Broadband Access Networks and Services in Korea

CHANGING ENVIRONMENTS AND FITL PLAN
In the early 90s, KT (former Korea Telecom) was facing an explosion of communication
demands. More than half of its capital expenditures, totaling approximately 1 billion US
dollars, were lavished on annual investments to lay copper cables, and CAPEX continued to
be snowballing down the road. The Korean economy was expanding at a fast rate, and large
buildings started to emerge day by day in the downtown of cities. Large buildings indicated
that there were huge communication demands, and new copper cables had to be installed
on the effected area. Although KT roll out copper cables based on 5-year demand predictions,
the fast booming economy made it impossible to project the 5-year demand estimation. It
was a yearly event that KT had to plan a new set of installations to meet the explosive
communication demands. So, empty conduits were being filled rapidly and new civil
engineering work took place frequently. However, it was extremely difficult to get the
permission to dig the ground. Even if KT was able to get the approval, it was too expensive
to lay new cables especially in the downtown areas.
To alleviate this problem, the adoption of loop carriers was proposed. By adopting loop
carriers, KT reduced the demands for copper pairs as well as the possibility of requirements
to lay new copper cables. Less cable installations meant less civil engineering work. As a
consequence KT got to expect significant cost savings. Despite the continued controversy,
the concept of Fiber Loop Carrier was defined.
So, KT established ‘Fiber in the Loop (FITL) evolution strategy’ in 1991. It comprised
three steps, that is, FTTO, FTTC, and FTTH. KT also projected a plan to develop FTTx
access systems, Fiber Loop Carrier (FLC) family, the concept of which is shown in
Figure 4.1. FLC family largely falls into three types as their target areas: Fiber to the
Office (FTTO) type for business areas, Fiber to the Curb (FTTC) type for densely populated
residential areas like apartment complexes, and Fiber to the Home (FTTH) type for homes,
which is the final destination of our plan. SERVICES CONSIDERED
The first version of fiber loop carriers was FLC-A. It was designed to provide the
functionality of copper cable links without any additional service features. In other words,
FLC-A was intended as a ‘pair gain’ system. Only existing services that had been provided
by copper pairs were included. Various communication needs from large office buildings,
such as simple black phone connections, enterprise PABX connections, and leased line
connections with speeds up to 45 Mbps were met with FLC-A. Sometimes these buildings
had Digital Subscriber Units (DSUs) for data communication, and they were accommodated
in FLC-A.
4.2.2 HARDWARE CONFIGURATION
The FLC-A access network system consisted of Central Office Terminal (COT) and Remote
Terminal (RT), and they are interconnected using high-speed optical links. The COT, which
is located in a central office, was connected to such systems as PSTN, X.25 network, etc.
The RT, which is installed in the customer’s site, provides connections to black phones,
ISDN, leased lines, pay phones, etc.
Both COT and RT are made of a common shelf and channel banks. The common shelf
provides clock synchronization, SDH multiplexing/de-multiplexing, equipment management
functions, and system power supply. A channel bank is equipped with a multitude of channel
cards that provide specific functions such as black phone services, PABX connections, pay
phone service, data services with speeds of 2.4–54 Kbps, and data services with speeds of n
multiple of 64 Kbps. The total capacity of the system is STM-1 (155 Mbps) and can
accommodate about 2000 voice channels. This capacity was determined to meet most
buildings’ demands with only one FLC system installation, while larger buildings could have
multiple systems. Each RT was connected to COT with four optical fiber cores; two cores for
one bidirectional connection while the other two were reserved for protection. MANAGEMENT SYSTEM
The FLC-A system has a point-to-point network configuration between a COT and an RT.
Inside the central office all COTs are interconnected via standard Ethernet 10baseT cables
for Operation, Administration, Maintenance, and Provisioning (OAM&P) so that each COT
and its corresponding RT may be managed from a central management system called FLC
Management System (FMS).
KT has its own unique telephone line testing system, Subscriber Line Monitoring and
Operating System (SLMOS). It is located at CO and monitors loop resistance, stray
capacitance, etc. By monitoring such parameters, SLMOS can distinguish line faults from
telephone hook-offs, thus eliminating unnecessary field trips for maintenance.
In order to utilize the end-to-end test capability of SLMOS even with the introduction of
FLC into the loop, we incorporated into the FLC design a function that can test the loop
portion between the RT and the customer terminal from CO locations. The FLC had features
to specify a protocol to choose the loop to be checked, activate tests, and report test results
back to SLMOS. This enabled subscriber lines connected using FLC-A to be serviced by
SLMOS.
4.2.4 FLC-B AN FLC-A UPGRADE
FLC-A was successful. However, its full STM-1 capacity was too big to economically serve
small-to-medium size buildings. And STM-1 capacity had to be shared between buildings.
Therefore, a point-to-multipoint network configuration was required, and it resulted in a plan
to develop FLC-B in 1995. FLC-B has point-to-multipoint configurability, that is, remote
terminals might be connected to a COT through a bus, ring, or star configuration making use
of SDH functionality. Also FLC-B has two new channel cards for ISDN BRI services. In
addition, IDLC was implemented to eliminate requirements of channel banks at COT in case
of connecting to public switches.
Although the loop carrier was generally deployed to connect a small number of residents
far distant from suburban areas, KT has utilized it as an access and aggregation technology
for providing services to a large pool of subscribers in metropolitan areas.
4.3 FLC-C
4.3.1 SERVICES CONSIDERED
FLC-A/B brought a tremendous success. FLC-A/B were produced with over 10 000 systems
in total. Meanwhile, the competitors including operators abroad also used roughly 6700
FLC-A/B systems. Encouraged by its great success of FLC-A/B, KT landed on a plan to
accelerate the development of FLC-C system. In 1996, KT started to develop FLC-C. At the
early development stage of the system several critical issues concerning ONU installation
environments were identified. Korea is relatively small by the size of its land but large in
population. Metropolitan cities like Seoul and Busan are estimated to have approximately
one-third to one-half of all their residents living in the area of apartment complexes.
Moreover, one apartment complex amounts to at least dozens of buildings in number, and
one building houses 50–300 households roughly. Therefore, we have chosen significantly
larger Optical Network Units (ONU), where one ONU covers up to a capacity of 180 POTS basic plus 64 Switched Digital Video (SDV) optional subscribers and etc. By choosing such
big ONU, KT had to abandon the grace of Passive Optical Network (PON), and had to
choose active optical network configuration. This decision also led to some new challenges
for KT, such as outdoor installation of bulky ONU cabinets, thermal management due to
more power consumptions, more reliable powering, and longer battery backups.
