Tài liệu GPON Migration Ensuring my Network is Ready to Migrate to GPON doc

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GPON Migration
Ensuring my Network is Ready to
Migrate to GPON
The demand for higher bandwidth by residential customers has led carriers to one
simple conclusion – optical fiber with its almost limitless bandwidth capability will
be a network necessity. The driver for such high consumer bandwidth usage is the
need for carriers to deliver video, along with voice and data services, to complete
the “triple play” package.
As traditional carriers experience declining revenue from voice services, they must
find ways to increase the revenue from data and video services. Since video is the
highest revenue generator, the ability to compete with cable companies is critical.
Delivering the same high-quality video customers have received from their cable
provider can only be accomplished using video-over-IP technologies.
Passive optical network (PON) architecture is the key element for allowing carriers to
support the demand for advanced broadband services today, while also providing the
flexibility to scale outside plane infrastructures to meet next generation broadband
requirements. But deploying PON technology requires one particularly careful
consideration – how easily will the network migrate from one PON flavor to the next
as bandwidth demand continues to rise.
GPON Migration
Ensuring my Network is Ready to Migrate to GPON
Ensuring my Network is Ready to Migrate to GPON
Page 3
Standardizing PON
As with many telecom technologies, standards have played
an important role in the development of PON protocols.
These standards drive the underlying protocols and the
basic specifications for specific telecom and data systems.
Ultimately, standards define the specifications to make
interoperability a reality and ensure product performance.

PON standards and recommendations were established
by the Full Service Access Network (FSAN) and the
International Telecommunications Union (ITU). ATM PON
(APON) was the first iteration of the technology for fiber-
to-the-premise (FTTP) solutions. However, APON lacked the
bandwidth required for more robust applications and its
popularity was short-lived.
Broadband PON (BPON) was standardized in 2001 as
the first viable PON flavor for general use in early FTTP
applications. It provided 622 Mbits/sec downstream and
155 Mbits/sec upstream. More importantly, it provided
carriers with the capability to overlay RF video. Still, as
bandwidth demand increases in the FTTP market with
newer services, BPON will struggle to meet the grade in
many deployments.
Ethernet PON (EPON), also referred to as Ethernet in
the First Mile (EFM), is an ongoing standard that uses
Ethernet protocol for packet data transport. Even with its
higher level protocols, offering 1.2 Gbits/sec symmetrical
bandwidth, it may not be enough to handle the
requirements of higher bandwidth applications.
It is the view of ADC that Gigabit PON (GPON),
standardized in 2003, will be the target for the majority
of PON migration paths while transitioning from one
PON to another for meeting higher bandwidth demand.
GPON combines the quality of service capabilities of BPON
with EPON’s ability to transport and interface on an all
IP network. It can address higher application bandwidth
requirements by offering 2.4 Gbits/sec downstream and
1.2 Gbits/sec upstream.

The promise of 1x64 split capabilities adds to the
attractiveness of GPON solutions in FTTP networks. This
enables carriers to double the number of customers served
from a single splitter in a fiber distribution hub (FDH).
PON architectures
Architectural decisions regarding any FTTP network buildout
are driven by initial and targeted take rates and a focused
design. This applies equally to overbuilds, Greenfield
applications, or even a migration network. To be profitable,
it’s imperative to understand the impact of the decisions
made early in the planning stages of the process.
The best example of how an early decision can affect
future operational costs can be found in the particular
point-to-multipoint PON architecture selected for the initial
FTTP outside plant. There are three possible scenarios and
each has advantages in certain situations. But network
architects should also be concerned with the aspects of
future-proofing the PON portion of the system to make
migrations to next-generation architectures as bandwidth
demands reach new levels.
The first PON architecture is the central switched, or home
run, whereby splitters are placed within the central office
(CO), headend, or remote terminal. A key advantage of this
design is that all changes, either in the electronics or split
ratios, can be done at one centralized location.
Next, there is the distributed or cascaded splitter
configuration which uses some combination of multiple
splitters (usually 1x4 and 1x8) at multiple locations. This
design is particularly effective in rural or widely spaced
networks where the number of customers per mile is

