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11institutetext: F. Author is working as an independent researcher
11email<EMAIL_ADDRESS>
# Optical Networking in Future-land: From Optical-bypass-enabled to Optical-
processing-enabled Paradigm

Dao Thanh Hai

(Received: date / Accepted: date)

###### Abstract

The evolution of optical networks is enabled by both technological and
architectural advances with the goal of reducing operational and capital
expenditure per transmitted bit. While the former one stimulates significant
system capacity, the latter paves the way for reducing the effective traffic
load in network so that more traffic can be carried. Accordingly, optical node
architectures have been transitioning from optical-electrical-optical mode to
all-optical one, leveraging the scalability and efficiency of fully optical
cross-connecting. Conventional wisdom in designing and architecting such
switching nodes is nevertheless rooted in the intuition that when an optical
channel crossing an intermediate node, it should be maximally isolated from
other optical channels in order to avoid interference which may result in
degrading signal quality. Such long-established paradigm perceiving the
interference of optical channels transiting at the same node as an adversarial
factor and should therefore circumvent, albeit reasonable, may leave vast
unexplored opportunities. Indeed, the rapid advances in all-optical signal
processing technologies has brought opportunities to re-define the optical
node architecture by upgrading its naive functionalities from simply add/drop
and cross-connecting to proactively mixing optical channels in photonic
domain. Specifically, all-optical channel aggregation and de-aggregation
technologies have been remarkably advancing in recent years, permitting two or
more optical channels at lower bit-rate and/or modulation formats could be
all-optically aggregated to a single channel of higher-rate and/or higher-
order modulation format and vice versa. Such evolutionary technique is poised
to disrupt the existing ecosystem for optical network design and planning, and
thus necessitates for a radical change to unlock new potentials. To that end,
this paper presents a new paradigm for future optical networks, namely,
optical-processing-enabled networks, which are powered by in-network all-
optical mixing capability. We introduce the operational principle of optical
channel aggregation (de-aggregation) and show how spectrally beneficial such
innovative operations could yield by an illustrative example. Next, in order
to maximize the aggregation opportunity, we present a mathematical model for
optimal routing based on integer linear programming model. Numerical results
on the realistic network topology COST239 are provided to quantify the
spectral gain of aggregation-aware routing compared to the conventional one.

###### Keywords:

optical-bypass network optical-processing networking optical aggregation
routing integer linear programming optical transport network

##  1 Introduction

Broadband Internet has become necessity for all people globally and most of us
might not imagine how we would work, interact with others and shopping without
it. However, less known is that the majority of Internet traffic have been
carried out in optical fiber and indeed, optical communication and networking
have become the foundation of global digital connectivity. In handling the
continued explosive Internet traffic growth that doubles roughly every two
years Cisco20 , numerous revolutionary technological and architectural
solutions have been proposed and implemented to increase the system capacity
on one hand, and on the other hand, to decrease the capital and operation
costs. Major breakthroughs include coherent transmission, higher-order
modulation formats, spectrally and spatially flexible optical transmission and
recently wide-band optical transmission 20years ; ir4 . For many years, the
capacity of an optical fiber has been considered as being nearly infinite and
indeed, the system capacity of a single fiber in optical core networks has
undergone leap-and-bound growth with a spectacular rise from the milestone of
1 Tb/s achieved in around 1996 to 319 Terabit/s across 3000 km recently set by
NICT in 2021 NICT . In a quarter of century, the system capacity has attained
more than 300-fold growth and such achievements have been owing to the
convergence of remarkable advances in electronic, photonic and digital signal
processing technologies. However, enlarging the transmission capacity is just
one side of operator’s solutions to accommodate unprecedented traffic
increases. The other side which is at least equally important to operators is
on a desire to exploit such continuously expanding capacity in a cost and
energy-efficient manner through innovative approaches in network
architectures.