4.3.2 HARDWARE CONFIGURATION
The Fiber Loop Carrier for Curb (FLC-C) system architecture is based on a Host Digital
Terminal (HDT) corresponding to COT in FLC-A/B, and Optical Network Unit (ONU)
comparable to RT in FLC-A/B. The HDT and ONU are interconnected using high-speed
fiber optical links. Each HDT shelf has 16 STM-4c interfaces, so 16 ONUs may be
connected to an HDT shelf. However, since each ONU is quite big, we usually use 1 : 1
protection. And eight ONUs can be connected to an HDT shelf in the case of 1 : 1 protection.
Basically, ONU may have star configuration. However, some ONUs can be a master ONU
and some ONUs can be linked to the master to form the double star configuration as shown
in Figure 4.2. ONUs can be configured to be ring, star, double star, or a mixture of ring and
star topologies.
The ONU interfaces to customer loops for providing POTS, ISDN, leased lines, and
interactive video services. For the ONU interface of broadband services, Very High-Speed
Digital Subscriber Line (VDSL) technology is employed so that existing conventional
subscriber copper pairs are used as a transport medium without new installation. The
HDT is connected to central office systems such as PSTN and ATM switches with
standard interfaces of DS1E for POTS and STM-1 for video services, respectively. The
optical link between the HDT and ONUs is based on STM-4c signals and carries
Asynchronous Transfer Mode (ATM) cells. The configuration of the FLC-C system is
shown in Figure 4.2. The ONU cabinet is installed outdoors in the vicinity of densely populated areas
like apartment complexes. The cabinet is made of aluminum and equipped with
telecommunications equipment, a power supply, and backup batteries, as shown in
Figure 4.3. On the front door inside is attached a heat exchanger which is used to control
the temperature inside the cabinet. The heat exchanger normally operates through AC
power source, but operates through backup batteries during the failure of AC commercial
power.
In installing ONU equipment outdoors, temperature and humidity are the two critical
factors to be handled effectively. For most telecommunications equipment it is difficult to
survive without using special electronic devices under a harsh environmental condition, for
example, operating temperature of 60
C or more. The thermal management problem of
outdoor cabinets has been, therefore, a big issue from the very early development stage. The
cabinet had to be designed to effectively remove internal heats generated from the
dissipation of electronic load as well as heats from the sun. OPERATION SUPPORT SYSTEM
The FLC-C system has a star network configuration from an HDT to each ONU. In this
configuration, one HDT shelf corresponds to a maximum of 16 ONUs as shown above in
Figure 4.2. Inside the telephone office all HDTs are interconnected through an Ethernet
Local Access Network (LAN) for Operation, Administration, Maintenance, and Provision
(OAM&P) so that each HDT and related ONUs can be managed from a central management
system, or FLC access network management system (FMS). The FMS operates
as a manager and each COT carries out its task as an agent. It is accordingly possible
that KT operators get the power and environment information of all ONUs from a
central maintenance center called Power Operation Center (POC) which is connected to
the FMS.
Through its standardized interfaces TMN gives a logical structure to exchange systems
management information defined as Managed Objects (MOs) between management systems
and networks equipment, and aims at interoperability, reusability, standardization, etc.,
between them. The exchange of management information is accomplished between a
manager and an agent, which is shown well in Figure 4.4.
In the operation and maintenance system of FLC-C, Common Management Information
Services/Common Management Information protocol (CMIS/CMIP) is used for the interface
between a manager and an agent. MOs are used as a means of the exchange of management
information between a manager and an agent. The definition of MOs is achieved through
GDMO/ASN.1 (Guidelines for the Definition of Managed Objects/Abstract Syntax Notation
One).
Figure 4.5 shows an example of GDMO template. MO Class templates are the most
important ones in GDMO, which conceptually represent resources including the information
of MOs. As shown in Figure 4.5, several items such as attributes, parameters, notifications,
etc., are defined in MO Class templates and the common relationships among MOs are
represented in Name Binding templates. retransmitted to the POC after the FMS processes it. The interfaces to notice here are: (1)
between the HDT (or ONU) telecommunications equipment and power supply, (2)
between the FMS and HDTs, and (3) between the FMS and POC. Through the first
interface which is internally specified in the form of Inter Processor Communication
(IPC), the power and environment information is exchanged. Here, the environment
information can be temperatures and humidity inside cabinets, door open, flood, etc. The
second interface is based on TMN where Common Management Information Protocol
(CMIP) is used. The last interface is X.25. The power monitoring terminal is used for
debugging in process of development and for backup monitoring after development.
Figure 4.8 shows schematically how the request and response messages are exchanged
between the POC and ONU power system. The POC uses a X.25 protocol to connect the
FMS for its remote operation and a CMIS/CMIP protocol is used between the FMS and
ONU power system.
4.3.4 MULTI-VENDOR INTEROPERABILITY
It is required to take into account how to exchange the management information in a
multi-vendor environment. ITU-T TMN standards recommends four factors to as interoperability: First of all, the management platform is one of the key requirements to cut the overheads
in constructing management systems. It helps to build a common view to implement
management systems. Second, the managed object modeling is crucial to get the interoperability
over access network systems among different vendors. For simultaneous provision of
narrowband and broadband services, a tremendous amount of efforts are required to make
relevant standards in the first place and to define consequent common managed objects for
the new features of network elements (NEs) that are on the way. In addition, well-established
managed objects modeling helps to provide smoother evolution. Third, the necessary
interfaces should be defined to connect manager(s) and agent(s). Here, based upon the
managed object model, Q3 interface is utilized to operate access network systems. And
lastly, the management functions have to provide a common way of controlling NEs.
Moreover, they should be designed to supply any requested information from upper layers.
All of the recommendations above must be fully addressed and well established in a multivendor
environment.
