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Open photonics DWDM systems deployment within the CESNET2 network, with emphasis to single fibre bidirectional operation

CESNET technical report 19/2008
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Josef Vojtěch, Václav Novák, Miloslav Hůla

Received 12.12.2008

Abstract

This paper describes an experience with the deployment of DWDM system, based on open photonic devices family called CzechLight, within the CESNET2 network optical infrastructure in 2008. Emphasis is given to single fibre bidirectional DWDM systems. The key motivation for CL DWDM based systems deployment is a cost effective solution for gigabit (1-10Gbps) connectivity provisioning for small PoPs with the backup connectivity based on pure optical circuits. A NIL approach, i.e. optical line design without the inline components (e.g. amplifiers and chromatic dispersion compensators), was implemented.

Keywords: photonic transmission system, DWDM, NIL, single fibre bidirectional transmission, open photonic system

1  Introduction

The CzechLight (CL) family devices and their application, especially CL Amplifiers (CLA), have been extensively described in the past, e.g. , and . The CLAs can be easily deployed for multi-gigabit transmission over single fibre bidirectional links, as reported in .The key motivation for single fibre systems is a provision of cost effective (both OPEX and CAPEX) DWDM (Dense Wavelength Division Multiplex) system. This approach was presented more time e.g. and also deployed in other National Research and Educational Networks (NREN) . DWDM system over single fibre functions similarly to a fibre pair system. Signals are multiplexed and demultiplexed or amplified in the same way as in fibre pair system, with the difference that output and input signals must be mixed together before entering and after leaving transmission line. This can be performed by different means, e.g. by usage optical circulators, wavelength interleavers or WDM couplers. Example setup of bidirectional DWDM transmission over 210km based on circulators reported in is shown in Figure 1.

When NIL (Nothing-in-line) setup over large distance must be deployed, the issue of high backscattered power raises. To overcome this issue, different channels for each transmission direction must be used which excludes deployment of circulators. From economical point of view, the best option is to use simple WDM couplers.

The average price of dark fibre pair lease is about 0.5 EUR/meter/year and price of single fibre lease is about 40% lower, 0.3 EUR/meter/year . The single fibre bidirectional transmission brings necessity of more complex hardware, but the average increase in price of transmission equipment is about 0.004 EUR/meter/year for NIL single fibre lines and about 0.002 EUR/meter/year for multi span lines. These average data are based on examples of lines deployed by open photonic transmission systems mentioned in . Together with light increase in price of hardware, the only drawback of a single fibre solution is the reduction of transmission channel (by one half comparing with fibre pair system). Nevertheless, while we are relying on mature EDFA technology, 32 or 40 bidirectional channels with 100 GHz spacing can be easily created exploiting both C and L bands.

Considering that present research networks typically operates small number of channels (below 10) and for “rural lines” there are no needs or plans to deploy over 32 channels this reduction should not be an obstacle.

2  “Regular” Fibre Pair Optical Lines

2.1  České Budějovice – Jihlava – Brno

In 2008 the CL DWDM deployment of České Budějovice – Jihlava – Brno line was successfully completed with the following goals:

-full 10Gbps backup for České Budějovice PoP.

-1Gbps backup connectivity for Jihlava PoP.

-1Gbps backup connectivity for Jindřichův Hradec PoP.

-simple provision(ing) of new optical channels between the any PoP

The line was designed in the form of two independent spans depicted in Figure 2. The Jihlava PoP is a full add/drop node allowing more freedom for future upgrades. It means that all wavelengths are dropped or added. Amplification is performed by CLA PB01F amplifiers and chromatic dispersion (CD) is compensated by un-channelized fibre Bragg gratings (FBGs). They support the easy upgrade in future, allowing deployment of spectrally broader signals. These signals could be induced by possible fast transmissions speeds, e.g. 40 or 100 Gbps. Signals are multiplexed and demultiplexed by 32 channel athermal arrayed waveguide gratings (AAWGs) which opens up the channel planning possibilities.

Currently the Jihlava is a 1Gbps PoP, with a future upgrade to 10G possible anytime, just by an exchange of transceivers in the client devices, i.e. IP/MPLS routers. Connectivity for Jindřichův Hradec PoP is provided by OADM node at Jindřichův Hradec railway station, where 3 lambdas are dropped and transported to/from Jindřichův Hradec PoP.

2.2  Brno - Ostrava

The upgrade of Brno – Ostrava line has brought a full 10Gbps backup connection for Ostrava PoP and reduction in latency of the line moderately loaded by almost 2.5 Gbps traffic.

The line deployment is shown in Figure 3. To overcome a huge attenuation of the 235km long line the CLA PB02F amplifiers (with saturated output power about 27 dBm) were deployed. CD is compensated by un-channelized FBGs for future upgradeability. Signals are multiplexed and demultiplexed by 32 channel AAWGs for achieving a freedom in channel planning.

