A Design Method of High-availability and
Low-optical-loss Optical Aggregation Network
Architecture
Takehiro Sato, Kunitaka Ashizawa, Kazumasa Tokuhashi,
Daisuke Ishii, Satoru Okamoto and Naoaki Yamanaka
Dept. of Information and Computer Science, Keio University 3-14-1 Hiyoshi, Kohoku, Yokohama, Kanagawa, Japan 223–8522Email: [email protected]
Eiji Oki
Dept. of Information and Communication Engineering, The University of Electro-Communications 1-5-1 Chofugaoka, Chofu, Tokyo, Japan 182–8585
Abstract—A highly-energy-efficient network using an optical aggregation network and a service cloud has been proposed to reduce the power consumption of today’s Internet greatly. The optical aggregation network connects a solitary giant router to users by logical tree topology. This paper proposes a novel optical aggregation network architecture. The proposed architecture uses 2x2 optical switches and combines the features of a tree topology and a ring topology. The paper also introduces a design method of the proposed network architecture. It is shown that the method can design high-availability and low-optical-loss network with fewer optical switches compared to a duplex tree topology.
I. INTRODUCTION
Recently, the number of broadband Internet subscribers is increasing rapidly, and is about 300 million all over the world in 2007 [1]. As the broadband Internet has spread, various services including contents delivery and video conference, are provided.
Today’s Internet has two big problems. The first problem is the increase of power consumption of network equipments. Worldwide power consumption of network equipments grows about 12% per year [2]. It was 25 GW in 2008, and is expected to 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 traffic increases drastically while Peer-to-Peer traffic shrinks [3]. However, the current Internet topology is almost mesh, as shown in Figure 1. Therefore, the number of hops between a user and a service server becomes large, so RTT (Round Trip Time) and delay jitter may affect the quality of services. To improve these problems, a highly-energy-efficient net-work using an optical aggregation netnet-work and a service cloud has been proposed [4]. Figure 2 shows its architecture. Routers and servers that the current Internet contains are collected up to a solitary power-scalable giant router. Together with service servers, this giant router constitutes the service cloud. All IP traffic is aggregated by the optical aggregation network and transferred to the giant router in one hop. The optical aggregation network consists of optical switches and connects the giant router to users transparently. The access method
Users IX ISP : Internet Service Provider IX : 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 RouterO
n
e
H
o
p
Fig. 2. Highly-energy-efficient network architecture.
is TDM (Time Division Multiplexing) like a PON (Passive Optical Network) system. According to the estimate, the power consumption of the highly-energy-efficient network is 1/1000 of that of the present Internet [4].
This paper proposes a novel architecture of the optical aggregation network that achieves high availability and low
optical loss. A design method of the proposed network ar-chitecture is also illustrated. An argument about availability is a great issue in the optical aggregation network, because thousands of users including business users and wireless base stations are contained in this network by logical tree topology. Low optical loss between the giant router and users is also important, because users are widespread, and no optical amplifiers should be utilized preferably in terms of deployment cost.
In this paper, we consider using Mach-Zehnder type 2x2 optical switches for the design of the optical aggregation network architecture. A PLZT (Plomb Lanthanum Zirconate Titanate) 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 voltage is applied to one of the electrodes.
The rest of the paper is organized as follows. Prior to the proposal, we discuss the features of some typical topologies in section II. The high-availability and low-optical-loss network architecture for the optical aggregation network is presented in 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 typical topologies that are constructed by 2x2 optical switches. We focus on the following parameters.
• Unavailability
It is the probability that a user cannot communicate with the giant router due to a failure of a 2x2 optical switch. It is equal to 1 minus availability. In the following section, the unavailability of user i(i = 0,1,2, . . . , N −1) is expressed as Ui.
• Optical loss
It is a sum of insertion losses of 2x2 optical switches that are placed between the giant router and a user. In the following section, the optical loss between the giant router and user i(i= 0,1,2, . . . , N−1) is expressed as Li.
• Number of optical switches
It is the number of 2x2 optical switches that is required
to connectNusers with the giant router. In the following section, it is expressed as S.
The values of Ui and Li are different among users
re-spectively, so we evaluate these parameters by worst-case conditions, maxiUi and maxiLi. We focus on the feature
of topology itself, and ignore any effect of other components or devices (e.g. an optical fiber cable that connects two 2x2 optical switches).
