Experimental Demonstration of Adaptive ... - Optics InfoBase

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CAI et al.: EXPERIMENTAL DEMONSTRATION OF ADAPTIVE COMBINATIONAL QOT 665

format and expanding the channel bandwidth on the impaired

channel [5], [9]. In the literature [5], a 360 Gb/s lightpath maintains

its QoT through 18 dB OSNR degradation by switching

the modulation format of the lightpath from 8 phase-shift keying

to quaternary phase-shift keying (QPSK) and binary phase-shift

keying (BPSK). In this scheme, since only the spectral resources

on the impaired link are utilized for QoT restoration, the load

conditions of other links will not be influenced and the restoration

is constrained to utilizing limited resources of the network.

However, MFS is not a solution for every case of QoT restoration

since, when impairment happens on a link, there might not

be vacant spectrum on that specific linkforalltheimpaired

channels to expand in bandwidth simultaneously.

In this paper, we propose and experimentally demonstrate

an adaptive combinational QoT restoration (ACQR) scheme,

which jointly uses both LR and MFS methods. Such scheme

is able to combat impairments and coordinate the restoration

of connections impaired at the same time within the common

risk group on the same physical link. Simulation results show a

significant decrease in the blocking probability (BP) (by more

than a factor of 2) and a reduction of the load increase caused

by QoT restoration in the bypass links down to 25% compared

with the rerouting-only method. A four-node elastic optical network

testbed with an adaptive control plane shows a successful

QoT restoration when impairments occur in the network.

The rest of this paper is organized as follows. Section II describes

two networking scenarios of a typical network topology

for detailed discussion of the ACQR application. Section III proposes

a hybrid objective algorithm that computes the optimal

way of jointly utilizing available resources on bypass lightpaths

and working paths for QoT restoration. Section IV describes

an experimental arrangement and shows experimental results of

the network testbed using the topology discussed in Section II.

Finally, Section V concludes this paper.

II. NETWORKING SCENARIO WITH ADAPTIVE CONTROL PLANE

Figs. 1 and 2 show the network topology used for algorithmic

study and experimental demonstration. The proof-of-principle

four-node network is connected with link – .Fig.3(a)shows

the node architecture [5]. The node consists of a wavelength selective

switch (WSS) that routes each input to the desired output

and an optical transponder (OTP) for transmission and detection

of data. The control plane of the network links WSS and

OTP for coordinating the configuration of the network. The OTP

can adopt various bandwidth scalable techniques for broadband

elastic channel generation. These techniques include optical arbitrary

waveform generation and measurement [15]–[17], coherent

wavelength division multiplexing [18], Nyquist WDM

[19], and coherent optical orthogonal frequency division multiplexing

[20]. To facilitate real-time adaptation, each input to

the node is equipped with a QoT monitor [see Fig. 3(b)]. The

input signal to WSS is tapped and enters another WSS to

filter out each flexpath. These flexpaths go through performance

monitoring modules which ideally consist of simple, low-speed

components [3]. Previous research [5], [21] has shown that a

supervisory channel overmodulation method can be used for

OSNR and CD performance monitoring using simple and low-

Fig. 1. Four-node network topology and case 1 scenario for demonstration of

ACQR. In case 1, Flexpath B (FP B) is rerouted and Flexpath A (FP A) undergoes

MFS.

Fig. 2. Case 2 scenario for demonstration of ACQR. In case 2, Flexpath A (FP

A) is rerouted and Flexpath B (FP B) undergoes MFS.

Fig. 3. (a) Node structure of elastic optical network and (b) detailed implementation

of QoT monitor.

speed components. The monitoring modules are connected to

a control plane that uses the monitor results as input for realtime

adaptive reconfiguration through field-programmable gate

array [5].

In our four-node topology, there are two original end-to-end

connections using QPSK modulation format established before

the impairment occurs, namely, Flexpath A and Flexpath B,

with 50 and 100 GHz bandwidth, respectively. These paths are

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