Saturday, June 9, 2012

Dense Wavelength Division Multiplexing (100 G Solution) adopted in SMW-5 Submarine Cable System - What is in the Laboratory for Higher Bandwidth Solution?


Md. Monwar Hossain *
Parvez M. Ashraf **

Introduction

Ever since the internet from the research lab of the scientists and engineers came into the people’s world during the 1990’s, there had been exponential growth of demand for bandwidth. The society and the economy started to be shaped by the wide spread use of internet. Today internet has put its spell on people with numerous features such as browsing, emailing, blogging, twitting, facebooking, online gaming, conferencing, audio/video streaming, internet TV, etc. Having access to high speed (or, in other term, high capacity) triple play (voice, data and video) communication for various business and non-business oriented activities has become the norm of today’s students, educators, researchers, professionals and other people.
So meeting the rapidly increasing demand for capacity in the global and national information superhighways is a great challenge as ever, making the telecommunication technology today to go through major innovations and developments to meet up the capacity requirements in the core communication systems and networks. As we have indicated in a Teletech article last year, the exponential growth of Bandwidth demand has made the 10 and 40 G technology of optical networking insufficient to meet future needs.
The SMW-4 Consortium’s Submarine Cable system was originally built with 10 G systems but it is now being upgraded with 40 G and 100 G technologies. BSCCL has been a member of the SMW-4 and also became a party of the newly formed SMW-5 Submarine cable consortium with a bid to join Bangladesh to a second submarine cable. SMW-5 cable has been planned based on the 100 G technology, which is now more matured and has become the standard since more than a couple of years. We hope that by the year 2014 Bangladesh will be able to take advantage of a technologically advanced system made althrough by using the 100 G technology.

2. Status of 100 G Technology

Fig. 1: A comparison between the Constellation points at different modulation schemes at bit rate 46 Gbps (aprrox. 40 Gbps)

The optical line terminal equipments of the present and near future need to be able to handle very high speed traffic transported to a long distance. Because of the notable technical

Fig. 2: A depiction of BER vs OSNR at different modulation schemes at bit rate 46 Gbps (aprrox. 40 Gbps)

developments on the DWDM components, it can be said that DWDM approaches have surpassed the time division multiplexing (TDM) for the high speed transmission over long distance which can be even on a single fiber instead of a pair of fibers for transmission and reception with a specific terminal. The economic and technical challenges associated with achieving a 100G transmission solution has been overcome within the last 4 years. 100Gbps error free transmission has been demonstrated in 2008 by companies such as the Nortel (now, Ciena), at the Optical Fiber Conference/National Fiber Optical Engineer Conference. The key breakthrough factor of the solution has been the coherent receiver. For the past three decades or so, optical system receivers have been working by detection of the transmitted signal’s intensity with on-off keying.
A coherent receiver operates by mixing a local oscillator and the incoming signal to be received. If the local oscillator is tuned into the frequency of the incoming signal, then only the information from the incoming signal is extracted, and neighboring channel information is ignored, thus unwanted signal elimination became much better. Most vendors basically applied this method towards solving optical transmission challenges at higher line rates.
The bit-rate of a channel can be described as the simple product of the baud rate or symbol-rate, bits per symbol and the number of carriers used. Recent commercial coherent systems at 40 Gbps and 100 Gbps have exploited all of these dimensions. The technology that made this

Fig. 3: Coherent 100 G System

100 G transmission possible is Dual Polarization QPSK modulation (DP-QPSK) with a coherent receiver. Modulation is required to ensure propagation, to perform multiple accesses and to enhance the SNR, as well as to achieve bandwidth compression. DP-QPSK modulation technique would decrease the baud or symbol rate of the system, using four bits per symbol, keeping the optical spectrum four times narrower than the unreduced baud rate. Because of the capability to pass through multiple Optical Add-Drop Multiplexers (OADMs) and its practical PMD (Polarization Mode Dispersion) tolerance, DP-QPSK is recognized as a viable format for deployment within 50GHz-spaced systems.

