Dan Sadot on coherent's role in the metro and the data centre
Gazettabyte went to visit Professor Dan Sadot, academic, entrepreneur and founder of chip start-up MultiPhy, to discuss his involvement in start-ups, his research interests and why he believes coherent technology will not only play an important role in the metro but also the data centre.
"Moore's Law is probably the most dangerous enemy of optics"
Professor Dan Sadot
The Ben-Gurion University campus in Beer-Sheba, Israel, is a mixture of brightly lit, sharp-edged glass-fronted buildings and decades-old Palm trees.
The first thing you notice on entering Dan Sadot's office is its tidiness; a paperless desk on which sits a MacBook Air. "For reading maybe the iPad could be better but I prefer a single device on which I can do everything," says Sadot, hinting at a need to be organised, unsurprising given his dual role as CTO of MultiPhy and chairman of Ben-Gurion University's Electrical and Computer Engineering Department.
The department, ranked in the country's top three, is multi-disciplinary. Just within the Electrical and Electronics Department there are eight tracks including signal processing, traditional communications and electro-optics. "That [system-oriented nature] is what gives you a clear advantage compared to experts in just optics," he says.
The same applies to optical companies: there are companies specialising in optics and ASIC companies that are expert in algorithms, but few have both. "Those that do are the giants: [Alcatel-Lucent's] Bell Labs, Nortel, Ciena," says Sadot. "But their business models don't necessarily fit that of start-ups so there is an opportunity here."
MultiPhy
MultiPhy is a fabless start-up that specialises in high-speed digital signal processing-based chips for optical transmission. In particular it is developing 100Gbps ICs for direct detection and coherent.
Sadot cites a rule of thumb that he adheres to religiously: "Everything you can do electronically, do not do optically. And vice versa: do optically only the things you can't do electronically." This is because using optics turns out to be more expensive.
And it is this that MultiPhy wants to exploit by being an ASIC-only company with specialist knowledge of the algorithms required for optical transmission.
"Electronics is catching up," says Sadot. "Moore's Law is probably the most dangerous enemy of optics."
Ben-Gurion University Source: Gazettabyte
Direct detection
Not only have developments in electronics made coherent transmission possible but also advances in hardware. For coherent, accurate retrieval of phase information is needed and that was not possible with available hardware until recently. In particular the phase noise of lasers was too high, says Sadot. Now optics is enabling coherent, and the issues that arise with coherent transmission can be solved electronically using DSP.
MultiPhy has entered the market with its MP1100Q chip for 100Gbps direct detection. According to Sadot, 100Gbps is the boundary data rate between direct detection and coherent. Below 100Gbps coherent is not really needed, he says, even though some operators are using the technology for superior long-haul optical transmission performance at 40Gbps.
"Beyond 100 Gig you need the spectral efficiency, you need to do denser [data] constellations so you must have coherent," says Sadot. "You are also much more vulnerable to distortions such as chromatic dispersion and you must have the coherent capability to do that."
But at 100 Gig the two - coherent and direct detection - will co-exist.
MultiPhy's first device runs the maximum likelihood sequence estimation (MLSE) algorithm that is used to counter fibre transmission distortions. "MLSE offers the best possible theoretical solution on a statistical basis without retrieving the exact phase," says Sadot. "That is the maximum you can squeeze out of direct detection."
The MLSE algorithm benefits optical performance by extending the link's reach while allowing lower cost, reduced-bandwidth optical components to be used. MultiPhy claims 4x10Gbps can be used for the transmit and the receive path to implement the 4x28Gbps (100Gbps) design.
Sadot describes MLSE as a safety net in its ability to handle transmitter and/or receiver imperfections. "We have shown that performance is almost identical with a high quality transmitter and a lower quality transmitter; MLSE is an important addition." he says.
Ben-Gurion University Source: Gazettabyte
Coherent metro
System vendors such as Ciena and Alcatel-Lucent have recently announced their latest generation coherent ASICs designed to deliver long-haul transmission performance. But this, argues Sadot, is overkill for most applications when ultra-long haul is not needed: metro alone accounts for 75% of all the line side ports.
He also says that the power consumption of long-haul solutions is over 3x what is required for metro: 75W versus the CFP pluggable module's 24W. This means the power available solely for the ASIC would be 15W.
"This is not fine-tuning; you really need to design the [coherent metro ASIC] from scratch," says Sadot. "This is what we are doing."
To achieve this, MultiPhy has developed patents that involve “sub-Nyquist” sampling. This allows the analogue-to-digital converters and the DSP to operate at half the sampling rate, saving power. To use sub-Nyquist sampling, a low-pass anti-aliasing filter is applied but this harms the received signal. Using the filter, sampling at half the rate can occur and using the MLSE algorithm, the effects of the low-pass filtering can be countered. And because of the low pass filtering, reduced bandwidth opto-electronics can be used which reduces cost.