From architectural perspectives, optical networking has undergone a paradigm
shift in the 2000s time frame with a transition from optical-electrical-
optical mode to optical-bypass and/or all-optical operations Simmons . Such
transition has been first driven by the observation that the majority of the
traffic entering an optical node are transitional and therefore, can be kept
in optical domain as they traverse the intermediate node rather than be
electronically processed. Second the significant advances in optical switching
technologies coupled with long-haul transmission has been instrumental to turn
the observation into reality. Indeed, the arrival of optical-bypass-enabled
networks have permitted the elimination of massive optical-electrical-optical
interfaces and thus, resulting in substantial savings in both capital and
operational expenses. As such, optical-bypass networking has greatly matured
since the early 2000s and is now a well-adopted technology that have been
deployed widely by carriers in both metro and backbone networks all-optical .
In optical-bypass networks, it is noted that the key functionalities of an
optical node are simply to add/drop traffic and optically cross-connect
transitional lightpaths. Most importantly, the foundation of optical-bypass
networking paradigm is relied on the conventional wisdom that when an optical
channel crossing an intermediate node, it should be maximally isolated from
other optical channels in order to avoid interference which may result in
degrading signal quality. Such long-established paradigm perceiving the
interference of optical channels transiting at the same node as an adversarial
factor and should therefore circumvent, albeit reasonable, may leave vast
unexplored opportunities. In fact, the rapid advances in all-optical signal
processing technologies has brought up unique opportunities to re-define the
optical node architecture by upgrading its naive functionalities from simply
add/drop and cross-connecting to pro-actively combining transitional optical
channels in photonic domain so as to better utilize spectrum capacity
optical_processing_1 ; optical_processing_2 .

The capability to process signal directly in optical domain rather than
converting it to electrical has been attracting the research communities for
many years thanks to its super-efficiency in cost and energy and also its
scalability to higher bit-rates optical_processing_1 ; optical_processing_2 ;
optical_processing_3 . Although fiber nonlinearity has been usually referred
as a major transmission impairment in optical fiber communications suggested1
; suggested2 , its other uses have been extensively exploited for realizing
various optical signal processing techniques. Among optical signal processing
functions, all-optical aggregation of lower-capacity channels into a single
higher-capacity channel and de-aggregation of a higher capacity channel into
many lower-capacity channels have been advancing significantly in recent years
from both theoretical, experiments and proof-of-concept demonstrations
aggregation1 ; aggregation2 ; ref1_22 ; ref2_22 ; ref3_22 . The arrival of
such functionality performing fully in optical domain paves the new way for
improving the re-configurability and flexibility of optical networks in an
attempt to attain greater spectral efficiency. Specifically, incorporating
all-optical channel aggregation and de-aggregation at intermediate optical
nodes re-defines the networking architecture as it challenges the long-
established foundation of avoiding unwanted interference between transiting
optical channels. Rather than simply performing add/drop function and/or
cross-connecting individual optical channels as in optical-bypass networking,
optical channels transiting the same intermediate node have new opportunities
of being added together in photonic domain for greater spectral efficiency.

In this paper, we propose a future-forwarding architectural paradigm,
entitled, optical-processing-enabled for optical networks by leveraging the
in-network all-optical mixing capability. In Sect. 2, related works and
contributions are highlighted. We then introduce the operational principle of
optical channel aggregation de-aggregation in Sect. 3. How spectrally
beneficial such innovative operations could yield is featured by an
illustrative example in Sect. 4. Next, in order to maximize the aggregation
opportunity, we present a mathematical model for optimal routing based on
integer linear programming model in Sect. 5. Section 6 is dedicated for
numerical results on the realistic network topology COST239 quantifying the
spectral gain of aggregation-aware routing compared to the conventional one.
Finally, the conclusions and perspectives are discussed in Sect. 7.

