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In making the transition to FTTH, operators will be conditioned by their existing HFC topologies.

By Dr. Merrion Edwards, Corning Optical Fiber

In deploying optical fibre cabling close to and inside the customer house to enable high speed broadband services, CATV operators may face many challenges. Lessons can be learned and challenges avoided through exploring best practices in existing FFTH deployments by Telco operators.

for Next Generation Access Networks: Engineering the Future, Today

technicaltechnical

built HFC networks are evolving, with fibre being deployed

much deeper (homes served by localised nodes, with only

50~400 homes per node) in the network and much closer

to the customer home. Many cable operators serving high-

density environments are now deploying fibre directly to the

basements of high-rise apartment buildings.

In making the transition to FTTH, operators will be conditioned

by their existing HFC topologies. Whatever this topology,

however, their FTTH deployment is likely to be constructed

using a Passive Optical Network (PON) architecture. CATV

standards organisations have examined PON architectures

and this has led to the development of a Radio Frequency over

Glass (RFoG) standard [1], which has been proposed by the

Society of Cable Telecommunications Engineers (SCTE) in the

US for deploying a broadcast architecture of analogue signals

similar to PONs. Therefore, although some of the challenges

will be specific to these new deployments by CATV operators,

many of the lessons learned by their cousins from the world of

telephony could potentially apply to CATV as well.

Lessons learned – meeting the challenges in access networksIrrespective of whether you are a CATV network operator or a

telco, communications network design is generally driven by

the system’s power and loss budgets which determines the

maximum acceptable amount of transmission loss before the

system fails and the customer loses signal. This budget varies

depending on the technology used to design the system

(P2P, active Ethernet or GPON/EPON) and is determined by

calculating the combined loss of each component in the link,

along with some allowance for repair and maintenance.

For instance, the typical loss budget of a GPON (point-

multipoint) link using a 1:32 splitter and a Class B laser would

be 28dB (Figure 1), with the splitter taking 17dB of that budget

(21dB for a 64-way splitter). A key challenge in PON FTTH/B

networks is ensuring that the link budget does not exceed this

value not only on installation but, most importantly, during the

lifetime of the system.

Access transport systems that are used to deliver broadband

to the home are constantly evolving e.g. BPON to GPON and

Ethernet to gigabit Ethernet. This represents a challenge

in that the network you deploy today must be capable of

upgrading to the transmission systems of tomorrow. CATV

operators will also need to maximise subscriber coverage

to maximise revenue from their investment. And, due to the

current coax infrastructure that needs to be retained, some

operators will encounter issues regarding space in existing

closures and cabinets.

For the indoor cable installation segment,

balancing the burden of minimising signal

loss whilst providing aesthetically-pleasing

installation solutions for the customer

(naturally leading to tight cable bends) puts

significant pressure on fibre and system

solutions, meaning it is important to select

the right fibre for a given scenario.

Most of these challenges, both indoor and

outdoor, can be overcome through careful

selection of optical fibre parameters. In the

OSP, attenuation (the loss of optical signal

as the light travels down the fibre) needs to

be curbed whereas indoors, macrobend

loss (light leaking out of the fibre at

bends due to light scattering and photon

absorption, causing signal loss) should

be the focus of attention. Fortunately,

Cable television networks, then and nowThe telecommunications market has evolved

significantly since community access television

(CATV) operators first started offering broadband

using hybrid fibre coax (HFC) systems. Now,

subscribers want to live in a ‘super-connected’

world. The smart phones, tablets, laptops and

games consoles consumers use to communicate

are hungry for bandwidth and are driving a global

need for higher broadband speeds. Acknowledging

this trend, CATV operators have started to look at

Fibre to The Home (FTTH) technology by running

pilot projects in preparation for the future where new

applications will demand much higher data rates.

