1 NEGONET A new transmission paradigm in optical networks: Evolution from fixed grid to gridless Dr. Cicek Cavdar [email protected] Next Generation Optical.

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A new transmission paradigm in optical networks: Evolution from fixed grid to gridless

Dr. Cicek [email protected]

Next Generation Optical Networks (NEGONET) group

The Royal Institute of Technology (KTH)

Sweden

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Outline

• Introduction• Motivation• Enabling Technologies• From RWA to RSA• Network Design Shift• Conclusions

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Fixed Grid

• Signals multiplexed into one fiber with a method of “grid”

• Fixed intervals between center frequencies• Problem: Gap between neighboring Ws when

different line rates of Ws are multiplexed. Bandwidth wasted

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Flexible Grid (or Gridless?)

• Central frequencies are not fixed. Enables squeezing!

• Under limited tuning resolutions mini grid is more practical. Gridless is possible if fine-tuning mechanisms are developed for tunable optical components.

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a) ITU-T DWDM frequency grid b) Single slot on the grid approach c) Double sided half slot approach

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Enabling Technologies

• The ongoing advances in photonic technologies make it possible to treat optical fiber as a sharable continuous resource pool.– Optical multilevel modulation, optical

orthogonal frequency-division multiplexing (O-OFDM), and

– Seamlessly bandwidth-variable wavelength selective switch (WSS) .

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Optical-transport networks in 2015• In September 2006, about five years after the

telecom bubble burst, Nippon Telegraph and Telephone Corporation (NTT) conducted an experiment that set a new world record for optical-fibre communication, by sending 14 Tbit s- 1 over 160 km of optical fibre. This experiment, which involved simultaneously transmitting 140 wavelength-division and polarization-division multiplexed (WDM/PDM) signals each with a bit rate of 111 Gbit s- 1, delivers two important statements about the probable direction of optical-fibre communication technologies.

Masahiko Jinno, Yutaka Miyamoto & Yoshinori Hibino, Nature Photonics, 2007

A wavelength in a link is no longer a static path between two specific connection end-points, but, now, part of a pool of flexible wavelengths or bandwidths that can be reconfigured on request

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State of the Art: Worst case design in Modulation Format

• For example, once the modulation format to transmit the 40 Gb/s client data streams is determined to be, say, differential quadrature phase shift keying (DQPSK), the format is applied to every 40 Gb/s optical path. In such a case every path occupies the same spectral width, regardless of each path’s distance or the number of transmitted hop nodes.

• As a result, most optical paths whose path lengths are far less than that of the worst case have large transport margins at the receiving end.

• If, instead, an adaptation mechanism for various physical impairments were introduced into the optical networks, the utilization efficiency could further be enhanced.

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Flexible Spectrum Allocation

• Distance-adaptive spectrum resource allocation• Minimum necessary spectral resource is

adaptively allocated according to the end-to-end physical condition of an optical path

• Two important parameters to determine the necessary spectral resources to be allocated for an optical path– Modulation format and – optical filter width.

Masahiko Jinno, Bartlomiej Kozicki, Hidehiko Takara, Atsushi Watanabe, Yoshiaki Sone, Takafumi Tanaka, and Akira Hirano, NTT Corporation, “Distance-Adaptive Spectrum Resource Allocation in Spectrum-Sliced Elastic Optical Path Network”, Com. Mag. Aug. 2010.

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Link Adaptation Technologies

• Link adaptation is a widely used key technology that increases the spectral efficiency of broadband wireless data networks and digital subscriber lines.

• The basic idea behind link adaptation technologies is to adjust the transmission parameters, such as modulation and coding levels, to take advantage of prevailing channel conditions.

• The most efficient set of transmission parameters under a certain channel condition is selected based on criteria such as maximizing data rate or minimizing transmit power

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Link adaptation

• Under good channel conditions an efficient set of parameters optimized for spectral efficiency (i.e., more bit loading per symbol using multilevel modulation schemes and less redundant error correction) is used to increase throughput.

• In contrast, under poor channel conditions a set of parameters optimized for robustness (i.e., less bit loading and more redundant data adding) is used to ensure connectivity.

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Modulation Format Adaptation• In good channel conditions: 16-ary quadrature-

amplitude modulation (QAM), despite having worse receiver sensitivity than quadrature phase shift keying (QPSK), provides – better spectral efficiency and – able to transmit double the data rate under

good channel conditions.• Some attempts to introduce link adaptation

technologies into optical networks[1] Q. Yang, W. Shieh, and Y. Ma, “Bit and Power Loading for

Coherent Optical OFDM,” IEEE Photonics Tech. Lett., vol. 20, no. 15, 2008, pp. 1305–7.

[2] O. Rival, A. Morea, and J. Antona, “Optical Network Planning with Rate Tunable NRZ Transponders,” Proc. ECOC ‘09, 2009.