##  2 Related Works and Our Contributions

Optical-bypass networking have been well-developed in the last two decades
with several research works for sustaining explosive traffic growth all-
optical . A central problem to solve in optical-bypass-enabled networks is the
routing and resources allocation Algorithm1  and algorithms for solving such
problem have been comprehensively investigated Algorithm2 ; Algorithm3 ;
hai_iet  addressing various objectives such as spectrum efficiency,
throughput, congestion level and energy consumption. The elastic optical
technologies with optical-bypass architecture have been proposed and studied
EON3 ; EON4  as a substitution for fixed-grid optical networks toward greater
spectrum efficiency. Various optical layer protection techniques including
dedicated and shared protection have been developed and implemented by relying
on optical-bypass architecture hai_csndsp ; hai_ps2 ; hai_wiley . Recently,
space division multiplexing (SDM) technologies such as multicore fiber (MCF)
and multi-mode fiber (MMF) have been intensively researched and in the context
of optical-bypass framework, various algorithms for solving routing, spectrum,
and core and/or mode assignment problem have been proposed to optimally
exploit the expanded capacity enabled by SDM sdm1 ; sdm2 . In an effort for
enhancing optical-bypass-enabled networks, proposals have been made by
incorporating simple optical signal processing including wavelength/spectrum
conversion, regeneration, and format conversion and these changes have been
helpful in boosting the network performance regenerator1 ; regenerator2 ;
regenerator3 . However, the performance improvement has come at a cost of more
complicated network design problems. Moreover, it should be noticed that such
adjustment has been limited to the optical manipulation of single individual
transiting lightpath and therefore, missed the potential benefits derived from
a wide range of signal processing enabled by controlled interference between
through lightpaths.

In an attempt to introduce various signal processing functions for optical
channels crossing an intermediate nodes, network coding NC  (NC) has been used
in optical networks to attain greater throughput, security and capacity. The
underlying idea is that intermediate nodes, instead of simply storing and
broadcasting data as in conventional networking paradigm, is allowed to
manipulate data and then forward such (non-) linearly mixed data to its
output. Different from broadcast nature of signals in wireless networks,
mining the bandwidth benefits of NC in optical networks have nevertheless
required multiple signals that can be favorably mixed together and such
condition often emerges from multi-cast scenarios nc-general3 ; nc-ofdm ,
passive optical networks NC-PON , or the case of disjoint paths in protection
Kamal  which are prime targets of network coding research in optical networks.
The majority of these works were however restricted to electronic network
coding in combination with optical-electrical-optical operations. Proposals
from xor3 ; all-optical-nc  have examined the enabling technologies for
performing network coding in photonic domain. Next the works in hai_comletter
; hai_access ; hai_oft ; hai_comcom ; hai_comcom2 ; hai_springer ; hai_rtuwo ;
hai_systems ; hai_springer2 ; nc_others9 ; nc_others7  have renewed the
interests and demonstrated potential benefits of adding photonic network
coding layer to optical networks. The use of digital all-optical physical-
layer network coding has also been extended to mm-wave radio-over-fiber
networks nc_others1 . In the realm of visible light communication, there have
been increasing works addressing the use of network coding nc_others2 ;
nc_others3  for boosting capacity and/or transmission qualities. A lately
published work n3  demonstrating channel-aware network coding has achieved
around two orders of magnitude reductions in the bit-error-rate (BER). Data-
center networking has also been an active area for applying NC to reduce
effective traffic nc_others5 ; nc_others6 . Recently, physical layer
encryption with NC has first been addressed in nc_security1 ; nc_security2
and marked an interesting avenue for re-considering the encryption at scale.

Optical aggregation and de-aggregation have been progressing fast in recent
years, permitting two or more lower bit-rate channels being added to a single
higher bit-rate optical_processing_1 ; optical_processing_2 ; aggregation1 ;
aggregation2 . The maturing of such optical signal processing technology
offers unique opportunities to re-imagine the functions of optical switching
node in optical-bypass realm. Specifically, integrating the all-optical
aggregation function to optical nodes is expected to reverse the conventional
assumption governing optical-bypass networking that each transiting lightpath
must be isolated from others to avoid unwanted interference. Indeed, by
allowing the optical mixing of transitional optical channels, a new optical
networking paradigm, namely, optical-processing-enabled networks, has been
emerged. To the best of our survey, no previously published works have
addressed the concept of optical-processing-enabled architecture. Our main
contributions are therefore highlighted as followed:

  * •

We present an architectural proposal for future optical networks by leveraging
the optical mixing operations between transitional lightpaths crossing the
same intermediate node.

