NX4000 40-Gbit/S Next-Generation Optical IP Transport Analyzer

YANAGISAWA Yoshiki¹ DAIRI Kenji¹ OMATA Hiroshi¹ SATOU Hiroyuki¹

With the advent of the next-generation optical transport network that provides the foundation for building up a Next Generation Network (NGN), a 40-Gbit/s class ultra-high-speed and large-capacity DWDM system is entering into the phase of commercial development. In order to realize 40-Gbit/s WDM long-distance transmission, to respond to a growing number of data packets, and to improve transmission efficiency and economic efficiency, various technologies are being developed and compatibility testing for diversifying services is underway. In order to help evaluate next-generation optical transport systems, we have developed 40-Gbit/s Next-generation optical IP Transport Analyzer, which responds to the new Optical Transport Network (OTN) testing needs, such as optical phase shift keying, 10-Gbit/s Ethernet direct mapping, and e nhanced FEC, in addition to basic SONET/SDH testing.

1. Communication and Measurement Business Headquarters

INTRODUCTION

Figure 1 NX4000 Transport Analyzer

Trends to build up a NGN (Next Generation Network) are intensifying worldwide. High expectations are being placed on NGNs, and trials are underway aiming to provide high added-value services such as high quality visual communication equivalent to the quality-assured HDTV, and high speed highly reliable economical corporate data services.

The next-generation optical transport networks of telecommunications carriers supporting these increasingly require ultra-high-speed and large-capacity transmission systems which can cope sufficiently with rapidly increasing traffic anticipated with the spread of NGN services such as image data transmission, etc., and the ability to transmit packet-based traffic flexibly, efficiently and economically. Needs for the commercial development of 40-Gbit/s optical transmission systems are growing with the innovation of transmission technology in recent years and urgent traffic demands. 40-Gbit/s transmission is therefore expected to spread in the several years from 2008.

To answer to these various needs, we have developed a 40-Gbit/s transport analyzer. Figure 1 shows its external view.

TECHNICAL TRENDS OF 40-GBIT/S NEXT-GENERATION TRANSMISSION METHOD

Various key technologies are being developed to economically realize 40-Gbit/s long distance transmission.

In 40-Gbit/s WDM transmission, signal quality is deteriorated by the influence of dispersion (wavelength, polarization) and nonlinear optical effects. For this reason, an optical phase shift keying method is applied. Key optical phase shift keying methods used include ODB (Optical Duo Binary), DPSK (Differential Phase Shift Keying), and DQPSK (Differential Quadrature Phase Shift Keying). In addition, the OTN (Optical Transport Network) frame which enables optical path settings, monitoring, and switching for each WDM signal wavelength is capable of synchronous/asynchronous multiple transmission of various client signals. In some cases, advanced transmission methods are also being applied, such as Ethernet-over-OTN which directly intakes all bits of 10-GbE (Giga bit Ethernet) client signals to the payload and transparently transmits the signals. Various enhanced FEC (Forward Error Correction) methods are also adopted to enhance error correction performance.

For the evaluation of 40-Gbit/s optical transport systems, in addition to SONET/SDH physical layer test, testing needs are diversifying, ranging from optical phase shift keying method, to Ethernet new capsule method, and support for enhanced FEC. Subsequently, optical transport measuring devices in the age of NGNs are required to support such new testing needs.

SYSTEM

Figure 2 System Block Diagram

Transport analyzers support a broad range of testing needs, from mainly low cost 40-Gbit/s SONET/SDH tests for maintenance and production to advanced Ethernet-over-OTN tests for research and development. The NX4000 has therefore been designed as the optimum system with scalability meeting customer needs. Scalability advantages include the following.

