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83 lines
3.2 KiB
Text
83 lines
3.2 KiB
Text
/*! \page page_rtp Radio Transport Protocols
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\tableofcontents
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Radio transport protocols are used to exchange samples (or other items) between host and devices.
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If one were to sniff Ethernet traffic between a USRP and a PC, the packets would conform to a
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radio transport protocol.
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For USRP devices, two radio transport protocols are relevant: VRT (the VITA Radio Transport protocol)
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and CVITA (compressed VITA), also known as CHDR. Generation-3 devices and the B200 use CHDR, the rest
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use VRT.
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\section rtp_vrt VRT
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VRT is an open protocol defined by the VITA-49 standard. It was designed for interoperability,
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and to allow different device types to work with different software stacks.
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VRT is a very verbose standard, and only a subset is implemented in UHD/USRPs.
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The full standard is available from the VITA website: http://www.vita.com .
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\section rtp_chdr CVITA (CHDR)
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For the third generation of Ettus devices, a new type transport protocol was designed.
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It reduces the complexity of the original standard and uses a fixed-length 64-Bit header
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for everything except the timestamp. Because this is a "compressed" form of VITA, it
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was dubbed "Compressed VITA" (CVITA). The compressed header is called CHDR, which is why
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the protocol is often called CHDR itself (pronounced like the cheese "cheddar").
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By compressing all information into a 64-bit line, the header can efficiently be parsed
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in newer FPGAs, where the common streaming protocol is 64-Bit AXI. The first line in a
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packet already provides all necessary information to proceed.
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Some CHDR-specific functions can be found in: uhd::transport::vrt::chdr.
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The form of a CVITA packet is the following:
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Address (Bytes) | Length (Bytes) | Payload
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----------------|----------------|----------------------------
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0 | 8 | Compressed Header (CHDR)
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8 | 8 | Fractional Time (Optional!)
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8/16 | - | Data
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If there is no timestamp present, the data starts at address 8, otherwise, it starts at 16.
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The 64 Bits in the compressed header have the following meaning:
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Bits | Meaning
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-------|--------------------------------------------------
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63:62 | Packet Type
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61 | Has fractional time stamp (1: Yes)
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60 | End-of-burst or error flag
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59:48 | 12-bit sequence number
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47:32 | Total packet length in Bytes
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31:0 | Stream ID (SID)
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The packet type is determined mainly by the first two bits, although
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the EOB or error flag are also taken into consideration:
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Bit 63 | Bit 62 | Bit 60 | Packet Type
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-------|--------|--------|--------------
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0 | 0 | 0 | Data
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0 | 0 | 1 | Data (End-of-burst)
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0 | 1 | 0 | Flow Control
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1 | 0 | 0 | Command Packet
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1 | 1 | 0 | Command Response
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1 | 1 | 1 | Command Response (Error)
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\section vrt_tools Tools
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For CHDR, we provide a Wireshark dissector under tools/chdr_dissector. It can be used
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for Ethernet links as well as USB (e.g., for the B210).
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\section vrt_code Code
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Relevant code sections for the radio transport layer are:
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* uhd::transport::vrt - Namespace for radio transport protocol related functions and definitions
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* uhd::transport::vrt::chdr - Sub-namespace specifically for CVITA/CHDR
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* uhd::sid_t - Datatype to represent SIDs
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*/
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// vim:ft=doxygen:
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