  * •

We propose the integration of all-optical channel aggregation and de-
aggregation operation into optical nodes to attain greater network efficiency

  * •

We develop a mathematical framework for maximizing the spectral benefits in
aggregation-aware optical networks

  * •

We showcase numerical results on the realistic network topology COST239 to
demonstrate the spectral efficacy of optical-processing-enabled networks

##  3 Optical Channel Aggregation and De-aggregation: Operational Principles
and Perspectives

In recent years, we have been witnessing a transition in computing moving from
electronics to the emerging field of photonics and this has been driven by the
massive investment in photonic computing with the promise of solving the
Moore’s law. Essentially, photonic processing can overcome the fundamental
physical limitation of modern electronic processing much in the way that
electronic systems surpassing the physical limitations of mechanics. Different
from electronic, photonic can make use of all four physical dimensions
including amplitude, wavelength, phase and polarization to achieve a wide
range of signal processing and computing capabilities. All-optical processing
is envisioned therefore as the blueprint for future communication networks to
overcome electronic bottleneck and to get in Tb/s regime in a super-efficient
way.

In line with unprecedented advances in photonic processing, all-optical
aggregation and de-aggregation technologies have been proposed, investigated
and implemented. It has to be noted that the operation of aggregating lower-
speed channels into a single higher bit-rate one and de-aggregation
constitutes the major task in optical networks and traditionally, this
important operation has been performed in the electrical domain. In overcoming
the scalability issues of electronic aggregation, various all-optical methods
have been proposed based on nonlinear effects. Specifically, the goal of
channel aggregation are realized through processes such as four-wave mixing,
cross-phase modulation, self-phase modulation and cross-gain modulation which
are generated from the second or third-order susceptibility of nonlinear
mediums, particularly, highly nonlinear fiber and semiconductor optical
amplifiers. There have been increasing experimental works for optical
aggregation; for instance, aggregation4  demonstrated combination of multiple
on-off keying (OOK) signals to a MPSK signal, optical_processing_2  performed
adding of two QPSK signals to a 16-QAM signal and aggregation3  implemented
four OOK signals to a polarization division multiplexing (PDM) QPSK signal.

A recent experimental works demonstrating remarkable results for 10 Gbaud
QAM-16 signal generation from aggregation of two 10 Gbaud QPSK signals has
been presented in aggregation5 . Different from traditional approaches relying
on nonlinear effect, this work has marked a departure with a completely linear
method for the vector summation of the input channels which is realized by
Kerr combs using ring resonators and coherent comb sources like mode-locked
lasers. This technique is called coherent vector addition whose merits are
notably operational reconfigurability, less system complexity and less power
consumption compared to nonlinear-based ones. Figure 1 shows the schematic
diagram of adding two QPSK signals into one 16-QAM and decomposing such 16-QAM
again into two separate QPSK signals. This operation helps to increase the
channel utilization by freeing up the low-speed channels in favorable
conditions and thus doubling spectrum efficiency.

Figure 1: Schematic Illustration of the Aggregation of two QPSK signals into a
single 16-QAM one and vice versa Figure 2: A generic model for optical
aggregation and de-aggregation

In a forward-looking perspective, accelerated investments and advances in
photonic signal processing will be expected to create more possibilities for
aggregating optical channels in a programmable manner. Functionally speaking,
the aggregation between two (or more) optical channels of different bit-rates
and/or formats will be performed in a way that the transmission properties for
output signal including wavelength/spectrum and/or modulation format could be
subject to selection. As shown in Fig. 2, two optical channels $X$ and $Y$
with transmission parameter $i$ and $j$ (i.e., wavelength/spectrum and
modulation format) respectively could be optically aggregated to a single
optical channel $Z$ whose line rate is equal to the sum of $X$ and $Y$ and
with selectable transmission properties $k$ (e.g., higher-order modulation
format). It is expected that the arrival of such optical functionality will
have massive impacts to future optical networks including design, planning and
management. In the next part, we will highlight the networking impact of
incorporating optical aggregation and de-aggregation to optical nodes.

