The invention relates to a method for transmitting data in a telecommunication system that comprises at least one packet-switched transmission network, in which data is transmitted in data packets of a standard length, the number of such standard lengths being determined in advance. To transfer information in as compact form as possible, a unique length code (L) is maintained for each standard length, and to each packet is added a length code (L) corresponding to the header of the packet to be transferred to indicate the length (3) of the packet. The invention further relates to a telecommunication system in which the method of the invention can be utilized.