While many CATV operators have experience

deploying metro and long-haul terrestrial optical

networks, FTTH represents an unprecedented

deployment scenario. Typically, original HFC

networks use six fibres per optical distribution node

placed within one mile of the customer and reaching

up to 3,000 homes per node. However, newly-

Dr. Merrion Edwards is currently Regional

Marketing Manager, India & EMEA, for Corning

Optical Fiber. She has over 20 years of

experience in the field of telecommunications

and is the author of 45 papers in industry

journals, covering a broad range of applications

including FTTx, access, premises, long-haul,

submarine and photonic devices.

During her 12-year career at Corning, Dr Edwards

has become a knowledge expert on emerging

telecommunications trends and synergetic

innovation of optical fibre and components to

enable advancement of telecommunications

transport systems. Before joining Corning,

Edwards conducted research into photonic

devices for telecommunications and sensing

with BICC Cables, Ltd. Edwards holds a PhD in

Optoelectronics from Southampton University in

the United Kingdom.

Dr. Merrion Edwards

Advanced Optical Fibre

Figure 1. Power budget is calculated as the cumulative loss of each individual component in a link

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optical fibre technology innovation has delivered new fibre

products with optimised attenuation and macrobending loss

specifications that offer significant advantages to the FTTH

operator. We will now consider the impact of these fibre

innovations in both indoor and outdoor deployment scenarios.

Fiber innovation for outside plant cablingRecent innovations in optical fibre offer solutions in the

following areas that could become concerns for CATV

operators deploying FTTH/B:

l System evolution

Fibre deployed today is expected to operate reliably for

20 years or more to maintain revenues and protect return

on investment. Whatever network topology the network

architects decide to adopt on day one, the standards

governing that network topology and its associated

communication systems will evolve over time.

When a system is designed, the power budget is usually

determined, from the upper attenuation limit of the operating

window with additional allowances for components

which contribute to signal losses such as splitters, for

example. Traditional GPON/EPON and BPON systems

use wavelengths from 1290nm to 1330nm for upstream

transmission, 1480nm to 1500nm for the downstream

with 1550nm being reserved as an “enhancement band”,

typically for video broadcast.

With the advent of next generation PON systems and, in

particular, 10GPON, new transmission bands within the

operating window have been allocated so that the new and

old transmission protocols can co-exist on the same fibre

connection. Consequently, 10GPON operators will need to

use an even broader spectrum of the fibre, from 1260nm up

to 1600nm (Figure 2). As intrinsic fibre attenuation increases

significantly at shorter wavelengths below 1310nm (due to

Rayleigh scattering) and macrobend sensitivity is greater at

longer wavelengths, this presents new challenges in terms

of increased power budget loss management.

An operator can prepare for the advent of 10GPON

and greatly mitigate the impact of moving to a 1260nm

transmission window by deploying a low-attenuation

G.652D fibre at the outset (such as Corning® SMF-

28e+® LL fibre) with reduced attenuation across the

whole spectrum. Such forward thinking and planning

will increase the technology robustness of the network,

ensuring that the network can be upgraded in the future

with minimum or no impact in terms of system design.

l Coverage

FTTH servicing of subscriber areas might be a

considerable distance from the closest hub or head-end

(typically 1-20km) resulting in much longer link lengths in

the access network than originally conceived in the HFC

network design. To avoid a potential increase in areas

without coverage (or “not-spots”) standards for extended

reach systems (e.g. Class C GPON) with higher power

loss budgets have been developed. However, even these

are limited in total reach and require incremental capital

expenditure on advanced electronics.

Advanced optical fibre technology can help here by

extending the coverage area from a particular exchange.

