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USRP Hardware Driver and USRP Manual Version: 4.1.0.4
UHD and USRP Manual
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USRP Hardware Driver and USRP Manual Version: 4.1.0.4
UHD and USRP Manual
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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, three radio transport protocols are relevant: CHDR, Legacy CHDR (from UHD 3.x) and VRT (the VITA Radio Transport protocol).
All Generation-3 devices use CHDR. Prior to UHD 4.0, Generation-3 devices used legacy CHDR. The B200 continues to use the legacy CHDR format. All other Generation-1 and Generation-2 devices use VRT.
The CHDR protocol is described in detail in the RFNoC Specification. Please refer to the specification document for protocol format information.
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 it is "compressed" into a 64-bit header, it was dubbed CHDR (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 CHDR 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). For the format of SID, see uhd::sid_t. |
The packet type is determined mainly by the first two bits, although the EOB or error flag are also taken into consideration (the third bit does not affect the packet type):
| Bit 63 | Bit 62 | Bit 61 | Bit 60 | Packet Type |
|---|---|---|---|---|
| 0 | 0 | x | 0 | Data |
| 0 | 0 | x | 1 | Data (End-of-burst) |
| 0 | 1 | x | 0 | Flow Control |
| 1 | 0 | x | 0 | Command Packet |
| 1 | 1 | x | 0 | Command Response |
| 1 | 1 | x | 1 | Command Response (Error) |
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 .
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).
Relevant 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 CHDR uhd::sid_t - Datatype to represent SIDs
1.9.3