##  4 A New Networking Paradigm: Optical-Processing-Enabled Network

The recent major revolution in optical networking architecture has started in
the year 2000s with the advent of optical-bypass-enabled networks, permitting
elimination of massive optical-electrical-optical interfaces and therefore,
generating substantial cost savings. In optical-bypass networking, the basic
functions of an optical node includes add/drop and cross-connect optical
channels. Initiatives for improving optical-bypass networking have been
proposed by integrating simple optical signal processing functions including
regeneration and wavelength/format conversion for each individual transitional
lightpath. However, the foundation of optical-bypass networking paradigm
remain immutable that each lightpath crossing an intermediate node should be
separately from others to mitigate unwanted interference which may degrade the
optical signal qualities. In light of tremendous progresses in optical signal
processing permitting precisely controlled interference between optical
channels, superposition of transiting lightpaths traversing the same optical
node pave the way for redefining the optical network architecture. We
highlight this perspective by leveraging the integration of all-optical
aggregation and de-aggregation capability through the following example.

Assuming that there are two traffic demands $a$ and $b$ with 100 Gbps from
node $A$ to node $C$ and from node $B$ to node $C$ respectively. In optical-
bypass networking, accommodating such two requests involves finding the route
and assigning the appropriate wavelength for each demand and it is shown in
Fig. 3. It is noted that the two lightpaths carrying traffic for demand $a$
and $b$ are routed over the same link $XI$ and $IC$, and more importantly,
both are optically crossed node $X$. In terms of wavelength count across link
$XI$ and $IC$ two wavelengths are thus needed.

Figure 3: Traffic Provisioning in Optical-bypass Networking

We turn the attention to the optical-processing-enabled paradigm where all-
optical aggregation at node $X$ is enabled. In this paradigm, two transitional
lightpaths $a_{\lambda_{1}}$ and $b_{\lambda_{2}}$ could be optically mixed at
node $X$. Specifically, two 100G QPSK signals modulated on wavelength
$\lambda_{1}$ and $\lambda_{2}$ respectively are optically aggregated to a
single higher-order modulation format signal, that is, 200G 16-QAM, modulated
on wavelength $\lambda_{3}$. By aggregating lower-capacity channels into a
single higher-capacity channel of higher-order modulation format, greater
spectrum efficiency could be realized. At the common destination node $C$, the
aggregated signal could be optically decomposed into individual ones as shown
in Fig. 4.

Figure 4: Optical-processing-enabled Paradigm with Optical Aggregation and De-
aggregation

It should be noted that the optical-processing-enabled paradigm introduces
more networking flexibility by permitting the controlled interference among
two or more favorable lightpaths and this poses important ramifications for
network design algorithms to maximize the potential benefits. For the case of
optical aggregation above, the critical problem to be addressed is the
determination of which pair of demands could be optically aggregated, where
are the nodes for optical aggregation and what are the routes for aggregated
lightpaths. In the next section, we provide an optimal mathematical framework
for solving such problem.

##  5 A Mathematical Formulation for Optimal Aggregation-aware Routing

In order to leverage the use of optical aggregation and de-aggregation in
network design and planning, we restrict our studies to the practical and
simple setting where the aggregation is only performed between two demands of
the same line-rate and modulation format while the de-aggregation takes place
only at the common destination node. This can be achieved by making use of a
pretty maturing optical aggregator combing two QPSK signals to a single 16-QAM
channel aggregation5 ; optical_processing_2

The spectral saving enabled by optical aggregation and de-aggregation is
reliant on the underlying physical topology, the nature of traffic demands,
routing schemes together with the location of aggregation nodes. In this
section, we develop a mathematical formulation that could minimize the
wavelength link cost or alternatively maximize the impact of aggregation.
Inputs to the model are the physical topology, traffic demands and the output
is the optimal identification of routing for each demand, set of pair of
demand for aggregation and their respective aggregation nodes so that the
wavelength link cost is minimized.

