A system for transmitting packets over a network of communication channels, the system comprising a set of nodes comprising at least first and second nodes and a network access coordinator operative to coordinate the access of the set of nodes to a synchronous network of channels, wherein, if at least one individual packet has been transmitted from the first node to the second node which did not receive at least one packet, the second node is operative to send a retransmission request to the network access coordinator requesting retransmission of at least one individual packet.