The typical reach of a FTTH link with GPON topology

(Class B) using standard fibre is around 18km from the

central office to the customer’s house (Figure 3). Using

a low-attenuation G.652D fibre (such as Corning® SMF-

28e+® LL fibre) can extend this link length by 10% to

almost 20km, which in turn yields a 20% increase in

coverage area1 (+189 km²) which, depending upon

population density, might also be home to many potential

new customers.

l Speed of deployment

Installing the high volumes of fibre that need to be

deployed in access networks to deliver FTTH/B is a time-

consuming challenge and also requires significant in-field

skill and training. This is a key challenge for carriers who

need to make a quick return on their investment. ‘Plug

and play’ cable solutions with pre-connectorised fibre

provide a much faster and simpler alternative to installation

via fibre splicing and, although connectors do incur slightly

more connection loss than splicing, if they are used in

combination with a low-attenuation G.652.D fibre, the

connection loss can be offset by the reduced fibre cable

loss thereby mitigating the need to compromise on system

design or performance to remain within power budget.

l Small hardware and equipment

Re-utilising an existing HFC node infrastructure will yield

cost savings and dramatically reduce activation time by

employing the existing plant and hardware. However, as

the existing cabinets will already be heavily populated with

legacy coax network connections, a key issue for CATV

operators is cabinet/closure space. New cabinets often

have to be installed, so the smaller and more discrete they

are, the better.

However, in a cabinet where space is at a premium,

macrobending is an obvious problem. The solution is to

use a fibre that can be bent without incurring this loss, like

the Corning® ClearCurve® ZBL fibre which enables more

flexible cables to be tied more tightly in enclosed spaces,

yielding a significant reduction in the size of the hardware

and equipment in a cabinet.

Indoor cablingBack in the world of telecommunications, a proliferation

of indoor cabling standards in Europe seeks to encourage

competitive broadband markets via open access architecture

regulation. This has a striking impact on indoor optical fibre

cabling performance requirements. Open access architecture

regulation requires that broadband networks are configured

so as to ensure that competitive broadband service providers

are not (within reason) precluded from connecting to a

customer because of low signal power levels (either due to the

operator’s central office being further away from the customer,

or a difference in broadband transmission technologies used).

To date, open access architecture initiatives in Europe

have resulted in such indoor cabling standards being put

in place, with maximum indoor loss figures (measured from

Most of these challenges, both indoor and outdoor, can be overcome through careful selection of optical fibre parameters.

...the network you deploy today must be capable of upgrading to the transmission systems of tomorrow.

Figure 2. Optical spectrum usage is being broadened by the evolution to new PON technologies

Figure 3. Using a low loss single-mode ITU-T G.652.D optical fibre, the coverage area of a hub or central office in a typical GPON architecture can be increased by ~20%

1 Comparing SMF-28e+ LL fibre with an attenuation of 0.32dB/km at 1310nm, to a typical fibre with an attenuation of 0.35dB/km results in extension of the maximum reach of a typical VDSL feeder cable from 10km to circa 11km, or extension of an FTTH feeder link from 18km to almost 20km. This increase in feeder cable length increases the area that a central office can cover by almost 20%.

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the distribution unit in the point of entry to the building up to

the customer’s terminal) being set at a maximum of 1.2dB

in Germany, 1.5-2dB in France and a maximum of 0.9dB in

Switzerland.

Although open access may not be a direct concern for CATV

operators, the idea behind it should be considered carefully

if operators have any plans for future network upgrades; for

example, upgrades in speed or node equipment consolidation,

both of which might have a bearing on power budget. On

installation, operators may not be concerned about indoor

loss because of the property’s proximity to their central office

or node.

However, should they decide to consolidate equipment later

and reduce the number of central offices or nodes, the node

which provides internet access may then be too far away from

the customer premises for the signal level to meet the power

budget requirement. Then, either more expensive transmission

equipment would have to be installed or the customer or

the consolidation itself would have to be sacrificed. Using a

bend-insensitive fibre to minimise indoor bend loss enables

more system power budget margin, margin that acts as an

insurance policy against such a situation by enabling future

upgrades with minimum impact on link design or transmission

equipment.

However, indoor optical fibre cabling is a complex task. Until

recently, the home environment was uncharted territory for

optical fibre; an environment where, for the first time, the

general public has access to fibre cable. Here, cables must

be installed quickly (often by a workforce unfamiliar with fibre)

with minimal aesthetic impact, resulting in cables being bent

around wall corners and doorframes and even stapled.