Inputs:

  * •

$G(V,E)$: A fiber-optic network composes $|V|$ nodes and $|E|$ optical fiber
links. Each link $e\in E$ is identified by its beginning node $s(e)$ and its
ending node $r(e)$.

  * •

$D$: Set of traffic demands; each demand $d\in D$ consists of its source node
$s(d)$ and destination node $r(d)$ respectively. All demands $d$ request
equally an amount of traffic equivalent to a wavelength capacity (e.g., 100
Gbps)

Variables:

  * •

$x_{e}^{d}\in\\{0,1\\}$: equals 1 if link $e$ is used for routing traffic of
demand $d$, 0 otherwise.

  * •

$z_{e}^{d,v}\in\\{0,1\\}$: equals 1 if demand $d$ is aggregated at node $v$
and link $e$ belongs to the routing path of the aggregated lightpath, 0
otherwise

  * •

$\theta_{v}^{d}\in\\{0,1\\}$: equals 1 if demand $d$ is aggregated at node
$v$, 0 otherwise

  * •

$f_{d_{1}}^{d_{2}}\in\\{0,1\\}$: equals 1 if demand $d_{1}$ is aggregated with
demand $d_{2}$, 0 otherwise

Objective Function: Minimize the following equation

$\sum_{d\in D}\sum_{e\in E}x_{e}^{d}-\sum_{d\in D}\sum_{e\in E}\sum_{v\in
V}\frac{z_{e}^{d,v}}{2}\\\ $ (1)

subject to the following constraints:

$\begin{split}\sum_{e\in{E}:v\equiv s(e)}x_{e}^{d}-\sum_{e\in{E}:v\equiv
r(e)}x_{e}^{d}=\\\ \begin{cases}1&\mbox{if }v\equiv s(d)\\\ -1&\mbox{if
}v\equiv r(d)\\\ $0$&otherwise\\\ \end{cases}\qquad\qquad\forall d\in
D,\forall v\in V\hfill\end{split}$ (2)

$\displaystyle{\sum_{v\in V}\theta_{v}^{d}\leq 1\qquad
and\qquad\theta_{v}^{d}=0\qquad\mbox{if }v\equiv r(d)\qquad\forall d\in D}$
(3)

$\displaystyle{\sum_{d_{2}\in D}f_{d_{1}}^{d_{2}}\leq 1\qquad\forall d_{1}\in
D}$ (4)

$\sum_{d_{2}\in D:r(d_{2})\neq r(d_{1})}{f^{d_{1}}_{d_{2}}}=0\qquad\forall
d_{1}\in D$ (5)

$\displaystyle{f_{d_{1}}^{d_{2}}=f_{d_{2}}^{d_{1}}\qquad\forall d_{1},d_{2}\in
D}$ (6)

$\displaystyle{\sum_{v\in V}z_{e}^{d_{1},v}\leq\sum_{d_{2}\in
D}f_{d_{1}}^{d_{2}}\qquad\forall d_{1}\in D,\forall e\in E}$ (7)

$\displaystyle{\sum_{d_{2}\in D}f_{d_{1}}^{d_{2}}=\sum_{v\in
V}\theta_{v}^{d_{1}}\qquad\forall d_{1}\in D}$ (8)

$\displaystyle{\theta_{v}^{d_{1}}-\theta_{v}^{d_{2}}+f_{d_{1}}^{d_{2}}\leq
1\qquad\forall d_{1},d_{2}\in D,\forall v\in V}$ (9)

$\displaystyle{\theta_{v}^{d_{2}}-\theta_{v}^{d_{1}}+f_{d_{1}}^{d_{2}}\leq
1\qquad\forall d_{1},d_{2}\in D,\forall v\in V}$ (10)

$\displaystyle{z_{e}^{d,v}\leq x_{e}^{d}\qquad\forall d\in D,\forall v\in
V,\forall e\in E}$ (11)