A. Analysis
1) Tree topology: A tree topology is often used in access networks (e.g. PON). Figure 4 shows the tree topology that is constructed by 2x2 optical switches. When it is a complete binary tree, each parameter is expressed as follows.
max i Ui = 1−(1−u) ⌈log2N⌉ (1) max i Li = ⌈log2N⌉l (2) S = N−1 (3)
uis the unavailability of a 2x2 optical switch itself, andl is the insertion loss of the optical switch. WhenN is power of two,UiandLiof all users are equal tomaxiUi andmaxiLi
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 unable to communicate with the giant router inevitably when any one of 2x2 optical switches fails. A duplex configuration is referred as the protection architecture “Type C” to enhance the reliability of PON in ITU-T recommendation G.983 [6]. Figure 5 shows the duplex tree topology that is constructed by 2x2 optical switches. When it is a complete binary tree, each parameter is expressed as follows.
max i Ui = (1−(1−u) ⌈log2N⌉)2 (4) max i Li = ⌈log2N⌉l (5) S = 2(N−1) (6)
Router LT(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 a metropolitan area network (e.g. ROADM (Reconfigurable Optical Add/Drop Multiplexer) ring, SONET (Synchronous Optical Network) ring). Figure 6 shows the ring topology that is constructed by 2x2 optical switches. Each parameter is expressed as follows. max i Ui = u+(1−u)(1−(1−u) ⌈N−1 2 ⌉)(1−(1−u)⌊N−12 ⌋) (7) max i Li = N l (8) S = N (9)
The user that meets Ui= maxiUi isi=N2 −1,N2 (whenN
is even) or N−21 (whenN is odd). Equation (7) means that the user cannot communicate with the giant router when the 2x2 optical switch that is connected directly with the user fails, or when both of its routes to the giant router (clockwise or counterclockwise) are unavailable concurrently due to failures of switches on each route. The user that meets Li= maxiLi
is i= 0, N−1. B. Comparison
Figure 7, 8 and 9 show maxiUi, maxiLi and S of each
topology respectively. We assume the unavailability of a 2x2 optical switch uis 10−6 and the insertion loss of the optical switch l is−1(dB).
maxiUi of the ring topology is lower than that of the tree
topology, except when N is extremely large. This is because users can communicate with the giant router by using one of the two routes (clockwise or counterclockwise) in the ring topology.
By contrast,maxiLi of the tree topology is lower than that
of the ring topology. This is because the number of optical switches that are placed between the giant router and the worst-case user is N in the ring topology, and ⌈log2N⌉ in the tree topology.
Router LT(0) LT(1) Users 6 7 8 9 0 1 2 3 4 15 14 13 12 11 10 5 LT(1) LT(0)
Fig. 6. Ring topology (N= 16).
The duplex tree topology achieves very lowmaxiUidue to
setting two different routes for every user that do not share any optical switch between the giant router and the user. However, Sof this topology is twice as many as that of the tree topology and the ring topology.
Based on above argument, we propose a novel optical ag-gregation network architecture that achieves high availability and low optical loss together in the next section.
III. PROPOSED ARCHITECTURE
Figure 10 shows the proposed architecture for the optical aggregation network. In this architecture, small-sized rings are
Fig. 7. maxiUiof typical topologies.
Fig. 8. maxiLiof typical topologies.
connected like a tree topology by using 2x2 optical switches. Figure 10 is the proposed architecture whose number of ring stages M is 2. With the introduction of tree structure, the proposed architecture reduces the optical loss between the giant router and users, maintaining low unavailability of the ring topology. In addition, two different routes that do not share any optical switch are set to every user. Therefore, all users can continue to communicate with the giant router when 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 two different routes to user who connects with these ports. Due to using all ports of a 2x2 optical switch (except ×-marked ports), this architecture reducesScompared to the duplex tree topology.
IV. DESIGN METHOD
In this section, a design method of the proposed architecture is shown.
Fig. 9. Sof 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 i Ui≤ U (i= 0,1,2, . . . , N−1) (11) max i Li≥ L(i= 0,1,2, . . . , N−1) (12)
smis the number of 2x2 optical switches of amth-stage ring.