The result is a low power, cost-conscious design suited for metro networks.
Coherent elsewhere
Next-generation PON is also a likely user of coherent technology for such schemes as ultra-dense WDM-PON.
Sadot believes coherent will also find its way into the data centre. "Again you will have to optimise the technology to fit the environment - you will not find an over-design here," he says.
Why would coherent, a technology associated with metro and long-haul, be needed in the data centre?
"Even though there is the 10x10 MSA, eventually you will be limited by spectral efficiency," he says. Although there is a tremendous amount of fibre in the data centre, there will be a need to use this resource to the maximum. "Here it will be all about spectral efficiency, not reach and optical signal-to-noise," says Sadot.
Sadot's start-ups
Sadot had a research posting at the optical communications lab at Stanford University. The inter-disciplinary and systems-oriented nature of the lab was an influence on Sadot when he founded the optical communications lab at Ben-Gurion University around the time of the optical boom. "A pleasant time to come up with ideas," is how he describes that period - 1999-2000.
The lab's research focus is split between optical and signal processing topics. Work there resulted in two start-ups during the optical bubble which Sadot was involved in: Xlight Photonics and TeraCross.
Xlight focused on ultra-fast lasers as part of a tunable transponder. Xlight eventually merged with another Israeli start-up Civcom, which in turn was acquired by Padtek.
The second start-up, TeraCross, looked at scheduling issues to improve throughput in Terabit routers. "The start-up led to a reference design that was plugged into routers in Cisco's Labs in Santa Clara [California]," says Sadot. "It was the first time a scheduler showed the capability to support a one Terabit data stream, and route in a sophisticated, global manner."
But with the downturn of the market, the need for terabit routers disappeared and the company folded.
Sadot's third and latest start-up, MultiPhy, also has its origins in Ben-Gurion's optical communications lab's work on enabling system upgrades without adding to system cost.
MultiPhy started as a PON company looking at how to upgrade GPON and EPON to 10 Gigabit PON without changing the hardware. "The magic was to use previous-generation hardware which introduces distortion as it doesn't really fit this upgrade speed, and then to compensate by signal processing," says Sadot.
After several rounds of venture funding the company shifted its focus from PON, applying the concept to 100 Gigabit optical transmission instead.
100 Gigabit direct detection gains wider backing
More vendors are coming to market with 100 Gigabit direct detection products for metro and private networks.
The emergence of a second de-facto 100 Gigabit standard, a complement to 100 Gigabit coherent, has gained credence with 4x28 Gigabit-per-second (Gbps) direct detection announcements from Finisar and Oclaro, as well as backing from system vendor, ECI Telecom.
"We believe that in some cases operators will prefer to go with this technology instead of coherent"
Shai Stein, CTO, ECI Telecom
ECI Telecom and chip vendor MultiPhy announced at OFC/NFOEC that they have been collaborating to develop a 168-pin MSA, 5x7-inch 100 Gigabit-per-second (Gbps) direct detection module. Finisar and Oclaro used the show held in Los Angeles to announce their market entry with 100Gbps direct detection CFP pluggable optical modules.
Late last year ADVA Optical Networking announced the industry's first 100Gbps direct detection product. At the same time, MultiPhy detailed its MP1100Q receiver chip designed for 100Gbps direct detection.
According to ECI, by having the 168-pin MSA interface, one line card can support a 100Gbps coherent transponder or the 100Gbps direct detection. "This is important as it enables us to fit the technology and price to the needs of end customers," says Shai Stern, CTO of ECI Telecom.
100 Gigabit transmission
Coherent technology has become the de-facto standard for 100Gbps long-haul transmission. Using dense wavelength division multiplexing (DWDM), system vendors can achieve 1,500km and greater reaches using a 50GHz channel.
But coherent designs are relatively costly and 100Gbps direct detection offers a cost-conscious alternative for metro networks and for linking data centres, achieving a reach of up to 800km.
"It [100 Gig direct detection] provides needed performance at an attractive cost, in particular when you are looking at private optical networks," says Per Hansen, vice president of product marketing, optical networks solutions at Oclaro.
Such networks need not be owned by private enterprises, they can belong to operators, says Hansen, but they are typically simple point-to-point connections or 3- to 4-node rings serving enterprises. "Bonding adjacent [4x28Gbps] wavelengths to create a 100Gbps channel that connects efficiently to your [IP] router is very attractive in such networks," says Hansen.
For more complex mesh metro networks, coherent is more attractive. "Simply because of the spectral resources being taken up through the mesh [with 4x28Gbps], and the operational aspect of routeing that," says Hansen.
ECI Telecom says that it has yet to decide whether it will adopt 100Gbps direct detection. But it does see a role for the technology in the metro since the 100Gbps technology works well alongside networks with 10 and 40 Gigabit on-off keying (OOK) channels. "We believe that in some cases operators will prefer to go with this technology instead of coherent," says Stein.
