ITU-X224

Reference standard: ISO 8073 and ITU X224

TPDU structure: fields

TPDU length field
1 octet Length indicator (fixed part + variable part)
(n-1) octets (last n) Fixed part
(p-n-1) octets (last p) Variable part
(z-p) octets (last z) Data field (user data)
 
bits 8 _ _ _ _ _ _ 1 Fixed part
octet 2, bits 8-5 type of TPDU
octet 2, bits 4-1 CDT, CREDIT field in TPDU CR, CC, RJ, AK
octets 3-n Parameters as DST-REF (Destination reference) and SRC-REF(Source reference)
 
bits 8 _ _ _ _ _ _ 1 Variable part
octet n+1 Code of parameter
octet n+2 Parameter length indicator
octets (n+3)-p Parameter value

Field designation

field mnemonic designation
CDT Credit
DST-REF Destination reference
SRC-REF Source reference
EOT The current TPDU is the last unit of a sequence (EOT=1) of a complete DT TPDU sequence (end of TSDU)
TPDU-NR TPDU Send Sequence Number;
(Normal = 1octet, Extended = 4 octets)
ED-TPDU-NR Expedited TPDU Identification Number,
(Normal = 1octet, Extended = 4 octets)
YR-TU-NR Sequence Number indicating the next expected DT TPDU number,
(Normal = 1octet, Extended = 4 octets)

TPDU structure: list of types

bits 8 _ _ _ _ _ _ 1 hexa type designation
1 1 1 0 x x x x D0 CR Connection Request
1 1 0 1 x x x x E0 CC Connection Confirm
1 0 0 0 0 0 0 0 80 DR Disconnect Request
1 1 0 0 0 0 0 0 C0 DC Disconnect Confirm
1 1 1 1 0 0 0 0 F0 DT Data
0 0 0 1 0 0 0 0 10 ED Expedited Data
0 1 1 0 z z z z 61 AK Data Acknowledge
0 0 1 0 0 0 0 0 40 EA Expedited Acknowledge
0 1 0 1 z z z z 51 RJ Reject
0 1 1 1 0 0 0 0 70 ER Error
0 0 0 0 0 0 0 1 01 PI Protocol Identifier

Note : xxxx (bits 4 to 1): used to signal the CDT in classes 2,3,4; set to 0000 in classes 0,1

zzzz (bits 4 to 1): used to signal the CDT in classes 2,3,4; set to 1111 in classes 1

List of types ordered according to hexa code

hexa bits 8 _ _ _ _ _ _ 1 type designation
01 0 0 0 0 0 0 0 1 PI Protocol Identifier
10 0 0 0 1 0 0 0 0 ED Expedited Data
40 0 0 1 0 0 0 0 0 EA Expedited Acknowledge
51 0 1 0 1 z z z z RJ Reject
61 0 1 1 0 z z z z AK Data Acknowledge
70 0 1 1 1 0 0 0 0 ER Error
80 1 0 0 0 0 0 0 0 DR Disconnect Request
C0 1 1 0 0 0 0 0 0 DC Disconnect Confirm
D0 1 1 1 0 x x x x CR Connection Request
E0 1 1 0 1 x x x x CC Connection Confirm
F0 1 1 1 1 0 0 0 0 DT Data

TPDU format: Data

Normal format in class 0 and 1
bits 8 _ _ _ _ _ _ 1 field
octet 1, bits 8-5 Length Indicator
octet 2, bits 8-5 TPDU type (1111 0000)
octet 3 TPDU-NR and EOT
octets 4-z User data
Normal format in class 2,3 and 4
bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2 TPDU type (1111 0000)
octets 3-4 Destination reference
octet 5 TPDU-NR and EOT
octets 6-p Variable part
octets (p+1)-z User data
Extended format in class 2,3 and 4
bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2 TPDU type (1111 0000)
octets 3-4 Destination reference
octets 5-8 TPDU-NR and EOT
octets 9-p Variable part
octets (p+1)-z User data

TPDU format: Data Acknowledge

Normal format in class 1,2,3 and 4
bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2, bits 8-5 TPDU type (0110)
octet 2, bits 4-1 CDT
octets 3-4 Destination reference
octet 5 YR-TU-NR
octets 6-p Variable part
octets (p+1)-z User data
Extended format in class 2,3 and 4
bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2 TPDU type (0110 0000)
octets 3-4 Destination reference
octets 5-8 YR-TU-NR
octets 9-p Variable part
octets (p+1)-z User data

TPDU format: Expedited Data

Normal format in class 1,2,3 and 4
bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2 TPDU type (0001 0000)
octets 3-4 Destination reference
octet 5 TPDU-NR and EOT
octets 6-p Variable part
octets (p+1)-z User data
Extended format in class 2,3 and 4
bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2 TPDU type (0001 0000)
octets 3-4 Destination reference
octets 5-8 TPDU-NR and EOT
octets 9-p Variable part
octets (p+1)-z User data

TPDU format: Expedited Data Acknowledge

Normal format in class 1,2,3 and 4
bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2 TPDU type (0010 0000)
octets 3-4 Destination reference
octet 5 YR-TU-NR
octets 6-p Variable part
Extended format in class 2,3 and 4
bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2 TPDU type (0010 0000)
octets 3-4 Destination reference
octets 5-8 YR-TU-NR
octets 9-p Variable part

TPDU format: Connection request

bits 8 _ _ _ _ _ _ 1 field
octet 1, bits 8-5 Length Indicator
octet 2, bits 8-5 TPDU type (1110)
octet 2, bits 4-1 CDT
octets 3-4 Destination reference
octets 5-6 Source reference
octet 7 Class options
octets 8-p Variable part
octets (p+1)-z User data

TPDU format: Disconnection request

bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2 TPDU type (1000 0000)
octets 3-4 Destination reference
octets 5-6 Source reference
octet 7 Cause
octets 8-p Variable part
octets (p+1)-z User data

TPDU format: Connection confirm

bits 8 _ _ _ _ _ _ 1 field
octet 1, bits 8-5 Length Indicator
octet 2, bits 8-5 TPDU type (1101)
octet 2, bits 4-1 CDT
octets 3-4 Destination reference
octets 5-6 Source reference
octet 7 Class options
octets 8-p Variable part
octets (p+1)-z User data

TPDU format: Disconnection confirm

bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2 TPDU type (1100 0000)
octets 3-4 Destination reference
octets 5-6 Source reference
octets 7-p Variable part

TPDU format: Reject

Normal format in class 1 and 3
bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2, bits 8-5 TPDU type (0101)
octet 2, bits 4-1 CDT
octets 3-4 Destination reference
octet 5 YR-TU-NR
Extended format in class 3
bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2 TPDU type (0101 0000)
octets 3-4 Destination reference
octets 5-8 YR-TU-NR
octets 9-10 CDT

TPDU format: Error

bits 8 _ _ _ _ _ _ 1 field
octet 1 Length Indicator
octet 2 TPDU type (0111 0000)
octets 3-4 Destination reference
octet 5 Reject cause
octets 6-p Variable part
 

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updated:  27-Feb-04