TPDU Code (4 bits)

PDU Part: Fixed Part

Field: TPDU code

Length: 4 bits

Contents: indicates the type of TPDU.

TPDU Codes

TPDU Codes:

Code

Class 0 1 2 3 4 MSB LSB

CR - connection request.......... x x x x x 1110 xxxx

CC - connection confirm.......... x x x x x 1101 xxxx

DR - disconnect request.......... x x x x x 1000 0000

DC - disconnect confirm.......... x x x x 1100 0000

DT - data........................ x x x x x 1111 0000

ED - expedited data.............. x NF x x 0001 0000

AK - data acknowledgement........ NRC NF x x 0100 zzzz

EA - expedited data acknowledgmt. x NR x x 0010 0000

RJ - reject...................... x x 0101 zzzz

ER - TPDU error.................. x x x x x 0111 0000

Code

xxxx (bits 4 to 1): used to signal the CDT; set to 0000 in

classes 0 and 1.

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

set to 1111 in class 1.

NF: not available when the not-explicit flow control option is selected.

NRC: not available when the receipt confirmation option is selected.

CDT: credit.