Envoy ACP/XF Application Programming Manual

Features Common to Bit-Synchronous Protocols
EnvoyACP/XF Application Programming Manual–132179
B-8
Control Field
U-Frame Control Field
A U-frame provides additional link-control functions. The control field of a U-frame
contains the following parameters:
•
Control code (bit 0 = 1, bit 1 = 1)
•
Command/response code (unnumbered message code)
•
Poll/final bit
In a U-frame, the control field is similar to that of an S-frame, except that there is no Nr
parameter. Thus, the U-frame is the only frame type that has neither the Ns nor the Nr
parameter, which is why it is called an “unnumbered frame.”
The control code that identifies the frame type is two bits, which is the same length as
the control code used by the S-frames. However, the U-frame command/response codes
differ from the command/response codes used by the S-frames. U-frame
command/response codes control mode setting, information transfer, and recovery.
U-frame command/response codes also control some miscellaneous and nonreserved
command/responses. The U-frame command/response codes vary somewhat from one
protocol to another. For example, to set the mode of operation, HDLC uses the U-frame
command code, Set Asynchronous Balanced Mode (SABM); whereas SDLC uses the U-
frame command code, Set Normal Response Mode (SNRM). All four protocols have a
DISC command to disconnect the line.
In general, the U-frame command codes differ from the U-frame response codes.
Table B-2
lists the possible commands and responses. See “U-Frame Commands” and
“U-Frame Responses” later in this section for further information on these codes.
Table B-2. U-Frame Command/Response Codes
U-Frame Category Command
Mode-setting commands Set Normal Response Mode (SNRM)
Set Asynchronous Balanced Mode (SABM)
Set Normal Response Mode Extended (SNRME)
Set Asynchronous Balanced Mode Extended (SABME)
Set Initialization Mode (SIM)
Disconnect (DISC)
Mode-setting responses Unnumbered acknowledgment (UA)
Disconnected mode (DM)
Request initialization mode (RIM)
Information-transfer commands Unnumbered information (UI)
Unnumbered poll (UP)
Information-transfer responses Unnumbered information (UI)
Recovery commands Reset (RSET)