A method of starting a session in a synchronization system. The maximum size of a message that is to be sent from a synchronization server to a mobile station for the request, and coding instructions, by which at least one of the identifiers can be coded into a bit sequence requiring substantially fewer bits than its ASCII presentation, are determined in the synchronization server. Decoding instructions, by means of which the original identifier is obtained from the bit sequence, are determined in the mobile station. When the purpose is to transmit a request indicating the need for starting a session to at least one mobile station, a message is formed, is shorter or as long as the maximum size and includes the pre-selected identifiers, at least one of which is presented as a bit sequence defined according to the coding instructions. The mobile station forms a session initialization message on the basis of the information included in the message received from the server, at least part of information being defined from the received bit sequence according to the decoding instructions.