1. A method of starting a session in a synchronization system which comprises at least one electronic device acting as a client device and comprising communication means, at least one synchronization server and a telecommunication network providing a message transmission service, the method comprising the steps of: configuring the synchronization server to determine for a request, indicating the need for starting a session and to be transmitted to the client device, the identifier of the synchronization server, a version identifier and the identifier of the requested synchronization session, determining in the synchronization server the maximum size for a message that is to be sent from the synchronization server to the client device for the request, determining 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, in the synchronization server and decoding instructions, by means of which the original identifier is obtained from the bit sequence, in the client device, in response to the need of transmitting the request indicating the need for starting a session to at least one client device, forming one message, which message is shorter or as long as said maximum size and comprises at least said identifiers, at least one of which is presented as a bit sequence defined according to the coding instructions, transmitting the message to the client device by using said message transmission service, forming a session initialization message on the basis of the information included in the received message, at least part of said information being defined from the received bit sequence by means of said decoding instructions, and transmitting the session initialization message from the client device to the synchronization server.