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Rate tunable planning [1][2]

• Data rate dynamic adjustment according to the quality of channels was experimentally demonstrated by using bit and power loading of optical OFDM subcarriers without modifying the channel bandwidth and launch power. [1]

• Network performance of symbol rate tuning in non-return-to-zero (NRZ) modulation formats in an opaque optical network was also studied [2]

• It was shown that reach-dependent link capacity adjustment can benefit from the added available capacity for short-distance demands and from the saved optoelectronic interfaces on lowrate long-distance demands .

• Such studies are based on the idea of increasing data rate for shorter links with large signal-to-noise ratio (SNR) and nonlinear effect (NLE) margins in opaque optical networks based on the fixed International Telecommunication Union — Telecommunication Standardization Sector (ITU-T) frequency grid.

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Distance-Adaptive Spectral Resource Allocation• 1. Bits per symbol adjustment for spectrum

resource saving• 2. Filter Bandwidth adjustment for spectrum

resource saving

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1. Bits per symbol adjustment for spectrum resource saving• The traditional link adaptation technologies that maximize

channel data rates may also be considered in the following manner: the unused SNR and NLE margins for shorter connections can be used for spectrum resource saving, while ensuring a constant data rate.

• For the same data rate 16-QAM carries twice the number of bits per symbol of QPSK, therefore requiring half the symbol rate and, consequently, half the spectral bandwidth.

• Similarly, 64-QAM carries three times the number of bits per symbol of QPSK, and requires 1/3 the spectral bandwidth.

• Thus, spectral bandwidth can be saved by reducing the symbol rate and increasing the number of bits per symbol to transmit the same data rate. Since higher-level bit loading decreases the distance between the two closest constellation points, 16-QAM and 64-QAM suffer from SNR penalty per bit of 4 dB and 8.5 dB, respectively, when compared with QPSK [3].

• [3] K.-P. Ho, Phase-Modulated Optical Communication Systems, Springer, 2005.

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Single Carrier and Multi Carrier Difference• In the single-carrier modulation approach, the

symbol rate is reduced to obtain a narrower spectral width while increasing the number of bits per symbol to keep the data rate constant. Conversely,

• In the multicarrier approach the number of subcarriers, with uniform symbol rate and bits per symbol, is changed to adjust the spectral bandwidth

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Parameters in tunable spectral width modulation format

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2. Filter Bandwidth adjustment for spectrum resource saving • When the scope of discussion is extended from point-to-point to

wavelength-routed transparent optical networks, it is necessary to consider the effect of waveform distortion due to cascaded optical filters in ROADMs and WXCs.

• Cascaded optical filters introduce significant narrowing of the passband, the filtered optical signal suffers distortion due to unwanted spectral clipping.

• The effect is stronger for optical paths experiencing larger numbers of node hops. The current design of filters in optical nodes allows for the accumulated filter narrowing effect by assuming wide filter bandwidth to accommodate the worst case (i.e., the paths with the largest numbers of node hops). As a result, most optical paths have large spectral clipping margins and are assigned redundant spectral resources.

• Spectral resource saving is achieved by adaptively choosing filter bandwidth according to the numbers of node hops. The necessary minimum bandwidth of optical filters for an optical path is determined to ensure that the effective passband of cascaded filters measured at the end of the optical path maintains acceptable performance.

• This functionality can be realized using bandwidth-variable optical filters employing a spatial light modulator, such as liquid crystal on silicon (LCoS), configured with a dispersive element to separate WDM signals.[4][4] G. Baxter et al., “Highly Programmable Wavelength Selective Switch Based on Liquid Crystal on

Silicon Switching Elements,” Proc. OFC/NFOEC ‘06, 2006, paper no. OTuF2.

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Transmission parameters

• Modulation format and coding levels• Adjusting the number of bits per symbol =

Choosing the right modulation format• Adjusting the filter bandwidth• --> To ensure constant data rate

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Available spectral resources in an optial fiber• Restricted by the gain bandwidth of the optical

amplifier used. • When Erbium-doped fiber amplifiers for C-band

or L-band are used, the available spectral width ranges from about 4 to 5 THz.

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Spectrum resource allocation in distance adaptive SLICE

• In distance-adaptive SLICE the most efficient set of transport parameters is chosen to minimize the allocated spectrum resources under a certain optical path condition while keeping the data rate unchanged. The parameters to be adapted include

– modulation level and– optical filter bandwidth.

• Path A: 16-QAM and filter width of 37.5 GHz) is selected.

• For path C or D having a larger number of node hops, a more robust set of parameters (e.g., QPSK and 50 GHz) is utilized.

• Since the filter narrowing effect is most crucial for the longest path, B, the broadest filter bandwidth (e.g., 62.5 GHz) should be assigned to ensure an acceptable passband at the egress optical node.

• From the viewpoint of practical implementations:

– Spectral frequency resource in optical fibers may be quantized to an appropriate unit, which we refer to as a frequency slot.

– If a 12.5 GHz slot width is assumed, the 37.5, 50, and 62.5 GHz spectra correspond to 3, 4, and 5 slots, respectively.

– The channel spacing is standardized by the ITUT frequency grid with granularities of 12.5, 25, 50, or 100 GHz

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New Network Problems

• Wavelength assignment Spectrum Assignment• Wavelength Continuity Spectrum Continuity

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Wavelength and Spectrum Assignment• RWA + Spectrum Continuity Constraint [1]• Given: Fixed routes from s to d.

