Introduction to Tandem X.25 Capabilities

Frame Types
What is X.25?
065307 Tandem Computers Incorporated 1–33
U-Frames Unnumbered frames are used primarily to establish and disconnect the link . The
U-frames used by LAPB are:
SABM — Set asynchronous balanced mode (SABM). A SABM command is sent to
establish the link. In asynchronous balanced mode communications, either station
can establish the link.
UA — Unnumbered acknowledgement (UA). A UA is sent in response to a
SABM, indicating that the other end has received the SABM and is ready to accept
further communications. A UA is also sent in response to a DISC, indicating that
the other end has received the disconnect command.
DISC — Disconnect (DISC). A DISC command is sent to disconnect a link. In
asynchronous balanced mode, either station can disconnect the link. The station
receiving the DISC frame acknowledges its receipt by returning a UA response. In
the disconnect state, a station may initiate link setup. However, a station may not
transmit an I-frame while it is in the disconnect state.
DM — Disconnect mode (DM). A DM is sent in response to a SABM, indicating
that the other end is off-line and cannot accept communications.
FRMR — Frame reject response (FRMR). A FRMR is sent to report an error
condition that is not recoverable by retransmission of the identical frame (see
Figure 1-22). This response includes an information field (or “diagnostic field”),
which provides a reason for the response.