Secure and Scalable Optical Access Networkusing PLZT High-speed Optical Switches

Kazumasa Tokuhashi†, Kunitaka Ashizawa†,Daisuke Ishii†, Yutaka Arakawa†, Naoaki Yamanaka† and Koji Wakayama‡†Dept. of Information and Computer Science, Faculty of Science and Technology,

Keio University, Yokohama, 223-8522, JapanEmail: tokuhashi@yamanaka.ics.keio.ac.jp

‡Hitachi Ltd., Central Research Laboratory, Tokyo, Japan

Abstract—We are proposing a new optical accessnetwork architecture using PLZT 10 nsec high-speedoptical switches, called Active Optical Network (ActiON).Our architecture can increase four times the number ofsubscribers as well as double the distance between OLTand ONUs than GE-PON. Moreover, a user can establishsecure communication in ActiON. Because ActiON usesnot broadcasting system but slot based switching system.ActiON has to overcome a Discovery process issue be-cause an optical switch can not broadcast. In this paper,we propose a new Discovery process technique. Throughsimulation results and experimental results, we show thatour proposed architecture is able to complete Discoveryprocess with accuracy.

I. I NTRODUCTION

IP traffic in a network increases rapidly in recentyears, and the demand for the high-speed access net-work has risen. Today, Passive Optical Network (PON)system is widely prevalent for access network. GigabitEthernet Passive Optical Network (GE-PON) [1] withthe transmission rate of 1 Gbps is the representativeexample of the access network.PON consists of threecontents, Optical Line Terminal (OLT) which connectsto backbone network, Optical Network Unit (ONU)which communicates with a user terminal and anoptical splitter, as shown in Figure 1. Currently, torealize 10 Gbps access network, 10 Gigabit EthernetPassive Optical Network (10GE-PON) [2] is becomingthe standard now.

However, PON system has limitations of the maxi-mum number of ONUs and the maximum transmissiondistance between OLT and ONUs. This is because themore ONUs increases, the weaker the optical powerdue to dividing optical power (broadcast) at a splitter.Moreover, PON system is a low-security architecturein principle because all ONUs receive all signals fromOLT. A malicious user can intercept all data. To resolvethese problems, access network systems using not anoptical splitter but an optical switch are proposed [3],[4]. But, these conventional systems don’t scale aboutwider bandwidth because these systems have electrical

control for reading each single Ethernet frame in orderto determine a destination.

In this paper, we propose a secure and scalable activeoptical access network architecture using PLZT high-speed optical switches [5] without electrical control.We call this proposed network Active Optical Net-work (ActiON). ActiON can increase four times thenumber of subscribers per OLT as well as double thedistance between OLT and ONUs. Moreover, a usercan establish secure communication. Because ActiONuses not broadcasting system but switching system andthe optical switch can greatly reduce optical powerlossthan an optical splitter. ActiON can contain a lot ofONUs than PON. So, by using ActiON , the numberof access network systems is expected to be decreased.Therefore, ActiON can contribute to energy saving.

The PLZT high-speed optical switch is able toswitch its output port with under 10 nsec, and to setthe switching guardtime for 10 nsec order. By usingPLZT optical switches, the access network with highutilization efficiency of bandwidth can be constructed.

When ActiON is materialized, a high-speed accessnetwork system can be set up in the areas where PONsystem is difficult to be constructed due to geographicaldefect such as mountain-ringed region. The ActiONproject aims to popularize a high-speed access networkthroughout all over the world, and to resolve regionaldifferences such as digital divide issues. The ActiONproject is an industry-university joint research, andbeing promoted by the national foundation.

In the ActiON project, there are many challenges, forexample, downsizing of OLT and ONU, bidirectionalactive channel control and developing PLZT opticalswitch. In this paper, we focus the bidirectional activechannel control. A major challenge in the channelcontrol of ActiON is a Discovery process becauseActiON doesn’t broadcast unlike in the case of PON.We propose and implement a new Discovery process.

These remaining sections of this paper are organizedas follows. Section II describes the PON system and its

Discovery process, Section III describes the proposedActiON system and its Discovery process. Section IVshows the simulation results for the Discovery processof ActiON, and Section V shows the implementationand experimentation of ActiON. Section VI summa-rizes the current progress of ActiON.

II. PASSIVE OPTICAL NETWORK (PON)

A. Architecture

Figure 1 shows PON architecture. The signals fromONUs (Upstream) are combined on an optical fiber atan optical splitter, and arrive at OLT without Ethernetframe collisions by accurate bandwidth assignment. Onthe other hand, the signals from OLT (Downstream) arereplicated in the number of ONUs at the optical splitter,and are broadcasted to all ONUs. ONUs receive all sig-nals from OLT and accept the only Ethernet frame witheach assigned identifier called Logical Link Identifier(LLID).

