AOCs – Active Optical Cables Why are 2 transceivers and fibers priced less than one connectorized transceiver?

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Quick Summary

This blog is Part 2 in a 3-part series on Mellanox’s high-speed interconnect products. My first blog covered Direct Attach Copper (DAC) cables which are the least expensive way to create a high-speed 25G/50G/100Gb/s interconnect, but have a maximum reach of about 3-5 meters. Active Optical Cables (AOCs) pick it up from there and can reach up to 100-200 meters and are the least expensive, optical links available. They are widely used in HPCs and more recently became popular in hyperscale, enterprise and storage systems.


What is an AOC?

Optical transceivers convert electrical data signals into blinking laser light which is then transmitted over an optical fiber. Optical transceivers have an optical connector to disconnect the fiber from the transceiver. AOCs bond the fiber connection inside the transceiver end, creating a complete cable assembly much like a DAC cable, only with a 3-200-meter reach capability. AOCs main benefit is the very long reach of optical technology, while acting like a simple, “plug & play” copper cable.

What are AOC Features and Advantages?

Compared to less expensive DAC cables, AOCs offer:

  • Longer reach capability than DAC 3-7 meter limits
  • 3m – 100-meters multi-mode technology
  • 100-200 meters with single-mode, Silicon Photonics
  • Lower weight, thinner cable and bend radius enabling increased airflow cooling and easier system

Compared to more expensive optical transceivers, AOCs offer:

  • Dramatically lower priced solution than two optical transceivers and connectorized fiber based links
  • Lower power consumption at 2.2 Watts versus up to 4.5 Watts for optical transceivers (4-channel)
  • Lower operational and maintenance cost

How is it Different from Two Transceivers with connectorized fibers?

Permanently attaching the fibers is a seemingly simple change but yields a surprisingly large number of technical benefits and cost advantages; enough to create an entirely new category of interconnect products. Since the optics is contained inside the cable, designers do not have to comply to IEEE or IBTA industry standards for transceiver interoperability with other vendors. Hence, customers can pick and choose the lowest cost, best performing technology. Here are some of the results this simple change enables:

  • Lowest priced optical interconnect available – almost half the price of optical transceivers – much more than just the cost of losing the optical connectors. (more on this later)
  • Plug & play: Ease-of-use “cable” features – like DAC cables
  • Long reach: Up to 100 and 200-meter reach depending on the technology
  • Lowest optical power consumption per end – significantly lower than connectorized transceivers – saves operating expenses
  • No optical connectors to clean and maintain – saves operating expenses and increases reliability
  • Optically isolates electrical systems from ground loops as with copper DAC cables – a technical advantage

How Does an AOC Achieve a Lower Product Cost Than Two Transceivers?

An AOC uses two optical transceivers with integrated fiber. So, how on earth can it cost less than a single optical transceiver with an optical connector?

  1. Testing Costs: Optical testing accounts for 40-60 percent of the cost of manufacturing a transceiver. AOCs are tested in switch systems as an electrical test. Cable is plugged in; test patterns and data run; come back later and look at the results. If good, ship! If not, scrap. Optical transceivers, on the other hand, require $500,000 of optical test equipment per station, a very experienced (e.g. expensive) test engineer, and a lot of time on the test bench. AOCs do away with all of this since the testing is only in the electrical domain. Mellanox uses its “scratch & dent” switches to test AOCs and is one way we achieve a bit error ratio (BER) feature of 1E-15 versus 1E-12 IEEE standard, about 1,000 times fewer number of bit errors induced by the AOC link.
  2. Design Freedom: Since the optics is contained inside the AOC cable, designers can utilize the lowest cost materials and transceiver designs. Besides deleting four MPO optical connectors (2 per end), for example, Mellanox’s Silicon Photonics AOC uses only one laser versus four lasers for a multi-mode AOC. Low cost, but short reach orange colored OM2 fiber can be used for
  3. Freedom from Industry Standards: AOCs must comply to the IEEE, IBTA and SFF industry standards for the electrical, mechanical, and thermal requirements but he hardest part are the optical requirements. Since the optics are contained inside the cable, they do not have to meet any standards hence allows a lot more design freedom and eliminates the costly optical testing.

AOCs Offer Lower Operational Costs Too

  1. AOCs do not have optical connectors to manually clean every time they are removed as a single speck of dust inside the connector can completely block the 50-um or 9-um diameter fiber light transmission area. In a transceiver link, there are two fiber ends and two transceiver ends to clean. Besides, the personnel cost the connector cleaners can cost upwards of $250 each and stocked.
  2. AOCs don’t use MPO optical connectors which, in crowded racks, can be dropped and the fiber end scratched rendering them useless.
  3. Optical connectors can channel an electrical static charge that builds up on a long plastic cable and can destroy the sensitive optical transceivers electronics.
  4. AOC is a “plug and play” cable solution rather than a “plug, assemble and clean” solution as with optical transceivers. Optical transceivers, fibers and connectors also have many different and complicated product variances. All these must all be exactly matched to the specific transceiver used and spares kept and a technician trained in the specifications.
  5. AOC cables have a short bend radius and much thinner able thickness than most DAC cables. This makes them easier to deploy and frees up a lot of space for increased air flow cooling in crowded systems.
  6. Lastly, there are big operational power cost savings. One Watt saved at the component level translates to 3-5 Watts at the data center facility level. This is when all the chassis, row, room and facility fans and air conditioning equipment is included along with the electrical power to drive them – not counting the repair and maintenance! AOCs are less complex than optical transceivers and offer lower power consumption. Mellanox’s multi-mode AOCs draw about 2.2Watts per end compared to 2.8-4.5W for optical transceivers.


