Abstract: A method of synchronizing data transmission between a host computer system and a transmitter across an interface with variable delay or latency. The host computer system marks transition frames between successive transmission intervals and transfers the outgoing frames across the variable interface to the transmitter. The transmitter enqueues outgoing frames into one or more FIFO transmission queue(s) and processes the enqueued frames as appropriate for the communication protocol in use. Marked frames are detected as they reach the head of the appropriate transmit queue. In particular, while bypassing is not active, the transmitter transmits unmarked frames until the end of the current interval, or until there is insufficient time in the interval to transmit another frame or until a marked frame is detected. While bypassing is not active, the transmitter terminates transmission from the transmit queue when a marked frame is detected during each interval.