A contention protocol permitting transmission of isochronous data. A device (104) wishing to transmit isochronous data can reserve periodic time slots for the transmission. Either the device (104) or a network controller (101) transmits the reservation information to other devices (106, 108) on the network. When non-isochronous transmissions are desired, the devices (104, 106, 108) contend for access using the CSMA/CD protocols, but taking into account the already reserved time slots. For example, the reserved period may be added to the randomly chosen backoff window. Alternatively, if a backoff window expires during a reserved period, the backoff window may be restarted upon completion of the reserved period. If not all the devices (104, 106, 108) on the network employ this modified CSMA/CD protocol, a "dummy" frame is provided at the beginning of each isochronous transmission. If another device is contending for access, the dummy frame collides, and the other device backs off. The isochronous transmission then proceeds.