Manuscript ID : 268716

1

Abstract—Varying the symbol rate is an alternative

or complementary approach to varying the modulation format or the channel spacing, in order to turn optical networks into elastic networks. We propose to allocate just-enough bandwidth for each optical connection by adjusting the symbol rate such that penalty originating from long cascades of optical filters is contained. This helps reducing over-provisioning for lightpaths where full capacity is not needed, by (i) eliminating unnecessary regenerators and (ii) reducing the power consumption of terminals, when the clock rate of electronics is reduced along with the Baud rate. We propose a novel architecture for an elastic optical interface combining a variable bitrate transceiver, paired with an elastic aggregation stage, with software-defined control. We then report a real-time FPGA-based prototype, delivering flexible transport frames to be sent with a PDM-QPSK modulation format. We interconnect this prototype with a commercial OTN switch and a centralized controller. We demonstrate fast and hitless reconfiguration of the interface and measure the reconfiguration time of hardware logic (

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ent traffic bato a just-enoujusting the syeates flex-framlivers the justndwidth occunstant. This

ghtpaths whentering cascadeal-time FPGAhich has been e under the suwell-suited fortical transmis

nd impairmentThe rest of t

tails the flex andards; Sectigether with obtless behaviorntroller. Finalork.

II. ARCHITECThe elastic o

ta rate in thelecting the apon the receptipability allowsjust the opticxible aggregatThe OTN sta

b-layers: amonnit (ODU) whitical transpor

ne rate since ib-layer alreadultiplexing thagher order Ogregated flow l the gap of teded at the O

ansported line

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ased on tributgh bandwidth

ymbol rate [16ming (e.g. flet-enough bitraupancy as wes helps redn full capacity e penalty. TheA prototype a

presented in upervision of ar an easy and ssion accordins.

the paper is orframing arch

ion III and Ibtained resultr and end-to-lly Section V d

CTURE OF ELptical interfac

e WDM line inppropriate nuion of SDN cons (i) to better fcal pipe to thtion and variabandard containg them thereich encapsulart unit (OTU) it adds forwar

dy exhibits somat allows to enODU or ODis mapped int

the ODU hierOTU sub-laye rate and flexi

Fig. 1: Flexlanes are th

taries (e.g. OTh for each optic6]. The elastic exible OTU siate to take adve keep the cducing over-p

is not neededproof-of-conce

and commerci[17]. These n

an open SDN cprecise manag

ng to light pa

rganized as fohitecture and IV detail the ts characterizi-end reconfigudraws some co

LASTIC OPTICAce not only delnterface, but aumber of activntrol messagefill the opticalhe exact traffble symbol ratins in its digite is the opticaates the client

corresponds rd error correme flexibility wncapsulate lowDUFlex(GFP) to a resizable archy. Now, f

er to bridge tible ODU hier

xible aggregatiohen running at

TN or Etherncal connectionoptical interfa

ignals [18]) avantage of lowchannel spaciprovisioning d and containiept is based onial OTN switetwork elemencontroller, whgement of elas

ath requiremen

ollows: Sectionreview existiproof-of-conc

ing the proposuration to SDonclusions of t

AL INTERFACEivers an adapt

also is capableve clients’ lan. This multilayl pipes and (ii)fic demand wte. tal layer seve

al channel digit frame and tto the resulti

ection. The ODwith either OD

w order ODU infor which t

container [18]flexibility is athe gap betwerarchy. The OT

on principles whhalf the maximu

net) by ace and wer ing

of ing n a tch nts

hich stic nts

n II ing ept sed DN his

E ted e of nes yer ) to

with

eral ital the ing DU DU nto the ] to

also een TN

standarflexibiliwill fulof the pdata ra

Thembeddof 100G400GbEnot nepossibilwhere nof 5G tsignal ccarries by a fulnot comalso actanalyzeevolutiobecomeTarget FlexE s

In

implemspeed dbest evwith XiQPSK sframes implemeach Ointerleaflows. Wy are inmarkernumberphysicalanes polarizaidentifialignme

hen 5 input OTUum baudrate.

