Broadband Optical Access Technologies and FTTH Deployment in NTT

INTRODUCTION
The NTT access network encompasses NTT central offices and customer terminals and
networks, and consists of transmission devices for individual customers in each NTT central
office, transmission lines, and corresponding devices on the customers’ premises. Unlike
trunk relay lines between NTT central offices, access networks are not designed to carry
concentrated communication traffic. Since cost reduction strategies such as multiplexing and
task splitting cannot be employed, it is important that the component parts are as inexpensive
as possible.
Providing an access network for ordinary customers that covers the length and breadth of
the nation represents an enormous undertaking. For instance, the process of upgrading
facilities to alleviate congestion in telephone services necessitated a substantial 20-year
investment program that started in the 1950s, when NTT was known as the Nippon
Telegraph and Telephone Public Corporation. Given the scale of the task, it is especially
important to develop simple, well-designed technologies and systems for the construction,
operation, and maintenance of the access network.
The transmission characteristics of the access network have a direct bearing on service
provision. Equipment and system designs must strike an appropriate balance between the
need to keep costs down at the initial stage and the need for the scalability and flexibility to
accommodate the future diversification and modernization of the services.
Thus, the access network is governed by a wide range of conditions and a different
level of requirements. Much is expected from NTT’s optical technology research
program, which began with the trunk network. In particular, it is hoped that the optical  technology will provide NTT with greater flexibility to respond to the very rapid
changes in the marketplace that followed privatization in 1985. These changes have
included the introduction of competition, which has forced prices down; the shift towards
mobile phones at the expense of landline services, which has affected equipment investment;
and the paradigm shift brought about by the advent of Internet and broadband
services.
The technologies involved in access networks are many and varied. They include optical
transmission systems and optical cable and associated hardware, optical wiring based on
demand and environmental considerations, and construction, cable laying, and maintenance
systems. These technologies have evolved in line with changes in demand patterns and the
socio-economic environment. Similarly, NTT’s equipment investment and service development
strategies are both influenced by technological progress.
This chapter provides a brief overview of the development of optical access technology
in Japan, before moving on to a discussion of modern broadband services. Finally,
it will examine the current state of optical access systems used in broadband services,
and consider the use of access network technology in the broadband services of the
future.
1.2 HISTORY OF OPTICAL TECHNOLOGY IN JAPAN
1.2.1 THE FIRST RESEARCH ON SUBSCRIBER OPTICAL
TRANSMISSION SYSTEMS
Research into optical transmission technology for subscriber systems and existing access
systems began in 1977, and research into trunk relay optical transmission technology began
shortly thereafter. Trunk relay optical transmission research was geared towards such
objectives as multiplexing, long-distance transmission, and relay-free system design. On
the other hand, the goals of optical transmission technology for access systems were to
achieve overall economic targets, design outdoor facilities whose simplicity of construction
and operation made them suitable for mass production, and develop services that fully utilize
the benefits of optical access networks.
1.2.2 FROM MULTI-MODE FIBER TO SINGLE-MODE FIBER
Multi-mode fiber was originally used as subscriber optical fiber. In 1988, NTT decided to
introduce single-mode fiber, which had originally been developed for trunk networks, into
the optical access network as well. This decision followed research into the best way to
introduce the fiber into the subscriber network [1].
The decision to go with the potentially more costly single-mode fiber in the subscriber
system, where cost considerations were paramount, was indeed a bold one, especially
considering that most other countries were considering multi-mode fiber for subscriber
systems. However, given the low-loss characteristics of single-mode fiber and its suitability
for broadband applications, it was definitely the correct decision for the future. The
introduction of single-mode fiber was made possible through the development of new
technology in a number of areas. In particular, these developments related to high-precision manufacturing (using a process known as total synthesis), the more economical production
of long-wavelength light sources, photoreceptors, and fiber interconnection and installation
techniques that enabled optical fiber cable to be laid more efficiently. There were also
significant economies of scale to be derived from the standardization of fiber with the trunk
network.
The focus of research subsequently shifted to more advanced areas, including simple, lowcost
wiring configurations such as multi-core cables, thinner wires and multi-core optical
fiber connectors; low-maintenance, remote-operated gas-less systems; and fast, low-cost
multi-core alternatives.
