100 Gig coherent

Arista Networks has developed a coherent optical transport line card for its 7500 high-end switch series. The line card hosts six 100 gigabit CFP2-ACO (analogue coherent optics) and has a reach of up to 5,000 km.

 

Martin Hull

Several optical equipment makers have announced ‘stackable’ platforms specifically to link data centres in the last year.

Infinera’s Cloud Xpress was the first while Coriant recently detailed its Groove G30 platform. Arista’s announcement offers data centre managers an alternative to such data centre interconnect platforms by adding dense wavelength-division multiplexing (DWDM) optics directly onto its switch. 

For customers investing in an optical solution, they now have an all-in-one alternative to an optical transport chassis or the newer stackable data centre interconnnect products, says Martin Hull, senior director product management at Arista Networks. Insert two such line cards into the 7500 and you have 12 ports of 100 gigabit coherent optics, eliminating the need for the separate optical transport platform, he says. 

The larger 11RU 7500 chassis has eight card slots such that the likely maximum number of coherent cards used in one chassis is four or five - 24 or 30 wavelengths - given that 40 or 100 Gigabit Ethernet client-side interfaces are also needed. The 7500 can support up to 96, 100 Gigabit Ethernet (GbE) interfaces. 

Arista says the coherent line card meets a variety of customer needs. Large enterprises such as financial companies may want two to four 100 gigabit wavelengths to connect their sites in a metro region. In contrast, cloud providers require a dozen or more wavelengths. “They talk about terabit bandwidth,” says Hull.

 

With the CFP2-ACO, the DSP is outside the module. That allows us to multi-source the optics

 

As well as the CFP2-ACO modules, the card also features six coherent DSP-ASICs. The DSPs support 100 gigabit dual-polarisation, quadrature phase-shift keying (DP-QPSK) modulation but do not support the more advanced quadrature amplitude modulation (QAM) schemes that carry more bits per wavelength.  The CFP2-ACO line card has a spectral efficiency that enables up to 96 wavelengths across the fibre's C-band.

Did Arista consider using CFP coherent optical modules that support 200 gigabit, and even 300 and 400 gigabit line rates using 8- and 16-QAM? “With the CFP2-ACO, the DSP is outside the module,” says Hull. “That allows us to multi-source the optics.”

The line card also includes 256-bit MACsec encryption. “Enterprises and cloud providers would love to encrypt everything - it is a requirement,” says Hull. “The problem is getting hold of 100-gigabit encryptors.” The MACsec silicon encrypts each packet sent, avoiding having to use a separate encryption platform.   

 

CFP4-ACO and COBO

As for denser CFP4-ACO coherent modules, the next development after the CFP2-ACO, Hull says it is still too early, as it is with for 400 gigabit on-board optics being developed by COBO and which is also intended to support coherent. “There is a lot of potential but it is still very early for COBO,” he says.

“Where we are today, we think we are on the cutting edge of what can be delivered on a line card,” says Hull. “Getting everything onto that line card is an engineering achievement.”    

 

Future developments

Arista does not make its own custom ASICs or develop optics for its switch platforms. Instead, the company uses merchant switch silicon from the likes of Broadcom and Intel.  

According to Hull, such merchant silicon continues to improve, adding capabilities to Arista’s top-of-rack ‘leaf’ switches and its more powerful ‘spine’ switches such as the 7500. This allows the company to make denser, higher-performance platforms that also scale when coupled with software and networking protocol developments. 

Arista claims many of the roles performed by traditional routers can now be fulfilled by the 7500 such as peering, the exchange of large routing table information between routers using the Border Gateway Protocol (BGP). “[With the 7500], we can have that peering session; we can exchange a full set of routes with that other device,” says Hull. 

 

"We think we are on the cutting edge of what can be delivered on a line card” 

 

The company uses what it calls selective route download where the long list of routes is filtered such that the switch hardware is only programmed with the routes to be communicated with. Hull cites as an example a content delivery site that sends content to subscribers. The subscribers are typically confined to a known geographic region. “I don’t need to have every single Internet route in my hardware, I just need the routes to reach that state or metro region,” says Hull. 

By having merchant silicon that supports large routing tables coupled with software such as selective route download, customers can use a switch to do the router’s job, he says.     

Arista says that in 2016 and 2017 it will continue to introduce leaf and spine switches that enable data centre customers to further scale their networks. In September Arista launched Broadcom Tomahawk-based switches that enable the transition from 10 gigabit server interfaces to 25 gigabit and the transition from 40 to 100 gigabit uplinks.

Longer term, there will be 50 GbE and iterations of 400 and one terabit Ethernet, says Hull. And all this relates to the switch silicon. At present 3.2 terabit switch chips are common and already there is a roadmap to 6.4 and even 12.8 terabits by increasing both the chip’s pin count and using PAM-4 alongside the 25 gigabit signalling to double input/ output again. A 12.8 terabit switch may be a single chip, says Hull, or it could be multiple 3.2 terabit building blocks integrated together.  

“It is not just a case of more ports on a box,” says Hull. “The boxes have to be more capable from a hardware perspective so that the software can harness that.”