hledat:

Design and Deployment of Phase 4 of the CESNET2 DWDM Optical Transport Core Network

CESNET technical report number 15/2006
also available in PDF, PostScript, and XML formats.

Václav Novák, Karel Slavíček
18.12.2006

1   Abstract

This paper describes design and implementation of the optical DWDM backbone of CESNET2 network in 2006 year. The article is divided into following logical parts: technology, topology and design. In the first part requirements of CESNET2 and properties of selected technology are described. The second part relates to technology solution itself and the process of implementation. It is enclosed by description of the current CESNET2 DWDM topology and prospective development.

Keywords: DWDM, Reconfigurable Optical Add/Drop Multiplexor, Cross-border Fiber, Performance Monitoring, Forward Error Correction

2   Introduction

The CESNET2 optical transport network backbone design and implementation started in 2004 year as the transformation of the point-to-point optical lines between the PoPs. The optical lines were designed with the OC-48/STM-16 optical regenerators and EDFA optical amplifiers as the preferred protocol-independent solution. The previous optical transport topology which utilised single-channel "grey" data transmission didn't allow easy capacity scaling and upgrade to 10 Gbps and was limiting factor for optical lambda services. It also represented a limiting factor for end-to-end service provisioning at the level of optical transmission channels.

The objective of DWDM optical transport network deployment is integration with the existing IP network and possible transition to the hybrid IP/optical backbone-based network controlled by protocols such as Generalized Multiprotocol Label Switching (GMPLS), including the implementation of optical switches for dynamic wavelength path switching.

3   DWDM technology and network deployment

The DWDM network deployment and technology selection started by pilot project (formerly Phase1) on the optical line Prague - Brno with the distance of 320 km. The main requirements for DWDM technology were as follows :

• Support of at least 16 traffic channels with 100 GHz spacing, according to the ITU-T G.694.1 recommendation

• Possibility to connect "colored" input signals (i.e., signals with a wavelength according to the ITU-T G.694.1 recommendation) without using a transponder

• Guaranteed bit-error rate (BER) better then 10 15 for all channels

• Automatic laser shutdown by optical fiber cutoff, according to the ITU-T G.694 recommendation

• Add/drop or rerouting of any optical channel in all terminal nodes

• Add/remove or rerouting of optical channels not causing traffic outage

• Automatic power control of optical amplifiers, including automatic reaction to optical channel add/remove and slow changes in optical parameters (e.g., fiber ageing)

• Multi-rate transponder supporting networking protocols used or planed in CESNET2

• Suitable management system for full control over nodes and network as well as out-of-band management

The winner of the public tender was Cisco Systems technology with a solution based on DWDM system ONS 15454 Multiservice Transport Platform (MSTP) with the ROADM (Reconfigurable Optical Add/Drop Multiplexing) allowing the evolution from point-to-point to ring networks. The Prague DWDM nodes are shown in Figure.

The first DWDM line Prague-Brno was deployed with the fixed 4-channel OADM as the pilot project. The migration to full 2-way ROADM (formerly known as Phase2) finished during the first half-year of 2005 as the main condition of the ring based DWDM topology deployment.

ONS 15454 MSTP offers a ROADM feature that allows for software configuration of zero up to 32 channels of pass-through or add/drop in every ROADM node. ROADM itself consists of an optical switch based on planar lightwave circuit (PLC) technology. The schema of ROADM utilization is depicted in Figure.

The amplifiers and chromatic dispersion compensation units are located in the intermediate nodes. They are needed for ensuring the proper function of DWDM system, required number of transmission channels (32) and guarantee of optical channel quality (BER value). The amplifiers are also based on the ONS 15454 platform. Though transponders used in the network provide rather high dispersion tolerance, the chromatic dispersion of all lines is compensated. The resulting chromatic dispersion is near zero. The main benefit of this approach is easy addition or reconfiguration of any number of channels up to the size of MUX/DEMUX matrix, easier scaling of DWDM network to new nodes and easier utilisation of "coloured" sign. In all of these cases we do not need to recalculate and check chromatic dispersion of the whole system.

The following are the two options for end-system (client) connection to DWDM:

• Using transponders that convert the standard "grey" 1310 or 1550 nm signal into a "colored" DWDM channel according to the ITU-T (OEO conversion) and perform reshaping retiming regeneration/amplification (3R) functionality. Transponders are an integral part of DWDM systems and are software-tunable to more wavelengths (across all 32 channels in C-band with the latest hardware versions).

