A Design Method of High-availability andLow-optical-loss Optical Aggregation Network

ArchitectureTakehiro Sato, Kunitaka Ashizawa, Kazumasa Tokuhashi,

Daisuke Ishii, Satoru Okamoto and Naoaki YamanakaDept. of Information and Computer Science, Keio University

3-14-1 Hiyoshi, Kohoku, Yokohama, Kanagawa, Japan 223–8522Email: t-sato@yamanaka.ics.keio.ac.jp

Eiji OkiDept. of Information and Communication Engineering,

The University of Electro-Communications1-5-1 Chofugaoka, Chofu, Tokyo, Japan 182–8585

Abstract—A highly-energy-efficient network using an opticalaggregation network and a service cloud has been proposed toreduce the power consumption of today’s Internet greatly. Theoptical aggregation network connects a solitary giant router tousers by logical tree topology. This paper proposes a novel opticalaggregation network architecture. The proposed architecture uses2x2 optical switches and combines the features of a tree topologyand a ring topology. The paper also introduces a design method ofthe proposed network architecture. It is shown that the methodcan design high-availability and low-optical-loss network withfewer optical switches compared to a duplex tree topology.

I. INTRODUCTION

Recently, the number of broadband Internet subscribers isincreasing rapidly, and is about 300 million all over the worldin 2007 [1]. As the broadband Internet has spread, variousservices including contents delivery and video conference, areprovided.

Today’s Internet has two big problems. The first problemis the increase of power consumption of network equipments.Worldwide power consumption of network equipments growsabout 12% per year [2]. It was 25 GW in 2008, and is expectedto be over 97 GW in 2020.

The second problem is the centralizing of the Internet traffic.With the rise of “Hyper Giants” including Google and Akamai,or the growth of cloud computing, Client-to-Data-center trafficincreases drastically while Peer-to-Peer traffic shrinks [3].However, the current Internet topology is almost mesh, asshown in Figure 1. Therefore, the number of hops betweena user and a service server becomes large, so RTT (RoundTrip Time) and delay jitter may affect the quality of services.

To improve these problems, a highly-energy-efficient net-work using an optical aggregation network and a service cloudhas been proposed [4]. Figure 2 shows its architecture. Routersand servers that the current Internet contains are collected upto a solitary power-scalable giant router. Together with serviceservers, this giant router constitutes the service cloud. AllIP traffic is aggregated by the optical aggregation networkand transferred to the giant router in one hop. The opticalaggregation network consists of optical switches and connectsthe giant router to users transparently. The access method

Users

IX

ISP : Internet Service ProviderIX : Internet eXchange

ISP

: Router

Service Servers

Fig. 1. Schematic view of current Internet.

Service Cloud

Optical

Aggregation

Network

Users

Service Servers

Optical Switches

Solitary Power-scalable

Giant Router

One

Ho

p

Fig. 2. Highly-energy-efficient network architecture.

is TDM (Time Division Multiplexing) like a PON (PassiveOptical Network) system. According to the estimate, the powerconsumption of the highly-energy-efficient network is 1/1000of that of the present Internet [4].

This paper proposes a novel architecture of the opticalaggregation network that achieves high availability and low

optical loss. A design method of the proposed network ar-chitecture is also illustrated. An argument about availabilityis a great issue in the optical aggregation network, becausethousands of users including business users and wirelessbase stations are contained in this network by logical treetopology. Low optical loss between the giant router and usersis also important, because users are widespread, and no opticalamplifiers should be utilized preferably in terms of deploymentcost.

In this paper, we consider using Mach-Zehnder type 2x2optical switches for the design of the optical aggregationnetwork architecture. A PLZT (Plomb Lanthanum ZirconateTitanate) optical switch [5] is a typical example of such switch.Figure 3 shows the architecture of 2x2 PLZT optical switch.It switches optical signals in less than 10 ns when a voltageis applied to one of the electrodes.

The rest of the paper is organized as follows. Prior to theproposal, we discuss the features of some typical topologies insection II. The high-availability and low-optical-loss networkarchitecture for the optical aggregation network is presentedin section III, and its design method is shown in section IV.Finally, we conclude the paper in section V.

On

Off

(b) Cross Mode

Off

On

(c) Bar Mode

Waveguide

(a) Architecture

Electrode P

Electrode Q

Fig. 3. Architecture of 2x2 PLZT optical switch.

II. FEATURES OF TYPICAL TOPOLOGIES

In this section, we evaluate the features of some typicaltopologies that are constructed by 2x2 optical switches. Wefocus on the following parameters.

• UnavailabilityIt is the probability that a user cannot communicate withthe giant router due to a failure of a 2x2 optical switch. Itis equal to 1 minus availability. In the following section,the unavailability of user i(i = 0, 1, 2, . . . , N − 1) isexpressed as Ui.

• Optical lossIt is a sum of insertion losses of 2x2 optical switchesthat are placed between the giant router and a user. Inthe following section, the optical loss between the giantrouter and user i(i = 0, 1, 2, . . . , N − 1) is expressed asLi.

