Monday, May 27, 2013

Use of Advanced Optical Transmission Technologies for Redundancy in the Submarine Cable Systems of Bangladesh

Md. Monwar Hossain                                                                       Parvez M. Ashraf  


Broadband internet has become a bare necessity for people of all walks and global
communication has become highly incorporated into our daily life, professional, social or
personal. High rates of bandwidth growth have been forecasted throughout the world as
people today are using numerous features of the web such as browsing, emailing, blogging,
twitting, social networking (such as, facebooking), online gaming, audio/video conferencing,
audio/video streaming, listening to internet radio, watching internet TV, VPN, etc. Meeting
the rapidly increasing demand for capacity in the global and national information
superhighways is a great challenge as ever. Moreover, with the advent of 3G Mobile services
and LTE, people are and will be using their cell phones and mobile devices to get all kinds of
internet  facilities whenever and wherever they want. Thus, it is a great challenge to address
the exponential growth of global bandwidth; and more so to ensure the availability on a 7 x 24
basis, taking into account the natural and manmade calamities that can occur anywhere at
any time.

The transmission technologies today are going through major innovations and developments
to meet up the requirements of redundancy as well as capacity in the core communication
systems and networks. Various products, components and technology solutions are available
today  for strengthening the international core network infrastructure. Telecom  service
providers  are benefited by such technologies in addressing the  bandwidth  requirements as
well as ensuring the  availability  of links and networks. The  solutions or products can be
applied to the whole or parts of the systems according specific requirements, such as cutting
down operating costs, reducing latency, enhancing QoS, reducing complexity, enabling more
protection and restorability, and of course, ensuring redundancy.

Some of the new developments in submarine and terrestrial optical transmission technologies
are briefly described in this article. A number of these features might be implemented with the new submarine cable (for example, SEA-ME-WE-5), Bangladesh has planned to join and also, with the interconnecting submarine cable between the existing and the new one. 

Submarine & Terrestrial Optical  Cable Systems Using Coherent Detection Technology and Integrated on Simplified Architecture

The  global network  of today  is made of both undersea  submarine  and terrestrial network
segments linked together to connect almost all continents of the world.

In the present systems,  a submarine cable would land on the  Cable Landing Station (CLS),
which houses  equipment  pertaining to both  submarine  (or, wet)  and terrestrial  segments.
Usually, the major Point-of-Presence (POP)s would be placed in the terrestrial/metro network,
with a backhaul link running from the CLS to the said POPs. Traditionally, the submarine and
terrestrial systems had adopted different networking architectures and technologies.
However, the inefficient network demarcation point which is typically situated between the
SIE (SDH Interconnection Equipment) and Terrestrial LTE (Line Terminal Equipment) regarding the separation of submarine (wet) and  terrestrial (dry) segments could be  eliminated to facilitate an integrated system of core networking.

Use of relatively new optical transport technologies specially the coherent detection and the
Reconfigurable  Optical Add/Drop Multiplexers  (ROADMs) can  significantly compact  the CLS
network configuration, with most of the remaining functionality and associated equipment
physically relocated into the inland Metro/Terrestrial PoP making it a major data center. This
equipment relocation  can be  made possible  by virtue of the significantly increased
interconnections  offered  by  these two technology products:  coherent  detection and the
photonic switching capabilities of ROADMs.

The proprietary (related to specific vendors) implementation of this  coherent transmission
technology  in DSP (Digital Signal Processing) chipsets typically incorporates other important
functionalities such as Soft Forward Error Correction (soft FEC) which is adjustable for latency
and signal transmission distance according to requirements. This would significantly  increase
achievable distances  for signal transmission. Therefore, it is possible to move  the submarine
wavelength termination point from the CLS to inland  metro PoP, which opens up  new
possibilities for CLS and related designs. Incoming wavelengths from the submarine cable can
be optically switched inland using either fixed filters or flexible ROADMs. ROADMs also offer
remote control feature and is a key enabler of agile photonic networks. Furthermore, keeping
wavelengths in the optical domain for as long as possible reduces latency. The equipment
consumes less power and are of small footprint. Therefore,  the overall operational network
complexity is also reduced and the  global network  simplification and its associated benefits
increases service provider’s  competiveness.  The inefficient submarine-to-terrestrial network
demarcation point is eliminated by relocated SLTE functionality inland while switching
wavelengths from the submarine cable to the inland PoP or the next CLS.
SLTE functionality can also be  physically  integrated  with an intelligent switch which will
further eliminate inefficient client-side handoffs between  the  previously distinct  SLTE and
intelligent switches. This simplifies the global network even further by integrating more
traditional CLS functionality directly into the inland PoP;  only the switching of light will be
performed by the Wavelength Selective  Switch (WSS) within the ROADM  and  the  power
management system (PFE & others) will be still  in the CLS. Integrating SLTE functionality into
inland switches running a control plane will prospectively facilitate the formation of intelligent
mesh networks, which ensures bandwidth availability and network connectivity/redundancy.
The result of this convergence is the elimination of the demarcation point between what was
traditionally  been  referred to as  the  “submarine” and “terrestrial” networks. The ability to
seamlessly interconnect networks overland and undersea helps  to  achieve global networks
that are simpler to design, manage, and maintain. Also, end-to-end network security is much
strengthened as the traffic is carried in the optical domain from PoPs to PoPs, specially when
coherent transmission technology is used, as it is far harder to tap and decode when
compared to network nodes that allow for access to traffic in the electrical domain. The SLTE
relocated directly into the inland PoP  could be  easier to manage and protect  in general as
compared to a CLS physically located in a rather sparsely populated and remote beach area or coastline location.

