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30 33 36 39 Building an efficient 100G transmission network Taking the correct “route” DPoE boosts enterprise access MSO evolution through small-cell networking Building an efficient 100G transmission network By Fang Guangxiang Trends and challenges for 100G transmission W ith surging bandwidth demands on the Internet backbone and a rapid growth of related services such as IPTV, VoD, and 3G applications, network operators and Internet service providers (ISPs) are looking for efficient high-bandwidth transport solutions, such as the 100GE interface and DWDM, which should provide greater bandwidth at a lower cost per bit. Currently, 40G WDM transmission systems are widely deployed, yet some backbone networks are expected to have bandwidth shortages by 2012; 100G would seem the next step forward. During the progression from 2.5 to 10Gbps, and later from 10 to 40Gbps, WDM transmission technologies faced physical limitations such as a higher optical signal-to-noise ratio (OSNR), lower dispersion tolerance, lower polarization mode dispersion (PMD) tolerance, and intensified nonlinear fiber effects. These physical limitations relate to data rate and transmission distance, and will further compromise transmission performance when line rates increase from 40 to 100Gbps. Key 100G technologies To address technical challenges and meet 100G transmission requirements, the telecom industry generally reduces the optical spectral width to improve the bit rate per spectrum width. Other measures include modulation formatting, which lowers the required optical signal-to-noise ratio (ROSNR), as well as better system resistance to transmission impairments. Another solution is to use digital signal processing (DSP) technology to reduce the chromatic dispersion and PMD effects of optical fiber. However, we need to consider cotransmission of 100G with other data rates, such as 40G and 10G wavelength; it is impossible for singlelevel modulation to meet 100G and 50GHz channel spacing requirements at the same time. Instead, the system must employ advanced multiplexing technologies so that an optical channel contains multiple binary channels. This reduces the baud rate while keeping the line bitrate unchanged; it also ensures that the spectral width is smaller than 50GHz, even after the line rate is increased to 100Gbps. A number of key technologies have been developed to meet these requirements. QPSK modulation As a multi-level modulation format, quadrature phase shift keying (QPSK) decreases the baud rate of optical signals by half while keeping the line rate NOV 2011 . ISSUE 62 30
Network Infrastructure Building an efficient 100G transmission network unchanged. This multiplexing technique also reduces the spectral width of an optical signal by half. Through QPSK, 40G ULH WDM transmission with 50GHz channel spacing becomes possible. The decrease in spectral width brings other benefits, such as reductions in the required OSNR and improved CD and polarization mode dispersion (PMD) tolerance. Polarization multiplexing Considering that the bit rate of a 100G transmission system is 112Gbps or higher, the bandwidth for multiplexing chips and Mach-Zehnder (MZ) modulators on optical transceivers must be as large as 56GHz, if QPSK is directly used for signal modulation. This is difficult for current photoelectric device manufacturing techniques to meet, so the industry has began to use polarization multiplexing to halve the baud rate for propagation of optical signals. However, polarization multiplexing has some potential hazards. Because the PDM-QPSK format independently carries service information on the X-pol and Y-pol, their optical signals may be superposed during fiber transmission, which causes bit errors due to the PMD effect. Therefore, the most outstanding challenge resulting from PDM-QPSK is to separate polarization and resolve the PMD penalty at the receiving end. To address these challenges, coherent detection and DSP technologies are key to 100G transmission. Coherent detection A PDM-QPSK signal has two orthogonal polarizations, each consisting of an I channel and a Q channel in the optical field. Traditional detection cannot distinguish between the X-pol and Y-pol or between the I and Q channels, which makes coherent detection the obvious choice. In addition to simultaneously obtaining polarization, amplitude, and phase information for the optical field, coherent detection offers a higher OSNR sensitivity than those for noncoherent systems, which contributes significantly to extending the distance of a 100G transmission system. Digital signal processing Since both CD and PMD effects are linear, a single filter can be used to mitigate each effect. Therefore, the digital receiver only needs to simulate two filters to eliminate the distortion and ISI caused by the CD and PMD on the eye pattern and finally recover the “clean” symbol elements. In general, CD equalization is performed in the frequency domain and is called frequency domain equalization (FDEQ), while PMD equalization is performed in the time domain and is called time domain equalization (TDEQ). Currently, PDM-QPSK, coherent detection, and DSP can be all used together (coherent PDM-QPSK); the combination of these technologies has become the most popular configuration scheme for 100G transmission. With these technologies, it will be possible for operators and ISPs to deploy 100G transmission systems on existing fiber networks, which will significantly reduce deployment costs. Huawei 100G ePDM- QPSK technology Compared with 10G and 40G, 100G transmission distance is much shorter and requires more regenerators, which could mean high CAPEX/OPEX. To avoid this, Huawei has done years of research on ultra-long distance 100G solutions. In 2008, Huawei demonstrated its 100G non-coherent system. In 2010, the company launched a 100G coherent prototype that attained 3000km transmission without electrical regeneration or Raman amplification. In June 2011, they released their 100G coherent solution. One highlight of the Huawei 100G transmission solution is the 100G ePDM-QPSK scheme, which features 100G PDM-QPSK and DSP, both of which have been refined by Huawei. In addition to robust suppression of nonlinear fiber effects, the 100G ePDM- QPSK scheme uses a Huawei-developed ASIC for strict error correction and high transmission efficiency. Another highlight of Huawei 100G transmission solution is 100G coherent ePDM-QPSK, which is important for deployment in metro core and longhaul backbone networks. Huawei’s 100G coherent PDM-QPSK system provides unique features and superior performance. Other features are listed below. Self-developed DSP chip Commercially released in 2011, the Huawei-developed DSP chip is highly integrated and manufactured using ASIC techniques; it ensures operational stability and low latency, while consuming less power than any competing DSP chip. Service providers can reduce the cost of optical transceivers, as 100G ePDM- QPSK transmission equipment will no longer need TDCMs and preamplifiers at the receiving end. Huawei’s 100G ePDM-QPSK transmission equipment also narrows the cost gap between 100G coherent and 40G non-coherent optical modules, which helps service providers to reduce CAPEX. Service providers can also reduce network construction costs and eliminate DCM latency, as the new 100G system will not require DCM. Thanks to this low latency, 100G ePDM-QPSK transmission is especially suitable for provision of transport networks for various business sectors such as finance, data centers, and cloud computing. Advanced single-carrier 100G technology Currently, 100G coherent PDM- QPSK technology is implemented in 31 NOV 2011 . ISSUE 62
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