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Reprinted with revisions to format, from the March/April 2012 edition of LIGHTWAVECopyright 2012 by PennWell Corporation

By GEOFF BENNETT

Superchannels will save

carriers from the dilemma

of how to flexibly scale

capacity, particularly as

requirements exceed

100 Gbps.

A S THE NEED for ever-

increasing amounts of

DWDM transmission

capacity shows no sign of waning,

the optical transport industry is

moving toward a new type of DWDM

technology – the “superchannel.”

A superchannel is a set of DWDM

wavelengths generated from the

same optical line card, brought

into service in one operational

cycle, and whose capacity can be

combined into a higher-data-rate

aggregate channel. It’s the DWDM

industry’s answer to the question,

“What comes next after 100 Gbps?”

Scaling optical-fiber capacityThe capacity and service flexibility

of optical fiber is remarkable, but

still governed by strict rules of

physics and engineering practicality.

Although written in 2006, Emmanuel

Desurvire’s paper still gives an excel-

lent overview of those limits, while

a more recent paper by Adel Saleh

and Jane Simmons points out that

increases in the spectral efficiency of

optical transport systems ultimately

provides the biggest “bang for the

buck” in terms of capacity scaling

to meet growing internet demand.1,2

But what neither of these papers

covers is that, despite 40% compound

growth in demand over the past

five years (equivalent to a factor of

five increase), service providers

are not able to hire an army of extra

network engineers. In fact, in most

cases headcount will be frozen.

So it’s clear that the optical

transport networks of the future

must be capable of turning up

much larger amounts of DWDM

capacity for a given operational

effort without sacrificing optical

Superchannelsto the rescue!

GEOFF BENNETT is the director of solutions and technology for Infinera. He has more than 20 years’ experience in the data communications industry, including IP routing with Proteon and Wellfleet, ATM and MPLS with FORE Systems, and optical transmission and switching with Marconi as distinguished engineer in the CTO Office.

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FEATURE Superchannels to the rescue!

reach or total fiber capacity. Today

that capacity unit in long-haul

networks is 100 Gbps – a data rate

enabled by a series of advances

in optical transmission, namely:

• High-order phase modulation

(typically polarization-multi-

plexed quadrature phase-shift

keying, or PM-QPSK).

• Coherent detection using a very

stable local oscillator laser.

• Advanced digital signal process-

ing in the receiver to compensate

for fiber impairments.

• High-gain forward error correc-

tion (FEC), including soft-decision

FEC that can offer more than 11

dB of gain for a typical span.

Let’s refer to the combination of

these four items as “coherent techno-

logy,” which offers a quantum leap

in terms of optical performance

compared to non-coherent systems.

While there will likely be incre-

mental improvements in future

coherent technology, these advan-

ces alone are unlikely to keep

up with bandwidth demands.

It’s interesting to note that

computer manufacturers are

facing a similar problem.

You may be aware that CPU

clock speeds appeared to

stop getting faster about

five years ago. Yet the

famous Moore’s law remains

valid in that the number of

transistors on a chip is still

increasing. CPU and GPU

(graphics- processing-unit)

manufacturers are using

those additional transistors

to build multiple cores, rather than

running individual cores at faster

data rates. But the chips they produce

appear as a single unit of processing

capacity to the operating system.

Likewise, a DWDM superchannel

consisting of multiple wavelengths

appears as a single unit of operatio-

nal capacity to the network engineer.

This analogy is shown in Figure 1.

Implementing superchannelsSo what’s the best way to implement

coherent superchannels? Let’s

assume that a service provider

needs to turn up a terabit of optical

capacity in a single operatio-

nal cycle. Today that would mean

installing ten 100G transponders

– an approach that actually takes

more than 10X the effort of a single

transponder because each time

a transponder is added it affects

the existing wavelengths in the

fiber. Since this approach offers

no value for operational scaling,

we will not consider it further.

Instead, Figure 2 shows three

engineering options – A, B, and C

– that we will consider. All three

ServicesO/S

Bandwidthvirtualization

layer

Processingvirtualization

layer

Multi-coreCPU Multi-carrier

superchannel

FIGURE 1. Virtualized parallel processing in the CPU and GPU world (left) and virtualized multi-carrier superchannel in the DWDM transport world (right).