4.3.5 FLC-C EXPERIENCE
Although the development of FLC-C was able to reach nice completion, FLC-C did not
achieve the successful result. It was too heavy to carry out POTS and the SDV service, which
was expected as a killer application of FLC-C, so it never got started. Later on, despite the
addition of internet access capability to FLC-C, only very limited number of FLC-C were
deployed. The system was a beautiful build, but was too expensive to accommodate any new
services. The failure of FLC-C was mainly doomed due to the failure of ATM. If there were
any native ATM services at the time, FLC-C would have been of more use. ATM native
services had never succeeded, and every new service had to be implemented on IP protocols
rather than ATM, so FLC-C was not heading towards success. BROADBAND ACCESS-XDSL
4.4.1 ENVIRONMENT
In the late 90s PC communication services emerged. It was required to have modems up to
56 Kbps to get connected, and people spent most of their time to use the telnet-like services,
Hitel, Chollian, Nownuri, Unitel, Netsgo, Channeli, etc. Users got connected through
modems, and mostly created and gathered in their own cyber-space communities. Bulletin
Board Service (BBS) and FTP service were among the popular to share their interests and
opinions. In 1999, more than 6 million PC communication users got logged on, and more
than a million subscribers among them spent above $30 USD per month to get dial-up
connections. Internet services were also expanding in a very fast pace.
4.4.2 INTERNET SERVICES IN KOREA
According to Korea Network Information Center (KRNIC), the number of high-speed
Internet connections, based largely on digital subscriber lines (xDSL) and cable modems
(CATV), surpassed 10 millions by the end of 2002 [1]. This figure corresponds to higher than
70 % of households and almost coincides with the 80 % penetration of personal computers.
Out of these 10 million users, 6.1 million subscribed to xDSL services. KT alone covers 5.4
million xDSL subscribers, including 0.8 million users subscribed to very high-data-rate DSL
(VDSL) services. The large number of xDSL subscriptions may be attributed to copper pairs
preinstalled for POTS, which covers the entire country with no additional cable installation
required. Cable modems over CATV networks is the second popular solution now with 3.7
million subscribers. These statistics are remarkable considering the current stagnation of
economy in the telecommunications industry worldwide.
The widespread availability of broadband Internet is now affecting every aspect of our
daily life. A significant portion of offline activities have shifted to online networks. Nearly
all transactions of banking, finance, tax payment, and stock trading are available at home,
and Internet shopping has already grown to 10 % of the total retail market. The successful
deployment of broadband Internet is the result of harmonization of network infrastructure,
public policy, demographic profiles, and other factors. From the policy side, regulators
strongly encourage controlled competition in the marketplace. This enables the normal user
to get high-quality service at a low price. The service providers, in turn, are constantly
striving to keep the cost low while increasing the quality of services. This has resulted in a
wide range of service portfolios based not only on ADSL but also on other advanced
technologies such as VDSL, Ethernet, and wireless LAN. KT has a broadband service
portfolio under the umbrella brand Megapass. The service consists of several subcategories
according to data speed and technology used, all with a monthly flat rate. Two-thirds of
subscribers selected Megapass Lite rather than Megapass Premium because of its relatively
high speed and reasonable price. On the other hand, Megapass Special and NESPOT users
are fast growing because of the significantly higher speed and wireless access, respectively,
of these two services. Another reason for this success is the high population density in the
distinctive residential segments in Korea. Two distinctive segments are the apartment area
and nonapartment area, which hereafter we call housing areas. The apartment area, where
nearly 40 % of the total population lives, is especially suitable for providing economical
fiber-based services. High population density and a distinctive residential environment contribute to shortening the copper loop length. Accordingly, half of the subscribers are
within 2 km from the central office (CO) and 80 % within 3 km. The average distance from
CO to subscriber is around 2.5 km. These factors enabled cost-effective broadband deployment.
KT, now serving almost 50 % of the Korean broadband market, has been in a pivotal
position in the introduction and expansion of broadband services in Korea.
4.4.3 COMPETING TECHNOLOGIES
To cope with an excessive amount of demands for data communications towards the end of
90s, KT landed on a project based on public switches, Integrated Communication Process
System (ICPS), and embarked offering 014 services upon its completion. Figure 4.9
depicts its architectural layout and constituents including dial-up modem banks. The service
provisioning processes are as follows. A customer dials up 014 and public switches route
the call to a corresponding modem bank of ICPS TNA. Followed by ICPS establishing a
session to packet networks or frame relay networks, a data network connects the session
through PNA to a specific host computer. This series of processes allow KT to bypass data
traffic from trunks and makes use of mature packet backbone networks. However, modems
can deliver only 56 Kbps and KT needed faster solutions.
In 1998 KT was hesitating between ISDN and ADSL. Although KT owned 40 % of the
national hybrid-fiber-coax (HFC) networks, KT was not interested in cable modems as
the future data communication solutions. KT was suffering from maintenance problems of
the HFC network, and was very doubtful for uplink performance of cable modems due to the
ingress noise. So, only ISDN and ADSL were the candidates for future broadband solutions.
KT had already spent plentiful resources in developing and deploying the ISDN, and it
was very difficult to totally abandon ISDN investments. Peoples in R&D department pushed
ADSL as their future solution; however, people from the financial department hesitated
because of the cost to deploy ADSL. In 1998, equipment cost of ADSL was expected to be higher than $1200 USD per subscriber. It looked hopeless to an ADSL party to persuade
financial people. Because it was a doubt that there are any applications or services that
require high speed such as ADSL in comparison to ISDN, KT hesitated to deploy ADSL
technology at that time.
However, everything changed suddenly. Thurunet begun the broadband access service in
June of 1998 by using cable modems over Hybrid Fiber Coax (HFC) network, and Hanaro
followed in February 1999 using Lucent’s ‘Any media’ ADSL solution. There were already
millions of subscribers who demand to be connected. Because both Thurunet and Hanaro
adopted a flat rate of <$30 USD, the customers could have significant savings if they
subscribe to the broadband access services. Higher speed and lower spending were perfect
answers to customers, and broadband service proliferated. KT was alerted by these
circumstances, ceased controversy, and changed his strategy quickly from deployment of
ISDN to adoption of ADSL as main broadband solutions.
4.4.4 BROADBAND ACCESS NETWORK
The decrease of local and long-distance phone call traffics, mainly resulting from the
introduction of cellular services, forced incumbent service providers to look for new growth
engines beyond telephone services. KT started by introducing Internet access service, which
was initially based on ISDN with a speed of 128 kbps. The service however proved
insufficient for multimedia purposes and could not satisfy customer demands. Accordingly,
ADSL was chosen as the broadband Internet technology, and intensive investment took place
in 2000. ADSL is now a matured technology, and other enhanced services based on new
technologies are emerging. Nevertheless, ADSL-based access service still occupies the
largest portion of the current broadband access market.