M is the number of ring stages of the network. Constraint (10) means that the number of users that can connect with optical switches of Mth-stage rings is equal to or larger thanN. U is the guaranteed unavailability that the network provider set based on SLA (Service Level Agreement).Lis the loss budget between the giant router and users.
Under these constraints, the following objective function is set 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 the number of users N is210, the unavailability of a 2x2 optical switch uis10−6 and the insertion loss of the optical switch l 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 of IEEE802.3av 10GE-PON (10Gigabit Ethernet-PON) [7].
Networks whose number of ring stages is M (M = 1,2,3, . . . ,⌈log2N⌉) are created as shown in Figure 11. For simplicity, the number of 2x2 optical switches of amth-stage ring is set to2x (x is a positive integer), with the exception
that that of a 1st-stage ring is set to 2x+ 1. The difference
of the number of optical switches betweenmth-stage rings is minimized. All networks are created to meet constraint (10) in this example.
Figure 12, 13 and 14 showmaxiUi,maxiLiandS of each
2 n d -s ta g e r in g 1st-stage ring
2nd-stage ring 2nd-stage ring
ձ ղ ճ մ յ ն շ ո չ պ ջ ռ ս վ տ ’ ձ’ ղ’ ճ’ մ ’ յ’ ն’ շ’ ո’ չ’ պ’ ջ’ ռ’ ս’ վ’ տ’ ’ձձ’ ղղ’ ճճ’մմ’յյ’նն’շշ’ոո’չչ’պպ’ջջ’ռռ’սս’վվ’տտ’ Users 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 LT(1) LT(0) Router LT(0) LT(1)
Fig. 10. Proposed architecture (N= 16,M= 2).
Router Router Router
䈈
(a) M=1 (S1=17) (b) M=2 (S1=5, S2=4) (c) M=3 (S1=3, S2=2, S3=4)
Fig. 11. Networks havingMring stages (N= 16).
meet constraint (11). Figure 13 shows that networks whose number of ring stages is 4 ≤M ≤10 meet constraint (12). Therefore, the network of M = 4 is selected as the solution because it has the smallestS among4≤M ≤10.
C. Comparison of the number of switchesS
Figure 15 shows S of the duplex tree topology and that of the network created by using the proposed design method. Assumed conditions except N are same as the above design example. Notice that the tree topology and the ring topology reduce S compared to the proposed design method, but they cannot create the network which meets all constraints under these assumed conditions.
The proposed design method can create the network which meets all constraints when the number of users is 2 < N <
214. The network created by using the method reduces S compared to the duplex tree topology, except when N = 2. For example, it reduces S by 28% whenN = 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 rings that are connected like a tree topology. The design method of the proposed network architecture is also introduced in this paper. 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 topology when N= 210,U = 10−6 andL=−29(dB).
Constraint( =10-6) N=210
Fig. 12. maxiUiof created networks.
Constraint( =-29)
N=210
Fig. 13. maxiLiof created networks.
ACKNOWLEDGEMENT
This work was supported by the Japan Society for the Promotion of Science’s (JSPS) Grant-in-aid for Scientific Research (C)22500068.
Duplex tree( =2046)
N=210
Fig. 14. Sof created networks.
Fig. 15. Sof duplex tree topology and that of created network.
REFERENCES [1] http://www.internetworldstats.com/
[2] M. Pickavet, et al, “Worldwide Energy Needs for ICT: the RISE of Power-Aware Networking,” IEEE ANTS 2008, Bombay, India, December 2008.
[3] D. McPherson, “ATLAS Internet Observatory,” ISOC Researchers, IETF76, Hiroshima, Japan, November 2009.
[4] H. Takeshita, et al, “High-energy Efficient Layer-3 Network Architec-ture based on Solitary Universal Cloud Router and Optical Aggregation Network,” COIN 2010, TuC1-2, pp. 138-140, Jeju, Korea, July 2010. [5] K. Nashimoto, “PLZT Waveguide Devices for High Speed Switching
and Filtering,” OFC/NFOEC 2008, OThE4, San Diego, USA, April 2008.
[6] ITU-T Recommendation G.983.1, “Broadband optical access systems based 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 Layer Specifications,” October 2009.
