ISDN - Computer Busses - page 541

01111110 Address Control Info FCS 01111110

8/16 bits

M - modifier function bits
S - supervisory function bits

Information frame

0 N(S) P/F N(R)

1 0 S 0 0 0 0 P/F N(R) Supervisory frame

1 1 M P/F M

Unnumbered frame

Figure G.8 ISDN control field

Table G.3 Supervisory frame types and control field settings

Type Control field setting

10000000PRRRRRRR Receiver ready (RR)

10100000PRRRRRRR Receiver not ready (RNR)

Reject (REJ)

10010000PRRRRRRR

Unnumbered frames set up and clear connections between a node and the network. Table G.4
lists the unnumbered frame commands and Table G.5 lists the unnumbered frame responses.

Table G.4 Unnumbered frame commands and control field settings

Type Control field setting

Set asynchronous balance mode extended (SABME)

1111P110

1100F000 Unnumbered information (UI)

Disconnect mode (DISC) 1100P010

Table G.5 Unnumbered frame responses and control field settings

Type Control field setting

1111P110 Disconnect mode (DM)

Unnumbered acknowledgment (UA) 1100F000

Frame reject (FRMR)

1110P001

In ISDN all connected nodes and the network connection can send commands and re-
ceive responses. Figure G.9 shows a sample connection of an incoming call to an ISDN node
(address TEI_1 ). The SABME mode is set up initially using the SABME command
( U[SABME,TEI_1,P=1] ), followed by an acknowledgement from the ISDN node
( U[UA,TEI_1,F=1] ). At any time, either the network or the node can disconnect the con-
nection. In this case the ISDN node disconnects the connection with the command
U[DISC,TEI_1,P=1] . The network connection acknowledges this with an unnumbered ac-
knowledgement ( U[UA,TEI_1,F=1] ).