The need for a fibre insensitive to bending then is obvious

and, recognising this, the optical fibre industry established

the ITU-T Recommendation G.657 fibre standard, as shown

in Figure 4 opposite. Within the standard are two sub-

categories: category A fibres have to comply with the ITU-T

Recommendation G.652.D standard to ensure compatibility

with existing legacy networks; category B fibres do not have

to comply with the Rec. G.652.D standard.

Under these two main categories, the G.657 standard further

classifies the fibres according to their respective tolerance

to bends: whether they are ‘bend-improved’ (G.657.A1 –

0.75dB loss per 10mm radius bend), ‘bend-tolerant’ (G.657.

A2/ – 0.5dB per 7.5mm bend) or ‘bend-insensitive’ (G.657.

B3 – 0.08dB per 7.5mm radius bend).

The next obvious question is which G.657 fibre should be

used and where? Let’s consider the real case of a European

operator who installs the building drop cable on the outer

façade of the building and has set a maximum loss for this

drop cable throughout the façade of the building at 2dB. It is

a fair assumption that at least four tight 90o degree bends will

occur (at circa 7.5mm in radius and equivalent to one full 360o

turn). If we compare the bend loss performance of G.657.

A1, G.657.A2 and G.657.B3 fibres, we can see from Figure 5

above that only the more robust G.657 categories (G.657.B3

and G.657.A2) ensure the very low levels of indoor cabling

loss necessary to achieve compliance with the operator’s 2dB

cabling loss requirement and provide technology robustness

for the network.

G.657.B3 and G.657.A2 fibres, like Corning® ClearCurve®

ZBL fibre (which is also G.652.D-compliant) and Corning®

ClearCurve® LBL fibre, offer significant network reliability

and subscriber revenue protection benefits. Without a bend-

insensitive fibre, the accidental bends that are likely to occur

within a customer’s house throughout the lifetime of the

network could mean that the customer could suffer signal

failure and lose network connectivity.

The operator cost of rectifying such problems results in

higher network OpEx expense and the event itself can

lower customer satisfaction and increase churn leading to

significant reductions in subscriber revenues. The use of

bend-insensitive fibre cabling for in-building cabling provides

the ultimate in protection against higher OpEx due to signal

failure and revenue losses due to associated customer churn.Figure 4. ITU-T G.657 optical fibre standard

A proliferation of indoor cabling standards in Europe seeks to encourage competitive broadband markets via open access architecture regulation. This has a striking impact on indoor optical fibre cabling performance requirements.

Figure 5. Performance comparison of all G.657 fibre types in a typical FTTH indoor installation

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By investing in advanced optical fibre today, CATV operators can secure network reliability and subscriber revenue protection for years to come.

ConclusionIn deploying optical fibre cabling close to and inside the

customer house to enable high speed broadband services,

CATV operators may face many challenges. Lessons can

be learned and challenges avoided through exploring best

practices in existing FFTH deployments by telco operators.

Many of the key challenges can be addressed by using

innovative fibre-optic technology. Transmission systems, for

instance, will continue to evolve so deployment of a low-

attenuation fibre, such as Corning® SMF-28e+® LL fibre, in

OSP cabling can ensure the flexibility needed to adapt to new

standards in access networks, cover new areas or simply

facilitate the use of faster and more flexible solutions such as

pre-connectorised cables.

Good system tolerance to macrobends, both indoors and

out, is of great importance so that the deployment of bend-

insensitive fibres (such as ITU-T Rec. G.652.D-compliant

Corning® ClearCurve® ZBL or Corning® ClearCurve® LBL fibre)

in areas of high bend probability is recommended.

Outdoors, such bend-insensitive fibres can enable compact

and discrete street cabinets. For in-building applications,

given the proliferation of installation challenges the fibre will

face and the likelihood of bends, only the more robust ITU-T

Rec. G.657 fibres (ie B3 and A2/B2) will truly protect the

system. By investing in advanced optical fibre today, CATV

operators can secure network reliability and subscriber

revenue protection for years to come.