Some operators have chosen to deploy coherent over new overlay networks, to avoid the non-linear transmission effects that result from mixing old and new technologies on the one network. "With this technology, operators can stay with their existing networks yet benefit from 100 Gig high capacity links," says Stein.
Finisar says 100Gbps direct detection is also suited to low-latency applications. "The fact that it is not coherent means it doesn't include a DSP chip, enabling it to be used for low latency applications," says Rafik Ward, vice president of marketing at Finisar.
Implementation
The announced 100Gbps direct detection designs all use 4x28Gbps channels and optical duo-binary (ODB) modulation, although MultiPhy also promotes an 80km point-to-point OOK version (see Table).
Source: Gazettabyte
The module input is a 10x10Gbps electrical interface: a CFP interface or the 168-pin line side MSA. A 'gearbox' IC is used to translate between the 10x10Gbps electrical interface and the four 28Gbps channels feeding the optics.
"There are a few suppliers that are offering that [gearbox IC]," says Robert Blum, director of product marketing for Oclaro's photonic components. AppliedMicro recently announced a duplex multiplexer-demultiplexer IC.
MultiPhy's receiver chip has a digital signal processor (DSP) that implements the maximum likelihood sequence estimation (MLSE) algorithm, which is says enables 10 Gig opto-electronics to be used for each channel. The result is a 100Gbps module based on the cost of 4x10Gbps optics. However, over-driving the 10Gbps opto-electronics creates inter-symbol interference, where the energy of a transmitted bit leaks into neighbouring signals. MultiPhy's DSP using MLSE counters the inter-symbol interference.
100G direct detection module showing MultiPhy's MP1100Q chip. Source: MultiPhy
Oclaro and Finisar claim that using ODB alone enables the use of lower-speed opto-electronics. "This is irrespective of whether you use MLSE or hard decision," says Blum. "The advantage of using optical duo-binary modulation is that you can use 10G-type optics."
Finisar's Ward points out that by using ODB, the 100Gbps direct-detection module avoids the price/ power penalty associated with a receiver DSP running MLSE to compensate for sub-optimal optical components.
Oclaro, however, has not ruled out using MLSE in future. The company endorsed MultiPhy's MLSE device when the product was first announced but its first 100G transceiver is not using the IC.
Finisar and Oclaro's modules require 200GHz to transmit the 100Gbps signal: 4x50GHz channels, each carrying the 28Gbps signal. "This architecture will enable 2.5x the spectral efficiency of tunable XFPs," says Ward. Using XFPs, ten would be needed for a 100Gbps throughput, each channel requiring 50GHz or 500GHz in total.
MultiPhy claims that it can implement the 100Gbps in a 100GHz channel, 5x the efficiency but still twice the spectrum used for 100Gbps coherent.
Finisar demonstrated its 100Gbps CFP module with SpectraWave, a 1 rack unit (1U) DWDM transport chassis, at OFC/NFOEC. "It provides all the things you need in line to enable a metro Ethernet link: an optical multiplexer and demultiplexer, amplification and dispersion compensation," says Ward. Up to four CFPs can be plugged into the SpectraWave unit.
Operator interest
In a recent survey published by Infonetics Research, operators had yet to show interest in 100Gbps direct detection. Infonetics attributed the finding to the technology still being unavailable and that operators hadn't yet assessed its merits.
"Operators are aware of this technology," says ECI's Stein. "It is true they are waiting to get a proof-of-concept and to test it in their networks and see the value they can get.
"That is why ECI has not yet decided to go for a generally-available product: we will deliver to potential customers, get their feedback and then take a decision regarding a commercial product," says Stein.
However MultiPhy claims that this is the first technology that enables 100Gbps in a pluggable module to achieve a reach beyond 40km. That fact coupled with the technology's unmatched cost-performance is what is getting the interest. "Every time you show a potential user some way they can save on cost, they are interested," says Neal Neslusan, vice president of sales and marketing at MultiPhy.
Direct detection roadmap
Recent announcements by Cisco Systems, Ciena, Alcatel-Lucent and Huawei highlight how the system vendors will use advanced modulation and super-channels to evolve coherent to speeds beyond 100Gbps. Does direct detection have a similar roadmap?
"I don't think that this on-off keying technology is coming instead of coherent," says Stein. "Once we move to super-channel and the spectral densities it can achieve, coherent technology is a must and will be used." But for 40Gbps and 100Gbps, what ECI calls intermediate rates, direct detection extends the life of OOK and existing network infrastructure.
ECI and MultiPhy are members of the Tera Santa Consortium developing 1 Terabit coherent technology, and MultiPhy stresses that as well as its direct detection DSP chips, it is also developing coherent ICs.
Further reading: 100 Gigabit: The coming metro opportunity