Abstract: A telecommunication system has a sending entity, a receiving entity, and an ATM link between the sending and receiving entities for transferring ATM cells on specific separate ATM connections. The ATM cells include user ATM cells having a header and payload containing data in the form of a determined number of data units which carry data in microcells. The header includes an ATM connection pointer. The microcells have payload for carrying a variable size user data package and a microcell header containing code identifying a microcell connection. A microcell unable to fit in its entirety in a remaining space of an ATM cell belonging to a certain ATM connection is divided into a first part located in a least on following ATM cell belonging to the same ATM connection. Microcell size information is associated with each microcell. Methods for finding microcell boundaries are provided.