• Number of optical switchesIt is the number of 2x2 optical switches that is required

to connect N users with the giant router. In the followingsection, it is expressed as S.

The values of Ui and Li are different among users re-spectively, so we evaluate these parameters by worst-caseconditions, maxi Ui and maxi Li. We focus on the featureof topology itself, and ignore any effect of other componentsor devices (e.g. an optical fiber cable that connects two 2x2optical switches).

A. Analysis

1) Tree topology: A tree topology is often used in accessnetworks (e.g. PON). Figure 4 shows the tree topology thatis constructed by 2x2 optical switches. When it is a completebinary tree, each parameter is expressed as follows.

maxi

Ui = 1− (1− u)⌈log2 N⌉ (1)

maxi

Li = ⌈log2 N⌉l (2)

S = N − 1 (3)

u is the unavailability of a 2x2 optical switch itself, and l isthe insertion loss of the optical switch. When N is power oftwo, Ui and Li of all users are equal to maxi Ui and maxi Li

respectively.

Router

LT

Users

0

2x2 Optical Switch

LT

1 2 3 4 5 6 7 8 9 1011 1213 1415

LT : Line Terminal

Fig. 4. Tree topology (N = 16).

In the simple tree topology, at least one user becomes unableto communicate with the giant router inevitably when anyone of 2x2 optical switches fails. A duplex configuration isreferred as the protection architecture “Type C” to enhancethe reliability of PON in ITU-T recommendation G.983 [6].Figure 5 shows the duplex tree topology that is constructed by2x2 optical switches. When it is a complete binary tree, eachparameter is expressed as follows.

maxi

Ui = (1− (1− u)⌈log2 N⌉)2 (4)

maxi

Li = ⌈log2 N⌉l (5)

S = 2(N − 1) (6)

RouterLT(0) LT(1)

LT(1)

Users

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

LT(0)

Fig. 5. Duplex tree topology (N = 16).

2) Ring topology: A ring topology is often used in ametropolitan area network (e.g. ROADM (ReconfigurableOptical Add/Drop Multiplexer) ring, SONET (SynchronousOptical Network) ring). Figure 6 shows the ring topologythat is constructed by 2x2 optical switches. Each parameteris expressed as follows.

maxi

Ui = u+(1−u)(1−(1−u)⌈N−12 ⌉)(1−(1−u)⌊

N−12 ⌋) (7)

maxi

Li = Nl (8)

S = N (9)

The user that meets Ui = maxi Ui is i = N2 − 1, N

2 (when Nis even) or N−1

2 (when N is odd). Equation (7) means that theuser cannot communicate with the giant router when the 2x2optical switch that is connected directly with the user fails,or when both of its routes to the giant router (clockwise orcounterclockwise) are unavailable concurrently due to failuresof switches on each route. The user that meets Li = maxi Li

is i = 0, N − 1.

B. Comparison

Figure 7, 8 and 9 show maxi Ui, maxi Li and S of eachtopology respectively. We assume the unavailability of a 2x2optical switch u is 10−6 and the insertion loss of the opticalswitch l is −1(dB).maxi Ui of the ring topology is lower than that of the tree

topology, except when N is extremely large. This is becauseusers can communicate with the giant router by using oneof the two routes (clockwise or counterclockwise) in the ringtopology.

By contrast, maxi Li of the tree topology is lower than thatof the ring topology. This is because the number of opticalswitches that are placed between the giant router and theworst-case user is N in the ring topology, and ⌈log2 N⌉ inthe tree topology.

RouterLT(0) LT(1)

Users

6 7 8 9

0

1

2

3

4

15

14

13

12

11

105LT(1)

LT(0)

Fig. 6. Ring topology (N = 16).

The duplex tree topology achieves very low maxi Ui due tosetting two different routes for every user that do not share anyoptical switch between the giant router and the user. However,S of this topology is twice as many as that of the tree topologyand the ring topology.

Based on above argument, we propose a novel optical ag-gregation network architecture that achieves high availabilityand low optical loss together in the next section.

III. PROPOSED ARCHITECTURE

Figure 10 shows the proposed architecture for the opticalaggregation network. In this architecture, small-sized rings are

Fig. 7. maxi Ui of typical topologies.

Fig. 8. maxi Li of typical topologies.

connected like a tree topology by using 2x2 optical switches.Figure 10 is the proposed architecture whose number of ringstages M is 2. With the introduction of tree structure, theproposed architecture reduces the optical loss between thegiant router and users, maintaining low unavailability of thering topology. In addition, two different routes that do notshare any optical switch are set to every user. Therefore,all users can continue to communicate with the giant routerwhen any one of 2x2 optical switches fails. In Figure 10,two different routes for user 2 are shown as dotted arrows.×-marked ports are unused because it is unable to set twodifferent routes to user who connects with these ports. Dueto using all ports of a 2x2 optical switch (except ×-markedports), this architecture reduces S compared to the duplex treetopology.

IV. DESIGN METHOD

In this section, a design method of the proposed architectureis shown.