10 lasers40 modulators32-Gbaud electronicsPhotonic ICsTime to market: ~2 years

375 GHz375 GHz

2 lasers8 modulators160-Gbaud electronics~16-nm siliconTime to market: ~7 years

1 TbpsPM-QPSK

375 GHz

1 laser4 modulators320-Gbaud electronics~ 11-nm siliconTime to market: ~10 years

Option A Option B Option C

C-band

FIGURE 2. Comparison of spectral efficiency and electronic-component performance for single-carrier, dual-carrier superchannel, and 10-carrier superchannel implementations.

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FEATURE Superchannels to the rescue!

examples will use PM-QPSK as

the modulation technique:

• Option A is a single-carrier

(i.e., one wavelength) transpon-

der operating at 1 Tbps.

That’s effectively a 100G

transponder where the

electronics run 10X faster.

Unfortunately, electronics

(particularly the analog-

to-digital converter and

DSP chips) that run at the

320-Gbaud rate required

will not be available for

another decade, according to

certain industry roadmaps.

• Option B is a superchannel imple-

mentation consisting of two

500-Gbps “subcarriers,” which

are electronically combined in the

transponder card to appear as a

1-Tbps superchannel. The advan-

tage is that the performance of

the electronics is halved to 160

Gbaud. Unfortunately, we still

have to wait about seven years

before chips with this performance

level are available for products

(they may be available for hero

experiments before this, of course).

• So let’s take that to the next step

with Option C, a superchannel with

10 subcarriers, which divides the

electronics performance by 10

also – and 32-Gbaud electronics is

actually available today. However,

10 subcarriers imply 10 optical

circuits, and coherent technology

already requires a rather large

number of high-quality and there-

fore expensive optical components

even for a single optical circuit.

In fact, a 10-carrier 1-Tbps super-

channel line card would involve

around 600 optical functions in total

for the transmitter and receiver

circuit – quite impractical if built

using discrete optical chips.

Fortunately, DWDM systems

based on large-scale (i.e., multi-

carrier) photonic integrated circuits

(PICs) have been commercially

available since 2004. These PICs

predate the more recent move

toward coherent technologies,

and many skeptics in the DWDM

industry had initially expressed

their doubts that such an advanced

level of optical performance could

be delivered in a commercial PIC.

During the course of 2010 and 2011,

however, a series of incremental field

trials was completed culminating in

a terabit of superchannel capacity

being transmitted over a production

DWDM fiber link between San Jose

and San Diego on the TeliaSonera

International Carrier network last

November. The TeliaSonera trial used

twin pre-production 500G coherent

superchannel line cards, thanks to

large-scale PIC technology. Turning

up this 1 Tbps of capacity took two

operational cycles, one for each

500 Gbps of capacity. This imple-

mentation compares much more

favorably to the “multiple rack”

implementation that’s typical for a

discrete-component superchan-

nel demonstration requiring 10 line

cards of 100 Gbps of capacity.

High-order modulationThose of you with a cable, wireless,

or xDSL technology background

may already be familiar with higher-

order phase modulation. Figure 3

shows the basic principle. Binary

phase-shift keying (BPSK) uses two

phase states per modulation symbol,

which encodes 1 bit in that symbol.

By adding polarization multiple-

xing, PM-BPSK encodes 2 bits per

symbol. We can add phase states to

each symbol to encode additional

bits, enabling us to transmit higher

data rates with much better spectral

efficiency. PM-BPSK will deliver 4

Tbps in the C-band, while PM-16QAM

increases that to about 16 Tbps.

But higher-order modulation comes

at a price. Because optical fiber is a

non-linear medium, each modula-

tion symbol can only be transmitted

at a certain power level before

non-linear effects are triggered.

A series of incremental field

trials culminated in 1 Tbit of

superchannel capacity transmitted

over a production DWDM

fiber link.

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FEATURE Superchannels to the rescue!

While PM-BPSK superchannels could

well be used for transpacific subma-

rine links, the reach of a PM-16QAM

superchannel may be limited.

