Capacity Efficiency of Sub-Wavelength Traffic Grooming in IP

over Quasi-CWDM Optical Networks (Invited)

Gangxiang Shen*, Senior Member, IEEE, Yongcheng Li, Mingyi Gao, Member, IEEE

School of Electronic and Information Engineering, Soochow University, Suzhou 215006, China

Tel: 86-512-65221537; *Email: shengx@suda.edu.cn

ABSTRACT

We proposed a new Quasi-CWDM optical network architecture which is spectrum efficient and cost effective [1].

Similar to the traditional DWDM network, the new network architecture does not require expensive flex-grid

wavelength selective switches (WSSs), but can support optical super-channels using fixed quasi-coarse WDM

grids, of which each spans at least 200 GHz. For each optical super-channel in the Quasi-CWDM network,

different modulation formats can be adaptively used based on their bit-rate requirements and channel physical

conditions. In [1], we have evaluated the performance of the Quasi-CWDM network architecture from the cost

perspective. In this study, we will evaluate the performance for the Quasi-CWDM optical network in terms of

capacity efficiency. We consider sub-wavelength traffic grooming in an IP over Quasi-CWDM network. A

mixed integer linear programming (MILP) optimization model is developed to maximize the total served IP

traffic demand. In addition, the capacity efficiency is evaluated through discrete event simulations to find

bandwidth blocking probability when using the Quasi-CWDM network to provision dynamic sub-wavelength

traffic demand. The results show that the Quasi-CWDM network is more capacity-efficient than the DWDM

optical network to accommodate more sub-wavelength traffic demand and shows the lowest bandwidth blocking

probability under different minimum elastic channel bandwidths while having a lower hardware cost than the

elastic optical network (EON).

Keywords: Quasi-CWDM, traffic grooming, elastic optical network

1. INTRODUCTION

The optical transmission system has been evolved from past coarse wavelength division multiplexing (CWDM)

to today’s dense wavelength division multiplexing (DWDM). Recently, a more flexible and effective network

architecture, called elastic optical network (EON) [2], was proposed to provide higher transmission capacity and

better meet ever-increasing traffic demand. EON provides high spectrum efficiency by reducing the granularity

of each frequency slot (FS) and adding flexibility in spectrum allocation for optical channels. However, the

compatibility between the current DWDM optical network and the EON is still a challenging issue. To enable

today’s DWDM optical network to evolve to EON, it is required to upgrade network hardware such as

reconfigurable optical add/drop multiplexer (ROADM) and to use expensive flex-grid wavelength selective

switches (WSSs). Thus, it is still under debate whether the DWDM network should and will be eventually

evolved to EON. Meanwhile, because of the requirement of a guard band between two neighbouring optical

channels, for an EON it is very inefficient to establish optical channels with few FSs.

Figure 1. Optical channel

spectrum layouts under

different technologies.

Figure 2. IP over Quasi-CWDM network

architecture.

Figure 3. Tradeoff between

modulation formats and

transparent reaches.

Viewing the potential disadvantages of EON and foreseeing that super-channels will dominate the future

optical transport network, we proposed a new optical transport network architecture that is based on the

Quasi-CWDM transmission technology [1] as a candidate for the next generation optical transport network. In

this type of network, all the established channels are super-channels with fixed bandwidth, whose frequency

spacing as shown in Fig. 1 is much coarser than the current DWDM technology. However, on each optical

channel, different modulation formats can be used such that the adaptability between the bit rate and the

transparent reach of each channel can be maintained. As a key advantage compared to EON, the ROADM node

of the Quasi-CWDM network is much simpler, which is similar to the one in the DWDM network except that the

filtering frequency spacing of each contained array waveguide grating (AWG) is much coarser under the

Quasi-CWDM technique, e.g., 200 GHz or even 400 GHz. The node however does not require any costly

flex-grid WSSs. It should also be noted that though the Quasi-CWDM technology is similar to the mixed line

rate technique in the DWDM network [3-4] due to the channel bit rate adaptation capability, the Quasi-CWDM