One Watt saved at the component level translates
into 3-5 Watts at the data center facility level!

How Are AOC used in Modern Data Centers?

While AOCs reaches can extend to the limits of the optical technology used (100-200 meters), installing a long 100-meter (328 foot) cable, complete with an expensive transceiver end, is difficult in crowded data center racks so the average reach typically used is between 3-30 meters. Only one “oops” per cable allowed. Damaging the cable means replacing it as it cannot be repaired. AOCs are typically deployed in open access areas such as within racks or in open cable trays for this reason.

Mellanox’s InfiniBand AOCs started out in about 2005 with DDR (4 x 5Gb/s) for use in the Top10 HPCs and quickly became the preferred solution for the large InfiniBand HPCs in the Top100. Today, it is the norm for QDR, FDR and EDR and in 2017 the newly announced LinkX® 200G HDR announced at SC’16 event in November.

The power and cost savings caught the eye of the Ethernet hyper scale and enterprise data center builders and has since become a popular way to link Top-of-Rack switches upwards to aggregation layer switches such as End-of-Row and leaf switches. Several hyperscale companies have publically stated their preferred use of AOCs for linking Top-of-Rack switches. Additionally, single channel (SFP) AOCs have become very popular in storage subsystems and some hyperscale builders who often run 10Gb/s or 25Gb/s AOCs from a Top-of-Rack switch to subsystems at reaches greater than DAC limits of 3-7 meters.

Here is an example of how AOCs are typically used inside systems racks to link subsystems together and between switches and systems in rows:

2016-12-12-12 Racks

Here is a more detailed view in Ethernet configurations showing the LinkX® 10Gb/s and 25Gb/s based AOCs, Spectrum switches and ConnectX-3, 4, 5 QSFP and SFP network adapters.

Here is an example at the Texas A&M HPC center and as you can see, nothing but orange AOCs and open access!

Typical AOC Use in HPC Super Computers

2016-12-12-14 Texas A&M pic


Newly Announced 200Gb/s HDR InfiniBand AOCs

At the HPC supercomputer conference SC’16 in Salt Lake City Utah, Mellanox announced its 40-port, 200Gb/s HDR InfiniBand line of Quantum-based switches and dual-port ConnectX®-6 host bus adapters. To link them all together, we also announced, a line LinkX® Direct Attach Copper (DAC) cables and splitters and Mellanox’s own design, Silicon Photonics-based, HDR200 Active Optical Cables (AOCs) running at a whopping 200Gb/s in a QSFP56, 4x50Gb/s configuration!

2016-12-12-15 HDR AOC

These will be available in Low-Smoke, Zero-Halogen (LSZH) jackets and reaches from 3 meters up to 100 meters. Available in mid 2017, watch our new LinkX® website for more detailed information coming!

Key Take-A-Ways

Mellanox LinkX® AOC cables offer the lowest-cost optical links with the lowest-power and latency in the industry. This enables the most cost efficient, highest ROI, lowest Capex and Opex interconnect solution. The full portfolio of 10Gb/s, 25Gb/s and 50Gb/s line rates and SFP and QSFP form-factors enable customers the ability to build a wide variety of configurations that meet every application in both InfiniBand and Ethernet. Everything is manufactured by Mellanox and uses Mellanox-designed Silicon Photonics and control ICs.

Check out the Mellanox LinkX® website and Mellanox Academy for more detailed DAC, AOC and transceiver white papers and articles in the future. LinkX® is Mellanox’s brand name for its cables and transceivers product line.

Contact your Mellanox sales representative for availability and pricing options and stay tuned to my blog more interconnect news and tips.

More Information:

Mellanox InfiniBand & Ethernet AOCs & DAC cables and transceivers
Mellanox HDR switches
Mellanox HDR ConnectX-6 host bus adapters


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About Brad Smith

Brad is the Director of Marketing at Mellanox, based in Silicon Valley for the LinkX cables and transceivers business focusing on hyperscale, Web 2.0, enterprise, storage and telco markets. Recently, Brad was Product Line Manager for Intel’s Silicon Photonics group for CWDM4/CLR4 and QSFP28 product lines and ran the 100G CLR4 Alliance. Director of Marketing & BusDev at OpSIS MPW Silicon Photonics foundry. President/COO of LuxSonar Semiconductors ( Cirrus Logic) and co-founder & Director of Product Marketing of NexGen, a X86-compatible CPU company sold to AMD - now the X86 product line. Brad also has ~15 years in technology market research as Vice president of the Computer Systems group at Dataquest/Gartner; VP/Chief Analyst at RHK and Light Counting networking research firms. Brad started his career at Digital Equipment near Boston with the VAX 11/780 and has served as CEO, president/COO and on the board-of-directors of three start-up companies. Brad has a BSEE degree from the University of Massachusetts; MBA from University of Phoenix and holds 2 optical patents

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