$\begin{split}\sum_{e\in E:i\equiv s(e)}z_{e}^{d,v}-\sum_{e\in E:i\equiv
r(e)}z_{e}^{d,v}=\\\ \begin{cases}\theta_{v}^{d}&\mbox{if }i\equiv v\\\
-\theta_{v}^{d}&\mbox{if }i\equiv r(d)\\\ 0&\mbox{otherwise}\\\
\end{cases}\qquad\qquad\forall d\in D,\forall v\in V,\forall i\in
V\hfill\end{split}$ (12)

The objective in Eq. 1 aims to minimize the wavelength link cost for
accommodating all demands. Note that whenever there is an aggregation
operation, it results in a saving of wavelength links. Equation 2 is the
conventional flow conservation constraint for each demand. Constraint in Eq. 3
captures the rule that each demand could be aggregated at most one times at a
certain aggregation node and that node has to be different from the
destination. The condition that each demand is aggregated with no more than
one another demand having the same destination is guaranteed by Eqs. 4, 5, and
6. Constraints in Eqs. 7, 8 are for coherence purpose connecting each
aggregated demand with the respective aggregation node and aggregation links.
Constraints (9, 10) ensures the observation that if two demands are
aggregated, the aggregation operation has to be occurred at the same node.
Constraint in Eq. 11 specifies the routing for aggregated lightpaths. Final
constraint (12) is for conservation of aggregated flows.

##  6 Numerical Results

This section presents numerical evaluations comparing our proposal that
leverages the use of optical aggregation within the framework of optical-
processing-enabled networks to the traditional optical-bypass networking. The
comparison is drawn on a realistic COST239 network topology shown in Fig. 5.
In order to exploit aggregation opportunities among demands, the traffic under
consideration is generated following the two-to-many model where two nodes are
randomly selected as sources and many remaining nodes are also arbitrarily
chosen as the destinations. For this traffic scenario, favorable conditions
for aggregation operation have been created as there have been multiple pair
of demands sharing the same destination. We consider three increasing loads
corresponding to 5, 7 and 9 destination nodes and for each load, 10 traffic
samples are simulated. Each traffic request is served by allocating a
dedicated wavelength channel capacity in the standard C-band where channel
spacing is 50 $GHz$. The performance metric for comparison is the wavelength
link cost for accommodating all demands.

Figure 5: Network Topology under Test

Figure 6, 7, and 8 report the performance comparison between our proposal of
aggregation-aware routing in optical-processing-enabled networks and the
traditional routing in optical-bypass networks for three traffic loads. As a
general trend to observe, the aggregation-aware scheme is highly more
spectrum-efficient than the non-aggregation counterpart. In all simulated
scenarios, the maximum relative gain is up to slightly more than $30\%$.

Figure 6: Routing Cost Comparison for Low-load Traffic Figure 7: Routing Cost
Comparison for Medium-load Traffic

An interesting observation is that there are cases that aggregation-aware
routing does not result in spectral gain compared with conventional routing.
This can be observed at traffic index 3 for the low-load scenario in Fig. 6,
traffic index 6 and 7 for the medium-load cases in Fig. 7. It might be
explained that in order to perform optical aggregation between two demands, in
addition to the necessary condition of having the same destination, two
demands must share a segment of its route. Such constraint might cause their
routing diverged from the shortest ones and if the aggregation benefit could
not compensate for the divergence from the shortest routes, no gain will be
realized.

Figure 8: Routing Cost Comparison for High-load Traffic

For the high-load traffic in Fig. 8, aggregation-aware mechanism is strictly
better than the traditional routing. Nevertheless, the average gain across
three traffic loads are quite comparable, being $20.5\%$, $17.3\%$ and
$18.5\%$ for low-load, medium-load and high-load respectively.

