Radio Transport Protocols

Table of Contents

• VRT
• CHDR
• Tools
• Code

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 relevant: VRT (the VITA Radio Transport protocol) and CHDR (compressed header, an Ettus-specific protocol). Generation-3 devices and the B200 use CHDR, the rest use 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 .

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 it is "compressed" into a 64-bit heaer, 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)

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)

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).

Code

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