1.2.3 DEVELOPMENT OF CT/RT SYSTEM
Research into the use of optical technology in ordinary subscriber telephone services began
in the late 1980s with the Central Terminal/Remote Terminal (CT/RT) system, which is
employed in the section of the network equivalent to a feeder line. CT/RT delivers
economies of scale through subscriber multiplexing, and allows systems to be scaled to
suit line capacity. It also permits the diversification of services into areas such as Integrated
Services Digital Networks (ISDN) and dedicated service support. As the results of research
and development began to translate into cost reductions, it became possible to design
smaller RT cabinets suitable for installation along roadways, on telegraph poles and inside
buildings. The first indoor model, installed at the Chiyoda pilot plant in Tokyo in
February 1993, featured 1000-strand multi-core optical fiber cable. The CT/RT system,
which was later named FALCON, was deployed nationwide in Japan as a typical optical
access system.
1.2.4 MOVING TOWARDS FTTH
Fiber-To-The-Home, or FTTH, cannot be achieved until we develop new forms of
technology capable of both delivering substantial cost reductions across the board and
facilitating a seamless transition from metal to optical cable systems. FTTH will require
advances in a range of areas from hardware, such as cables and transmission system
components, to nonhardware items, such as network design, construction, operation, and
ongoing maintenance. Following the successful development of CT/RT, NTT continued
research and development with the goal of realizing FTTH.
The Passive Double Star (PDS), which makes it possible for NTT central offices and
multiple customers to share cables, was considered particularly promising with respect to
cost reduction. The system consists entirely of passive components, and so unlike the CT/RT
system it does not need an electricity supply (or the associated space requirements).
Furthermore, the same cost-sharing benefits as the CT/RT system are complemented by
impressive cost reductions in terms of the components used at the RT end. Another reason
for the appeal of PDS was the network topology, which provides excellent compatibility
with distribution services such as CATV.
A number of two-way communication systems were developed, including Synchronous
Transfer Mode PDS (STM-PDS) [2,3], Asynchronous Transfer Mode PDS (ATM-PDS)
[4–7], and Sub-Carrier Multiplexing PDS (SCM-PDS) [8,9]. 1.2.5 OPTICAL SYSTEMS AT METAL-WIRE COSTS
In 1994, NTT announced its intention to accelerate the development of optical fiber access
networks and provide an upgraded communication infrastructure for future broadband
expansion. To achieve the objectives set out in the published statements, it would be
necessary to achieve further economies with FTTH technology. To this end, in 1995 NTT
embarked on research into cost reduction initiatives, particularly in relation to distribution
systems and user-end optical distribution line systems. When the research program began, the
cost per subscriber of optical access was some six to eight times greater than that for access
via conventional metal wire; by 1997–1998, this had been reduced to approximately double
the cost, and by 2000 the costs were the same. The core technological developments that
enabled NTT to achieve its cost-reduction objectives were based on five key concepts:
(1) new approach to network configurations;
(2) low-cost optical PDS;
(3) low-cost optical fiber cables and associated technology;
(4) simplicity of installation and maintenance; and
(5) optical operation systems.
The newly developed technology first appeared in the optical access system,
commonly known as the p system [10], which essentially involved grafting an optical
access network onto an existing metal-wire telephone infrastructure. The optical
network was brought as close as possible to subscriber homes (usually to the nearest
telephone pole or the outer wall of an apartment building). It was terminated at a p-ONU
device carrying the traffic of 10–20 subscribers, which was then linked to the existing metal
cables. In July 1996, NTT announced its plan to deploy the p system to upgrade the access
network.
1.2.6 ACCESS NETWORK OPTICAL UPGRADING PROGRAM
The construction of an optical access network requires a huge investment. It is therefore
necessary to prepare a proper equipment investment program that takes account of service
supply and demand considerations. In addition to upgrading metal-wire equipment to optical
fiber, efforts were made to introduce technology developed for new services tailored to meet
unknown demand levels and business requirements. The combination of the p system and
optical fiber cable at the same cost as metal cable meant that optical access systems could
also be extended to ordinary user districts where the upgrading of aging metal cable
equipment was required.
