowledgment (UA). The UA frame confirms that the DCE has accepted the request and is ready to exchange data frames in the Asynchronous Balanced Mode.

The SABM command is also used during the termination of a connection. When a DTE wants to terminate the logical connection, it sends an SABM frame with a "DISC" (Disconnect) command code. This indicates to the DCE that the DTE wishes to end the connection. The DCE responds with a UA frame to acknowledge the termination request.

In summary, SABM (Set Asynchronous Balanced Mode) is a control command used in the LAPB protocol to establish and terminate logical connections between DTE and DCE in packet-switched networks. It plays a crucial role in initiating the Asynchronous Balanced Mode, where data can be reliably exchanged between the communicating devices.