Former Compass Networks staff look to silicon photonics

The Compass Networks team that designed a novel chip with optical input-output is exploring new opportunities now that the IP core router venture has closed it doors. 

The team plans to develop chips using silicon photonics for input-output and is involved in a European Commission (EC) Horizon 2020 project dubbed L3Matrix that will make such a chip for the data centre. 

Kobi HasharoniCompass Network was the first company to sell a commercial product - an IP core router - that used an ASIC co-packaged with optics. The IP router was sold to several leading service providers including NTT Communications and Comcast but the venture ultimately failed.

Compass Networks has now become a software company, while its chip R&D team decided to spin off to keep the co-packaged IC and photonics technology alive. 

Compass Networks

The ambitious Israeli start-up, Compass Networks, developed its IP core router to compete with the likes of Cisco Systems, Juniper Networks, Alcatel-Lucent (now Nokia) and Chinese giant, Huawei.

Using the chip - a traffic manager with optical input-output - resulted in a smaller, lower-power IP core router design. However, despite the compact platform enabled by the chip, the company failed commercially. The main issue was not the router hardware but the size of Compass Networks’ software team: its 60 engineers could not compete with its much larger IP core router rivals, says Kobi Hasharoni, who was director of electro-optics at Compass Networks.

An IP router takes traffic in the form of packets on its input ports and forwards them to their destination via its output ports. To do this, two functions are used: a network processor unit and a traffic manager. The two functions can be integrated in a single chip or, typically for core routers, implemented using two devices.

The network processor chip performs the packet processing, taking each packet’s header and using a look-up routing table to update the header with the destination address before sending the packet on its way.

The second chip, the traffic manager, oversees billions of packets. The chip implements the queueing protocols and, based on a set of rules, determines which packets have priority on what ports. In a conventional IP router there is also a switch fabric which connects the router cards to be able to send the packets to the required output port.

Compass Networks designed the router between 2007 and 2010. The design team chose the EZchip 100-gigabit NP-4 network processor for the router but developed its own complex traffic manager ASIC, adding the twist of optics for the chip’s input-output.

We didn’t have a backplane; our backplane was just fibres 

The resulting chip - referred to as icPhotonics or the D-chip - performed the roles of both traffic manager and switch fabric.

Instead of the traffic manager going through switch fabrics chips and an electrical backplane to a traffic manager on another card, each traffic manager had sufficient bandwidth due to the optics to connect to all the other traffic managers in a mesh configuration.

“We didn’t have a backplane,” says Hasharoni. “Our backplane was just fibres.” Avoiding a backplane resulted in a more compact, lower-power IP core router that saved on operational costs.

D-chip

To make the D-chip, Compass developed a mixed signal ASIC. The 21x21 mm chip comprised the traffic manager and a matrix of analogue circuitry to interface to the optics.

The company used 168 vertical-cavity surface-emitting lasers (VCSELs) and 168 photo-detectors in a 2D array that was positioned above the analogue circuitry; each optical device positioned above its own analogue driver or receiver circuitry. Two ribbon cables, one for the VCSELs and one for the photo-detectors, were then connected to the chip.  

VCSELs were at 10 gigabit-per-second (Gbps) at the time and Compass Networks chose to operate them at 8Gbps. “Going to 8 gigabit-per-second seemed reasonable,” says Hasharoni.

Each NP-4 processed 100Gbps of traffic and sent out 160Gbps to the D-chip. The extra traffic included forward error correction and overhead bits to speed up queueing.

The core router platform comprised four line cards, each card having two 100-gigabit NP-4s and two D-chips.

The total optical input-output bandwidth of each D-chip was 1.34 terabits in each direction. The 168 VCSELs were used in such a way that each group of 20 VCSELs supported the 160-gigabit stream of packets, enabling each D-chip to connect directly to the seven other D-chips in a fully connected mesh, while the 28 remaining VCSELs were used for redundancy.

At some point you will not get all this input-output into the ASIC

Silicon photonics

Were the team to tackle a similar design today, the designers would use silicon photonics instead of VCSELs, says Hasharoni. A silicon photonics design would support single-mode fibre and its associated longer reach, while the co-packaging would be easier given both the ASIC and the optics are silicon-based.

Hasharoni points to the rapid development in the capacity of switch chips used in the data centre. Current Ethernet switch silicon from the likes of Broadcom support 3.2 terabits of capacity and this will double in 2017 and double again to 12.8 terabits in 2018. There is even talk of 25.6 terabits switching silicon by 2020.

The issue, however, is that the input-output required for these higher-capacity chips consume more and more power; at 12.8 terabits it will be over half of chip's overall power consumption. "At some point you will not get all this input-output into the ASIC," says Hasharoni.

Using a co-packaged electronics and silicon photonics design, the input-output's power consumption will be halved, says Hasharoni. The optical density is also an order of magnitude higher, thus only a fraction of the ASIC area is used for chip input-output compared to conventional electrical input-ouput. And the resulting switch will not need optical transceivers. "The fibre goes out directly from the IC; the power saving is huge," says Hasharoni.

The EC Horizon 2020 L3Matrix project also includes IBM Research, the Fraunhofer Institute for Reliability and Microintergration (Fraunhofer IZM) and several universities. The project will use embedded III-V light sources on a silicon substrate along with optical modulators. The aim of the design is to develop low-latency, high-radix switch elements using 25Gbps single-mode fibres and waveguides.

"The novel thing here is the use of two-dimensional silicon photonics matrices on an ASIC," says Hasharoni.