On an access network, there are two transport pro-cesses, 1) Discovery process and 2) Report and Gateprocess. These processes are controlled by Multi PointControl Protocol (MPCP) to avoid conflicts of data atthe optical splitter. The Discovery process is to discoverunregistered ONUs, to assign LLID to the ONUs, andto measure the distance between ONUs and OLT. TheReport and Gate process is to control and schedulecommunication band without collision on the upstreamdata transfer. In this paper, we focus the Discoveryprocess.

B. Discovery process

Figure 2 shows the Discovery process in PON. TheMPCP message which is used for the Discovery pro-cess is of four types, ”GATE and DISCOVERYGATEmessage”, ”REGISTERREQUEST message”, ”REG-ISTER message” and ”REGISTERACK message”.First of all, at T0, OLT broadcasts the DISCOV-ERY GATE message with Grant Start TimeT1 toONUs. Grant Start Time denotes the time at whichONU can start sending the messages. After receivingthe DISCOVERY GATE message, unregistered ONUsset their own clock time atT0, and send the REG-ISTER REQUEST messages to OLT atT2 (equalT1

and random time). The reason why ONU waits randomtime is to reduce collision probability at the opticalsplitter. After receiving this REGISTERREQUESTmessage, OLT assigns LLID to this ONU by sendingthe REGISTER message which contains LLID. ONUonly accepts the REGISTER message with its MACaddress, and marks its LLID. Next, OLT sends theGATE message with a Grant Start TimeT6 calculated

#1

#X

#1

#1

#X

#X

Copy

ONU#X

WDM

OLT

Downstream: 1490 nm

Upstream: 1310 nm

ONU#2

ONU#1

Optical Splitter

Fig. 1. PON architecture.

OLT ONUDiscovery Gate

DA= MAC Control, SA = OLT MAC address, Contents = Sync Time, Grant

Register

DA= ONU MAC Address, SA = OLT MAC address,

Contents = Sync Time, Flag, LLID

Gate

DA= MAC Control, SA = OLT MAC address, Contents = Grant

Register Request

DA= MAC Control, SA = ONU MAC address, Contents = Flag, Timestamp#1

Register Ack

DA= MAC Control, SA = ONU MAC address, Contents = Flag

T0 T0

T2T3

T4 T4

T5 T5

T6T7

T1

Random Delay

Fig. 2. Discovery process in PON.

to avoid collision. After receiving the GATE message,ONU sends the REGISTERACK message at the GrantStart Time T6. OLT determines the round trip timeT7 − T6. This is the end of the Discovery process.

III. A CTIVE OPTICAL NETWORK (ACTION)

A. Architecture

The target of ActiON is the access optical networkwhich can accomplish 128 subscribers and 40 km ofthe maximum transmission distance with user commu-nication security. We utilize MPCP for compatibilitywith 10GE-PON (IEEE 802.3av) [2]. Figure 3 showsActiON architecture. Two optical switches (Upstreamand Downstream switch) are used between OLT andONUs. It is a big difference from PON system.

B. Transmission method

ActiON employs Optical Slot Switching (OSS)which we proposed previously with PLZT high-speedoptical switches [6]. In OSS, bandwidth is allocatedand reserved to each ONU by fixed-length time foreasy control. This fixed-length time is called ”slot”, and

the slot size is discretionary. In ActiON, we use PLZThigh-speed optical switches which we used heretofore[7], because a high-speed switching allows to micrifya slot.

Figure 4 shows the effect of the guard time betweenslots. As shown in Figure 4(a), the conventional MEMSswitch needs the guard time of msec order. On the otherhand, the PLZT optical switch can shorten the guardtime down to 10 nsec as shown in Figure 4(b). Becauseof reduction of the guard time, data transfer with TimeDivision method is efficient and realistic.

C. Discovery process

The Discovery process in PON is based on broadcastin the optical splitter, therefore, ActiON needs a newmodified Discovery process without broadcast. On thissystem, OLT sends the DISCOVERYGATE messagefor each ONUs by unicast.

In ActiON, ONUs can receive MPCP frames fromOLT without frame collisions because optical switchesare placed within carrier’s network and synchronizedwith OLT. Whereas, it is very difficult for OLT toreceive MPCP frames from ONUs because MPCPframes can’t be combined unlike PON using the op-tical splitter. Therefore, the conventional optical accessnetwork with optical switches was proposed in theliterature [3], [4]. But, this conventional architecturedecides a switching optical port by electrically readingthe destination LLID of MPCP frames. Therefore,it is disadvantageous in that this electrical controlis bottleneck in 10 Gbps network, and it loses thetransparent transmission. Then, the Discovery processwithout electrical control is accomplished by setting upCommunication Channel (CC) and the grasp of CC.The details about our proposed Discovery process aredescribed in the following section.