1.3 TRENDS IN BROADBAND SERVICES
1.3.1 GROWTH OF BROADBAND SERVICES IN JAPAN
We first describe the trend of broadband services in Japan. Figure 1.1 shows the recent
growth in the number of broadband users in Japan. We have three categories of broadband
communications. The dominant broadband medium is the ADSL service with more than
13.6 million users. CATV comes next, but its growth is slackening off, and has become nearly flat. The last is FTTH, which is growing rapidly and the number of users has exceeded
2.8 million and is still growing. Figure 1.2 shows the quarterly increment in new broadband
access users. The number of new FTTH users surpassed the number of new ADSL for the
first time in the first quarter of 2005. This is the Japanese network environment for which we
are now developing technologies.
Another issue with the FTTH service is so-called ‘regional disparities.’ Although ADSL
broadband services are steadily spreading throughout Japan, FTTH (which brings optical
fiber directly to the home) is still confined to Tokyo and Osaka and their immediate
surroundings, with little penetration into regional cities. Research and development programs
will also need to focus on the issue of eliminating regional disparities with respect to
broadband services. 1.3.2 VISION FOR A NEW OPTICAL GENERATION
1.3.2.1 Broadband Leading to the World of Resonant Communication
In November 2002, NTT released a statement entitled ‘Vision for a new optical generation:
Broadband leading to the world of resonant communication,’ describing its outlook for the
ubiquitous broadband era of around 2008 based on optical technology [11].
In the optical fiber ‘resonant communication environment’ envisaged by NTT for the year
2008, video and related technology will provide more natural, real-time communication than
has been possible with narrowband, thereby achieving the ultimate aim of communication,
which is to demolish the barriers of time and distance. Breaking down the time barrier
translates into more free time for people, while breaking down the distance barrier will allow
individuals and corporations to expand their spheres of activity. This in turn promotes human
activity on a global scale, transcending national and regional boundaries as well as industries
and generations, and enables available knowledge and wisdom to be shared among
individuals, companies, and borderless business operations. In this way, individuals and
corporations will be able to access knowledge from around the world in what is termed the
‘cooperative creation of knowledge.’ This will revolutionize both the way we live and the
way we do business.
The announced vision has been adopted as a common theme for the entire NTT Group,
which is working to develop advanced business models by stimulating demand for optical
services and creating business solutions. At the same time, partnerships with both domestic
and overseas businesses in a variety of different fields are helping to promote and develop
the resonant communication environment in industry.
1.4 OPTICAL ACCESS TECHNOLOGY BEHIND
BROADBAND SERVICES
In this section, we describe the access network technology that is currently used to provide
broadband services. The key requirements of the access network are economy, performance,
and ease of use for both installation technicians and customers. Ease of use has a
direct bearing on cost, since a well-designed network is easier and hence less costly to set
up and maintain. And lower network setup and operating costs mean that services can be
provided in both rural and metropolitan areas, thereby reducing regional disparities. Below,
we discuss some of the technologies that have been developed with these objectives in
mind.
1.4.1 OPTICAL ACCESS TECHNOLOGY FOR CURRENT
BROADBAND SERVICES
1.4.1.1 Optical Access Systems
Figure 1.3 shows the configuration of an optical access network. Optical technology
provides an economical way to transmit large volumes of data. Thus, an optical fiber system
extends from NTT substations to points near customers’ homes, such as footpaths and
telephone poles. At the telephone pole or its equivalent, optical signals are converted into
electrical signals, which are then fed down metal cables to individual households. With FTTH, optical fiber is connected right through to customers’ homes to provide faster
broadband services. The FTTH system shown in the diagram employs the PON configuration,
where a signal from one device at a substation is used by many customers. The signal is
distributed to different customers via an optical coupler, which is a simple low-cost device
made from glass. There are substantial economic benefits when a single substation device
can service multiple customers.
NTT has finished developing a carrier-grade Gigabit EPON system for commercial
deployment, as shown in Figure 1.4. The GE-PON system [12,13] is one example of an
optical access system based on FTTH in a PON configuration. It provides a transmission
capacity of 1 Gb/s through optical fiber at the substation and has an Ethernet interface. Key
technologies provided by the GE-PON system include encryption and burst reception, both
vital components of the PON setup, and bandwidth control and delay suppression, which are
used to maximize system performance. NTT is also involved in the development of key
carrier-grade operation functions.
