This file specifies the format of USB packets used for in-band data
transmission and signaling on the USRP.  All packets are 512-byte long,
and are transfered using USB "bulk" transfers.

IN packets are sent towards the host.
OUT packets are sent away from the host.

The layout is 32-bits wide.  All data is transmitted in little-endian
format across the USB.


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |O|U|D|S|E|    mbz    |  Chan   | mbz |  Tag  |   Payload Len   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                            Payload                            |
   .                                                               .
   .                                                               .
   .                                                               .
   |                                                               |
   +             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      ...    |                                                 .
   +-+-+-+-+-+-+-+                                                 .
   .                                                               .
   .                            Padding                            .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   

  mbz   Must be Zero: these bits must be zero in both IN and OUT packets.

  O     Overrun Flag: set in an IN packet if an overrun condition was
        detected.  Must be zero in OUT packets.  Overrun occurs when
        the FPGA has data to transmit to the host and there is no
        buffer space available.  This generally indicates a problem on
        the host.  Either it is not keeping up, or it has configured
        the FPGA to transmit data at a higher rate than the transport
        (USB) can support.

  U     Underrun Flag: set in an IN packet if an underrun condition
        was detected.  Must be zero in OUT packets.  Underrun occurs
        when the FPGA runs out of samples, and it's not between
        bursts.  See the "End of Burst flag" below.

  D     Dropped Packet Flag: Set in an IN packet if the FPGA
	discarded an OUT packet because its timestamp had already
	passed.

  S     Start of Burst Flag:  Set in an OUT packet if the data is the
        first segment of what is logically a continuous burst of data.
        Must be zero in IN packets.

  E     End of Burst Flag:  Set in an OUT packet if the data is the
        last segment of what is logically a continuous burst of data.
        Must be zero in IN packets.  Underruns are not reported
        when the FPGA runs out of samples between bursts.


  Chan  5-bit logical channel number.  Channel number 0x1f is reserved
        for control information.  See "Control Channel" below.  Other
        channels are "data channels."  Each data channel is logically
        independent of the others.  A data channel payload field
        contains a sequence of homogeneous samples.  The format of the
        samples is determined by the configuration associated with the
        given channel.  It is often the case that the payload field
        contains 32-bit complex samples, each containing 16-bit real
        and imaginary components.

  Tag   4-bit tag for matching IN packets with OUT packets.
        [FIXME, write more...]

  Payload Len: 9-bit field that specifies the length of the payload
        field in bytes.  Must be in the range 0 to 504 inclusive.

  Timestamp: 32-bit timestamp.  [FIXME, Write more]
	On OUT packets, 0xffffffff means "Now".

  Payload: Variable length field.  Length is specified by the
        Payload Len field.

  Padding: This field is 504 - Payload Len bytes long, and its content
        is unspecified.  This field pads the packet out to a constant
        512 bytes.