200GHz

CWDM

DWDM

Flexi-grid

Quasi-CWDM8-QAM CO-OFDM QPSK CO-OFDM

(a)

(b)

(c)

(d)

Guard-band

IP layer

Optical layerTransponder

QAM QPSK

Physical link

OXC

Router

Router port

A B C

BPSK

N1 N2

QPSK

8-QAM

1800 km

TX

RX

1800 km

1000 km 800 km

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optical network is innovative to propose much coarser frequency spacing than that of the DWDM network, e.g.,

200 GHz vs. 50 GHz. This can greatly enhance spectrum efficiency for the optical network.

In [1], we have evaluated the Quasi-CWDM optical network from the cost perspective compared to the other

network architectures including the DWDM network and EON. In this paper, based on the work in [1], we

consider an IP over Quasi-CWDM optical network and evaluate the benefit of the Quasi-CWDM network from

the perspective of capacity efficiency when used for provisioning sub-wavelength traffic demand. An MILP

optimization model is proposed for IP traffic grooming and the bandwidth blocking performance is evaluated

under dynamic sub-wavelength traffic demand.

2. IP OVER QUASI-CWDM OPTICAL NETWORK

Fig. 2 shows the architecture of an IP over Quasi-CWDM network, which includes IP layer and Quasi-CWDM

optical layer. Each node consists of a pair of core router and Quasi-CWDM ROADM. The IP layer consists of

IP router nodes and virtual links which are the lightpaths established between node pairs. The optical layer is

made up of ROADM nodes and fiber links. To establish a lightpath, we need two IP router ports in the IP layer

and zero or multiple signal regenerators in the optical layer. The Quasi-CWDM network provisions

super-channels whose spectrum spacing is at least 200 GHz. The modulation format of each optical

super-channel can be adaptively chosen. Under 200-GHz frequency spacing, Table 1 shows the relationship

between the transparent reach and the used modulation format for each super-channel, and the relative costs of

regenerators and router ports with different modulation formats, where the cost of a BPSK regenerator is

normalized to 1.0 unit and the cost of a router port is double of a regenerator for any modulation format.

Different modulation formats require different number of regenerators for a certain lightpath distance

because of different transparent reaches as shown in Table 1. For example in Fig. 3, we assume that there is

800-Gb/s traffic demand to be served between nodes N1 and N2. We also assume that the Quasi-CWDM

technique has 200-GHz frequency spacing and requires 25-GHz guard band between two neighbouring

super-channels. Based on the transparent reach information in Table 1, BPSK does not require any regenerators,

but it has the lowest channel capacity (i.e., 175 Gb/s). In contrast, 8 QAM has the highest channel capacity (i.e.,

525 Gb/s), but needs the largest number of signal regenerators. Table 2 shows the required numbers of IP router

ports, regenerators, and total costs under the different modulation formats, from which we can see that it is

important to optimally choose the modulation format to balance the cost and the spectrum usage.

Table 1. Transparent reaches [5] and costs of

different modulation formats

Table 2. Required number of IP router ports, regenerators,

and total costs for different modulation formats

Modulation

format

Spectrum

efficiency

Cost of

regenerator

Cost of

IP router

Transparent

reach (km)

BPSK 1 1 2 4000

QPSK 2 1.3 2.6 2000

8 QAM 3 1.5 3 1000

Modulation

format

# of

regens

# of IP router

ports

# of channels

(bidirectional)

Total

cost

BPSK 0 10 5 20

QPSK 0 6 3 15.6

8 QAM 4 4 2 18

For the IP over Quasi-CWDM network, signal regeneration can be implemented either in the IP layer or in

the optical layer. Fig. 4 shows signal regeneration in the IP layer, which interrupts and regenerates the signal of

an optical channel by a router. For this type of signal regeneration, more traffic demand can be added to the

regenerated optical channel if it is not fully filled. However, it needs two additional IP router ports at the

intermediate node, which is more expensive. Another approach for signal regeneration is to regenerate the signal

in the optical layer as shown in Fig. 5, which needs only a pair of OEO converters and is generally cheaper than

IP layer regeneration. However, the signal regeneration in the optical layer does not allow grooming additional

traffic at the regeneration node even though the channel may be under-utilized, which is therefore inefficient.