1. Optical I/F supporting various modulation formats which the customer can easily replace to
2. SONET/SDH tests can be realized at the minimum configuration and provided to customers at low costs
3. Anticipatory functions such as Ethernet-over-OTN, enhanced-FEC, etc. can be mounted

To realize scalability, modules are established for each function, and the required modules are built-in to configure one system. For instance, for 40-Gbit/s SONET/SDH tests, two types of modules, optical I/F (NRZ) module and SONET/SDH BASE module, are used. For OTN tests, the OTN module is added to the optical I/F (NRZ) module and SONET/SDH BASE module to realize testing functions.

Inter-Module Interface

For functional partition, signals are transmitted via a backboard by 40 Gbit/s (transmission capacity of 80 Gbit/s for both transmission and reception) between modules. Like SFI-5 (Serdes Framer Interface Level 5), the interface is realized by sixteen 2.5-Gbit/s data and deskew signals (Figure 2). However, because the transmission path length becomes long via the backboard, there is a need to take into account countermeasures against deterioration of waveform quality. In NX4000, this deterioration is controlled by configuring strip lines using low permittivity substrate material for transmission loss on the substrate, and using impedance-controlled high speed differential connector for connection between substrates.

Optical I/F Module

Since the 40-Gbit/s WDM system adopts optical modulation formats such as ODB, DPSK, DQPSK, etc. best suited to the system based on transmission distance, characteristics of fiber used, and transponder costs, the measuring devices used must support the same optical modulation formats. The OIF(Optical Internetworking Forum) SFI-5 compliant 300PIN 40-Gb Transponder has therefore been adopted for the transponder mounted on the optical I/F module to maintain compatibility as much as possible. Two types of optical modulation formats, NRZ and DQPSK, are supported, and in the future, ODB and DPSK will also be supported.

The optical I/F (NRZ) module supports VSR2000-3R2 (ITU-T G.693).

The optical I/F (DQPSK) module has a C-band or L-band full-grid wavelength-variable function to support ROADM tests as well.

Figure 3 OTN Mapping Configuration

SONET/SDH BASE Module

The SONET/SDH BASE module functions as a basic framer (SONET/SDH, OTN). One FPGA is used for transmitter and one for receiver. On the SONET/SDH frame, processing can be performed using this module only.

To support the OTN frame, the following OTN module is required. By making the SONET/SDH function independent, costs of the SONET/SDH analyzer are optimized.

OTN Module

By adding the OTN module to the slot next to the SONET/ SDH BASE module, the OTN framer is supported. One feature of the OTN framer is that it has a built-in optical phase shift keying precode function as support even if the precode circuit for optical phase shift keying is not mounted in the optical I/F module. The Payload Framer not only serves to map STS-768/STM-256 to OPU-3 but also supports ODTU-23 multiplexing (Figure 3). STS-192/STM-64 can be mapped to four OPU-2 Payloads, and pattern generation and measurement can be performed at each individual transmitter and receiver. Furthermore, it also supports 10G LANPHY, enabling bit error measurement and delay time measurement by the MAC frame.

EXAMPLES OF APPLICATIONS

Figure 4 Loopback Test

The transport analyzer is used for various applications ranging from system evaluation to device (transponder) evaluation in the 40-Gbit/s network.

In transmission device and system evaluation, SONET/SDH or OTN error, alarm tests, Payload BER test, and delay time measurement are performed. In device (transponder) evaluation, BER test of nonframe signals, BER test of SONET/SDH or OTN frame signal, and CDR (Clock Data Recovery) 0 continuous strength evaluation are mainly performed. The following shows examples of applications of these measurements as well as introduces characteristic test functions.

Transmission System Loopback Test

The following are 40-Gbit/s SONET/SDH test functions.

• OH alarm/error evaluation
  Various alarm generations, error addition and measurement are available. In alarm generation, the number of generation frames and cycle period can be set, which is useful for the evaluation of alarm detection conditions.
• Payload BER evaluation
  PRBS pattern can be inserted into the payload to measure BER at random patterns close to actual lines.
• Reception frequency strength evaluation
  This is a transmission rate offset variable function for evaluating the reception frequency strength of the machine.
• APS function evaluation
  Sequential patterns can be programmed for K1/K2 byte data for evaluating APS switching operations and measuring switching time.
• SONET/SDH frame through tests
  By inserting the transport analyzer between opposing devices, alarms/errors can be inserted, and OH and payload can be partially monitored.
• Delay time evaluation
  By embedding time stamp data in the payload and sending it, and reading the time stamp from the received signal, the delay time from transmission to reception can be measured.
  Figure 4 shows the test example where the line side of the transmission device is returned (loopback) and the transmission device is evaluated from the client side.

