Format Message Type Control/Response
Information Transfer I = Information Control
Supervisory RR = Receive Ready Control/Response
Supervisory RNR = Receive Not Ready Control/Response
Supervisory REJ = Reject Control/Response
Unnumbered SABME = Set Asynchronous Balanced
Mode Extended
Control
Unnumbered DM = Disconnected Mode Response
Unnumbered UI = Unnumbered Information Control
Unnumbered DISC = Disconnect Control
Unnumbered UA = Unnumbered Acknowledgment Response
Unnumbered FRMR = Frame Reject Response
Unnumbered XID = Exchange Identifier Control/Response
Information This field carries the Q.931 protocol data. Figure 26.5 illustrates how it is laid
out. This is where the user data is carried.
The following information describes the field format as shown in Figure 26.5:
Protocol discriminator One octet. Identifies the layer 3 protocol.
Length One octet. Indicates the length of the call reference value (CRV).
Call reference value (CRV) One or two octets. This value is assigned to each call at the beginning,
is used to distinguish between other simultaneous calls, and is released after the call is torn down.
Message type One octet.
Mandatory and optional information elements (variable length) Options based on the message
type.
CRC Contains the cyclic redundancy check derived value from the Address, Control, and
Information fields. This is also known as the frame check sequence (FCS) field.
Q.921/Q.931 Information field format
Information Field
1
Protocol Discriminator
0
Call Reference Value (1 or 2 octets)
0
2 3 4 5
0 0 0 Length of CRV
6 7 8
Message Type (SETUP, CONNECT, etc.)
Mandatory and Optional Information Elements (Variable)