A transmitter transmits a data frame as an uninterrupted stream of codeblocks of predefined size on a first 10 Gigabit/second data path between a MAC and PHY. It inserts a first idle block of predefined size within the data frame if there is insufficient data. A receiver receives a second idle block on a second data path, the second idle block including a request to slow down the transmission on the first data path. The receiver causes the transmitter to insert a third idle block in response to receiving the second idle block. The transmitter may further send a stream identifier including an identifier for a data stream and a bandwidth factor. The transmitter may send one codeblock chosen from data blocks for the data stream and idle blocks, and then send the bandwidth factor number of codeblocks chosen from data blocks for other data streams and idle blocks.