hledat:

Deployment of CL ROADM and CL WSS Devices in the CESNET2 Network and the CESNET Experimental Facility

CESNET technical report 18/2010

Jan Radil, Lada Altmannová, Michal Altmann, Miloslav Hůla, Jan Nejman, Stanislav Šíma, Pavel Škoda, Tomáš Uhlář, Josef Vojtěch

Received 1. 12. 2010

Other formats: PDF, EPUB

Abstract

This technical report deals with practical results of deployment of new advanced photonic devices from the CzechLight (CL) family of open photonic devices. CL ROADMs (Reconfigurable Optical Add/Drop Multiplexer) deployment will solve some issues on the single fibre link (bidirectional transmission over one fibre). The main goals are elimination both of optical signal noise propagation and backreflections, which are major issues of the single fibre transmission. The ROADM technology enables dynamic and flexible (touch-less) provisioning of new lambda services. A CL WSS (Wavelength Selective Switch) was tested and deployed in the CESNET EF to verify not only switching capabilities but also new SW enhancements and developments. Analysis of various technologies of WSSs/ROADMs is not in the scope of this technical report.

Keywords: all-optical processing, experimental facility, ROADM, WSS

1  Introduction

New photonic devices like ROADM and WSS are widely deployed and considered by many networks operators today, including commercial Internet Service Providers (ISP) and national research and educational networks (NREN). Modern DWDM optical networks are transitioning from static wavelength provisioning to dynamic one, allowing so called touch-less lambda provisioning.  In other words, new optical channels can be added or dropped as requested without time consuming and error-prone manual re-patching of optical cables. Photonic devices like ROADMs and WSSs eliminate OEO conversions and therefore are transparent to bit rates and data protocols. WSSs can be considered as a basic building block of ROADMs. Standalone WSSs are not deployed that often but can be useful for some special applications where more than one channel are to be adder or dropped, for example all optical multicast [1] or all optical wavelength conversions [2]. Usually only one channel is used to connect routers, switches or other devices (one channel per one port).

The CzechLight family includes open photonic devices, allowing free modifications and improvements, both in HW and SW. Some types of CL devices (e.g. optical amplifiers) have been developed 5 years ago and deployed since both in the CESNET2 network and some other networks too. New advanced CL devices are under continuous development based on new components availability and advanced users requests.

There are many articles and reports on various technologies used to produce WSSs and ROADMs and for this reason are not repeated here and for those more interested in such technical details (e.g. channel plan and centre wavelength accuracy, insertion loss, switching time, channel crosstalk) we can recommend to use public search engines like Google. Less is published on some special cases of practical deployment and this is the intent of this report.

2  CL ROADM deployment in the CESNET2

CL ROADMs are going to be deployed in the CESNET2 network, on one dark fibre link between two cities Most and Plzeň. Cheb is the city in the middle of this link, where optical channels will be added and dropped as well. This dark fibre link is a single fibre one, meaning that bidirectional data transmission uses only one fibre. This design reduces costs related to leasing of fibres, on the other hand halves the total number of channels and is more complicated than standard two fibre transmission More details are available online [3].

This fibre link is somewhat more complicated because the single fibre is carrying more optical channels which are terminated beyond Most a Plzeň. For this reason, any failure or unexpected condition (amplifier outage, added channels resulting in additional crosstalks) is more difficult to troubleshoot and correctly identify the place of failure.

CL ROADM (or any other ROADM, of course) can help to improve the situation because unused channels on ROADMs can be attenuated individually. This is very interesting feature because usually variable attenuators are used for gain flattening (e.i. signals power equalization) only. But in this particular case, built-in attenuators are used to switch the unused channels off completely, thus avoiding ‘noise leaking’. This noise is generated by optical amplifiers (EDFAs in this case) and is known as amplified spontaneous emission (ASE) and is not usually problem for standard bidirectional fibre links, but single fibre links are more sensitive to crosstalks and backreflections and any parasitic noise can be a much more serious problem.

The CL ROADM has all the features as other CL devices. Can be accessed over an IP network and other ways are possible too (e.g. GSM modem). Secure shell (SSH) is implemented and access is limited to particular IP addresses or networks. For operation (setting and reading of any parameter), the shell scripts are prepared and used. Web access is available too and is shown in Figure 3. Red are added channels, green are channels passing through and other important parameters are displayed too. For every channel, attenuation can be set from 0 to 20 dB with variable optical attenuators (VOA). This feature can stop to propagate noise (ASE) from input to output ports.

As stated before, any ROADM can be used but we have decided to deploy CL ROADMs because of the open approach which results in lower costs (both CAPEX and OPEX) and any required future additions can be done very easy (no problems with roadmaps and committed or uncommitted features).

3  CL WSS deployment in the CESNET Experimental Facility

WSSs are basic building blocks of ROADMs and as stated in Introduction, WSSs are not deployed as standalone devices that often because each port usually carries more than one wavelength. And because we’re going to connect individual ports of routers or switches, only one wavelength must be transmitted at a time. For this reason, standalone WSSs are required in special situations, for example adding or dropping many channels at a time in some backbone deployment (although this is usually done ’within’ ROADMs). WSSs can be very interesting devices when used for some not so typical applications like routing or switching of IP packets, for example all optical multicast or all optical wavelength conversions, where working with more than one wavelength per port is quite common. Of course, optical filters or demultiplexers have to be used in conjunction with WSSs when connecting individual optical signals to end devices, PCs with DWDM interfaces in many cases.

ASE from EDFAs was used for testing, to overcome problems with many required DWDM transceivers. For our purposes, ASE is used to feed all possible channels between individual ports.

More details on CL WSS cannot be given here because of a utility model pending but this report is focused on practical deployment from user’s perspective. As with any other CL devices, CL WSS can be accessed over an IP network and all featured described in CL ROADM deployment in the CESNET2 apply here too. CL WSSs are configured via command line interface (CLI) now but web access can be implemented if needed. The CL WSS offers commands to connect channels between ports, set attenuation, show alarms etc. More details can be seen from the following screenshots.

4  Conclusions

New advanced open photonic devices CL ROADM and CL WSS have been successfully tested and deployed in the CESNET2 network and the CESNET Experimental Facility, respectively. All stabile and proven SW features known from previous deployment of CL amplifiers are embedded in these devices too.

Deployment of these devices can be recommended for Open DWDM systems or in addition to DWDM systems from traditional big vendors, because of economical reasons and possible customization for every network applications.

5  Acknowledgements

This work has been supported by the research intent Optical Network of National Research and Its New Applications, MŠMT 6383917201.

References

[1]	RADIL, J.; ŠÍMA, S. Customized Approaches to Fibre-based E2E Services. In 1st E2E Workshop – Establishing Lightpaths, 1-2 December 2008. TERENA. Available online.
[2]	ŠKODA, P. et al. All-optical Wavelength Converter. Technical Report 6/2009, Praha: CESNET, 2009.
[3]	VOJTĚCH, J.; NOVÁK, V.; HŮLA, M. Open photonics DWDM systems deployment within the CESNET2 network, with emphasis to single fibre bidirectional operation. Technical Report 19/2008, Praha: CESNET, 2008.