• Transmitting directly to the "colored" DWDM channel. End equipment must transmit at a wavelength specified in the ITU-T G.694.1 recommendation (typically pluggable optics such as Xenpak and SFP). This option is cheaper in comparison to the transponder but has a number of limitations (mostly caused by the absence of FEC functionality) and can be used for shorter distances only.

The DWDM technology and implementation of ROADM represents a significant shift in the development of the optical network toward provision of bandwidth on-demand and creation of logically independent networks on common optical fiber infrastructure. The added benefit is the capability to satisfy demanding and potentially aggressive applications with specific requests on dynamic response of the network (e.g., low latency) and bandwidth - such as grids - without affecting services provided to traditional applications.

4   Topology

The main DWDM ring topology (Phase3) was finished at the end of 2005. Based on the public tender condition (primarily functional and management compatibility with the previous DWDM solution), the Cisco ONST15454 MSTP platform was selected. ROADM nodes that support software-managed optical path add/drop are based at optical nodes in Prague, Brno, Olomouc, and Hradec Králové. The scheme of core DWDM ring topology is shown in Figure.

In cases where termination is necessary, the ROADM hands off the optical wavelength, keeping it in the optical domain without any prior electrical conversion to the native DWDM interface of the router, where the electrical conversion is used only for IP processing. Pure optical transmission is inherently more tolerant to bit-rate variations where moves to higher rates and new protocols may still be required in the future, and hence more robust because photonic processing is intrinsically insensitive to protocol changes, unlike typical electrical processing elements. When the filling ratio of wavelength is high enough, ROADMs allow transit at the lambda level, which results in transit traffic bypassing the IP/MPLS routers.The ROADM's per-channel automatic power control allows a "self healing" intelligent approach to DWDM for trouble-free wavelength transmission. It also support per-channel optical power monitoring.

In 2006, the expansion to new locations was planned as Phase4. New spans Prague - Pilsen and Olomouc -Ostrava were interconnected with the main DWDM ring at optical level to enable optical channel provisioning between any CESNET2 optical PoPs (see Figure). In this phase we had to change DWDM node topology in PoPs Prague and Olomouc from the ROADM ring to hub topology in order to connect new spans to the main DWDM ring. As a workaround solution, optical patch cables are used to interconnect optical paths from terminal nodes (ET1) to ROADM. This solution is inexpensive but breaks flexible optical channels provisioning within the DWDM network (for the optical channel which are crossing hub node within the ring is manual patching between EAST and WEST ROADM node sides needed as well). Technical solution using the optical switches or three-way ROADM based on wavelength cross-connects (WXCs) is planned for next phases of CESNET2 DWDM core network deployment.

As the important part of Phase 4 was included CBF (Cross-border Fiber) interconnection to Poland network Pionier node in Cieszyn. CESNET2 DWDM node is interconnected with the DWDM network in Poland (based on ADVA Optical Networking technology). Because there are technology and optical limitations, we selected as the technical solution back-to-back connection between the transponders (at L2 on client side with the 10GE LAN PHY framing). We measured optical channel parameters Prague - Cieszyn with the length about 609 km (with the local loop on client side of 10GE transponder in Cieszyn) by Ethernet tester (see utilization on Figure):

• measured BER values 10^-15 or better, no E-FEC error correction founded in transponders statistics

• average latency 6777,99 microseconds (maximal 6782.36 and minimum 6768,54 microseconds)

• no losses at full rate

• frame size 12000 bytes

• Tx Status

  • TX Rate 9,983,282,176 bps

  • TX Line Rate 9,999,921,128 bps

  • TX Utilization 100.00%

• Rx Status

  • RX Rate 9,983,279,104 bps

  • RX Line Rate 9,999,917,289 bps

  • RX Utilization 100.00%

The results of this testing confirmed the channel quality and projected parameters in Table.

5   "Alien Wavelength" Support

The transport of "alien" wavelength is the transport of any wavelength specified in the ITU-T G.694.1 without the transponders (transport-less operation) installed in the DWDM system. The "alien" wavelength could be terminated on client side using the pluggable optics (DWDM SFP, Xenpak or XFP). This optics is available for specified wavelength only (in the C-band), doesn't support FEC or E-FEC features and is more sensitive to chromatic dispersion in comparison with the transponders. Based on these limiting factors is recommended length of 10 Gbps wavelength about the 200 km within the DWDM system. We are using "alien" wavelength transport on the line Prague - Brno (about 320 km) for 1 Gbps speed. It serves for MEDIMED project. The aim of MEDIMED project is processing of multimedia medicine data like CT, X-ray, ultrasound etc. This application has high demand for bandwidth and security. Utilisation of "alien" wavelength technology allows us to offer L1 gigabit connectivity for this application. Topology of this service is in Figure. "Coloured" signal is transported from Thomayer Teaching Hospital via passive 8-channel DWDM system over local loop which is about 26km in length. The source of "coloured" signal is mediaconverter with DWDM SFP module. Incoming signal is amplified via EDFA preamplifier at Cesnet PoP in Prague. At Cesnet PoP in Prague this signal is connected to production DWDM backbone. The amplification is needed for signal equalisation.

