IEEE 802.2 LLC
Standard reference: ISO/IEC 8802-2
PDU structure: fields
| length |
field |
| 8 bits |
DSAP Address
Destination Service Access Point |
| 8 bits |
SSAP Address
Source Service Access Point |
| 8 or 16 bits |
Control |
| M*8 bits |
Information |
Address field
The address field identifies a PDU by its SAP as either source or destination
and by its mode as a command or a response.
| - 1rst octet: |
DSAP address |
| bit 1 |
I/G |
Individual/Group I=0 G=1 |
| bits 2-8 |
DDDDDDD |
DSAP Address |
| - 2nd octet: |
SSAP address |
| bit 1 |
C/R |
Command/Response C=0 R=1 |
| bits 2-8 |
S S S S S S S |
DSAP Address |
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.
| - 1rst octet: |
|
| bit 1 |
0 |
|
| bits 2-8 |
N(S) |
Transmitter send sequence number |
| bit 1 |
Command |
Response |
Frame designation |
| 0 |
I |
|
Information |
| - 2nd octet: |
|
| bit 1 |
P |
Poll bit (1 = Poll) |
| bits 2-8 |
N(R) |
Transmit. receive sequence number |
| - 1rst octet: |
|
| bits 1-2 |
1 0 |
|
| bits 3-4 |
S S |
S: supervisory function bit |
| bits 5-8 |
X X X X |
X: reserved and set to 0 |
| 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: |
|
| bit 1 |
P/F |
Poll bit when issued as a command Final bit when
issued as a response
(1 = Poll/Final)
|
| bits 2-8 |
N(R) |
Transmit. receive sequence number |
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.
| - 1rst octet: |
|
| bits 1-2 |
1 1 |
|
| bits 3-4 |
M M |
M: modifier function bit |
| bit 5 |
P/F |
Poll bit when issued as a command Final bit when
issued as a response
(1 = Poll/Final)
|
| bits 6-8 |
M M M |
M: modifier function bit |
| |
|
|
| bits M : 34 - 678 |
Command |
Response |
Frame designation |
| 0 0 - 0 0 0 |
UI |
UI |
Unnumberd Information |
| 1 1 - 1 1 0 |
SABME |
|
Set Asynchronous Balanced Mode |
| 0 0 - 0 1 0 |
DISC |
|
Disconnect |
| 0 0 - 1 1 0 |
|
UA |
Unnumbered Acknowledgement |
| 1 1 - 0 0 0 |
|
DM |
Disconnected Mode |
| 1 0 - 0 0 1 |
|
FRMR |
Frame Reject |
| 1 1 - 1 0 1 |
XID |
XID |
Exchange Identification |
| 0 0 - 1 1 1 |
TEST |
TEST |
Test |
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.
| 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 |
| E3 |
F3 |
TEST |
C/R |
Test |
Note : Hexadecimal code corresponds to b8-b1
with P/F bit=0 or P/F bit=1.
| 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 |
| TEST |
E3 |
F3 |
C/R |
Test |
| UA |
63 |
73 |
R |
Unnumbered Acknowledgement |
| UI |
03 |
13 |
C/R |
Unnumberd 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.
| 