Thus, for an IP over Quasi-CWDM network, it is important to choose signal regeneration modes for different

optical channels so as to achieve an optimal network design.

3. STATIC SUB-WAVELENGTH TRAFFIC GROOMING

We aim to design an IP over Quasi-CWDM network with the objectives of maximizing served traffic demand,

and meanwhile minimizing the total hardware cost that is made up of IP router ports and signal regenerators. The

given inputs are as follows: (1) The physical topology of a network 𝑮𝒑 = (𝑵, 𝑬), where 𝑵 is the set of network

nodes and 𝑬 is the set of network links. (2) Traffic demand matrix [𝑇𝑠𝑑] in units of Gb/s between each node

pair (s, d). (3) The cost of a regenerator 𝐶𝑅𝑓 and the cost of an IP router port 𝐶𝐼𝑃

𝑓 for the 𝑓𝑡ℎ modulation

format. (4) The limited spectrum resource in each fiber link. To solve the problem, we develop an MILP model.

We present the notations of indexes, sets, parameters, and variables of the model as follows: s and d are the

indexes of the source and destination nodes of an IP traffic flow, which is routed over the lightpath virtual

topology. i and j are the indexes of the nodes in the lightpath virtual topology. A lightpath established between

the two nodes connects a pair of router ports each at the nodes. 𝑵𝒊 is the set of neighbouring nodes of node i in

physical topology 𝑮𝒑. 𝑭 is the set of modulation formats, which include BPSK, QPSK, and 8 QAM. 𝑾 is the

set of (coarse) wavelengths in each fiber link. 𝜆𝑠𝑑 is served traffic demand in units of Gb/s between each node

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pair. 𝑅𝑖𝑗𝑓

is number of required regenerators (in the optical layer) along the fixed shortest route of lightpath

virtual link (𝑖, 𝑗) when the 𝑓𝑡ℎ modulation format is applied. We assume that each virtual link takes the shortest

route. 𝐶𝑓 is bit rate of a Quasi-CWDM optical channel with the 𝑓𝑡ℎ modulation format. 𝜃𝑚𝑛𝑖𝑗

equals one if

physical link (𝑚, 𝑛) is traversed by the shortest route of lightpath virtual link (𝑖, 𝑗); zero, otherwise. 𝛼 is a

weight factor. 𝜆𝑖𝑗𝑠𝑑 is a variable to indicate the traffic demand between node pair (𝑠, 𝑑) that traverses virtual link

(𝑖, 𝑗). 𝑉𝑖𝑗𝑓 is a variable to indicate the number of Quasi-CWDM optical channels with the 𝑓𝑡ℎ modulation

format on virtual link (𝑖, 𝑗). 𝑁𝐼𝑃𝑖,𝑓

is the number of IP router ports with the 𝑓𝑡ℎ modulation format at node i.

𝑁𝑅 𝑖𝑗,𝑓

is the number of signal regenerators with the 𝑓𝑡ℎ modulation format on virtual link (𝑖, 𝑗). 𝛿𝑤𝑖𝑗,𝑓

is a binary

variable that equals 1 if an optical channel on virtual link (𝑖, 𝑗) uses the 𝑓𝑡ℎ modulation format on wavelength w.

