Synchronising network nodes

A network node comprising a radio and a clock, the network node being configured to:

• receive a first message from a primary node;
• receive an indication of a transmission time of the first message by the primary node from the primary node;
• transmit a second message to the primary node;
• receive a third message from the primary node;
• receive an indication of a transmission time of the third message by the primary node from the primary node;
• calculate a rate of a clock of a network node relative to the clock of the primary node using a ratio of a time difference between the transmission time of the first message and the transmission time of the third message by the primary node to a time difference between a reception time of the first message and a reception time of the third message by the secondary network node;
• calculate a time offset of the clock of the secondary network node relative to the clock of the primary node using the calculated rate, a value of a propagation delay between the primary node and the secondary network node and one of the indication of the transmission time of the first message or the indication of the transmission time of the third message;
• and
• synchronizing the clock of the network node with the clock of the primary node.

A method comprising:

• transmitting a packet from one of a first node and a second node to the other of the first node and the second node;
• a transmitting node counting a respective number of clock cycles of a clock of the transmitting node that occur during the transmission of the packet;
• a receiving node counting a respective number of clock cycles of a clock of the receiving node that occur during the receipt of the packet;
• the first node transmitting a respective number of clock cycles of a clock of the first node to the second node;
• the second node calculating a rate of a clock of the second node relative to a rate of the clock of the first node using a ratio between the respective number of clock cycles counted by the first node and the respective number of clock cycles counted by the second node;
• and
• synchronizing rates of clocks of the first node and the second node.