1) Communication Channel and Grasp of the po-sition of Communication Channel:The slot that isswitched to a predetermined optical port is called a CC.Figure 5 shows the detail of CC. A size of CC is oneslot. CCs periodically are set and rotate from ONU#1to ONU#128, and they are used without reservation.Data Channel (DC) between CCs consists of 128 slotsthat are used for data communication. OLT allocatesDC to ONUs based on the request from 128 ONUs.We define the 129 slots consisting of a CC and DCas a periodic time. As a result, OLT certainly allocatesCC to each ONU once every 16512 slots (8.5 ms).

We propose a new modified Discovery process usingCCs. Our proposed Discovery process features thatONU finds out its own CCs by consecutive sendingof the REGISTERREQUEST messages, unlike PON

ONU#X

Optical Switch

WDM

OLT

Downstream: 1490 nm

Upstream: 1310 nm

#1

#X

SW ONU#2

ONU#1

Fig. 3. ActiON architecture.

Data Data

Guard Time = nsec order

Data

SlotGuard Time = msec order

Slot

(a) MEMS Switch

(b) PLZT Switch

Data

Fig. 4. Effect of the guard time between slots.

OLT

ONU#1

OLT

ONU#2

OLT

ONU#X

CC

Data Channel

periodic time

1 2 X

CC : Communication Channel

OLT

ONU#1

1

Fig. 5. Periodical Communication Channels.

system with broadcast. The new Discovery processconsists of two phases, the rough ranging phase andthe precise ranging phase.

2) Rough ranging phase:Figure 6 shows the roughranging phase. Firstly, atT0, OLT sends the DIS-COVERY GATE message with Grant Start TimeT1

to ONU#X’s through CC at the downstream switch.When ONU#1 is unregistered, ONU#1 sends continu-ously the REGISTERREQUEST messages to OLT at

OLT ONU#1Upstream SW

Communication Channels

Discovery Gate for ONU#1

Downstream SW

Register Requests

from ONU#1

T0

T0

T1 T3T2 T4 T5T6

Next Communication Channels

Gate for ONU#1

Register Ack

from ONU#1

T7T7

T8

T9 = T8 + (T4 - T0)

Grant Start Time

Register for ONU#1

T8

T9T10

Grant Start Time

(T4)

(T5)

( T9 = T8 + (T4 - T0) )

Receiving

(T4) (T5) time stampConsecutive

sending

Fig. 6. Rough ranging phase.

T1 after receiving the DISCOVERYGATE message.Some REGISTERREQUEST message can reach OLTthrough CC of ONU#1 period at upstream switch.

After receiving the REGISTERREQUEST mes-sage, OLT keeps the TimestampT4 of the messagebecause OLT wants to receive the messages withoutconsecutive sending messages for the next time. In thecase of receiving a number of REGISTERREQUESTmessages, OLT adopts the first message.

Next, OLT sends the REGISTER message to ONU#1through the next ONU#1’s CC at the downstreamswitch. OLT assigns LLID to ONU#1 by sending theREGISTER message.

Next, OLT sends the GATE message with ONU#1’sGrant Start TimeT9(= T8 + (T4 − T0)) throughthe next ONU#1’s CC. After receiving the GATEmessage, ONU#1 sends the REGISTERACK messageat the specified Grant Start TimeT9. On this occasion,ONU#1 sends only one REGISTERACK message un-like in the case of sending the REGISTERREQUESTmessages. OLT determines the round trip timeT10−T9.This is the end of the rough ranging phase. Move tothe precise ranging phase.

3) Precise ranging phase:With just the rough rang-ing phase, OLT can’t grasp the accurate position ofCC. To grasp the accurate position of CC, it is impor-tant that the REGISTERREQUEST message passesthrough the CC at the instant of switching CC. Thus,the precise ranging phase is used as shown in Fig-ure 7. OLT repeatedly sends the DISCOVERYGATEmessage with the Grant Start Time. OLT staggers theGrant Start Time when ONU#1 starts to send theREGISTER REQUEST message by several tens ofnano seconds, and the Grant Start Time changes to

T1, T1 + ∆1, T1 + ∆2, · · · (∆1 < ∆2 < · · ·). ∆x isdefined asx×∆. We describe this∆ value as Preciseranging value.

Therefore, OLT can grasp the accurate position ofCC when the number of the first received REGIS-TER REQUEST message changes (for example, from#4 to #3).

OLT ONU#1SW(UP)

T0

#1 #2 #3…

T1+∆1

T1+∆2

T0

T0

Discovery Gate

Discovery Gate

Discovery Gate T1

Receive#4

Receive#4

Receive

#3

Register Req

T1

T1+∆1

T1+∆2

Register Req

Register Req

#4

#4

#3

Fig. 7. Precise ranging phase.