Objective: Maximize ∑ 𝜆𝑖𝑗𝑠𝑑

𝑗∈𝑵:𝑖≠𝑗 − 𝛼(∑ 𝐶𝐼𝑃𝑓

∙ 𝑁𝐼𝑃𝑖,𝑓

𝑖∈𝑵,𝑓∈𝑭 + ∑ 𝐶𝑅𝑓

∙ 𝑁𝑅𝑖𝑗,𝑓

𝑓∈𝑭,𝑖,𝑗∈𝑵:𝑖≠𝑗 ). The first objective is to

maximize total served traffic demand and the second objective is to minimize the total cost of router ports and

regenerators. The first objective has a higher priority by setting 𝛼 to be a small value.

Subject to:

𝜆𝑠𝑑 ≤ 𝑇𝑠𝑑 ∀𝑠, 𝑑 ∈ 𝑵: 𝑠 ≠ 𝑑 (1)

∑ 𝜆𝑖𝑗𝑠𝑑

𝑗∈𝑵:𝑖≠𝑗 − ∑ 𝜆𝑗𝑖𝑠𝑑

𝑗∈𝑵:𝑖≠𝑗 = {𝜆𝑠𝑑 𝑖 = 𝑠

−𝜆𝑠𝑑 𝑖 = 𝑑

0 𝑜𝑡ℎ𝑒𝑟𝑤𝑖𝑠𝑒

∀𝑠, 𝑑, 𝑖 ∈ 𝑵: 𝑠 ≠ 𝑑

(2) 𝜆𝑖𝑗

𝑠𝑑 = 𝜆𝑗𝑖𝑑𝑠 ∀𝑠, 𝑑, 𝑖, 𝑗 ∈ 𝑵: 𝑠 ≠ 𝑑, 𝑖 ≠ 𝑗 (3)

∑ 𝑉𝑖𝑗

𝑓𝑗∈𝑵 = 𝑁𝐼𝑃

𝑖,𝑓 ∀𝑖 ∈ 𝑵, 𝑓 ∈ 𝑭 (4)

𝑉𝑖𝑗

𝑓= ∑ 𝛿𝑤

𝑖𝑗,𝑓𝑤∈𝑾 ∀𝑖, 𝑗 ∈ 𝑵: 𝑖 ≠ 𝑗, ∀𝑓 ∈ 𝑭 (5) ∑ 𝜆𝑖𝑗

𝑠𝑑𝑠,𝑑∈𝑵:𝑠≠𝑑 ≤ ∑ 𝐶𝑓

𝑓∈𝑭 ∙ 𝑉𝑖𝑗

𝑓 ∀𝑖, 𝑗 ∈ 𝑵: 𝑖 ≠ 𝑗 (6)

𝑁𝑅𝑖𝑗,𝑓

= 𝑅𝑖𝑗𝑓

∙ 𝑉𝑖𝑗𝑓 ∀𝑖, 𝑗 ∈ 𝑵: 𝑖 ≠ 𝑗, ∀𝑓 ∈ 𝑭 (7) ∑ 𝛿𝑤

𝑖𝑗,𝑓 ∙ 𝜃𝑚𝑛𝑖𝑗

𝑓∈𝑭,𝑖,𝑗∈𝑵:𝑖≠𝑗 ≤ 1 ∀𝑤 ∈ 𝑾, 𝑚 ∈ 𝑵, 𝑛 ∈ 𝑵𝒊 (8)

Due to the page limit, we do not explicitly explain each of the constraints.

4. DYNAMIC SUB-WAVELENGTH TRAFFIC GROOMING

To evaluate capacity efficiency of the IP over Quasi-CWDM optical network, we also consider using the

Quasi-CWDM optical network to provision dynamic sub-wavelength traffic. The arrival of IP traffic service

requests with different bandwidth follows a Poisson distribution and the holding time of each established IP

traffic flow follows an exponential distribution. As a performance evaluation criterion, we evaluate bandwidth

blocking probability for IP traffic flows, which is defined as a ratio of total blocked IP traffic bandwidth to the

total bandwidth of arrived IP traffic flow requests.