Figure 4.10 shows two kinds of ADSL networks. The first one is ADSL directly from CO,
the central office, called CO-ADSL. In this scheme, DSL access multiplexers (DSLAMs) are
located in CO, and subscribers are directly connected to DSLAMs only through twisted
pairs. In its early years, about 90% of installed ADSL lines are COADSL, mainly provided
for housing areas. COADSL, however, may not be effective for remote subscribers more
than 3 km away from CO. About 20 % of all subscribers fall into this category, and they are
the first candidates to be connected by fiber-based broadband access.
The second type of ADSL is based on fiber to the curb (FTTC). FTTC-ADSL mainly
serves subscribers in apartment areas or, to avoid degradation of ADSL quality, in residential
areas where the loop length is 3 km or more away from the CO. In this scheme, the optical
line termination (OLT) is placed in CO, and the optical network unit (ONU) in apartment
areas. So far, current access networks are based on asynchronous transfer mode (ATM) and
ADSL. On the contrary, emerging broadband access networks are based on IP, Ethernet,
and VDSL.
The successful deployment of an ADSL network depends heavily on the provisioning and
maintenance capabilities of the service. For the provisioning side, shortening the delay
between service request and actual provisioning becomes a key issue. On the maintenance
side, timely and/or proactive service quality maintenance activities are crucial. For this
purpose, KT has newly developed some operations support systems such as Telecom-
Integration Management System (TIMS) and Access Network Support & Warranty for End
Resources (ANSWERS) for successful OAM&P of ADSL networks. For example, when customer called for installation, the call receptionist put customer telephone number into the
TIMS, then TIMS makes a query to Telephone line Operation and Management System
(TOMS) to get the cable information. TOMS has the whole information about KT’s access
network. It has the geographic information of the conduits, cables, and so on. It also has the
information of the wire gauge and the length of the cable. TOMS can give the cable length
and the wire gauges of the usable copper pair between central office and the customer, and
then TIMS can estimate the maximum speed of the ADSL connection. Therefore, TIMS can
be used to determine whether KT can provide the broadband service or not. And if service is
possible, TIMS can estimate the maximum speed of the service by inter-working with ADSL
Transmission Line Analysis System (ATLAS) that provides service availability information
and network quality. Before subscription, customers can get service availability information
and network quality with the help of the ATLAS. KT called this function as ‘preordering.’
TIMS even has the configuration, performance, and fault management functions of the
ADSL network.
An ANSWERS provides the customer care service and service assurance functions for
maintaining xDSL service. It provides systematic line testing, problem localization and
identification, management of trouble ticket, and dispatching outside field technician for
problem handling at residential site.
For proper maintenance of the service, interoperability between devices from diverse
vendors is crucial. Lack of interoperability can incur unnecessary cost when a device needs
to be replaced. The customer’s PC is another source of maintenance cost. Statistics show that
a significant portion of customer requests for after-sale service come from malfunctions of
customer PCs rather than problems related to Internet access functions. Since this incurs
unnecessary cost, remote maintenance of customer PCs becomes an important approach for
reducing service cost. NETWORK ARCHITECTURES AND PROTOCOL
As a service provider network, KORea-telecom interNET (KORNET) provides Internet
access services and contents. Now this network interconnects major nodes with 10 Gbps
transmission lines. With the rapid increase of ADSL subscribers, user traffic is increasing
rapidly as well and IP network gets more complicated. In order to provide better services
to subscribers, KT increases network bandwidth and is considering to adopt Multi-
Protocol Label Switching (MPLS) and IPv6 to improve routing structures, network
security, and IP address deficiencies. Broadband Remote Access Server (B-RAS) is
located in the Regional Operation Center, which has KORNET Point of present (POP). It
concentrates DSLAMs from each CO, terminates ATM PVCs, and routes user traffic to
KORNET.
DSL Forum recommended four architectures for access core networks: the transparent
ATM core network architecture, the Layer Two Tunneling Protocol (L2TP) Access
Aggregation (LAA) architecture, the PPP Terminated Aggregation (PTA) architecture, and
Virtual Path Tunneling (VPT) architecture. KT selected PPP as a transport protocol to route
IP traffic because this protocol can go with preexisting infrastructure for dial-up users. In
addition, the protocol provides benefits of subscriber management, AAA functions, etc. To
transport PPP protocol, two kinds of transport technology, PPPoA and PPPoE, are required
according to CPE types. For core network access, we adopted the PTA architecture with
PPPoE and PPPoA, since KT’s ADSL subscribers have to get connected to KORNET in
order to access Internet using ADSL and POP.
ATU-R comes in two types. The first one is a plug-in type and the other is a stand-alone
type. As the plug-in type ATU-R, Network Interface Card (NIC) type with Peripheral
Component Interconnect (PCI) interface is used. This type of ATU-R can provide IPoA (IP
over ATM), PPPoA (PPP over ATM), and bridged mode by device drivers. As the standalone
ATU-R, 10BaseT interface is used instead of ATM-25 because Ethernet card is much
cheaper than that of ATM NIC and is prevalent. In case of NIC-type ATU-R, most of users
can be supported with PPPoA architecture, but if users want to use PPPoE architecture, they
should install a PPPoE driver for ATU-R and PPPoE software to their PC.
4.4.6 VDSL
Despite the widespread of broadband Internet services, the future success of broadband
access businesses in Korea is not ensured. As the market becomes saturated, growth of new
subscribers is leveling off, and broadband Internet service providers now face severe
competitions. In terms of competitions, consumers have been asking for faster and faster
speeds, and providers are always looking for the most cost-effective way to increase
bandwidth. Termed post-ADSL, various newly emerging technologies are now being
deployed to provide more advanced services in a cost-effective manner. These post-ADSL
technologies include VDSL, Ethernet to the home, Metro Ethernet, and emerging wireless
LAN.
In order to overcome sluggish growth, create new revenue sources utilizing existing
copper, and maintain initiatives in the future broadband market, KT started to deploy
VDSL in areas where churn is very heavy. Using VDSL means shortening the copper
portion and moving the fiber closer to the neighborhood. This way we can expand our
coverage and provide DSL in new areas. In July 2002, KT launched 13 Mbps symmetric VDSL for the first time in Korea and also announced 25 Mbps asymmetric VDSL in the
first half of 2003. Fifty megabit per second asymmetric VDSL service was launched in the
second half of 2003.
4.4.7 SERVICES
The VDSL down and upstream bandwidth enables telcos to provide triple-play service of
digital TV, video on demand, (VOD) and higher speed Internet and telephone service.