Going gridlessIn explaining Figure 2, I had said

that 1 Tbps of capacity will require

about the same amount of fiber

spectrum regardless of how many

subcarriers make up the super-

channel. That’s not true if the

channels are “forced apart” to

comply with a fixed grid ITU-T

G.694.1 spacing. This recommenda-

tion defines several grid spacings,

including 25 and 50 GHz. If we

assume a 10-carrier superchan-

nel of 100G per subcarrier, using

PM-QPSK, then the carrier width is

about 37 GHz. That’s too wide for

a 25-GHz grid, yet using a 50-GHz

grid will “waste” about 25% of

the available fiber spectrum.

For this reason ITU-T has updated

G.694.1 to include a “flex grid”

option based on a 12.5-GHz granu-

larity. The spectral width for a

1-Tbps gridless superchannel varies

from about 750 GHz (PM-BPSK)

to about 200 GHz (PM-16QAM).

But all of these superchannels

can be accommodated efficiently

using a multiple of 12.5 GHz.

In the short term, however,

service providers will need a super-

channel that can be deployed on

an existing grid-based DWDM line

system. So the first generation of

commercial superchannel products

will use “split spectrum” super-

channels, a term coined by the IETF.

Split spectrum superchannels could

potentially be designed to operate

on 25- or 50-GHz G694.1 grid line

systems and will provide a seamless

migration from a grid-based to

gridless architecture. Meanwhile,

they’ll also offer the required opera-

tional scaling benefits and only

sacrifice about 20–25% of the

maximum ideal fiber spectrum

(assuming PM-QPSK modulation).

OTN flexibilityAn interesting technical challenge

that results from superchannel

architectures is the need for more

f lexibility for Optical Transport

Network (OTN) transport contai-

ners. The current OTN hierarchy

defines ODU0 (1.25G), ODU1 (2.5G),

ODU2 (10G), ODU3 (40G), ODU4

(100G), and ODUflex (n x 1.25G).

ODUflex was ITU-T’s response

for more f lexible, lower-data-rate

containers. Since superchannels

may vary in their total capacity,

depending on the balance of

capacity and reach needed by the

network designer, it’s necessary to

define an “adaptable” OTN contai-

ner that can be sized accordingly.

At last December’s ITU Study

Group 15 meeting, an “OTUadapt”

proposal gained widespread

support from vendors, component

companies, and service provi-

ders. This f lexibility would help

to solve the nagging problem that

OTN containers are often “out of

sync” with next generation Ethernet

services. Gigabit Ethernet (GbE),

10GbE, and 40GbE all had diffe-

rent but significant issues in their

OTN mapping. OTUadapt will

avoid these issues in the future –

especially since the data rate for

Ethernet services beyond 100GbE

has not yet been defined (note

the IEEE standard is expected

in the 2016-17 timeframe).

Flexible capacityDWDM superchannels potenti-

ally offer an ideal solution to the

twin problems of increasing optical

BPSK1 bit per symbolper polarization

QPSK

8QAM

16QAM

2 bits persymbol perpolarization

3 bits persymbol perpolarization

4 bits persymbol perpolarization

FIGURE 3. Adding more bits to a symbol increases spectral efficiency, but the total power per symbol (before non-linear threshold is reached) is shown by the thick black circle.

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FEATURE Superchannels to the rescue!

transport capacity beyond 100

Gbps and providing the f lexibi-

lity to maximize the combination

of optical capacity and reach. By

implementing a superchannel

with many optical carriers, we can

reduce the requirement for exotic

electronics, allowing this techno-

logy to be delivered much more

quickly than other options. The

key to a multi-carrier superchan-

nel is the use of large scale PICs to

reduce optical-circuit complexity

and offer the maximum f lexibility

for an engineering design.

References1. E. Desurvire, “Capacity Demand

and Technology Challenges

for Lightwave Systems in the

Next Two Decades,” Journal

of Lightwave Technology, Vol.

24, No. 12, December 2006.

2. A. Saleh, J. Simmons, “Technology

and Architecture to Enable

the Explosive Growth of the

Internet,” IEEE Communications

Magazine, January 2011.