Figure 5 WDM Transmission Test

WDM Unit Test

Figure 5 shows the test example where the transport analyzers are installed at the line side and client side of a WDM unit, and evaluation is carried in the opposing state.

Though limited to cases where the optical I/F is NRZ, as the transport analyzer can set the transmitter and receiver separately, the transmitter can be set to OTN and the receiver to SONET/ SDH, enabling multiplexing tests of OTN line signals and SONET/SDH client signals. The following are OTN test functions.

1. OTN frame
   • Alarm/error evaluation
   • Reception frequency strength evaluation
   • FEC evaluation
   By adding errors to FEC data, the FEC error correction ability of the reception side can be evaluated.
   • SONET/SDH client signal

Transponder (NRZ) Test

Generally, BERT (Bit Error Rate Tester) is used in the evaluation of transponders. However, as the BERT is expensive, use of the transport analyzer for optical performance offers high cost merits. In the case of transponders for transmission devices, BER tests on the SONET/SDH frame may also be required, and test functions for the transponder are realized using the transport analyzer. Figure 6 shows an example of transponder test.

 

Figure 6 Transponder Test

(1) Test item

1. BER evaluation
   • BER evaluation on non frame Pattern: Supports PRBS31 to PRBS7 Useful for BER evaluation taking into account periodicity on SFI-5
   • BER evaluation on SONET/SDH frame or OTN frame
2. 0 continuous strength evaluation
   CID (Consecutive Identical Digits) pattern inserting 0 continuous/1 continuous signals at the top of the SONET/ SDH frame payload is generated and the 0 continuous reception strength of the receiver is measured.
   In addition, the frequency dividing clock used as the trigger signal of the optical sampling oscilloscope for eye pattern mask tests and reference clock supplied to the transponder are provided.

Transponder (DQPSK) Test

DQPSK interface tests can be performed by replacing the standard NRZ optical I/F module with the DQPSK optical I/F module. For transponders intended for long distance transmission, there is a need to evaluate transmission performance such as dispersion tolerance combining the use of the FEC function. Tests include BER evaluation on the OTN frame, and evaluation of error correction ability at FEC ON/OFF. FEC supports the RS (255, 239) prescribed in ITU-T G.709 and non-standard enhanced FEC.

Figure 7 Ethernet-over-OTN Test

WDM Unit Test(Ethernet-over-OTN)

Figure 7 shows an example of multiplexing test of a WDM unit multiplexing 10 G LANPHY 4 channels.

(1) Test items

1. OTN frame test
   • Alarm/error evaluation
   • FEC evaluation w 10 G LANPHY client signal evaluation
   • Test patterns are set individually for 4 channels
   • Independent bitrate offset variability for 4 channels
   • Traffic load test
   • 4 channel simultaneous Packet BER test
   • Delay time measurement

CONCLUSION

40-Gbit/s next-generation transport network test needs are diversifying, ranging from SDH tests to OTN tests using multi-transmission formats and error correction technology, and we hope to provide optimum measurement solutions meeting customer needs.

REFERENCES

1. "OIF-SFI5-01.0", OPTICAL INTERNETWORKING FORUM (http://www.oiforum.com/)
2. "REFERENCE DOCUMENT FOR 300 PIN 40 Gb TRANSPONDER", 300PIN MSA (http://300pinmsa.org/)
3. ITU-T G.693(01/2005)
4. ITU-T G.709/Y.1331(03/2003)

• 'Ethernet' is the registered trademark of Fuji Xerox Co., Ltd.