relatively sparse. Transitioning to a GPON in a distributed
architecture presents significant challenges since carriers
are forced to migrate every subscriber or make costly
modifications to the OSP portion of the network.
Finally, there is the centralized splitter design where a
single coupler is placed within the central hub or cabinet.
From there, the distribution fiber interfaces with the entire
splitting scheme with drop cables extending directly to
each customer. This scheme offers great flexibility for future
migration to GPON and typically involves an upgrade of
electronics at each end of the PON.
Additional considerations
There are several additional considerations when designing
a PON for ease of migration to GPON. These include the
fiber optic cable characteristics, optics classes, and split ratio
implications. We’ll briefly address each of these topics.
Fiber optic cable
Different fiber cable from various manufacturers may have
similar loss characteristics – but they may also be quite
different. The spectral attenuation refers to the loss of
a signal as a direct correlation between the wavelength
and the distance traveled. The lower the wavelength, the
higher its spectral attenuation will be.
When applied to distances and calculating the link
loss budget for a given architecture, fiber optic cable
manufacturers must specify the spectral attenuation for their
products. When designing the FTTP network, the network
engineer typically designs for the wavelength with the highest
loss characteristics. For BPON and GPON, this will always be
1310 nm, but the loss will differ between manufacturers.

When increasing split ratios from 1x32 to 1x64 or even
higher, for example, spectral attenuation will become
an important factor to consider. Additionally, due to the
phenomena of hydrogen aging, attenuation increases
in fiber optic cables as they get older. The molecules of
hydrogen atoms in the silica or glass tend to break down
over time, making the fiber less clear for transporting light
pulses. This may be an additional consideration when re-
using fiber in an overlay scenario.
PON optics classes
Optical link budgets are determined by individual vendors’ active components – PON chips within the
electronics, lasers, and receivers. The loss range for each class is as follows:
Class A – Min. 5 dB to max. 20 dB Class B – Min. 10 dB to max. 25 dB
Class B+ -- Min. 10 dB to max. 28 dB Class C – Min. 15 dB to max. 30 dB
Traditional BPON equipment has typically used Class B optics, but it was determined that some of the
PON network of 20 km were actually stretching the budget to the limits, forcing active equipment
manufacturers to increase budgets to 26.5 dB. These increased budgets, coupled with a possible
requirement to increase the split ratios of GPON, resulted in an increase in the Class B receiver photo
detectors to allow for a 28 dB loss budget – thus, establishing the Class B+ optics category.
However, despite the increase, these Class B+ optics have not escalated to the higher Class C pricing while
maintaining better PON loss characteristics. In the future, however, the need to transport greater distances
(30 km or 40 km) and even higher split ratios (1:128) could eventually force equipment manufacturers to
use the Class C optics.
With Class A optics typically associated with fiber-to-the-curb (FTTC) applications, Class B and Class B+
optics provide today’s FTTP PONs with the best reach and split ratios. However, the need to reach longer
distances could make Class C optics tomorrow’s choice – with significant cost implications.
Split ratio implications
Splitter loss depends mainly on the number of output ports on the splitter and, contrary to some
expectations, adds about the same loss whether traveling in the downstream or upstream direction. Each
splitter configuration is assigned a particular maximum split ratio loss, including connectors, defined by the

ITU G.671 standard and Telcordia GR-1209.
Since the GPON standards have not yet defined the current split ratio maximum for 1x64 splitters, network
designers must use a single 1x2 splitter interfacing two 1x32 splitters to make up the 1x64 configuration.
Although this is allowable with today’s packaging, using Class B optics only leaves 5.35 dB of “head
room.” Therefore, even with the best fiber manufactured, where the spectral attenuation is 0.31 dB per
kilometer, only a 17.25 km PON network is achievable without including any of the connectors within the
CO or the splices in the OSP.
However, the design engineer does have some options. In designing the network, premium splitters and
low loss connectors can be deployed, and fusion splices must be kept well below 0.05 dB of loss per splice.
There are other techniques ADC will be using until the standards line up with the technology for 1x64 and
higher split ratios.
ADC is working closely with customers to ensure their networks can easily and cost-effectively migrate as
higher bandwidth demand dictates. At the end of the day, making the correct initial decisions regarding
PON implementation techniques, carriers will maximize the flexibility of their networks to enable smooth
and rapid migration to the next PON level – and provide customers with many years of reliable cutting
edge services and applications.
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improving our products, ADC reserves the right to change specifications without prior notice. At any time, you may
verify product specifications by contacting our headquarters office in Minneapolis. ADC Telecommunications, Inc.
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103132AE 7/06 Original © 2006 ADC Telecommunications, Inc. All Rights Reserved
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