To inspect more on the mechanism of aggregation-aware routing, Table 1 and 2
present the comprehensive simulation results for one traffic sample of low-
load scenario. It is noted that for aggregation-aware routing, in addition to
the routing information, the determination of aggregation node, pair of
demands for aggregation, and the links for routing aggregated lightpaths are
also revealed. Here we would like to draw the attention to the routing of
demand $9\rightarrow 2$ and $9\rightarrow 4$ as it uncovers an important
observation. As shown in Tab. 2, both two demands select the shortest path
composing of two links to minimize the wavelength link cost. However, in
aggregation-aware mechanism shown in Tab. 1, these two demands are routed over
longer paths consisting of three links to make them aggregated to other
demands $7\rightarrow 2$ and $7\rightarrow 4$. This is justifiable because the
spectral saving enabled by aggregation is higher than the cost of deviation
from shortest paths.

Table 1: Aggregation-aware Routing Demand | Routing | Aggregation Node | Aggregation Links | With Demand
---|---|---|---|---
7$\rightarrow$2 | (7-1-2) | 7 | (7-1-2) | 9$\rightarrow$2
7$\rightarrow$11 | (7-9-11) | 9 | (9-11) | 9$\rightarrow$11
7$\rightarrow$4 | (7-5-4) | 7 | (7-5-4) | 9$\rightarrow$4
7$\rightarrow$3 | (7-8-3) | 8 | (8-3) | 9$\rightarrow$3
7$\rightarrow$1 | (7-1) | 7 | (7-1) | 9$\rightarrow$1
9$\rightarrow$2 | (9-7-1-2) | 7 | (7-1-2) | 7$\rightarrow$2
9$\rightarrow$11 | (9-11) | 9 | (9-11) | 7$\rightarrow$11
9$\rightarrow$4 | (9-7-5-4) | 7 | (7-5-4) | 7$\rightarrow$4
9$\rightarrow$3 | (9-8-3) | 8 | (8-3) | 7$\rightarrow$3
9$\rightarrow$1 | (9-7-1) | 7 | (7-1) | 7$\rightarrow$1
Table 2: Conventional Routing in Optical-bypass Networks Demand | Routing
---|---
7$\rightarrow$2 | (7-1-2)
7$\rightarrow$11 | (7-9-11)
7$\rightarrow$4 | (7-5-4)
7$\rightarrow$3 | (7-8-3)
7$\rightarrow$1 | (7-1)
9$\rightarrow$2 | (9-8-2)
9$\rightarrow$11 | (9-11)
9$\rightarrow$4 | (9-10-4)
9$\rightarrow$3 | (9-8-3)
9$\rightarrow$1 | (9-8-1)

##  7 Conclusions

The aim of this paper was to propose a new architectural paradigm for future
optical networks that leverage the optical signal processing between
transiting lightpaths to attain greater capacity efficiency. As a departure
from simple functions of add/drop and cross-connect in optical-bypass
networking, our proposal—optical-processing-enabled networks—took advantage of
optical aggregation capability that permits adding lower-speed channels into a
single higher bit-rate one for capacity savings. Specifically, we made use of
an optical aggregator that can combine two QPSK channels into a single 16-QAM
one and developed an optimal mathematical model based on integer linear
programming to maximize the aggregation benefits. Numerical results on a
realistic topology were provided to verify the efficacy of our proposal
compared to the conventional approach and it was shown that up to more than
$30\%$ spectral saving could be attained in favorable conditions.

Albeit still at early-stage, this paper has identified an important
observation that by permitting optical superposition of transitional
lightpaths, new opportunities for capacity saving could be emerged thanks to
reducing the effective traffic load in the network. In fact, optical channel
aggregation and de-aggregation has begun to alter the way we think about
optical networks architecture, reversing the conventional assumption that
transitional lightpaths should be maximally separated to avoid unwanted
interferences. Driven by the rapid advances in photonic signal processing
technologies and the great incentive from operators in making the optical
networks more spectrum-efficient and cost-efficient, further innovations in
optical-processing-enabled realm from both enabling technologies to designing
algorithms should be pushed to fully unlock its practicability and
applicability.

## Conflict of interest

The authors declare that they have no conflict of interest.

## Data availability

The datasets generated during and/or analysed during the current study are
available from the corresponding author on reasonable request.

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