pdu as shown in fig5 b the se indicates whether another segment header is following or not the segment header 514 524 is optional and may be omitted when there is no padding as shown in fig5 a or the ciphered data block has a fixed size alternatively the outer arq entity 318 322 may concatenate several data blocks into one pdu when a ciphered data block size is smaller than a pdu size multiple ciphered data blocks may be concatenated in one pdu fig6 shows an exemplary concatenated pdu 600 in accordance with the present invention an optional arq sn 602 may be assigned by the outer arq entity 318 322 to the pdu 600 the concatenated pdu 600 is generated from multiple ciphered data blocks and includes multiple segment headers 604 a 604 n each segment header 604 a 604 n indicates the ending position of the corresponding ciphered data blocks 608 a 608 n in the pdu 600 a different generic sn is used for each of the data blocks and the generic sns 606 a 606 n are included in the pdu 600 the extension field 603 after the arq sn 602 indicates whether a segment header 604 a is following or not if the concatenation always supports in sequence concatenated sdus the generic sequence number may only be included in the first concatenated sdu alternatively the outer arq entity 318 322 may generate one pdu from one ciphered data block i e one to one mapping when the ciphered data block size is close or same to the pdu size the outer arq entity 318 322 may generate one pdu from one data block the one to one mapping may.