Evolution of Network Intelligence and the Control Plane

The control plane  functions as the brain of an intelligent network which  autonomously
maintains an accurate database of all network resources to decide on the optimal connection.
It is essentially comprised of special hardware and software that make a self-aware network.

Traditional networks employ centralized intelligence, or network management software  that
runs on  an external workstation for connection management.  Networks with  intelligent
control planes  possess a sort of  nervous system through which connection management is
autonomously performed by the network itself, within predefined conditions. Connections are
autonomously created and deleted via machine-to-machine signaling and routing protocols.
However, a  mesh network governed by intelligent control  plane technology will  make
connection management, protection, and restoration decisions based on the policies created
by human operators and  the network operator is in complete control of an intelligent mesh
network at all times, although connections are autonomously created and deleted.  This
fundamental paradigm  shift in connection management, from an external centralized model
to an  internal decentralized model, brings network operators  some important advantages in
terms of rapid service provision. Significant savings in capital and operational expenditures are
made possible.

Technology Advancement through DWDM Modulation Techniques & PM-QPSK

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. Advanced optical network technologies  
such as Dense Wavelength Division Multiplexing (DWDM) form the  foundation for
communications infrastructures of today, enabling worldwide traffic aggregation and metro
and regional network consolidation. Because of the notable technical 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 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.

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
100 G transmission possible is Polarization Multiplexed QPSK modulation  (PM-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.  PM-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, PM-QPSK is recognized as a
viable format for deployment within 50GHz-spaced systems. 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.

Technology of ROADM

Reconfigurable Optical Add/Drop Multiplexers (ROADMs) are to deliver new flexibility to
DWDM networks by enabling dynamic, transparent optical wavelength add/drop functioning.  

ROADMs add considerable agility and robustness to network architectures, vastly improving
service and lowering the Total Cost of Ownership (TCO). ROADM Technology delivers greater
flexibility and cost  savings on optical transport platforms and enables add/drop  of optical
channels anywhere within an optical network linear  span or ring. These channels can be
wavelengths with any bandwidth  rate  2.5, 10, and 40 Gbps currently; 100 Gbps. ROADM
technology also allows traffic to pass through a network  location transparently in the optical
domain without Optical Electrical-Optical (OEO) conversions, as shown in Figure 1. ROADMs
ease the planning process for DWDM-based networks by allowing the addition, removal, or
modification  of one or more wavelength  channels within a network  automatically, with
minimal user  intervention. Thus, the new ROADM technology are now-a-days used to design
flexible and efficient branching units used in large submarine cable systems.

Application of new Technologies in the Consortium Submarine 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 gets  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.

It is expected that Bangladesh would join a consortium submarine cable. The prospective
submarine cable would be built using high capacity handling DWDM and Coherent detection
technology, and highly flexible ROADM. Depending on the supplier’s equipment, it is likely
that sophisticated intelligent control plane technology will be implemented.

Bangladesh will join as a branch party to this submarine cable. While the cost per party will
depend on various factors, the cost can be reduced if the branch can be shared with another
party. There is a possibility that Myanmar might  join  the Consortium  and share the branch
cable with Bangladesh. In that case, Bangladesh could save as much as 10 million US Dollars.

The “Branch on Branch” architecture for Branch sharing  might  be implemented in various
ways. Some typical situations are as given in the figures (Fig.2, 3 & 4):

Interconnecting Submarine Cable Between the Landing Stations  and DWDM Transmission Backbone

A plan has been made to interconnect the two submarine cable systems through another
submarine cable. In this way, the two landing stations Cox’s Bazar (for SMW-4) and Kuakata
(proposed for the second submarine cable) would get interlinked.

The estimated length of this link would be 235 Km. It would be made of 2 (two) fiber pairs
with an initial capacity of 100 Gbps/per fiber pair and 16 Tbps as design capacity. The
modulation scheme to be used: PM-QPSK (Polarization Multiplexed- Quadrature Phase Shift
Keying). Other important feature is this submarine cable would be a repeater less system with
a design life of about 25 years.  

The Interconnecting Submarine cable is shown in Fig.5

Fig.5: Proposed Interconnecting Submarine Cable Between the Two Landing Stations: Kuakata and Cox’s Bazar

The two submarine cable systems will also be interconnected through a terrestrial  DWDM
backbone, shown in Fig.6

Therefore, interconnection through both the Submarine Cable and Terrestrial Systems will
ensure the strong security and redundancy in the Submarine Cable Infrastructure of Bangladesh which will keep the core communication infrastructure seamlessly connected to the Information Superhighway.


1.  Optical Fiber Telecommunications part V- vol. B (Systems and Networks) edited by I. P.
Kaminow,T. Li & A. E. Willner
2.  Fiber Optics Engineering by M. Azadeh
3.  Performance of Dual-Polarization QPSK for Optical Transport Systems by K. Roberts, M.
O’Sullivan, K. T. Wu, H. Sun, A. Awadalla, D. J. Krause, & C. Laperle
4.  Digital Coherent Receiver Technology for 100 Gb/s Optical Transport Systems by J. C.
Rasmussen, T. Hoshida & H. Nakashima
5.  Introduction to DWDM Technology by Cisco Systems
6.  Optical Internetworking Forum:
7.  Suboptic Forum:
11. Latest Technology of Optical Transmission System (40G/100G Solutions) Deployed in
SMW-4 Submarine Cable Upgrade and the Bandwidth Situation in Bangladesh by Md.
Monwar Hossain & Parvez M. Ashraf (published in Teletech 2011)

Md. Monwar Hossain: Managing Director, BSCCL

Parvez M. Ashraf: Deputy General Manager (Bandwidth Planning), BSCCL


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