Q922
Reference Standard: ITU Q922
Frame structure: fields
They are 2 possible structures:
| bits 8 _ _ _ _ _ _ 1 |
field |
| octets 1-i |
Address ( 2, 3 or 4 octets) |
| octet i+1 |
Control (1rst octet) |
| octet i+2 |
Control (2nd octet) |
| octets (i+3)-n |
Optional information |
| bits 8 _ _ _ _ _ _ 1 |
field |
| octets 1-i |
Address ( 2, 3 or 4 octets) |
| octet i+1 |
Control |
| octets (i+2)-n |
Optional information |
Reference Standard: ITU Q922 CORE Aspects (Q922 Annex A)
In Q922-CORE (Q922 Annex A) for use with frame relaying bearer service, the CLLM
(Consolidated Link Layer Management) message is based on ISO 8885 definition of the use of
XID frames for the transport of functional information (such as congestion sent in
backward direction by the network).
| bits 8 _ _ _ _ _ _ 1 |
field |
| octets 1-i |
Address ( 2, 3 or 4 octets) |
| octet i+1 |
AFh = XID control field |
| octet i+2 |
82h = Format identifier |
| octet i+3 |
0Fh = Group identifier |
| octet i+4 |
Group length octet 1 |
| octet i+5 |
Group length octet 2 |
| octet i+6 |
00h = Parameter identifier |
| octet i+7 |
04h = Parameter length |
| octet i+8 |
105d =Parameter value (IA5 character ‘I’) |
| octet i+9 |
49d =Parameter value (IA5 character ‘1’) |
| octet i+10 |
50d =Parameter value (IA5 character ‘2’) |
| octet i+11 |
50d =Parameter value (IA5 character ‘2’) |
| octet i+12 |
02h = Parameter identifier |
| octet i+13 |
01h = Parameter length |
| octet i+14 |
Cause value |
| octet i+15 |
03h =Parameter value (DLCI identifier) |
| octet i+16 |
Parameter length |
| octets i+17-n |
DLCI value octets (1rst DLCI) |
|
DLCI value octets (ntht DLCI) |
Q922 frame structure: address field
They are 3 possible formats for the address field:
Address field 2 octets
| - Octet 1: |
|
| bits 8-3 |
upper DLCI: Data Link Connection Identifier |
| bit 2 |
C/R: Command/Response |
| bit 1 |
Address field extension (0) |
| - Octet 2: |
|
| bits 8-5 |
lower DLCI: Data Link Connection Identifier |
| bit 4 |
FECN: forward explicit congestion notification |
| bit 3 |
BECN: backward explicit congestion notification |
| bit 2 |
DE: discard eligibility indicator |
| bit 1 |
Address field extension (1) |
Address field 3 octets
| - Octet 1: |
|
| bits 8-3 |
upper DLCI: Data Link Connection Identifier |
| bit 2 |
C/R: Command/Response |
| bit 1 |
Address field extension (0) |
| - Octet 2: |
|
| bits 8-5 |
DLCI: Data Link Connection Identifier |
| bit 4 |
FECN: forward explicit congestion notification |
| bit 3 |
BECN: backward explicit congestion notification |
| bit 2 |
DE: discard eligibility indicator |
| bit 1 |
Address field extension (0) |
| - Octet 3: |
|
| bits 8-3 |
lower DLCI or DL-CORE control (depends on bit 2) |
| bit 2 |
D/C: DLCI (0) /DL-CORE (1) control indicator |
| bit 1 |
Address field extension (1) |
Address field 4 octets
| - Octet 1: |
|
| bits 8-3 |
upper DLCI: Data Link Connection Identifier |
| bit 2 |
C/R: Command/Response |
| bit 1 |
Address field extension (0) |
| - Octet 2: |
|
| bits 8-5 |
DLCI: Data Link Connection Identifier |
| bit 4 |
FECN: forward explicit congestion notification |
| bit 3 |
BECN: backward explicit congestion notification |
| bit 2 |
DE: discard eligibility indicator |
| bit 1 |
Address field extension (0) |
| - Octet 3: |
|
| bits 8-2 |
DLCI: Data Link Connection Identifier |
| bit 1 |
Address field extension (0) |
| - Octet 4: |
|
| bits 8-3 |
lower DLCI or DL-CORE control (depends on bit 2) |
| bit 2 |
D/C: DLCI (0) /DL-CORE (1) control indicator |
| bit 1 |
Address field extension (1) |
Q922 frame structure: control field
The control field contains a command or a response, and sequence numbers where
applicable.
