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 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)

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