IV. D ISCUSSION ABOUTDISCOVERY PROCESS

To confirm the proposed system operation capa-bilities, we evaluate its performance by simulation.

Simulation parameters are shown below.

• Number of ONUs : 128• Bandwidth : 10 Gbps• Transfer time of 1 Ethernet frame (64 byte) : 51.2

nsec• Distance between OLT and optical switch : 10 km• Distance between OLT and ONU : 10 km∼ 40

km• Slot length : 520nsec (About 10 Ethernet frame)• Periodic time : CC + DC = 1 + 128 = 129• Precise ranging value∆ : 10 nsec

Fig. 8. Number of REGISTER REQUEST message.

Figure 8 shows the relationship betweentransmission distance and the number ofREGISTER REQUEST message. The minimumcontinuous transmission interval is 60 nsec includingGuard time which is 8.8 nsec (= 60 nsec - 51.2nsec). The position of CC is set up in a way that thenumber of continuous transmission is the smallest incases of ONU which is located at the 40 km. In shorttransmission interval, 60 nsec, OLT rapidly know theprecise position of CC. On the other hand, when thenumber of continuous transmission is smaller, 120nsec and 180 nsec, it has a low load on the opticalswitch and other devices.

Table I shows the relationship between transmissioninterval and the required time to complete the Discov-ery process. By the Precise ranging phase, the positoinof CC is optimally measured to an accuracy of 10 nsec.

TABLE ITHE COMPLETE CYCLES AND TIME OFDISCOVERY PROCESS

Transmission interval (nsec) 60 120 180

Average number of cycles 3.5 6.5 9.5Maximum number of cycles 6 12 18Average complete time (msec) 90.3 167.7 245.1Maximum complete time (msec) 258 516 774

ONU#3

PLZT optical switch

emulatorOLT ONU#2

#1

#3

ONU#1

Delay insertion

machine

20km

40km

ONU#1

Delay insertion machine

PLZT optical switch emulator

OLT

ONU#2

ONU#3

Fig. 9. Experimental setup of ActiON.

V. EXPERIMENTS

In the experimental system, the number of ONU isthree and Bandwidth is 1 Gbps. Figure 9 shows theexperimental setup of ActiON. There are one OLT,three ONUs, a delay insertion machine and one PLZToptical switch emulator. We developed OLT, ONU andthe PLZT optical switch emulator instead of PLZToptical switches because of the control mechanism ofthe PLZT optical switch being developed at present.

By placing the optical switch at this experimentalsystem, this system can easily change from electrical tooptical network. Each ONU is assigned with differentdistance by the delay insertion machine. In various de-lay situations, we verified that our proposed Discoveryprocess function well.

Figure 10 shows the success result of proposedDiscovery process. This data is captured between OLTand the optical switch emulator. By the delay insertion,the distance between OLT and ONU#2 is set to 20 km.

(

Discovery Gate(to ONU#1)

Register Request(from ONU#1)

Discovery Gate(to ONU#2)

Discovery Gate(to ONU#3)

Register Request(from ONU#2)

Register Request(from ONU#3)

Register (to ONU#1)Register (to ONU#2)Register (to ONU#3)

Gate (to ONU#1)

Gate (to ONU#2)

Gate (to ONU#3)

Register Ack (from ONU#1)

Register Ack (from ONU#2)

Register Ack (from ONU#3)

0002: Gate0004: Register Request0005: Register0006: Register Ack

Unit : clock

1clock = 6nsec

Fig. 10. Success result of proposed Discovery process.

Also, that between OLT and ONU#3 is set to 40km.MPCP frames to each ONU were able to be capturedin sequence through each CC. Whereat, it is provedthat the position of CC is found out by consecutivesending of the REGISTERREQUEST messages.

The new Discovery process is demonstrated at KEIOTECHNO-MALL, which is held in Tokyo InternationalForum in December, 2008.

VI. CONCLUSION

We have proposed an optical access network usingPLZT optical switches named ActiON. The ActiONproject which is being promoted by the national foun-dation aims to popularize a high-speed access network,and to resolve regional differences such as digitaldivide issues. There are many tasks in this project,and we focus the channel control of ActiON in thispaper. We proposed and implemented a new Discoveryprocess. Periodical CC between OLT and ONU canrealize the Discovery process for ActiON. For thefuture, we would like to propose and to implement anew Report and Gate process, and an optimal algorithmreserving DC. In addition, there are some tasks to doin this project, such as downsizing OLT and ONU,developing the PLZT optical switch module and soforth.

ACKNOWLEDGMENT

This work was a part of the R&D on photonicnetwork promoted by Ministry of Internal Affairs andCommunications, and supported by National Instituteof Information and Communications Technology.

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