Newly approved ITU-T Recommendation H.610 of the FS-VDSL Focus Group defines
triple-play service using DSL technologies and makes it possible for telcos to provide
video-centric full service [2]. To be successful as a triple-play operator, the network
should support multimedia content and communication services that require bandwidth
and quality management along the end-to-end path of the session. To exploit the most out
of the increased bandwidth at the access network, and to avoid the quality problem
associated with the backbone/peering traffic, the multimedia services are best provided at
the edge of the core network. For multimedia services, KT has launched HomeMedia
service that provides on-demand content service through a content delivery network
comprising 10 regional server farms. For communication services, the quality control and
management capability at the edge becomes crucial for delivering commercial-level
services.
4.4.8 INTERFERENCES
While preparing for VDSL deployment in 2002, we found that the interference between
VDSL and existing broadband access TLAN was severe. TLAN is one of the initial types of
broadband Internet access that uses time-division duplexing LAN technology. As one of the
early techniques, TLAN was based on proprietary technology and did not consider spectrum
regulatory issues. Unfortunately, the spectrum of TLAN overlapped significantly with that of
the newly deployed VDSL, and until it was fixed, it was the main cause of performance
degradation. Another problem was the use of VDSL technology that was not standardized at
deployment time. Mainly driven by the need to jumpstart the market, deployment of
nonstandard VDSL was helpful in maintaining leadership in a fast-changing broadband
market. In return, additional efforts were made to upgrade the nonstandard VDSL to prevent
potential frequency interference with standard VDSL.
4.4.9 IP-VDSL
International Telecommunication Union – Telecommunication Standardization Sector
(ITU-T) VDSL Recommendation G.993.1 specifies that the transfer mode of VDSL should
be either ATM with transmission convergence (ATM-TC) or plesiochronous transfer mode
with TC (PTM-TC) [3]. This means that while ADSL was based on ATM or synchronous
transfer mode (STM), VDSL can use ATM or Ethernet. PTM-TC VDSL can be called
simply Ethernet over VDSL. KT chose Ethernet as the next broadband protocol because of
its wider availability, cost-effectiveness, IP conformance, and expected future-proofing.
Thus, IP-VDSL was named in accordance with this viewpoint and represents Ethernet-based
VDSL for an IP-converged broadband network. MONUMENTAL SUCCESS OF THE XDSL
KT was the third to enter the broadband access service in Korea. However, KT planned
massive deployment. It was possible because KT had budgetary advantage comparing to
Thurunet and Hanaro. Also, KT had the advantage in the service area. Thurunet adopted
cable modems, and because they did not own the HFC network by themselves, they had to
negotiate with more than 30 individual cable TVoperators for unbundling the HFC network.
Hanaro had to install fiber cable first to deploy their fiber equipment. So, both Thurunet and
Hanaro could deploy their system only in restricted regions. However, KT adopted copper
base ADSL solution and virtually anywhere KT could begin the broadband service.
Budgetary advantage, larger service regions, and good operation support systems such as
TIMS, ANSWERS, TOMS, and ATLAS made KT win the broadband access market in
Korea. KT is still the biggest broadband service provider in Korea and has more than 6
million broadband subscribers.
4.5 ETHERNET TO THE HOME AND WLAN
4.5.1 ENVIRONMENTS
Besides VDSL, another post-ADSL solution is Ethernet to the home. In Korea, UTP cables
are already installed in some newly built apartments that enabled service providers to use
pure Ethernet solutions to those residential areas. By using Ethernet technology directly to
the home, we can connect at up to 100 Mbps line rate to customers while the speed of service
is limited to 10 Mbps by provision. This service is characterized as Megapass Ntopia in
Figure 4.11. Figure 4.12 shows IP-VDSL and Ethernet to the home for apartments. Both IPVDSL
and Ethernet to the home have the same network topology, except that the VDSL
DSLAM is substituted by a fast Ethernet switch. Both of them use a Metro Ethernet network
as the regional broadband network. METRO ETHERNET
Figure 4.12 shows the overall architecture of IP-VDSL in accordance with the Metro
Ethernet network. To overcome the limitation of loop length, the layer 3 switch and DSLAM
are placed in the apartment complex. It is a simple but effective deployment because 40 % of
the total population in Korea lives in apartment complexes where one apartment complex
normally has more than 1000 households. The DSLAM terminates the VDSL and implements
Ethernet switching, IGMP snooping, prioritized traffic handling, and so on. An L3
switch aggregates tens of DSLAMs and works as a gateway to the Internet. It implements L3
and L2 protocols including default routing, DHCP server/relay, multi-casting, and 802.1p/Q.
The Metro access switch, Metro edge switch, and Metro core switch are all L3 Ethernet
switches and hierarchically constitute the Metro Ethernet network. A Metro access switch is
located in every local CO and aggregates tens of L3 switches via Gigabit Ethernet. The
hierarchical position of ADSL DSLAM in Figure 4.10 corresponds to the Metro access
switch in Figure 4.12. The Metro edge switch is located in the regional CO and aggregates
several Metro access switches via Gigabit Ethernet. The Metro edge switch corresponds to
B-RAS in Figure 4.10.
4.5.3 WIRELESS LAN
So far, we have only covered wired broadband access networks. Considering the
increasing popularity of wireless information terminals, wireless LAN (WLAN)
becomes another important post-ADSL solution for new growth. KT started public
nationwide WLAN service in early 2002, and the customer base is about a half million
users by the end of 2004. The service, called NESPOT, is currently based on the
IEEE 802.11b standard, and it will evolve to 802.11a or 802.11 g for enhanced services.
Figure 4.13 shows the position of WLAN in the service spectrum from wired to mobile services. Wired broadband service such as ADSL/VDSL provides very high-speed
Internet connection but does not support mobility. 3G CDMA service enables
relatively low data rates, but supports mobility in wide areas. The position of WLAN
is located between the two, and the service enables easy and high-speed Internet
access through a notebook PC or PDA with restricted mobility in hotspot areas. Besides
public WLAN, it is noteworthy that home WLAN service is very promising because
Korea has a sufficient number of homes wired to high-speed Internet. With growing
numbers of homes having multiple PCs and notebooks, the home WLAN market is
emerging as a strong potential market combined with existing wired access. Using
access points integrated in ADSL or VDSL modems, home WLAN provides a wireless
home network that enables multi-PC connections while using one wired access line of
ADSL/VDSL. The service is available to existing customers for a small additional
charge. Current WLAN service has several drawbacks such as short coverage, limited
number of hotspots, and radio frequency (RF) interference in the industrial, scientific, and
medical (ISM) band. It also has to overcome problems such as ease of use, security,
mobility, and network management [4,5]. However, with advances in wireless communication
technology along with integration with other fixed and mobile technologies,
the wireless broadband Internet market is believed to be another major market in the
long term.
