X25-LAPB
X25 LAPB frame structure: fields
They are 2 possible structures:
| bits 1 _ _ _ _ _ _ 8 |
field |
| octet 1 |
Address |
| octet 2 |
Control (1rst octet) |
| octet 3 |
Control (2nd octet) |
| octets 4-n |
Optional information |
|
| bits 1 _ _ _ _ _ _ 8 |
field |
| octet 1 |
Address |
| octet 2 |
Control |
| octets 3-n |
Optional information |
|
X25 LAPB frame structure: address field
The address field identifies a frame as either a command or a response.
A command frame contains the address of a DCE or DTE to which the command
is being sent. A response frame contains the address of the DCE or DTE
sending the frame:
| Address field |
1 _ _ _ _ _ _ 8 bits |
| From DCE to DTE |
From DTE to DCE |
|
| Single link operation |
|
| Responses |
Commands |
1 0 0 0 0 0 0 0 |
| Commands |
Responses |
1 1 0 0 0 0 0 0 |
| Multilink operation |
|
| Responses |
Commands |
1 1 1 0 0 0 0 0 |
| Commands |
Responses |
1 1 1 1 0 0 0 0 |
|
X25 LAPB 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
The control field Information format is either for basic (modulo 8) operation or for
extended (modulo 128) operation:
Basic operation: I format
| - 1st octet: |
|
| bit 1 |
0 |
|
| bits 2-4 |
N(S) |
Transmitter send sequence number |
| bit 5 |
P |
Poll bit (1 = Poll) |
| bits 6-8 |
N(R) |
Transmit. receive sequence number |
| bit 1 |
Command |
Response |
Frame designation |
| 0 |
I |
|
Information |
|
Extended operation: I format
| - 1st 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 |
|
S format
The control field Supervisory format is either for basic (modulo 8) operation or for
extended (modulo 128) operation:
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.
Basic operation: S format
| - 1st octet: |
|
| bits 1-2 |
1 0 |
|
| bits 3-4 |
S S |
S: supervisory 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 |
N(R) |
Transmit. receive sequence number |
| 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 |
|
Extended operation: S format
| - 1st 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 |
|
| - 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.
U format
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).
| - 1st 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 |
| 1 1 - 1 0 0 |
SABM |
|
Set Asynchronous Balanced Mode |
| 0 0 - 0 1 0 |
DISC |
|
Disconnect |
| 1 1 - 0 0 0 |
|
DM |
Disconnected Mode |
| 0 0 - 1 1 0 |
|
UA |
Unnumbered Acknowledgement |
| 1 0 - 0 0 1 |
|
FRMR |
Frame Reject |
|
M format : codes
| P=0 |
P=1 |
Type |
C/R |
Frame designation |
| 0F |
1F |
DM |
R |
Disconnected Mode |
| 2F |
3F |
SABM |
C |
Set Asynch. Balanced Mode Extended |
| 43 |
53 |
DISC |
C |
Disconnect |
| 63 |
73 |
UA |
R |
Unnumbered Acknowledgement |
| 87 |
97 |
FRMR |
R |
Frame Reject |
|
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 |
| SABM |
2F |
3F |
C |
Set Asynch. Balanced Mode Extended |
| UA |
63 |
73 |
R |
Unnumbered Acknowledgement |
|
Note : Hexadecimal code corresponds to b8-b1 with P/F bit=0
or P/F bit=1.
X25 LAPB Parameters: numbers list
| N2 |
|
| Description |
Maximum number of attempts to complete the successful transmission
of a frame |
| Value |
10 |
| N1 |
|
| Description |
Maximum number of bits in an I frame |
| Value |
32824 |
| k |
|
| Description |
Maximum number of outstanding I frames (window)
Values for DTE and DCE have to be equal
|
| Value |
7 |
|
X25 LAPB Parameters: time-limit list
| T1 |
DCE and DTE side |
| Description |
Maximum time authorized for receiving acknowledgement
of a frame; time at the end of which retransmission of a frame
may be initiated |
| Value |
DTE Timer T1 and DCE Timer T1 may be different
800 ms
|
| T2 |
DCE and DTE side |
| Description |
Amount of time available at the DCE or DTE before
the acknowledging frame must be initiated in order to ensure
its receipt by the DTE or DCE, respectively, prior to Timer
T1 running out at the DTE or DCE |
| Value |
DTE Timer T1 and DCE Timer T1 may be different
50 ms (T2 < T1)
|
| T3 |
DCE side |
| Description |
Maximum time authorized, at the end of which an
indication of an observed excessively long idle channel state
condition is passed to the Packet layer.
The expiration of T3 results in a data link set-up before
normal data link operation can resume.
|
| Value |
3000 ms (T3 > T1) |
|
| 