Figure 1.5 is a diagram showing encryption technology. Encryption technology is
employed during transmission from a substation to ONU devices in customers’ homes. In
a PON system, an optical star coupler is used to split the optical signal from the substation to
the ONU devices. Since the entire signal reaches all ONU devices, a different form of
encryption is used for each ONU device. This ensures that each ONU reads only signals
intended for that customer Figure 1.6 is an illustration of burst transmission technology, which is employed during
the transmission of an upstream signal from an ONU to an OLT. One function of burst
technology is to prevent collisions between signals sent from customers’ homes. A single
receiver at the OLT accepts signals from multiple ONU devices. Since different signals
cannot be received simultaneously, times for transmitting signals to the OLTare allocated to
the ONU devices, and collisions are avoided by processing these transmissions in a set
order.
Also, the different lengths of optical fiber between the coupler and the various ONU
devices generate differences in path loss. This means that although each ONU device
generates optical signals at the same strength, by the time the signals reach the OLT the
power levels will vary somewhat. The receiver must be capable of processing signals of
various strengths in rapid succession. This is achieved through the use of burst transmission
technology.
Now let us look at some of the technologies used to maximize the performance of the
PON system. The first is bandwidth control. Figure 1.7 shows the dynamic bandwidth
allocation. Different users require different bandwidths at different times. Sometimes they
need to transfer large amounts of data, while at other times they may need to transfer only a
little. Rather than allocating the same bandwidth to every user, the system makes its ‘best effort’ to redirect unused bandwidth to users who need it the most. In this way, all users are
guaranteed a minimum level of bandwidth, and are also provided with additional bandwidth
when required, which is allocated economically on a best-effort basis.
The priority control, shown in Figure 1.8, is another form of technology used to maximize
the PON system performance. Video and audio data must be transmitted in real time. GEPON
gives priority to the regular transmission of signals associated with such services, and
transmits other signals as time permits. Information classified as ‘minimum delay class’ is
given priority, with regular transmissions at shorter cycles, while information classified as
‘normal delay class’ has lower priority. These functions are controlled equitably on a besteffort
basis in order to provide users with an appropriate environment in which to enjoy
audio and video services.
The technique of wavelength division multiplexing (WDM) significantly increases the
transmission capacity of optical fiber by enabling signals with different wavelengths to be
transmitted together along the same fiber. The ITU-T has established standards for the WDM
of up- and downstream signals and video signals in each optical fiber in an optical access
line. In accordance with these standards, the upstream line from the ONU to the substation uses the 1.3 mm wavelength band, while the downstream line from the substation to the ONU
uses the 1.48–1.5 mm wavelength band. For video signals, the standards specify up- and
downstream lines that use different wavelengths in the 1.55 mm band. In this way, a single
optical fiber can be used to carry upstream, downstream, and video signals, using prism-like
optical elements (coarse wavelength division multiplexing or CWDM elements) to split and
then recombine the various wavelengths.
NTT has developed the STM-PON, B-PON, and GE-PON systems to meet the increasing
transmission capacity requirements. In light of the anticipated need for even greater capacity
in the future, NTT continues to work on the development of new systems designed to harness
the broadband capabilities of optical fiber.
1.4.1.2 Optical Access Installation
In order to provide FTTH services to customers at affordable prices, it is necessary to reduce
both the cost of optical access systems and the expenses associated with the installation of
optical fiber and associated hardware by simplifying the installation of optical drop cables,
optical cabinets, and indoor wiring. NTT is working on developments in a number of areas
designed to improve the installation process.
One example is bend-resistant optical fiber. Conventional optical cable requires a
minimum bending radius of 30 mm during installation to avoid damage. NTT has developed
an optical cable that can be bent at 15 mm without fear of damage. Figure 1.9 shows
photographs of conventional and newly developed bend-insensitive indoor optical fiber
cables. Not only is the cable easier to work with, it has a cleaner look since the bent sections
are less prone to swelling. This improvement will have a significant impact on the
installation of optical cable in ordinary homes. Furthermore, the introduction of a curled
form of optical fiber with a low bending loss makes the cable as easy to handle as metal wire
cable such as telephone receiver cords. Curled cable will greatly facilitate the ONU
connection procedure shown in Figure 1.10 [14].
The installation of the optical cabinet used for the optical drop cable has been made easier.
Figure 1.11 shows photographs of conventional and newly developed cabinets [15]. It is no
longer necessary to provide a length of fiber with its sheath removed at the fiber termination.