3. SEA-ME-WE-5

Fig. 4: Proposed Route Diagram of SMW-5 (Bangladesh will join through a branch cable with the main cable)

Existing SMW-4 cable is the only submarine cable that has kept Bangladesh connected with the international information superhighway. Due to any calamity or other reasons, if this cable get into any kind of physical damage or disruption, country’s international long distance telecommunication would suffer badly. That’s why Bangladesh has been working for long to acquire a second submarine cable so that the international links can be maintained without outage. In this sequence of efforts, Bangladesh established contact with a new consortium,
SEA-ME-WE-5, and already signed a MoU (Memorandum of Understanding) with the Consortium. Initially, there will be 16 parties in the Consortium. The submarine cable this time will extend from Japan up to London for a total of 25000 Km. The estimated cost for joining this project is 48 million USD for Bangladesh. However, the cost will be reduced to 38 million USD if Myanmar joins and shares the branch cable with Bangladesh. Bangladesh might get 17 lambda of 100 Gbps capacity for each altogether coming to 1700 Gbps. Upto now, the Landing Station of the second submarine cable has been planned to be in Mongla of Bagerhat district. The physical infrastructure of the Landing Station is expected to be built by 2012-2013. It is expected that by the end of 2014, the process of joining of Bangladesh with a second submarine cable will be completed.

Fig. 5: System Configuration Diagram (proposed) of SMW-5

4. Beyond 100 G Technology: Coherent Systems, Super channels or Optical OFDM might be the solutions
In the recent years, due to the new developments in polarization multiplexed phase modulated DWDM transmission over long distance, optical coherent detection, sophisticated DSP (Digital Signal Processing) and high performance ICs (application specific integrated circuits or ASICs), the transceiver equipment for optical transmission is emerging with high
level of capacity & sophistication. Specially, coherent detection has made possible to choose among wide range of modulation formats, such as the use of dual polarization or multiple sub-carriers. Also, use of digital signal processing techniques for leveling out various linear and nonlinear impairments has become viable with coherent detection. It has been practically found that the coherent systems can provide robust tolerance against unwanted transient signals. Therefore, the future high speed and high performance transmission systems are expected to be based on coherent systems. Optical coherent systems are likely to bring future optical transmission systems at 200 G, 400 G, and later 1 (one) Terra or 1000 G systems.
DWDM is considered as an important technique enabling multiple optical carriers to travel in coexistence in parallel through a fiber that facilitates more efficient use of the expensive fibers over thousands of Kilometers. The present “state of the art” for DWDM in 2012 or 2013 may be still 100 Gbps. However, the growth in the internet has created requirement for new scale for bandwidth and that is preferably without adding any more complexity to the operations. It is clear that for a high capacity network beyond the 100G, in addition to a move toward larger, more powerful transport switches, the mechanisms of DWDM optical transmission may have to change.

Fig. 6: Bandwidth Virtualization with Super-channels

A new approach to DWDM capacity, the super-channel could be an effective solution to the challenges posed by today’s internet growth. In simple terms, the super-channel is an evolution in DWDM in which several optical carriers are combined to create a composite line side signal of the desired capacity, and which is provisioned in one operational cycle. It could be more practical to combine multiple carriers into a super-channel to move beyond 100 Gbps than it is to simply increase the data rate of an individual carrier. However super-channels are indistinguishable from a single carrier channel of the same data rate as long as normal end
users are concerned. Similar to CPU multi core processing the concept of super channels resemble to Bandwidth virtualization through multi-carrier techniques. DWDM super channels have the potential to offer an ideal solution to the problems of increasing optical transport capacity beyond 100 Gbps, up to 1 (one) Terra bps. This will also provide reduction in complexity with electronic circuitry by using large scale PICs (Photonic Integrated Circuit).

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