/*! \page page_rtp Radio Transport Protocols

\tableofcontents

Radio transport protocols are used to exchange samples (or other items) between host and devices.
If one were to sniff Ethernet traffic between a USRP and a PC, the packets would conform to a
radio transport protocol.

For USRP devices, two radio transport protocols are relevent: VRT (the VITA Radio Transport protocol)
and CVITA (compressed VITA), also known as CHDR. Generation-3 devices and the B200 use CHDR, the rest
use VRT.

\section rtp_vrt VRT

VRT is an open protocol defined by the VITA-49 standard. It was designed for interoperability,
and to allow different device types to work with different software stacks.

VRT is a very verbose standard, and only a subset is implemented in UHD/USRPs.
The full standard is available from the VITA website: http://www.vita.com .


\section rtp_chdr CVITA (CHDR)

For the third generation of Ettus devices, a new type transport protocol was designed.
It reduces the complexity of the original standard and uses a fixed-length 64-Bit header
for everything except the timestamp. Because this is a "compressed" form of VITA, it
was dubbed "Compressed VITA" (CVITA). The compressed header is called CHDR, which is why
the protocol is often called CHDR itself (pronounced like the cheese "cheddar").

By compressing all information into a 64-bit line, the header can efficiently be parsed
in newer FPGAs, where the common streaming protocol is 64-Bit AXI. The first line in a
packet already provides all necessary information to proceed.

Some CHDR-specific functions can be found in: uhd::transport::vrt::chdr.

The form of a CVITA packet is the following:

Address (Bytes) | Length (Bytes) | Payload
----------------|----------------|----------------------------
0               | 8              | Compressed Header (CHDR)
8               | 8              | Fractional Time (Optional!)
8/16            | -              | Data

If there is no timestamp present, the data starts at address 8, otherwise, it starts at 16.

The 64 Bits in the compressed header have the following meaning:

Bits   | Meaning
-------|--------------------------------------------------
63:62  | Packet Type
61     | Has fractional time stamp (1: Yes)
60     | End-of-burst or error flag
59:48  | 12-bit sequence number
47:32  | Total packet length in Bytes
31:0   | Stream ID (SID)


The packet type is determined mainly by the first two bits, although
the EOB or error flag are also taken into consideration:

Bit 63 | Bit 62 | Bit 60 | Packet Type
-------|--------|--------|--------------
0      | 0      | 0      | Data
0      | 0      | 1      | Data (End-of-burst)
0      | 1      | 0      | Flow Control
1      | 0      | 0      | Command Packet
1      | 1      | 0      | Command Response
1      | 1      | 1      | Command Response (Error)

\section vrt_tools Tools

For CHDR, we provide a Wireshark dissector under tools/chdr_dissector. It can be used
for Ethernet links as well as USB (e.g., for the B210).

\section vrt_code Code

Relevent code sections for the radio transport layer are:
* uhd::transport::vrt - Namespace for radio transport protocol related functions and definitions
* uhd::transport::vrt::chdr - Sub-namespace specifically for CVITA/CHDR
* uhd::sid_t - Datatype to represent SIDs

*/
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