A length of exposed optical fiber must be provided at the end of the drop cable for
connection to the indoor optical cord via a mechanical splice. The optical fiber can be
connected directly to the drop cable using a special connector, and this can be accomplished on site during the wiring process, eliminating the need for exposed optical fiber cord.
Meanwhile, the bend-resistant fiber cord has reduced the required length from 30 to 15 mm.
As a result, the volume of the new optical cabinet is around one-third that of the conventional
cabinet.
Conventional drop cable contains metal wire as a reinforcing tension member, and this
needs to be grounded to protect against, for example, lightning. The new cable uses nonconducting
Fiber Reinforced Plastic (FRP) as the reinforcing tension member. Grounding is
no longer required and this reduces the time needed for the installation of lead-in wires by
around 20%.
1.4.1.3 Wireless Access
In some cases it is not easy to service the demand for broadband access. Laying new optical
fiber throughout existing apartment buildings, for instance, can present difficulties. WIPAS
[16], a wide-area FWA system, was developed as a means of providing broadband services in
such situations. Figure 1.12 shows the WIPAS system. Access is provided from a small
wireless base station mounted on a telephone pole, which transmits wirelessly to outdoor terminals usually installed on the verandas or rooftops of user households. WIPAS provides
an inexpensive way to deliver broadband services tailored to local conditions in areas
lacking a fully developed optical fiber network. In this way, wireless technology can be used
to supplement FTTH in certain circumstances and provide a quick and inexpensive
alternative source of broadband services.
1.4.2 BROADBAND ACCESS NETWORK TECHNOLOGY
IN THE FUTURE
This section discusses the future of access technology. The target of the Medium-Term
Management Strategy of the NTT Group is to provide optical access to 30 million
customers in Japan by 2010. To achieve this target, it will be necessary to deliver further
improvements in cost, performance, and ease of handling. We shall now examine these
parameters with respect to optical access systems, optical access installation, and wireless
systems.
1.4.2.1 Optical Access Systems
The primary development objective for optical access systems is performance enhancement,
particularly with respect to cost, handling, and capacity. In the preceding section, we
discussed optical systems that use different wavelengths for the up- and downstream
communication lines. It should be possible to boost the cost-effectiveness and performance
further by utilizing the broad optical wavelength domain of optical transmission routes via
techniques such as WDM, which increases transmission capacity by transmitting multiple
wavelengths through a single fiber. In terms of ease of use, systems can be designed to utilize
existing networks and facilities. This approach minimizes installation costs and makes it
easier to introduce and add new services. In addition, the introduction of common
specifications for optical interface modules promotes component sharing for improved
economy and user-friendly design. There are two ways to harness the broad optical wavelength domain of optical fiber:
wavelength division multiplexing, WDM, as described above, and code division multiple
access, CDMA, which is already used for mobile telephone systems. We begin by looking at
the general features of WDM in access systems.
The only extra initial investment required for WDM, in addition to the optical transceiver
and transmission paths used in conventional systems, is the CWDM (coarse WDM) unit.
There are two types of CWDM unit: one which splits light from the reception side into
multiple wavelengths, and the other which combines optical signals of different wavelengths
from the transmission side. Normally, the aim is to keep initial investment to a minimum,
since there is rarely any demand for high-volume data transmission capacity at the outset.
Ideally, the system design should be able to accommodate future capacity expansion in line
with demand growth without great expense. WDM provides this feature. In most cases it is
possible to expand capacity simply by adding more optical transceivers; there is no need to
lay additional optical fiber. Similarly, if the original design capacity of 100 Mb/s is
insufficient, it can be upgraded to 1 Gb/s, for instance, simply by installing a new 1 Gb/s
transceiver. The changeover can be performed without affecting systems operating at other
wavelengths.
The same advantages apply equally in the case of access systems, making the future
expansion of system capacity an inexpensive proposition. WDM can be used in access
networks as an economical means of boosting transmission capacity by adding different
wavelength transceivers, but it also promises economic benefits in other areas. For instance,
if demand for Internet services is accompanied by demand for video and other content
delivery services, these new services can be accommodated on new wavelengths over the
existing optical fiber. It is also possible to allocate separate wavelengths to users who require
greater transmission capacity. In this way, WDM represents an economical way to utilize
existing networks, allowing the flexibility to upgrade performance and functionality by
increasing capacity and providing new services.
