ITU-Q921
Q921 frame structure: fields
|
|
Field (modulo128) |
| octet 1 |
Address (High order octet) |
|
| octet 2 |
Address (Low order octet) |
|
| octet 3 |
Control (1rst octet) |
|
| octet 4 |
Control (2nd octet) |
|
| octets 5-n |
Optional information |
|
|
|
Field (modulo8) |
| octet 1 |
Address (High order octet) |
|
| octet 2 |
Address (Low order octet) |
|
| octet 3 |
Control (1rst octet) |
|
| octets 4-n |
Optional information |
|
Q921 frame structure: address field
Octet 1
|
Designation |
Value |
|
|
SAPI |
Call control procedure |
0 |
Reserved to packet mode communications
using Q931 call control procedure |
1 |
Packet communications conforming
to X25 level 3 procedures |
16 |
| Layer 2 management procedures |
63 |
| Reserved for future standardization |
All others |
|
|
C/R: Command/Response |
From network to user |
From user to network |
|
| Response |
Command |
0 |
| Command |
Response |
1 |
|
|
Address field extension (continue= 0) |
0 |
Octet 2
|
Designation |
Value |
|
|
TEI |
Non automatic TEI assignment user equipment |
0-63 |
| Automatic TEI assignment user equipment |
64-126 |
| Group TEI (broadcast data link connection) |
127 |
|
|
Address field extension (end= 1) |
1 |
Q921 frame structure: control field
The control field contains a command or a response, and sequence numbers
where applicable.
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
| Octet |
Designation |
Value |
Octet 3
|
|
N(S)Transmitter
send sequence number0 |
|
|
Set to 0
= Frame type |
| |
Command |
Response |
Frame designation |
| 0 |
I |
|
Information |
|
Octet 4
|
|
N(R)Transmitter
receive sequence number0 |
|
|
P Poll
bit |
S format
| Octet |
Designation |
Value |
Octet 3
|
|
X X X X reserved
and set to 0 |
|
|
SS supervisory
function bits= Frame type |
| |
Command |
Response |
Frame designation |
| 00 |
RR |
RR |
Receive Ready |
| 01 |
RNR |
RNR |
Receive Not Ready |
| 10 |
REJ |
REJ |
Reject |
|
|
|
Set to 01 |
Octet 4
|
|
N(R)
Transmitter receive sequence number0 |
|
|
P/F
Poll/Final bit |
The encoding values of the two S bits included in the first octet of the control field
can also be listed and ordered according to their hexadecimal code.
| |
Command |
Response |
Frame designation |
| x1 |
RR |
RR |
Receive Ready |
| x9 |
RNR |
RNR |
Receive Not Ready |
| x5 |
REJ |
REJ |
Reject |
U format
| Octet |
Designation |
Value |
Octet 3
|
|
P/F
Poll/Final bit |
|
|
M M M M Mmodifier function bits =
Frame type |
| |
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 |
|
|
|
Set to 11 |
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
(mnemonic).
| 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 |
C/R |
Exchange Identification |
| 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 |
Unnumberd Information |
| XID |
AF |
BF |
C/R |
Exchange Identification |
Q921 Parameters
Numbers
|
Designation |
Value |
N200
|
Maximum number of frame re-transmissions |
3 |
N201
|
Maximum length of the I frames (in octets) |
260 |
N202
|
Maximum number of transmissions of an
identity request message |
3 |
k
|
Maximum number of Outstanding I frames (window) |
BRI: SAPI 0 |
1 |
| BRI: SAPI 16 |
3 |
| PRI: SAPI 0 |
7 |
| PRI: SAPI 16 |
7 |
Q921 Time-limits
Time-limit
|
Designation |
Value |
T200
|
Maximum time authorized for receiving acknowledgement
of a frame |
1 s |
T201
|
Maximum time between the identity check messages |
1 s |
T202
|
Maximum time between the identity request messages |
2 s |
T203
|
Maximum time allowed without frames being exchanged |
10 s |
| 