4.6 B-PON (BROADBAND PASSIVE OPTICAL NETWORKS)
4.6.1 ENVIRONMENTS
Although ADSL was very successful, the speed of the ADSL depends on the distance of the
local loop. Figure 4.14 shows its network coverage roughly. And an ever-increasing speed requirement makes KT engineers to remind the FITL plan once more. International standard
organizations already were preparing PON solutions, and KT also was interested in B-PON
system.
B-PON development was begun based on the Full Service Access Network (FSAN)
standards. PON architecture seemed to be advantageous in maintenance and believed to
reduce the operation cost. A major feature of the B-PON is that 32 customers can be
concentrated on a single fiber to the central office using a simple passive optical splitter.
Since this optical splitter requires no electrical power, it reduces a major cost and
maintenance element. The B-PON supports a bit rate of 622 or 155 Mbps in the downstream
and upstream direction, which is shared by the users through time division multiplexing. The
multi-service capability and support of different QoS classes of ATM are well suited to
efficiently deploy Ethernet data, video stream, as well as the legacy narrowband leased line
services over a single full service access network infrastructure.
Within B-PON passive optical components are used in the optical distribution network
(ODN) and at the optical interface connection of the termination equipment with the
ODN. B-PON system consists of optical network unit (ONU) or optical network
terminal (ONT) at the subscriber side and the optical line termination (OLT) at the
network or CO side. B-PON system is based on ITU-T G.983.1 and can configure into
various networks. OLT transmits the ATM cells to ONT/ONU after multiplexing the
ATM-based broadband digital signal. The ODN takes care of the connections between the
ONU/ONT and OLT. The ODN is splitting and combining the optical signal flows
between OLT and ONU/ONT. The main components of the ODN are therefore the optical
fiber and the optical couplers having, respectively, the transport and the splitting/
combining functions. MANAGEMENT
From the operator’s point of view, management system should be easily operated without
concerning the features of complex technology. Therefore, BPMS supports mirrored views
that are simulating real Network Elements (NEs), network topology view, OLT/ONU/ONT
rack view, rectifier view, etc. With these mirrored images, operator can look and feel
the views as a real system. Figure 4.15(a) shows B-PON system configuration, and
Figure 4.15(b) represents the management system configuration for B-PON in KT.
4.6.3 LEGACY OSS INTERFACES
B-PON management system comprises peripheral NMS interface, Management Modules,
and OLT interface. Peripheral NMS interface takes part in cooperation with NMSs, which
are TIMS, ANSWERS, and ATM-NMS at KT as follows. B-PON DEPLOYMENT
When KT planned B-PON, KT wanted to deploy the B-PON as the FTTC applications. In
other words, each ONU serves multiple homes and targeted services include the POTS,
internet access, VOD, etc. At the very early planning stage, FTTH seemed to be unrealistic.
B-PON had advantages to the FLC-C in architectural aspects; however, if B-PON is used as
FTTC system, the advantage of the passive optical network disappears because ONU must
be fed electric power and equipped with battery backup system. And also ONU must be
equipped with ATM to IP protocol conversion, IP encapsulation, and voice adaptation
functions, and so on. And, since B-PON uses time division multiple access technology for
uplink, B-PON implementation requires very complicated ranging algorithm and multiple
access technology. Also burst mode receiver is needed. So, adopting B-PON as a FTTC
solution resulted in abandoning the advantage of the PON configuration. The development
was completed in 2001. B-PON could not claim any merits and the cost of the B-PON was
much more expensive than the ADSL. And B-PON missed the chance to be deployed.
4.7 WDM-PON
4.7.1 NEXT GENERATION NETWORK
Spurred by the enthusiasm of Internet savvies and youngsters, broadband Internet service
in Korea has penetrated into the daily lives of common people [6]. A recent survey of
Internet usage patterns shows that users are now spending around 2 h/day on Internet
applications [1], which is comparable to the average daily TV viewing time. Although
encouraging, the survey also shows that the main Internet applications in Korea are still
Web surfing, e-mail, games, and chatting using PCs at home. This shows that even though
broadband Internet connectivity is becoming nearly universal in every household in
Korea, applications are still limited to sending and receiving traditional text, image, and
control information through the Internet, only at faster speeds. This is in contrast to
the predictions of wide penetration of multimedia entertainment and communication
applications, such as Internet broadcasting, video on demand (VoD), voice over IP (VoIP),
and multimedia conferencing. BANDWIDTH
Bandwidth demand is expected to grow rapidly as high-quality multimedia applications
begin to be used in daily life. A typical service configuration for the home includes several
high-quality video streams for broadcast and VoD sessions, a number of multimedia
communication sessions, high-bandwidth peer-to-peer (p2p) sessions, and normal Web
surfing. The bandwidth required for this service set can range from tens of megabits per
second to 100 Mbps depending on the quality requirements for multimedia streams. To
provide users with a multimedia experience comparable or superior to the current TV
experience, it is envisioned that Internet-based multimedia streaming service will support
high-definition TV (HDTV) quality for media broadcasting and VoD, and standard TV
quality for multimedia communication sessions. Considering that the typical high definition
media stream requires up to 20 Mbps, access network bandwidth of 50–100 Mbps is
considered adequate to accommodate future broadband Internet traffic. To provide the
required bandwidth as a universal service, service providers need to upgrade their access
network. KT is taking a phased approach to provid bandwidth for next-generation applications
– FTTC-based VDSL and fiber to the home (FTTH). VDSL has already achieved
50 Mbps within 300 m, and since fiber is available for apartment complexes, VDSL service
is currently applicable to apartment areas. Apartment complexes currently under construction
are also recommended to be equipped with UTP CAT 5 that enables 100 Mbps Fast
Ethernet speed. Moreover, upcoming apartments are expected to have optical fiber as basic
cables, and every home in those optical apartments will have optical connection. This means
that FTTH can be provided to apartment complexes in the near future. For nonapartment
areas (i.e., housing areas), VDSL deployment makes use of broadband access nodes that will
be placed on the street, on a wall, on a pole, or somewhere near the houses. The broadband
access node is connected by fiber from the CO and will be of varying size in accordance with
location. Each house is connected to the broadband access node through VDSL technology.