rd is now currity at the OTUlly leverage onphysical layer ate, the reach ahe beyond 100ding a variableG units, and E client when ecessarily a lity to describn is still the ntributary slotscould be tran30 tributary sll 200G OTUC

mpleted, this mtively discussied in [20] and ons are closely

e more dynamcompletion da

standard is ab

the elasticmented the OTdue to real-timvaluation boarilinx Virtex 7signal. We int

(i.e. OTU2 mented conceptOTU2 flow isaved at the byWe define a unntegers where rs at regular r of n varyingal output lane

correspondinations). At theication of theent procedure

U2 flows are act

rently under dU sub-layer fon the flexible in order to opt

and spectrum 0G OTN new e parameter nwould be abn = 4. Howevmultiple of

be this kind ofnumber of 100Gs supported. A

nsported in anslots out of theC2. However, umight change. ing the flexiblboth the Ethe

y linked to helmic with moate for beyondout 2018/2020

c optical inTUCn conceptme FPGA maxrd is capable 7 series, henceterleave a varframes). In Ft in real-time fs split into tyte level withniversal markex ∈ [0:9]; y ∈ intervals. Thi

g input client es (in PDM-Q

ng to the e reception, the received dat.

tive out of 10. T

discussion to por beyond 100transmission

timize simultaoccupancy. frame is call

n representing ble to carry thver, the line in100G, thus f frame as anG unit and MAs an exampl

n OTUC2-30 se 40 that wouluntil the OTNThe Ethernetle framing wiernet and OTNlp elastic opticore automatedd 100G OTN

0.

nterface prott, but scaled dximum speed. of running ate ~100Gbit/s

riable number Fig. 1, we illfor n = 5. We the 4 outputh the others aer of the form [0:3]) and we is allows to dflows from the

QPSK, we havtwo quadra

e markers alsota which hel

The output

2

provide more G [19]. This parameters

aneously the

led OTUCn, the number

he expected nterfaces are

there is a n OTUCn-M, M the number

le the 150G since it only ld be carried

N standard is t standard is ith FlexE as N standards cal networks d functions. standard or

totype, we down to low Indeed, the

t ~30GBaud for a PDM-n of 10Gb/s

lustrate the can see that

t lanes and active OTU2

“x.y” (x and insert these

decouple the e number of ve 4 output atures and o permit the p the skew

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validate tmplemented

ogrammable end reconfigurab

has been charceiver combineThe elastic gregation funly one SDN coveloped a resmmands of a NOS [26] via a

g. 2: Architectur

The incoming nd can be sent

optical controequency slot rameter n aultiplied to 1cupied by the e knowledge o

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g. 3: Experiment

268716

III. PROOF-he variable a transmitt

electronic (FPGble with SDNracterized fromed with digitaloptical interfction of an OT

ontroller (Fig. stful server, w

SDN controlla restful API.

re of Elastic Opt

signaling meby a smartph

ol. They incluvia nomin

nd slot widt2.5GHz as deelastic optical

of the hardware elastic optind has been r capabilities, the hardware

S-36. The lineen modified tver into regise the BVT bo

nd used by a l

tal set-up.

-OF-CONCEPT symbol rat

ter prototypGA), electro-op

N controller anm end-to-end ul processing [1face is assocTN switch, bo2). To realize

which is able ler like OpenD

ical Interface w

ssages are in hone or a appliude the inform

al central h with paraefined in [8] l signal. The rre state compical interfaceinterpreted anlow level com

e controller of nux kernel ofo adapt the coters access or

ox (Fig. 2), a local controlle

te concept, pe based ptical conversind restful servup to the off-l6][17].

ciated with toth controlled this function, to interpret t

DayLight [25]

ith controller

http/json formication dedicat

mation relatedfrequency wmeter m to and [27] to

estful server hosed of the OT. When a nnd is compati

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Fig. 4: F The coperformrestful network

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inite State Mas flow is stoppler manages th server whentful server can

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EXPERIMENTAof the BVT haand an off-linexer with a locaphotodiodes ath of 20 GHz oital processingcover the PDMor Rate on the een first charom its local co

m of the PDM-ed by the localof, respectivel

Wait for new frequency demand

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te State Machber of OTU2 land demultiplpeed interfacex-like multiplem 2.67 GBd to nal from 10.7 Go-optical modnding number

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Manuscript ID : 268716

4

We see directly the impact on the spectral occupancy.

