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diff --git a/host/docs/uhd.dox b/host/docs/uhd.dox index 5b0738969..5cbe51a90 100644 --- a/host/docs/uhd.dox +++ b/host/docs/uhd.dox @@ -11,6 +11,7 @@ Some additional pages on developing UHD are also available here: \li \subpage page_coding \li \subpage page_stream +\li \subpage page_rtp */ // vim:ft=doxygen: diff --git a/host/docs/vrt_chdr.dox b/host/docs/vrt_chdr.dox new file mode 100644 index 000000000..8ab177b21 --- /dev/null +++ b/host/docs/vrt_chdr.dox @@ -0,0 +1,83 @@ +/*! \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 + +*/ +// vim:ft=doxygen: |