ISO 4335 - HDLC

Reference standard: ISO 3309 and ISO 4335

HDLC frame structure: fields

length field
1or n octets Address
1 or 2 octets Control
n octets Optional information
2 octets Frame Control Sequence

HDLC frame structure: address field

Reference standard: ISO 3309

The address field identifies a frame as either a command or a response. A command frame contains the address of a station to which the command is being sent. A response frame contains the address of the station sending the frame.

bits 1 _ _ _ _ _ _ 8 Address field basic mode in octet 1
a a a a a a a a 1 octet - value 0 to 255

bits 1 _ _ _ _ _ _ 8 Address field extended in octets 1 to n
0 a a a a a a a octet 1 (bit 1 = 0)
0 a a a a a a a octet i (bit 1 = 0)
1 a a a a a a a last octet n (bit 1 = 1)


Frame structure: control field

Reference standatd: ISO 4335

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
- 1rst octet:
bit 1 0
bits 2-4 N(S) Transmitter send sequence number
bit 5 P Poll bit when issued as a command (1 = Poll)
bits 6-8 N(R) Transmit. receive sequence number
bit 1 Command Response Frame designation
0 I Information
Extended operation: I 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 when issued as a command (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
- 1rst 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
1 1 SREJ SREJ Selective Reject
Extended operation: S format
- 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
1 1 SREJ SREJ Selective 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 : hex 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
xD SREJ C/R Selective Reject

Note : Hexadecimal code corresponds to b8-b1.

U format 

The control field Unnumbered format is either for basic (modulo 8) operation or for extended (modulo 128) operation:

The encoding values of the five M bits included in the first octet of the control field are listed and ordered according to their hexadecimal code or their type (mnemonic).

Basic operation: U format 
- 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 1 SNRM Set Normal Response Mode
1 1 - 1 0 0 SARM Set Asynchronous Response Mode
1 1 - 1 0 0 SABM Set Asynchronous Balanced Mode
0 0 - 0 1 0 DISC Disconnect
1 1 - 0 1 1 SNRME Set Normal Response Mode Extended
1 1 - 0 1 0 SARME Set Asynchronous Response Mode Extended
1 1 - 1 1 0 SABME Set Asynchronous Balanced Mode Extended
1 0 - 0 1 0 SIM Set Initialization Mode
0 0 - 1 0 0 UP Unnumbered Poll
0 0 - 0 0 0 UI UI Unnumbered Information
1 1 - 1 0 1 XID XID Exchange Identification
1 1 - 0 0 1 RESET Reset
0 0 - 1 1 1 TEST TEST Test
1 1 - 0 0 0 DM Disconnected Mode
0 0 - 1 1 0 UA Unnumbered Acknowledgement
1 0 - 0 0 1 FRMR Frame Reject
0 0 - 0 1 0 RD Request Disconnect
1 0 - 0 0 0 RIM Request Initialization Mode
Extended operation: U format 
- 1rst octet:
bits 1-2 1 1
bits 3-4 M M M: modifier function bit
bit 5 U U: non specified
bits 6-8 M M M M: modifier function bit

 

- 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 xxxxxxx X: reserved and set to 0

 

bits M : 34 - 678 Command Response Frame designation
0 0 - 0 0 1 SNRM Set Normal Response Mode
1 1 - 1 0 0 SARM Set Asynchronous Response Mode
1 1 - 1 0 0 SABM Set Asynchronous Balanced Mode
0 0 - 0 1 0 DISC Disconnect
1 1 - 0 1 1 SNRME Set Normal Response Mode Extended
1 1 - 0 1 0 SARME Set Asynchronous Response Mode Extended
1 1 - 1 1 0 SABME Set Asynchronous Balanced Mode Extended
1 0 - 0 1 0 SIM Set Initialization Mode
0 0 - 1 0 0 UP Unnumbered Poll
0 0 - 0 0 0 UI UI Unnumbered Information
1 1 - 1 0 1 XID XID Exchange Identification
1 1 - 0 0 1 RESET Reset
0 0 - 1 1 1 TEST TEST Test
1 1 - 0 0 0 DM Disconnected Mode
0 0 - 1 1 0 UA Unnumbered Acknowledgement
1 0 - 0 0 1 FRMR Frame Reject
0 0 - 0 1 0 RD Request Disconnect
1 0 - 0 0 0 RIM Request Initialization Mode
M format : hex codes
P=0 P=1 Type C/R Frame designation
03 13 UI C/R Unnumbered Information
07 17 RIM R Request Initialization Mode
07 17 SIM C Set Initialization Mode
0F 1F SARM C Set Asynchronous Response Mode
0F 1F DM R Disconnected Mode
13 23 SNRM C Set Normal Response Mode
23 33 UP C Unnumbered Poll
2F 3F SABM C Set Asynchronous Balanced Mode
43 53 DISC C Disconnect
43 53 RD R Request Disconnect
4F 5F SARME C Set Asynch. Response Mode Extended
63 73 UA R Unnumbered Acknowledgement
6F 7F SABME C Set Asynch. Balanced Mode Extended
87 97 FRMR R Frame Reject
87 97 RESET C Reset
AF BF XID C/R Exchange Identification
CF DF SNRME C Set Normal Response Mode Extended
E3 F3 TEST C/R Test

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
RD 43 53 R Request Disconnect
RESET 87 97 C Reset
RIM 07 17 R Request Initialization Mode
SABM 2F 3F C Set Asynchronous Balanced Mode
SABME 6F 7F C Set Asynch. Balanced Mode Extended
SARM 0F 1F C Set Asynchronous Response Mode
SARME 4F 5F C Set Asynch. Response Mode Extended
SIM 07 17 C Set Initialization Mode
SNRM 13 23 C Set Normal Response Mode
SNRME CF DF C Set Normal Response Mode Extended
TEST E3 F3 C/R Test
UA 63 73 R Unnumbered Acknowledgement
UI 03 13 C/R Unnumbered Information
UP 23 33 C Unnumbered Poll
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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updated:  27-Feb-04