Fig. 5: Optical spectrum at BVT output after electro-optical modulation for different OTU2 client numbers and Baud rates.

Fig. 6 represents the electronic eye diagrams for two different configurations. With 10 OTU2 clients at the input of the transmitter, the total output bitrate is maximum with 107 Gbit/s corresponding to a one I/Q electrical signal at 26.7 GBauds (Fig. 6a). Fig. 6b shows the eye diagram when 4 OTU2 clients are multiplexed byte by byte, resulting in a 42.8 GBauds electrical signal on one I/Q arm of the modulator.

a) b) Fig. 6: BVT electrical outputs before E/O conversion for 10 clients (a) and 4 clients (b).

The BVT has been then characterized in terms of quality of transmission and switching time when a new frequency command arrives at the restful server. The switching time displayed in Fig. 7 is the measured time of the total reset sequence of high speed output serializer/deserializer interfaces (GTZ in Xilinx Virtex 7), in which the serializer is aligned, reconfigured and no data is transmitted. This switching time includes ~2 µs guard time, in which the reference clock for the data is unstable [23]. It is technology dependent and may be improved with the most recent programmable electronic technologies and ASIC progress. Fig. 7 illustrates all combinations of any starting bitrate to ending bitrate, both from 10.7 Gbit/s to 107 Gbit/s, corresponding to the number of multiplexed OTU2 clients, which is an integer in the range [1:10]. In all 100 configurations the switching time is below 450 µs and mainly depends of the ending bitrate. For example, case a (423µs) in Fig. 7 is the measured time of the switching from 75 Gbit/s to 107 Gbit/s. Case b (432 µs) is the switching time from 107 Gbit/s to 53.5 Gbit/s. During the reconfiguration phase, the high speed transmitters are reset, and when it’s finished, notification signal is sent to the local controller, which can start again the packets transmission. A maximum pause time of 450 µs on the data flow corresponds

to a maximum of 37 OTU2 frames to be stored and delayed in on-chip memories, which represents 74% of the total available block memories in this FPGA whitout need of external memory.

Fig. 7: Switching time for all starting and ending bitrates combinations (10x10 in total).

When the configuration for the new frequency request is

done, the stored frames can be sent again, avoiding the lost of any data, but with additional latency due to the traffic interruption. This latency is temporary because it can be absorbed after reconfiguration by a real-time adjustment of the buffer emptying and a synchronization with the knowledge of the real traffic load. Latency variation is also limited to the reconfiguration time and the buffer reading, and can be then cancelled due to synchronous and deterministic processing inside the FPGA. With the management of the QoS of the traffic inside the OTN switch and an accurate monitoring of the OTU2 frames number to be multiplexed and transported, service continuity can be performed without any packet loss.

The quality of the elastic optical signal has been measured with the Bit Error Rate vs the OSNR in 0.1nm for different transmitted bitrates (Fig. 8). The OSNR penalty for a Q-Factor of 8.5 dB (HD-FEC limit) remains below 1.1dB for the bitrates of 107 Gbit/s and 75 Gbit/s, compared to the theoretical limit for a PDM-QPSK signal. This is partially due to the skew between the IQ lines of the two polarizations which are not easy to tune in this FPGA prototype. The constellation after offline processing for the two polarization states is inserted in Fig. 8 for the maximum transmitted bitrate of 107 Gbit/s and has been recorded in the laboratory clear and open for all combinations of bitrates.

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Ghazisaeidi, Pnaudier, G. Chaering on the perAM over 37.5 G13. Kozicki H. Takaltering Charaectrum-Sliced Eoc OFC, paper JWRahman, A. Naonkwo, H. de

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ACKNOWLEDGpartly supporant agreementtners for the ring these last

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Manuscript ID : 268716

6

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