Mapping control protocol time onto a physical layer

1. A method comprising:

in a communication device comprising a Media Access Control (MAC) layer and a Physical (PHY) layer:

determining a MAC layer data rate of the MAC layer;

determining a PHY layer data rate at which communication will occur over a physical communication medium; and

at the MAC layer:

determining a rate difference between the MAC layer data rate and the PHY layer data rate;

communicating a data flow from the MAC layer to the PHY layer, the data flow comprising data for communication over the physical communication medium, including inserting IDLE characters into the data flow responsive to the rate difference; and

in the PHY layer, replacing, prior to forward error correction encoding, selected IDLE characters among those inserted into the data flow at the MAC layer with data bytes of the data flow received from the MAC layer to adapt to the rate difference between the MAC layer and the PHY layer.

2. The method of claim 1 , further comprising:

determining an error correcting overhead incurred in the PHY layer; and

accounting for the error correcting overhead in the rate difference used for inserting the IDLE characters.

3. The method of claim 1 , further comprising:

in the PHY layer, replacing selected IDLE characters among those inserted at the MAC layer with data bytes to adapt to the rate difference between the MAC layer and the PHY layer.

4. The method of claim 1 , further comprising:

in the PHY layer, replacing other selected IDLE characters among those inserted at the MAC layer with parity bits generated from the forward error correction coding.

5. The method of claim 1 , where inserting comprises:

inserting the IDLE characters for the PHY layer to perform rate conversion to account for the rate difference and run the physical communication medium at the PHY layer data rate.

6. The method of claim 1 , where:

the MAC layer data rate is a constant rate;

the PHY layer data rate is a subrated rate with respect to the MAC layer data rate; and

where inserting comprises:

inserting the IDLE characters to facilitate adaptation of the constant rate of the MAC layer to the subrated rate of PHY layer.

7. The method of claim 1 , where the data flow comprises a packet comprising:

a packet starting point followed by an interpacket gap.

8. The method of claim 7 , further comprising:

referencing packet timing to the packet starting point.

9. The method of claim 8 , where inserting comprises:

inserting the IDLE characters in the data flow after the packet starting point and prior to the interpacket gap so that packet timing is maintained after:

mapping the data flow to carriers on the physical communication medium and recovery of the data flow by applying an inverse of the mapping at a receiver.

10. The method of claim 9 , further comprising:

in the PHY layer, transferring specific bytes of the packet into locations held by specific IDLE characters among those inserted at the MAC layer with data.

11. The method of claim 9 , further comprising:

in the PHY layer, replacing selected IDLE characters among those inserted at the MAC layer with data accumulated from the data flow that is temporarily held to accommodate for the rate difference.

12. A system comprising:

control circuitry characterized by a control layer data rate; and

communication medium circuitry comprising:

a communication interface to a communication medium; where:

the communication medium characterized by a transmit data rate at which

the communication interface places data onto the communication medium; at the control circuitry:

determining a rate difference between the control layer data rate and the transmit data rate;

preparing a data flow for the communication medium circuitry; and

responsive to the rate difference, inserting spacing characters in the data flow to the communication medium circuitry, wherein the spacing characters are inserted into the data flow after a start of a packet and before an interpacket gap; and

the communication medium circuitry configured to:

replace selected spacing characters among those inserted into the data flow at the control circuitry with data bytes of the data flow received from the control circuitry to adapt to the rate difference between the control layer data rate and the transmit data rate.

13. The system of claim 12 , where the control circuitry is further configured to:

determine an error coding overhead incurred by the communication medium circuitry; and

where the rate difference accounts for the error coding overhead, the control layer data rate, and the transmit data rate.

14. The system of claim 12 , where:

the control circuitry comprises a Media Access Control (MAC) layer configured to receive data from an optical communication interface.

15. The system of claim 12 , where:

the communication medium circuitry comprises a Physical (PHY) layer and the communication interface comprises a coaxial cable interface.

16. The system of claim 12 , where the communication medium circuitry is configured to:

replace other selected spacing characters among those inserted by the control circuitry with parity bits generated from forward error correction coding.

17. The system of claim 12 , where the data flow includes a packet comprising:

a packet starting point and where the control circuitry is configured to reference packet timing to the packet starting point.

18. The system of claim 17 , where:

an interpacket gap follows the packet; and where:

the control circuitry is configured to insert spacing characters in the data flow after the packet starting point and prior to the interpacket gap so that packet timing is maintained after:

mapping the data flow to frequency sub-carriers on the communication medium and recovery of the data flow by applying an inverse of the mapping at a receiver.

19. A system comprising:

an optical communication interface;

a coaxial communication interface;

control circuitry in communication with the optical communication interface and the coaxial communication interface, the control circuitry comprising:

a Media Access Control (MAC) layer configured to operate at a fixed MAC layer data rate; and

a Physical (PHY) layer configured to transmit data through the coaxial communication interface at a PHY layer data rate subrated with respect to the fixed MAC layer data rate; where:

the MAC layer is configured to:

determine a rate difference between the fixed MAC layer data rate and the PHY layer data rate;

communicate a datastream from the MAC layer to the PHY layer, the datastream comprising data bits for communication through the coaxial communication interface; and

insert, into the datastream, IDLE characters for adapting the PHY layer to the rate difference; and

the PHY layer is configured to:

hold data from the datastream received from the MAC layer; and

replace selected IDLE characters among those inserted at the MAC layer with the data from the datastream that was held in the PHY layer due to the rate difference.

20. The system of claim 19 , where:

the MAC layer is further configured to:

determine an error coding overhead added by the PHY layer;

where the rate difference accounts for the error coding overhead, the MAC layer data rate, and the PHY layer data rate;

insert a packet into the datastream followed by an interpacket gap, the packet comprising a packet starting point to which the MAC layer references packet timing; and

insert the IDLE characters in the datastream after the packet starting point and prior to the interpacket gap so that packet timing is maintained after:

mapping the datastream to frequency sub-carriers in the PHY layer and recovering the datastream by applying an inverse of the mapping.