A multiple access telecommunication network wherein a primary station is coupled to a plurality of secondary stations by a shared transmission medium. Depending on traffic volume it is advantageous to be able to change the type of access protocol employed by the secondary stations in order to prevent deterioration of transmission efficiency. Accordingly, the access protocol is set by the primary station by transmitting to the secondary stations a channel indicator indicating where in an uplink frame a secondary station can submit its access request. The primary station may also specify the message key which a secondary station must use when transmitting an access request. Applicable to multimedia networks using CATV systems.