– All frequency slots are numbered.• (1) When a connection request arrives, select

route from the list.• (2) Search for contiguous frequency slots along

the path/route. • (3) Try all routes until you find one.

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Flexible optical WDM network

• In a FWDM network, the control plane must follow

• (1) the wavelength continuity constraint, which is defined as the allocation of the same wavelength on each fiber link along the route of a channel,

• (2) the spectral continuity constraint, which is defined as allocation of the same continuous spectrum on each fiber along the route of a channel, and

• (3) the spectral conflict constraint, which is defined as non-overlapping spectrum allocation to different channels on the same fiber.

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NEGONETDefragmentation of Transparent Flexible Optical WDM (FWDM) Networks• Network defragmentation problem for FWDM

networks is formulated, and heuristics are proposed minimizing the number of interrupted connections.

• Given: set of existing connections operating at a specific line rate (a spectrum) and a wavelength.

Ankitkumar N. Patel, Philip N. Ji, Jason P. Jue, Ting Wang, OFC 2011

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NEGONETDefragmentation of Transparent Flexible Optical WDM (FWDM) Networks

• D) Shifted to lower Ws without changing their routes

• C) request G can be rerouted on path B-C-A since both fiber link (B, C) and (C, A) have sufficient continuous spectrum (75 GHz) available at the same wavelength 191.65 GHz. However, the same connection cannot be routed on the same path at wavelength 191.65 GHz due to the spectral conflict constraint.

Ankitkumar N. Patel, Philip N. Ji, Jason P. Jue, Ting Wang, OFC 2011

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Bandwidth Allocation in Flexible OFDM based networks

• Dynamic case: OFC 2011 • Static case: ECOC 2010• Physical layer impairments are neglected• ILP for static: Candidate paths and routings are

given. • Objective: Min.utilized spectrum

I. Thomkos and Varvarigos, OFC 2011

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Bandwidth Allocation in Flexible OFDM based networks

• The spectrum is divided in subcarrier slots of F GHz, and each subcarrier is mapped to an integer number. To route the paths through the WXC a guardband of G subcarriers has to separate adjacent spectrum paths. Serving a connection i that requires Ti subcarriers is translated to finding a starting subcarrier frequency fi after which it can use Ti contiguous subcarriers (in addition to the guardbands).

• Spectrum traffic matrix is given. (number of subcarriers required, corresp. to tr. rate)

• Spectrum continuity constraint is translated to non-overlapping spectrum allocation. Thus, the starting frequencies of the connections that utilize a common link are ordered so that their allocated spectrums do not overlap.

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NEGONETDynamic Bandwidth Allocation in Flexible OFDM based networks

(1) No distance consideration [5]• For each source-destination pair, traffic model,

the biggest and smallest fluctuation rates are known together with current traffic rate.

• Two types of subcarriers in the network: – (a) those that are pre-reserved by the

connections (“guaranteed” or reserved subcarriers)

– (b) shared on a demand basis, and can be allocated/deallocated to the connections according to their time-varying requirements (“best effort” or shared subcarriers).

(1) Distance consideration [6]• Only batch arrivals are covered and same

approach as in [1][5] I. Thomkos et. al., “Dynamic Bandwidth Allocation in Felxible OFDM based networks”, OFC 2011[6] T. Takagi et. al, “Dynamic Routing and Frequency Slot Assignment for Elastic Optical Path Networks”, OFC 2011

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Traffic Grooming

• In order to reduce the overhead of filter guardband

• Gigher spectrum efficiency with TG

[7] Y.. Zhang et. al., “Traffic Grooming in Spectrum Elastic Optical Path Networks”, OFC 2011

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What we need for Flex-Grid?

• Control Plane which can do periodic “Garbage collection” by regrooming channel allocations to maximum contiguous free spectrum.

• Software defined optical transceivers• Ability to dynamically change channel

assignments for circuits in service • Any routing and channel assignment algorithm

must be rather sophisticated to take non-linearalities into account.

• We need WSS switch 1x20-ish ports in about the footprint of today’s 1x5 ports.

• It is very hard to calculate non-linear impairments in such a mixed line-rate, mixed-modulation format channel. No way to guarantee. (?) Maybe you start, monitor, and backoff and try again (??)

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Flexibility and adaptiveness

• What changes?– Modulation Format– Symbol rate– Data rate ( Ch. Capacity)– Spectral Width– Reach

• Why?– Adapt to

• Traffic• Impairments• Fiber types

• Adapt rate to traffic demand• Adapt modulation format to impairments

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Optical follows Radio Communication (?)• Can we learn from radio communication?• Emergence of small cell other then building huge

cell towers: Move from Macro cell to metro cell-pico cell-femto cell..

• SDR : Software defined radio (similarities with software defined transceivers)

• Flexibility in– Channel coding, – Modulation, – Multiple carriers, – Spatial Diversity

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Thank You!

Questions?