In the near term, it accommodates VDSL DSLAMs or access switches. VDSL or Ethernet on
CAT 5 will serve as a preFTTH solution in short range for the time being. In the long term,
the broadband access node will ultimately be responsible for FTTH. Regarding FTTH access
technologies, passive optical network (PON) is considered at present the best candidate for
an FTTH solution enabling point-to-multipoint fiber connection. While point-to-point fiber
connection for FTTH requires as many optical ports as the number of subscribers, PON has
an advantage in reducing this cost as well as cutting down fiber installation cost. Eventually,
wavelength division multiplexing PON (WDM-PON) is expected to provide a dedicated
100 Mbps fiber connection to houses (Figure 4.16).
4.7.3 QoS
Compared to the current practice of providing multimedia services over a best-effort
network, providing high-quality media services requires the network to meet the specific
QoS requirements of applications. For example, high-quality interactive communication
requires one-way delay of less than 150 ms and less than 1% packet loss. On the other hand,
a distributive streaming session (e.g., VoD) has less stringent delay requirements. From the
service provider’s standpoint, QoS provisioning implies building a managed IP network that
can transfer the media information in a managed way. Also, the end-to-end nature of QoS requires quality handling not only at the access network but also at other parts of the end-toend
path, including the core IP network, the customer premise network, and even at the
terminal itself. The control mechanism should be able to track the location/status of
customer terminals and manage network resources for guaranteed performance. Considering
that broadband Internet business until now has mainly focused on the access network part of
the end-to-end path, this approach means a significant change of direction for traditional
broadband Internet service providers.
Providing QoS at the network and premises sites has been a major technical issue for
network service providers seeking revenue sources other than network connectivity services
[7,8]. KT’s effort to meet future service requirements involves the buildup of a premium
network, as shown in Figure 4.17. The network consists of three parts: access, edge, and core
networks. The access network is responsible for delivering information between customer
equipment and edge nodes. For economic reasons, the network is shared between managed
service traffic and best-effort traffic, requiring differentiated traffic handling at the access
network. It has some differentiation mechanisms at the access network ranging from simple
priority handling to more sophisticated ones such as virtual tunnels supporting QoS and
security. The managed core network will consist of a set of MPLS tunnels that provides
simple but high-throughput transport of packets between edge nodes. The edge network
contains service intelligence in that it performs the mapping of QoS requirements between
the access network and the managed core network, and different route control for best effort
and managed service traffic. This requires the most sophisticated processing in edge nodes in
that each of the incoming packets from the access network should be mapped to the proper
tunnel in the core and vice versa, based on the QoS marking and destination address. Added
to the complexity is the requirement to process each packet to identify source address,
application type, destination address, and other information elements to perform proper
routing and traffic management functions. The edge network also contains value-added
service features that are best provided at the edge locations (e.g., managed security, content
filtering). Equally important is the QoS at the terminal where media encoding/decoding is
performed. Terminals with less processing capability may not deliver satisfactory quality
to end-users even though the network quality is above the required level. A typical problem
arises when performing multimedia conferencing using a PC where the processing delay at
the terminal is often more than 200 ms (compared to a 150 ms one-way delay requirement
for a high-quality interactive session). From the service provider’s point of view, providing a
high-quality media application can include controlling not only the network but also
equipment at customer premises.
4.7.4 DESIGN CONCEPT
Since the successful deployment of ADSL access network, the penetration of high-speed
access network is increasing worldwide. Also the demand for higher speed is ever
increasing. Initial application of the high-speed access network has been the internet access.
As the number of internet user increases and the bandwidth improves, contents of internet
web sites tend to shift from text-based page to picture and video-based pages. Also the usage
pattern of internet shifts from web surfing to download of large files and streaming of
multimedia contents.
A motivation for the implementation of higher speed network is the competition among
the network operators. While KT is competing with Hanaro and Thrunet, higher speed was
always the advertisement point. After introducing copper-based ADSL, in order to increase
the speed, KT deployed DSLAM deeper in the vicinity of the customers. Eventually, ADSL
modems were replaced by 13 Mbps VDSL, 26 Mbps VDSL, and now 52 Mbps VDSL
modems. These ever-increasing speed upgrades were not initiated by customers rather by
operators. This network upgrade was not followed by revenue increase. The next motivation for the higher speed is the convergence of broadcasting and
telecommunication. Currently, voice, TV broadcasting, and data are delivered to subscribers
in separate networks, that is, public switched telephone network (PSTN), CATV network,
and DSL. However, it is believed that the three separate services can be offered in the
integrated form of Triple-Play service (TPS) which can be delivered by a single network.
Furthermore, the services may be displayed by a single equipment, for example, a TV set.
CATVoperators are offering video and data service over their HFC network with almost half
the cost of the telephone operators. They offer as low as 18 000 Won per month for the
bundling service of CATV and internet access. POTS is not bundled with that package
because in Korea telephone service is already mature, and VoIP has very little room. The
same service will cost about 43 000 Won if offered by KT. So, CATV operators are very
competitive and they are taking significant shares in the market. KT is thus facing a serious
competition from the CATV operators, and needs a new strategy to compete effectively.
HFC is quite adequate for broadcasting service, and cable modem is also quite effective in
delivering internet data. KT have owned HFC network till 1999; however, KT sold its HFC
network partly because of the difficulty of maintenance and partly because of the regulations.
Korea has very unique regulations about CATV. CATV industry is divided into three
groups of companies: the first group is Program Providers (PP), second one is Service
operators (SO), and the last one is network operators (NO). KT was NO, so KT has the
responsibility to install and maintain the network to transport program to customers.
According to the CATV regulations, NO cannot make use of the excess channels which
are left after channels are assigned to specific programs. The right to use the remaining
network resource was given to SO. HFC network was therefore not attractive to KT, so
finally KT sold its HFC network. However, service operators eventually bought NO’s HFC
network and they are allowed to offer video and data bundled service. As mentioned above,
HFC was not attractive to KT, but HFC network becomes a spearhead against KT. CATV
operators now have more than 7.4 % of the broadband market share by the end of 2004, and
furthermore, their average growth ratio is much larger than any other broadband service
providers. To cope with this competition, KT needs new enhanced video services. Competing
with CATVoperators with the same services, KT will have no real advantage. However,
if KT can provide new, better, enhanced video services, KT could compete effectively
against CATV operators.