2.3  Praha – Pardubice

In 2008 leased fibre pair line was upgraded from a ”gray” single 1 Gbps to 10Gbps DWDM system using “lit fibre approach”, see . It means that deployment was in the hands of fibre pair owner, but all capacity remains with customer (i.e. CESNET). The line deployment is shown in Figure 4. To overcome attenuation of this nearly 190 km long line the CLA PB02F amplifiers were deployed. CD is compensated by un-channelized FBGs for future upgradeability. Signals are multiplexed and demultiplexed by 32 channel AAWGs for freedom in channel planning.

The Pardubice PoP is planned to be full ADD/DROP node and upgrade of span to Hradec Králové is in progress.

3  Single Fibre Bidirectional Optical Lines

3.1  Plzeň-Cheb-Most-Ústí n. Labem

The network of single fibre lines connecting small PoPs in West and North-West Bohemia (Cheb, Most, Děčín) has been extended and the DWDM system has been designed to provide backup connectivity for these PoP and also all-optical 10Gbps capable paths between Plzeň and Ústí nad Labem PoPs.

The planned deployment is shown in Figure 5. An amplification is provided by CLA PB01 F amplifiers as relatively low channel number is considered. Over Plzeň – Cheb – Most – Ústí n. Labem lines the CD is compensated by un-channelized FBGs for future upgradeability. As there is no demand for 10Gbps connections in Děčín, CD of Most – Děčín - Ústí n. Labem line remains uncompensated. 8-channel MUX/DEMUXes based on thin filter technology were used. Děčín PoP was designed as passive OADM nodes similar to the future planned node in Marianské Lázně. The Most PoP was designed as full ADD/DROP node to allow crossing of three directions.

“Lit fibre” approach was used and upgrades were requested from different fibre owners. In November 2008 the Plzeň - Cheb and Ústí nad Labem – Most lines were upgraded and came into operation. For example, the upgrade of Plzeň - Cheb line has allowed high speed interconnection of backup servers. Traffic invoked by large backup is shown in Figure 6. Upgrades of remaining lines are in progress.

3.2  Letohrad-Opava

The reason for deployment of Letohrad - Opava line was to provide the secondup connection for Opava PoP.

The line setup is shown in Figure 7. An amplification is provided by CLA PB01 F amplifiers as relatively low channel number is considered. As only 1Gbps speed is considered, CD remains uncompensated. 6-channel MUX/DEMUXes based on thin filter technology were used.

As Letohrad PoP is an optical transport (ROADM) node only (without the IP equipment), lambdas are therefore transported as alien wavelengths over the main DWDM system based on Cisco ONS15454 MSTP technology from/to Hradec Králové. Unfortunately, signal levels outgoing from ONS 15454 MSTP system are too week (about -12 dBm) which requires an additional amplification and the upgrade of Letohrad node has been ordered. The line currently operates in an experimental regime with limited monitoring possibilities.

4  Future plans

The CESNET2 backbone network optical topology is depicted in Figure 8. The main DWDM backbone system is based on a commercially available technology (Cisco ONS 15454 MSTP with the ROADM) in the optical network core. The DWDM system interconnects the most important PoPs at the optical level, when there is a need for more optical transport channels (between the PoPs or for research projects). It also allows the flexible optical channels provisioning using the ROADM “on-demand”. The CL DWDM systems on some backbone lines effectively complements the main DWDM system and are effective (in terms of CAPEX, OPEX including power consumption) in comparison with commercially available DWDM systems. Both ONS15454 MSTP and CL DWDM systems supports the “alien” wavelength transport via all the optical backbone network.

We plan to implement CL DWDM systems on new optical lines during the future CESNET2 deployment stages. In 2009 we plan to deliver a 10 Gbps connectivity and backup connectivity to Zlín PoP, and an upgrade of Brno – Zlín – Olomouc fibre pair lines has been designed. The “lit fibre service” approach will be used again.

Zlín was designed to be a full ADD/DROP node where signals will be muxltiplexed amd demultiplexed by 32-channel AAWGs for freedom in future channels planning. CD will be compensated by un-channelized FBGs for future upgradeability. Attenuation of longer Brno – Zlín span is compensated by CLAPB02F amplifiers, span Zlín – Olomouc will be operated by CLAPB01F devices.

For a 10 Gbps backup connectivity of Pardubice PoP, the upgrade of fibre pair Pardubice - Hradec Králové has also been designed.

Pardubice PoP is considered to be full ADD/DROP node and signals will be MUXed/DEMUXed by 32-channel AAWGs for freedom in future channels planning. As the line length is only about 30km, the CD will not be compensated and one-side amplification will be provided by single CLADI02F device.

In 2009 we plan to implement and test the new CL DWDM technology components in the CESNET2 operational network. There are the first CL DWDM ROADM and CLS (CL optical switches) which will be deployed on the CL DWDM optical lines.

References

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