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unnumbered frame (U-frame)

U-frames are Unnumbered Frames, non-numbered data frames. They are protocol elements that are transmitted in the control field of the HDLC protocol and are used for connection setup and disconnection at the link layer.

Structure of HDLC blocks

Structure of HDLC blocks

Since Unnumbered Frames have no reference to the I-Frame, the Information Frame, they also have no transmit and receive sequence counters.

U blocks perform control functions outside the control of data exchange between communicating systems. Commands and messages of this block group are used to establish and terminate connections between corresponding systems.

The group of U blocks includes two commands: Set Asynchronous Balanced Mode( SABM) and Disconnect( DISC), and three messages: Unnumbered Acknowledgement( UA), Disconnect Mode( DM) and Frame Reject( FRMR).

Englisch: unnumbered frame - U-frame
Updated at: 04.03.2021
#Words: 119
Links: data, protocol, field, high level data link control (HDLC), connection setup
Translations: DE

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