In Q922-LAPF, three types of control field formats are used to perform:
- numbered information transfer with I format,
- numbered supervisory functions with S format,
- unnumbered control functions with U
format.
I format
| - 1rst octet: |
|
| bits 8-2 |
N(S) |
Transmitter send sequence number |
| bit 1 |
0 |
|
| bit 1 |
Command |
Response |
Frame designation |
| 0 |
I |
|
Information |
| - 2nd octet: |
|
| bits 8-2 |
N(R) |
Transmit. receive sequence number |
| bit 1 |
P/F |
Poll bit when issued as a command Final bit when issued as a response |
S format
| - 1rst octet: |
|
| bits 8-5 |
X X X X |
X: reserved and set to 0 |
| bits 4-3 |
S S |
S: supervisory function bit |
| bits 2-1 |
0 1 |
|
| bits 3-4 = SS |
Command |
Response |
Frame designation |
| 0 0 |
RR |
RR |
Receive Ready |
| 0 1 |
RNR |
RNR |
Receive Not Ready |
| 1 0 |
REJ |
REJ |
Reject |
The encoding values of the two S bits included in the first octet of the control field
are listed and ordered according to their hexadecimal code.
| - 2nd octet: |
|
| bits 8-2 |
N(R) |
Transmit. receive sequence number |
| bit 1 |
P/F |
Poll bit when issued as a command Final bit when issued as a response |
S format : codes
| octet code |
Type |
C/R |
Frame designation |
| x1 |
RR |
C/R |
Receive Ready |
| x9 |
RNR |
C/R |
Receive Not Ready |
| x5 |
REJ |
C/R |
Reject |
Note : Hexadecimal code corresponds to b8-b1.
U format
| - 1rst octet: |
|
| bits 8-6 |
M M M |
M: modifier function bit |
| bit 5 |
P/F |
Poll bit when issued as a command Final bit when issued as a response |
| bits 4-3 |
M M |
M: modifier function bit |
| bits 2-1 |
1 1 |
|
| bits M : 876 - 43 |
Command |
Response |
Frame designation |
| 0 1 1 - 1 1 |
SABME |
|
Set Asynchronous Balanced Mode Extended |
| 0 0 0 - 1 1 |
|
DM |
Disconnected Mode |
| 0 0 0 - 0 0 |
UI |
|
Unnumbered Information |
| 0 1 0 - 0 0 |
DISC |
|
Disconnect |
| 0 1 1 - 0 0 |
|
UA |
Unnumbered Acknowledgement |
| 1 0 0 - 0 1 |
|
FRMR |
Frame Reject |
| 1 0 1 - 1 1 |
|
XID |
Exchange Identification |
The encoding values of the five M bits included in the first octet of the control field
can also be listed and ordered according to their hexadecimal code or their type.
M format : codes
| P=0 |
P=1 |
Type |
C/R |
Frame designation |
| 03 |
13 |
UI |
C/R |
Unnumberd Information |
| 0F |
1F |
DM |
R |
Disconnected Mode |
| 43 |
53 |
DISC |
C |
Disconnect |
| 63 |
73 |
UA |
R |
Unnumbered Acknowledgement |
| 6F |
7F |
SABME |
C |
Set Asynch. Balanced Mode Extended |
| 87 |
97 |
FRMR |
R |
Frame Reject |
| AF |
BF |
XID |
R |
Exchange Identification |
Note : Hexadecimal code corresponds to b8-b1 with P/F bit=0 or P/F bit=1.
M format : types
| Type |
P=0 |
P=1 |
C/R |
Frame designation |
| DISC |
43 |
53 |
C |
Disconnect |
| DM |
0F |
1F |
R |
Disconnected Mode |
| FRMR |
87 |
97 |
R |
Frame Reject |
| SABME |
6F |
7F |
C |
Set Asynch. Balanced Mode Extended |
| UA |
63 |
73 |
R |
Unnumbered Acknowledgement |
| UI |
03 |
13 |
C/R |
Unnumbered Information |
| XID |
AF |
BF |
C/R |
Exchange Identification |
Note : Hexadecimal code corresponds to b8-b1 with P/F bit=0 or P/F bit=1.
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