In October of 2017 the AXIe Consortium, VITA trade association, and six companies endorsed a new optical interface standard named Optical Data Interface, or ODI for short. The six companies were Conduant Corporation, Guzik Technical Enterprises, Intel Corporation, Keysight Technologies, Samtec Incorporated, and Xilinx.
The ODI standard is a new high-speed point-to-point interface for instrumentation and embedded systems. ODI breaks speed and distance barriers by optical communication between devices over a simple pluggable optical cable. With speeds up to 20 GBytes/s from a single optical port, and up to 80 GBytes/s through port aggregation, ODI is designed to address challenging applications in 5G communications, mil/aero systems, high-speed data acquisition, and communication research.
Why ODI is Needed
Advanced communication systems increasingly rely on digitized values in an RF stream to connect one device to another. The stream may come from one or more receivers, it is then processed in a digital signal processing (DSP) device, and finally demodulated digitally. This mathematical processing can perform certain operations such as beamforming, more accurately than analog methods. Alternatively, a signal may be generated by reversing this flow. That is, a baseband signal can be processed by DSP, and then sent to one or more signal generators.
However, high-bandwidth applications such as 5G wireless and phased-array radar challenge the bandwidth of electrical buses to transport the digital signals. 5G signals can easily require 15 GB/s and more for additional channels. Even a 16-lane Gen3 PCI Express bus struggles to achieve this, and can only do so over a very limited distance, often only inches. As speeds increase, connecting two devices over an electrical bus becomes problematic.
Enter ODI. Instead of electrical signals, the devices are connected optically. Each ODI port is capable of 20 GBytes/s, and multiple ports may be chained together through a technique called port aggregation. Aggregation of four ODI ports has already been announced, allowing 80GBytes/s between devices. Not only do the speeds increase, but the distance issues disappear as well. ODI allows cables up to 100 meters in length, enabling the devices to be placed anywhere in a rack or an engineering lab. Figure 1 shows how an instrumentation system might use ODI.
How ODI Works
ODI leverages three layers of technology, as shown in Figure 2. The optical layer consists of 12 lanes of up to 14.1 Gb/s each, enabling 20 GBytes/s per optical port. Multimode fiber cables connect ports together, using standard Multi-Fiber Push On (MPO) connectors. The protocol layer is defined by the Interlaken standard, a device interconnect standard common in data centers, conceived by Cortina Systems and Cisco Systems. Interlaken is supported by the major FPGA suppliers, and delivers arbitrary packets over any number of lanes. The top layer specifies packets defined by the VITA 49 family of standards, also known as VITA Radio Transport (VRT). VRT packets contain the digitized data, and can support a wide set of channel counts and data formats.
Figure 3 shows a conceptual implementation of an ODI link. A device that transmits data is called a producer and a device that receives data is a consumer. Each device has an MPO connector that houses two rows of 12 fibers each. The top row is for transmit functions, and the bottom row is for receive functions. The standard ODI cable is a 24-fiber multi-mode optical crossover cable, which automatically connects transmitters to receivers. This way, only a single ODI cable is defined, though at different lengths up to 100 meters.
There is no requirement that a device be bidirectional. A device may transmit-only, receive-only, or both. Often, digitizers are unidirectional, while signal generators such as Arbitrary Waveform Generators (AWGs) are bidirectional. Signal generators use flow control in the reverse direction to pace the incoming data to their precise speed, managing the data input buffer. Unidirectional devices are required to have both rows present within the MPO connector, even if only the Tx row or Rx row is active. With this rule, the standard ODI cable works with unidirectional devices as well.
Electrical to optical (E/O) transceivers connect between the FPGA and the MPO connector. Transmitters source 850nm optical signals on each lane at IEEE 802.3ba power levels. Samtec Firefly optical engines meet the ODI specifications, but so do any optics that meet IEEE 802.3ba optical levels and the specified line rates.
As stated earlier, the optical layer consists of 12 lanes of optical communication in each direction, at either 12.5 Gb/s or 14.1 Gb/s line rates. Two line rates were chosen to address economy and performance applications. The 12.5 Gb/s rate allows lower cost FPGAs and nearly free Interlaken IP licensing, while the 14.1 Gb/s rate allows higher data rates. Devices that operate at the higher speed must operate at the lower speed as well, enabling upward compatibility. Multiply 12 lanes by 14.1 Gb/s, and the ODI port is capable of over 160 Gb/s or 20 GBytes in each direction. There is no requirement that data samples be packed in a byte format, and can be any number of bits in length.
The optical transceivers are connected to the SerDes lines of the respective FPGAs. The Interlaken IP manages the 12 SerDes in each direction, and sends and receives packet data. The reverse path may be used for flow control, supported by the Interlaken protocol. Interlaken IP is available from the major FPGA vendors, such as Intel Corporation and Xilinx.
VRT packets, compliant with the VITA 49 specification, are sent from the producer to the consumer. Although they have an overhead of only 32 bytes in total, the packets have data payloads up to 256 Kbytes. The simplicity of the VRT protocol makes it very efficient, and ideal for implementation within an FPGA. By sending consecutive packets, continuous data may be streamed indefinitely, with no data gaps.
The particular data format and packing method is chosen by the device manufacturer. For an instrument, this is typically chosen to match the resolution, number of channels, and data rate of the device. Not all devices need significant packet comprehension to perform their operations. Storage devices, for example, can store or source data without knowledge of the meaning of the data.
As technologies change, standards must adapt. It is the intent of the ODI standard to follow the speed curves of FPGAs and optical devices. All future rates and implementations should be considered somewhat speculative at this point. Nevertheless, the ODI Technical Committee has published the roadmap shown in Figure 4 as a directional document. The figure shows the data speeds available from today's ODI specification. ODI-1 defines the per-port speed, while the ODI-2 specification uses port aggregation to multiply the ODI-1 speed by the number of aggregated ports. In general, higher speed devices must support the lower rates as well, delivering upwards compatibility. ODI-1.1, at a 28 GB/s line rate, will effectively double the throughput of today's systems.
Since the ODI standard is designed around a standard optical connector, which may be placed anywhere on a device, ODI works equally well with any product format, whether a traditional bench instrument or a plug-in module. It also works with instrumentation and embedded systems, such as those found in mil/aero applications. ODI enables high-speed communications between instruments, processors, storage, and embedded devices, through the standardized ports.
The ODI specifications are managed by the AXIe Consortium, and are available here. Any organization or vendor may develop and offer ODI-compatible devices without restriction. There are no license fees, and membership in the AXIe Consortium is not required.
The first ODI products have been announced by Guzik, consisting of a high-speed digitizer and companion processing module. As the speed limitations of electrical buses continue to increasingly inhibit next-generation applications, expect more announcements in 2018.