Thus, WDM makes maximum use of existing networks and equipment, and provides an
economical means of delivering advanced services simply by upgrading terminals as
required. The underlying system concept presupposes compatibility with existing networks
and equipment. Next, we consider economic and ease of handling issues as regards WDM
equipment and components.
Conventional WDM devices consist of integrated sets of electrical and optical components
for each separate wavelength, gathered together on a board. With this approach, the
specifications for each wavelength are slightly different. Fabrication costs are higher,
since it is necessary to produce a large number of slightly different devices to make up a
single unit.
In contrast, with the new devices the interfaces between the optical transceiver, electrical
processor, and CWDM are common to all manufacturers, and are designed as interchangeable
modules. This approach allows the mass production of fewer components for less total
cost. Furthermore, the optical transceivers are integrated into the body of the unit, making
the entire relay both smaller and cheaper to produce. The same principles also apply to the
CWDM module [17–19].
NTT is using this approach to develop WDM optical modules designed to reduce the cost
of WDM systems. The optical transceiver modules used to emit and receive light at various
wavelengths and the CWDM modules are designed to be readily interchangeable and thus allow greater flexibility in terms of system configuration. For instance, it is possible to
design a point-to-point transmission system, as used for relay transmission, consisting of the
light-to-electric conversion of the user signal, an electrical processor, electric-to-light
conversion, and an optical fiber transmission path. Awavelength converter can be configured
with similar ease. The cascade format, used extensively in LAN design, can be configured to
generate a number of different wavelengths (normally at the server), with successive
repetitions of the reception, transmission, splitting, and recombining of different wavelengths
for each user.
Further into the future, we can envisage the use of CDMA technology in optical
communication systems. The advantages of CDMA are already well known in relation to
mobile telephone systems. First, the code division format offers excellent resistance to signal
interference, or in the case of optical communication, to light interference. Thus, if a
customer inadvertently introduces a wavelength into the PON network that acts as
interference, this will have only minimal impact on other customers. As with wireless
communication, CDMA is a more efficient use of the available wavelengths (spectrum) than
WDM. This technology could potentially prove vital in the event of a shortage of optical
fiber bandwidth, as it will help us to make better use of the available wavelengths. Also,
allocating codes rather than separate wavelengths to individual users allows more flexibility
in planning user deployment.
In this way, the next-generation GE-PON, in the form of systems such as WDM that are
designed to utilize the broadband capabilities of optical fiber, will deliver economic and
performance benefits and be easier to handle.
1.4.2.2 Optical Access Installation
We now consider optical access installation technology. By creating small holes in the fiber,
it is possible to change the fiber properties [20,21]. This technology enables to develop fiber
with a very small bending radius and substantially reduced bending losses. The configuration
and bending loss characteristics of this fiber, which is known as Hole-Assisted Fiber, is
shown in Figures 1.13 and 1.1.4, respectively. Because such a fiber could easily be bent with
a radius of less than 15 mm, fiber cords containing such fiber could be laid quickly indoors in
any required configuration without compromising aesthetic requirements.
1.4.2.3 Wireless Access
Finally, we briefly discuss the wireless access technology used to complement optical access
services. Wireless systems on the so-called ‘last one mile’ segment are capable of transmission speeds of 50 Mb/s, which is about half that of optical drop fiber operating at
100 Mb/s. The aim is therefore to increase this speed to that of optical fiber. The deployment
of additional base station antennas must not be too expensive, since this will occasionally be
needed to ensure quality of service. In addition, it will of course be necessary to reduce the
overall costs. Future development of wireless access technology is geared towards these
objectives.
1.5 CONCLUSION
In this chapter, we briefly reviewed the history of optical communication systems
in NTT, Japan. In terms of broadband services, the current focus of research and
development is on servicing a range of user demands and reducing regional disparities
in service levels.We discussed optical access technology for broadband services in the form
of the GE-PON system, installation technology, and wireless access technology, and
analyzed NTT’s research and development with respect to the broadband environment.
Finally, we looked at the development of network access technologies for future broadband
services.
Such factors as the increasing interest in high-volume applications including Internet
video distribution services and large data file exchange, the expectations of guaranteed
services to complement existing best effort services, and the ongoing diversification of the
network environment emphasize the need for the further development and enhancement of
transmission services. Access networks represent the entrance through which customers gain
access to such services. As such, more effort must be poured into the research and
development of optical access networks in line with the advent of ever more advanced
broadband services tailored to the market demand.