6   Management and performance monitoring

Cisco ONS15145 MSTP node software includes management suite CTC (Cisco Transport Controller) as its integral part. The CTC is a Java-based application which performs management functions of the whole DWDM network and individual DWDM Network Elements as well (provisioning, maintenance, performance monitoring, alarms, etc.). The proprietary CTC system cannot be integrated with other CESNET2 network management systems. The ONS 15454 system supports TL 1 language and SNMP MIBs version 1 and 2c. The CESNET2 network management and monitoring is based on SNMP (Simple Network Management Protocol) functionality. Within the research activities is being developed network and traffic monitoring system called G3 which was extended to adopt monitoring of the ONS 15454 platform (system specific, optics and L2 SNMP statistics).

The performance monitoring (PM) includes status monitoring on the client connections to DWDM system (optical port of ROADM for "alien" wavelengths transport and client side of transponders) and performance statistics from the transponders, see Figure.

There are two basic types of Optics PM which can be from DWDM nodes retrieved :

• Current Optics PM are optical measurements taken in the last 15 minutes or 24 hours window

• Historical Optics PM are a set of 32 measurements taken every 15 minutes (the newest record overwrites the oldest) and 1 set of measurements taken in the 24 hours preceding the current 24 hours window

G3 measurement systems polls DWDM nodes every 15 minutes and retrieves Current Optics PM values (transmit and receive optical powers as the min/max/average values) from the ROADM system :

On the transponder DWDM trunk port are available more detailed statistics (laser temperature, laser bias current and others). For the trunk interface of the transponders that have the FEC functionality and that have it enabled can be retrieved FEC (Forward Error Correction) PM:

The FEC statistics can be included as pro-active optical channel monitoring.

OTN PM can be retrieved for the trunk interface of the transponders that have the OTN functionality and that have it enabled OTN PM are OTN related measurements taken in the last 15 minutes or 24 hours window. ONS 15454 system supports SDH G.709 statistics for transponders and OSCM modules.

Layer 2 PM can be retrieved for the client and trunk interface of the transponders that have the support Layer 2 functionality and that have it enabled. Current Layer 2 PM are instantaneous readings of the Layer 2 PM counters. The supported MIBs are RMON-MIB.my and HC-RMON-MIB.my.

7   Conclusion and future development

The CESNET2 optical network layer development, based on originally developed DWDM technology with ROADM, is the key requirement for new services introduction. It enables the support of on-demand end-to-end services on L1 and L2 network layers between the optical PoPs. It also provides an easy capacity scaling of the dark-fiber optical lines and optical channel provisioning. The objective of the DWDM deployment is an integration with an existing IP network and a step forward to a hybrid IP/optical network controlled by GMPLS or a similar protocol, including deployment of optical cross-connects for dynamic optical path switching.

For next phases of DWDM network deployment we are going to focus on following areas :

• SW provisioning end-to-end optical paths within the DWDM system using the optical switches or 3-way OADM based on wavelength cross-connects (WXCs).

• performance monitoring extension and visualization for E2E users

• native multipoint Ethernet services testing and implementation

• optical channel protection verification

• "alien" wavelength transport at 10 Gbps speeds

• research of possibility IP/DWDM integration

• DWDM network extension to new CESNET2 PoPs

8   Resources

Cisco Systems Inc.: http://www.cisco.com/en/US/products/hw/optical/ps2006/ps5320/index.html

Novák V., Slavíček K., Cihlář J., Forghieri A.: Design and Deployment of CESNET2 DWDM Core Network. In: Proceedings of CESNET Conference 2006, CESNET, 2006, str. 43-53, ISBN 80-239-6533-6.

Novák V., Slavíček K., Dostál O., Filka M.: DWDM in CESNET Backbone Network. In: Proceedings of Telecommunications and Signal Processing (TSP-2006), VUT Brno, 2006, str. 145-151, ISBN 80-214-3226-8.