KT performed many IP multi-casting trials before. IP multi-casting was believed to be the
solution for broadcasting over IP network for the telephone operators. While there have been
several proposals for video broadcasting overlay over PON [11,12], IP multi-cast technology
is bandwidth efficient and proved to be reliable and scalable. However, IP multi-casting is
only the duplicate of the CATV broadcasting service, and no more. No additional value is
given to the customers. That means KT can offer same service as the CATVoperators with a
more expensive infrastructure. Therefore, IP multi-casting alone is not enough to compete
with the CATV operators.
However, IP-based video delivery allows new video services. IP-based video delivery can
offer target advertisement (TA) by which advertisement is customized to each subscriber and
the advertisement effect is enhanced. Also, IP-based delivery of video content allows the
time-shifted viewing services. In the established broadcasting industry, the value of program
has been determined not only by the content but also by the scheduling of time. The timeshifted
viewing environment of TV programs eliminates the imposition of schedule and creates additional value in the broadcasting. Therefore, IP-based video delivery is in more
favorable position for the convergence service of telecommunication and broadcasting than
the data overlay over HFC. These time-shifted viewing services and target advertisement
service are basically unicast type services and are very difficult to implement over HFC. So,
if and only if these applications (TA and time-shifted viewing) can be the killer applications
of the TPS (triple-play service), telecommunication operators would have the chance to win
the game.
In the multimedia communication environment such as TPS, the statistical multiplexing
effect is not significant and the guaranteed bandwidth becomes more important rather than
the maximum bandwidth. The guaranteed bandwidth requirement per user is estimated to be
more than 75 Mbps in the near future as shown in Figure 4.18.
For the provisioning of the bandwidth required for TPS, the existing copper-based
infrastructure such as twisted pair and hybrid fiber coax (HFC) network is not appropriate
because of the limited bandwidth. Some very complicated VDSL technology can transmit
data at 100 Mbps speed but within a very short range. That means we need to install ONU
deeper and deeper toward the customers. That also means the total number of subscribers
served by each ONU is decreased, and we need much more ONUs in the outside plant. That
means fiber carries the signal almost all the way to the customer and only over the fraction of
the distance copper pair carries the data. KT wants to get rid of the active element from the
outside plant because of the maintenance problem, and if it cannot be avoided, KT wants to
keep the number of the active elements to minimum. This arrangement is a very expensive
solution too.
KT does not own the HFC. Even if KT owns the HFC network, KT does not have the
privilege to use the network. Therefore, KT prefers to ignore the cable modem solution.
However, the potential of the cable modem was carefully studied and economic analysis was
done. Cable modem uses single TV channel, 6 MHz bandwidth, for data transmission.
Downlink speed can be as fast as 40 Mbps. One hundred megabit per second transmission
can be achieved by using three channels and inverse multiplexing the output of three cable
modems. However, if we are considering streaming data, the whole bandwidth is occupied
by a single subscriber and bandwidth sharing cannot be expected. We need additional cable modem terminating systems (CMTS) to use additional channels. However, CMTS is a very
expensive network element (more than $10 000 USD per channel) and should be shared
between subscribers. Usually, one CMTS is shared by hundreds of subscribers. If a
subscriber needs to occupy the whole CMTS bandwidth, the system cost will be too high,
and cable modem is not applicable in that case. So, neither VDSL nor cable modem can
provide guaranteed 100 Mbps transmission. Therefore, FTTH development was started.
FTTH network architectures can be divided into two main categories according to fiber
distribution architecture [13]. One is point-to-point star (also known as home run)
architecture, and the other is double-star architecture. In the home run architecture, the
required number of fiber core is the same as that of the subscribers, and it is expensive to
install and handle the numerous fibers. Obviously, it is not appropriate for the massive
deployment. In the double-star architecture, however, many subscribers share one fiber line
through a remote node (RN) that performs one of active switching, passive power splitting,
or wavelength (de)multiplexing functions. The RN is located between subscribers and
central office (CO), and can be active or passive depending on whether the remote node is
electrically powered or not. The double-star architecture with an active RN is referred to as
active optical network (AON), and that with passive RN is referred to as passive optical
network (PON). PON has advantages over AON in terms of installation, operation, and
maintenance of network. PON can be divided into several categories according to multiple
access schemes such as sub-carrier multiple access (SCMA), time division multiple access
(TDMA), and wavelength division multiple access (WDMA).
In SCMA, the base-band signal of a subscriber modulates a radio-frequency (RF) carrier
with unique frequency to the subscriber, which subsequently modulates a light wave [14]. It
needs high optical power as the number of subscribers increases because of the clipping
induced noise.
In TDMA, the collision of signals is avoided by access control protocol including ranging
and cell allocation to each subscriber. TDMA has drawn interests from industries. ATMPON
was standardized by ITU-T G 983.3 [9] and Ethernet-PON (E-PON) is in the process of
standardization IEEE EFM [10]. TDMA has merits in that it can utilize the bandwidth of
optical link effectively by the statistical multiplexing of traffic for several subscribers. In
TDMA, the downstream signals are broadcasted and optical signal power is split at RN. The
upstream signals of subscribers are combined at RN. Therefore, security algorithms for
downstream signals and collision avoiding algorithms for upstream signals are required.
Even though TDMA has several advantages as an optical access network, it has several
problems. Since optical power splitter is used in RN, the optical power loss of both direction
signals increases as the number of ONU increases. And the splitting ratio is limited by the
optical power of transmitters. Performance and speed of TDMA is restricted by the inherent
characteristics of burst data transmission. First, the transmitter should turn off the signal
power during transmission of other channels because background light will degrade signal to
noise ratio (SNR). Also, there exists turn-on-delay before the data changes from low to high
level. Second, the level of optical signal varies depending on the distance from the ONU’s. It
increases the complexity of the optical transceiver. Therefore, the burst mode transmission of
TDM-PON makes it difficult to increase the speed high enough for TPS.
In WDMA, signal collision is avoided by allocating a dedicated wavelength for each
subscriber. As WDM technology has matured during successful applications in backbone
networks, there have been several suggestions for application of WDM to the access network. The proposals can be classified as hybrid WDM-PON solution and WDM-PON.
The former proposals attempt to take advantage of a combination of merits of WDM-PON
and those of other technologies. TDMA-PON partially employs WDM to add additional
services. WDM/SCM PON was proposed to increase the utilization of bandwidth [15].
WDM-FTTC was