Fig. 9. S of typical topologies.

A. Constraint and objective function

The following constraints are set in this method.

Subject to :(s1 − 1)× s2 × s3 × · · · × sM ≥ N (10)max

iUi ≤ U (i = 0, 1, 2, . . . , N − 1) (11)

maxi

Li ≥ L (i = 0, 1, 2, . . . , N − 1) (12)

sm is the number of 2x2 optical switches of a mth-stage ring.M is the number of ring stages of the network. Constraint (10)means that the number of users that can connect with opticalswitches of M th-stage rings is equal to or larger than N . Uis the guaranteed unavailability that the network provider setbased on SLA (Service Level Agreement). L is the loss budgetbetween the giant router and users.

Under these constraints, the following objective function isset in this method.

Objective :minS (i = 0, 1, 2, . . . , N − 1) (13)

S = s1 × (1 + s2 × (1 + s3 × · · · × (1 + sM ) . . . )) (14)

B. Design example

In the following design example, it is assumed that thenumber of users N is 210, the unavailability of a 2x2 opticalswitch u is 10−6 and the insertion loss of the optical switchl is −1(dB). The guaranteed unavailability U is set to 10−6.The loss budget L is set to −29(dB) in reference to that ofIEEE802.3av 10GE-PON (10Gigabit Ethernet-PON) [7].

Networks whose number of ring stages is M (M =1, 2, 3, . . . , ⌈log2 N⌉) are created as shown in Figure 11. Forsimplicity, the number of 2x2 optical switches of a mth-stagering is set to 2x (x is a positive integer), with the exceptionthat that of a 1st-stage ring is set to 2x + 1. The differenceof the number of optical switches between mth-stage rings isminimized. All networks are created to meet constraint (10)in this example.

Figure 12, 13 and 14 show maxi Ui, maxi Li and S of eachcreated network. Figure 12 shows that all created networks

2n

d-stag

e ring

1st-stage ring2nd-stage ring 2nd-stage ring

’

’

’

’’

’

’

’

’

’

’

’

’ ’

’’

’ ’

’

’ ’ ’ ’ ’ ’’ ’ ’ ’ ’ ’ ’

Users

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15LT(1)

LT(0)

RouterLT(0) LT(1)

Fig. 10. Proposed architecture (N = 16, M = 2).

Router RouterRouter

(a) M=1 (S1=17) (b) M=2 (S1=5, S2=4) (c) M=3 (S1=3, S2=2, S3=4)

Fig. 11. Networks having M ring stages (N = 16).

meet constraint (11). Figure 13 shows that networks whosenumber of ring stages is 4 ≤ M ≤ 10 meet constraint (12).Therefore, the network of M = 4 is selected as the solutionbecause it has the smallest S among 4 ≤ M ≤ 10.

C. Comparison of the number of switches S

Figure 15 shows S of the duplex tree topology and thatof the network created by using the proposed design method.Assumed conditions except N are same as the above designexample. Notice that the tree topology and the ring topologyreduce S compared to the proposed design method, but theycannot create the network which meets all constraints underthese assumed conditions.

The proposed design method can create the network whichmeets all constraints when the number of users is 2 < N <

214. The network created by using the method reduces Scompared to the duplex tree topology, except when N = 2.For example, it reduces S by 28% when N = 210.

V. CONCLUSION

In this paper, a novel optical aggregation network architec-ture using Mach-Zehnder type 2x2 optical switches is pro-posed. The proposed architecture consists of small-sized ringsthat are connected like a tree topology. The design method ofthe proposed network architecture is also introduced in thispaper. The example shows that a high-availability and low-optical-loss optical aggregation network is designed at 28%fewer optical switches compared to a duplex tree topologywhen N = 210, U = 10−6 and L = −29(dB).

Constraint( =10-6)

N=210

Fig. 12. maxi Ui of created networks.

Constraint( =-29)

N=210

Fig. 13. maxi Li of created networks.

ACKNOWLEDGEMENT

This work was supported by the Japan Society for thePromotion of Science’s (JSPS) Grant-in-aid for ScientificResearch (C)22500068.

Duplex tree( =2046)

N=210

Fig. 14. S of created networks.

Fig. 15. S of duplex tree topology and that of created network.

REFERENCES

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[4] H. Takeshita, et al, “High-energy Efficient Layer-3 Network Architec-ture based on Solitary Universal Cloud Router and Optical AggregationNetwork,” COIN 2010, TuC1-2, pp. 138-140, Jeju, Korea, July 2010.

[5] K. Nashimoto, “PLZT Waveguide Devices for High Speed Switchingand Filtering,” OFC/NFOEC 2008, OThE4, San Diego, USA, April2008.

[6] ITU-T Recommendation G.983.1, “Broadband optical access systemsbased on Passive Optical Networks (PON),” January 2005.

[7] IEEE Standard 802.3av, “Part 3: Carrier Sense Multiple Access with-Collision Detection (CSMA/CD) Access Methodand Physical LayerSpecifications,” October 2009.