We employ the traditional multiple-hop traffic grooming scheme to provision the IP service requests through

the existing remaining capacity on established optical channels. Only if the remaining capacity is not sufficient

to accommodate an IP traffic flow, would we seek to establish a new direct lightpath between a pair of nodes to

provision the IP traffic service. For simplicity, we establish the lightpath on the shortest route between the node

pair and choose the most efficient modulation format according to its physical distance (we do not allow signal

regeneration in the middle when establishing the lightpath). We search for enough idle spectrum resources to

establish the optical channel. If there are no sufficient idle spectrum resources along the shortest path, we block

the IP traffic demand request. In the traffic grooming process, the IP traffic flow can be split onto multiple

connections via different routes. For IP traffic demand release, we remove the IP traffic flows from the network

and release the consumed network resources. If the removal of an IP traffic flow makes an optical channel free

of any IP traffic, we will also release the optical channel in the optical layer.

5. PERFORMANCE EVALUATION

We considered two test networks: (1) a six-node, nine-link (n6s9) network and (2) the 14-node, 21-link NSFNET

network. A 4000-GHz fiber spectrum is assumed in each fiber link, which corresponds to 80 50-GHz optical

channels. A 25-GHz guard band is always required between two neighbouring optical channels for any optical

transmission techniques. Three modulation formats, i.e., BPSK, QPSK, and 8 QAM, are assumed, whose

corresponding IP router port and signal regenerator costs for 200-GHz frequency spacing are shown in Table 1.

a) Static traffic demand: for this case, the IP traffic demand between each node pair is randomly generated

within a range of (400, 2000) Gb/s for the n6s9 network and (400, 1000) Gb/s for the NSFNET network. We

solved the MILP model by the commercial software AMPL/Gurobi [6]. We assumed that frequency spacing can

vary from 50 GHz (DWDM), to 100 GHz (DWDM), to 200 GHz (Quasi-CWDM), and to 400 GHz

(Quasi-CWDM). The costs of IP router ports and signal regenerators of different modulation formats and under

different frequency spacing are from Table III in our previous work [1].

Fig. 6 shows the MILP results, in which legend “STD” means the total served traffic demand and “TC”

means the total network cost. We can see that with the increase of frequency spacing, the served traffic demands

for both the test networks increase. This is because larger frequency spacing requires fewer guard bands between

channels given the limited total fiber spectrum resource. For example, 200-GHz frequency spacing requires 20

guard bands in a 4000-GHz fiber spectrum, while 100-GHz frequency spacing needs 40 guard bands. Thus, the

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Quasi-CWDM network is more spectrum efficient than the DWDM network. We also observe a cost reduction in

Fig. 6, which means that larger frequency spacing is beneficial to reducing the network cost.

Figure 4. IP layer

regeneration.

Figure 5. Optical layer

regeneration.

Figure 6. Total network cost and

percentage of served traffic demand

Figure 7. Bandwidth blocking

probability vs.𝐹𝑆𝑚𝑖𝑛

b) Dynamic traffic demand: we also evaluated the performance of the IP over Quasi-CWDM network under

dynamic IP traffic demand. The performance is compared to the IP over EON network [7-8]. For EON, we

assume that there are 320 FSs in each fiber link with each FS having 12.5 GHz. The traffic load between each

node pair is assumed to be 5 Erlang. And for each arrived IP traffic flow, its bandwidth is assumed to be within a

range of (10, 100) Gb/s. For EON, each channel is assumed to be assigned a bandwidth between a minimal

𝐹𝑆𝑚𝑖𝑛 and a maximal 16 FSs (which corresponds to 200 GHz). The purpose of setting a minimum number of

FSs for each elastic optical channel is to avoid too much waste of guard bands because if a channel’s effective

bandwidth is too small, the relative waste of the guard band will be high. There are 105 IP traffic flow arrival

events simulated for each test point. Fig. 7 shows how bandwidth blocking probability changes with the increase

of the value of 𝐹𝑆𝑚𝑖𝑛. Note that when 𝐹𝑆𝑚𝑖𝑛 equals 16, which means that the minimal and maximal numbers

of the assigned FSs for an optical channel are equal, this just corresponds to the case of Quasi-CWDM, in which

every channel is a 200-GHz super-channel. We see that with an increasing 𝐹𝑆𝑚𝑖𝑛 , the IP traffic flow bandwidth

blocking probability decreases and when the network is configured as a Quasi-CWDM optical network (i.e.,

𝐹𝑆𝑚𝑖𝑛=16), the bandwidth blocking probability is the lowest, which implies higher capacity efficiency of the

Quasi-CWDM network compared to the EON.

6. CONCLUSIONS

The Quasi-CWDM optical network is promising to have a low hardware cost and flexible adaptation capability

between the bit rate and the transparent reach of optical super-channels. In this paper, we evaluated the capacity

efficiency of the IP over Quasi-CWDM optical network for sub-wavelength traffic flow provisioning. We

developed an MILP model to maximize the served traffic demand and to minimize the total network cost. We

also carried out a simulation study to evaluate the bandwidth blocking performance of the IP over Quasi-CWDM

optical network under dynamic sub-wavelength traffic demands. The results show that the IP over Quasi-CWDM

network can accommodate more sub-wavelength IP traffic flows subject to limited optical layer resources. The

study on bandwidth blocking probability under dynamic sub-wavelength traffic demand also shows that the IP

over Quasi-CWDM optical network is capacity efficient to demonstrate the lowest bandwidth blocking

probability.

ACKNOWLEDGEMENTS: This work was jointly supported by the NSFC (61322109, 61172057, and

61307082) and NSF of Jiangsu Prov. (BK20130003 and BK2012179).

REFERENCES

[1] G. Shen, Y. Li et al. “Quasi-CWDM optical network: cost effective and spectrum efficient architecture for

future optical networks”, accepted by ONDM 2015.

[2] M. Jinno et al., “Spectrum-efficient and scalable elastic optical path network: architecture, benefits, and

enabling technologies,” IEEE Communications Magazine, vol. 47, no. 11, pp. 66-73, Nov. 2009.

[3] A. Nag et al., “Optical network design with mixed line rates and multiple modulation formats,” IEEE/OSA

Journal of Lightwave Technology, vol. 28, no. 4, pp. 130-139, Feb. 2010.

[4] K. Christodoulopoulos et al., “Reach adapting algorithms for mixed line rate WDM transport networks,”

IEEE/OSA Journal of Lightwave Technology, vol. 29, no. 21, pp. 3350-3363, Nov. 2011.

[5] J. Lopez et al., “On the energy efficiency of survivable optical transport…,” in Proc. ECOC 2012.

[6] Gurobi [Online]. Available: http://www.gurobi.com.

[7] A. Cai et al., “Optimal planning for electronic traffic grooming in IP …,” in Proc. ACP 2012.

[8] J. Zhang et al., “Energy-efficient traffic grooming in sliceable-transponder-equipped IP-over-elastic …,”

IEEE/OSA Journal of Optical Communications and Networking, vol. 7, no. 1, pp. 142-152, Jan. 2015.

IP IP IP

OXC

OXC

OXC

Need two IP router ports

IP layer

Optical layer

Insert new

IP traffic flows

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400

0

2000

4000

6000

8000

Pe

rce

nta

ge

of

se

rve

d tra

ffic

de

ma

nd

Fixed spectrum grid granularity

To

tal co

st

MILP_TC_n6s9MILP_STD_n6s9

MILP_STD_NSFNETMILP_TC_NSFNET

1.00E-02

1.00E-01

1.00E+00

2 4 6 8 10 12 14 16

Ban

dw

idth

blo

ckin

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robabili

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Minimum number of FSs per channel FSmin

Dynamic_NSFNET

Quasi-CWDM

IP IP IP

regenerator

IP layer

Optical layer

No IP router ports

OXC

OXC

OXC

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