Today's Broadband Fiber Access Technologies and Deployment Considerations at SBC

INTRODUCTION
There is no doubt that the age of Fiber to the Home (FTTH) has arrived. The fits and starts of
the past are over. In North America, every major Telecom provider and many smaller service
providers are proceeding with deployments or trials of fiber technologies, predominately
Passive Optical Networks (PONs). Most providers in Asia are also deploying or are
extremely interested in doing so. Much of this activity can be attributed to achieving
installation cost points that are competitive with alternative copper-based products that can
deliver at least voice and data. The key to the realization of FTTH is the standardization of
PON, allowing component vendors and system vendors to focus on a single solution.
Favorable regulatory policies are also helping. In the United States, the Federal Communications
Commission recently clarified rulings about FTTH to the effect that no
unbundling will be required for new deployments. This decision reflects the competitive
environment for new home developments where builders are seeking providers of full
service networks with advanced capabilities.
In this chapter we review the background of the extensive work we have done in SBC on
the path to current deployment plans. The overall effort started with substantial investment in
standards work with the FSAN group of companies. Then the opportunity arose to use PON
technology as specified by ITU-T to meet the challenge of providing an FTTH network at a
new building site, Mission Bay, in San Francisco, CA.
After the success of the Mission Bay network, SBC moved forward, with Verizon and
BellSouth, to release a joint RFP for FTTH that specified compliance with ITU-T PON
standards. This joint effort was successful in giving direction to the industry about the
desired product standards.  While SBC is planning extensive FTTH deployment, the full story of fiber deployment
also includes extensive use of Fiber to the Node (FTTN). Use of the dual platforms is
expected to provide the most cost-effective strategy for delivering a ‘triple play’ of services,
including voice, high-speed Internet access (HSI), and video. In this chapter we review the
technologies of both fiber solutions.
In the following material we will discuss the FTTH and FTTN architectures, describe
some of the key fundamentals of PON technology, review our learnings from current
deployment efforts, show how the FTTH will complement our FTTN plans, and describe the
general solutions we are working on for home networking.
2.2 FIBER-TO-THE-NEIGHBORHOOD (FTTX)
ARCHITECTURE
Figure 2.1(a) provides a high-level illustration of the SBC strategy. For greenfield areas, we
will deploy FTTH. In brownfield/overbuild areas, we will deploy a FTTN platform that
utilizes VDSL in the last mile. Both networks will support switched digital video (SDV)
employing IP as the end-to-end protocol.
2.2.1 FTTH ACCESS ARCHITECTURE
Figure 2.1(b) provides a high-level illustration of the FTTH architecture that will be
deployed in SBC. It is an integrated platform capable of providing telephony, data, and
video services to residential areas, which may include a mix of single-family homes/units
(SFUs), multi-dwelling units (MDUs), small business offices/units (SBUs), and multi-tenant
offices/units (MTUs). The system contains seven basic building blocks:

The Optical Network Terminations (ONTs), which interface the system to customers’
home telephony, data, and video networks. The Optical Line Termination System (OLT), which manages the ONTs, aggregates/
cross-connects voice and data traffic from multiple PONs/services, and interfaces the
system to core transmission networks.

The Voice Gateway (VGW), which interfaces the system to the legacy PSTN/TDM
network.

The Video OLT (V-OLT), which receives and amplifies/regenerates video signals from a
video headend and inserts local video signals. (As described below, SBC has no plans to
deploy this element.)

The Element Management Systems (EMSs), which interface the different network
elements to SBC’s core operations network(s).

The ATM network, which aggregates/switches ATM traffic from multiple core networks
to the OLT(s).

The Passive Optical Network (PON) or Optical Distribution Network (ODN), which
connects the ONTs to the OLT and provides the optical paths over which they
communicate.
Currently, the FTTH architecture is based on the ITU-T B-PON access network, which is
standardized in the G.983 series of recommendations. Eventually, it will migrate to the ITUT
G-PON network standardized in the G.984 series of recommendations.
The B-PON network is an ATM-based, integrated platform capable of providing
telephony, data, and video services to residential and small business customers over a single fiber. One feature of this network is an overlay wavelength that can be used to provide
conventional video services. While this is a compelling feature, it will not be implemented in
SBC because of our desire to have a common product suite and transport network for both
FTTH and FTTN. Instead, video over FTTH will be based on the SDV IPTV format and
will be carried over the B-PON ‘basic’ bands.
2.2.2 FTTN ACCESS ARCHITECTURE
Figure 2.1(c) provides a high-level illustration of the FTTN architecture that will be
deployed. It is an integrated platform capable of providing telephony, data, and video
services to residential customers. The access network has basically one key new network
element: the FTTN Remote Node (RN). Broadband transport/services are provided to this
element from/to the Central Office (CO) by Gigabit Ethernet (GigE) fiber; these are then
cross-connected to existing twisted-pair copper in the Serving Access Interface (SAI), and
are transported to/from the customer using Ethernet-based VDSL.
2.3 ITU-T PON STANDARDS
The Full Service Access Network (FSAN) group, which consists of 22 operators and
approximately 30 vendors from around the world, has been highly instrumental in the
development and ongoing enhancements of the PON standards. Two of the FSAN Working
Groups provided the foundation and much detailed input on B-PON and G-PON to ITU-T.
The Optical Access Network (OAN) Working Group provided input to ITU-T Question
2/Study Group 15, under which the G.983 and G.984 series were developed; the Operations and Maintenance (OAM) Working Group detailed specifications to ITU-T Question 14/
Study Group 4, under which the Q.834 series was developed.
2.3.1 ITU-T G.983 B-PON STANDARDS SERIES
Standards pertaining to B-PON have been developed and published through two ITU-T
Recommendation series: ITU-T G.983 and ITU-T Q.834. The G.983 series began with
standardization of the physical and transmission convergence layers, and of the ONT
management and control interface (OMCI). Later, standards support was added for an
overlay wavelength, dynamic bandwidth assignment (DBA), survivability, increased line
rates, enhanced security, and enhanced ONT management. The Q.834 series of recommendations
pertains to management of B-PON networks. Table 2.1 provides a listing of key
features of the G.983 and Q.834 Recommendations.
Table 2.1 Key features of ITU-T G.983.x and Q.834.x Recommendations.
ITU-T Recommendation Key features
G.983.1 Broadband optical access
systems based on passive
optical networks (PON)

Provides specifications for
155/155 Mbps and 622/155 Mbps
systems with 20 km reach
G.983.1
Amendment 1
G.983.1 Amendment 1
Extends G.983.1 to support
622/622 Mbps systems
G.983.1
Amendment 2
G.983.1 Amendment 2
Extends G.983.1 to support
1.244 Gbps downstream and
to support AES security option
G.983.2 (2002) ONT management and
control interface specification
for ATM PON

Includes ONT management/
control support for: voice (POTS);
data (MAC bridged LAN); and
video (card/port functions)
G.983.3 A broadband optical access
system with increased service
capability by wavelength
allocation

Enhanced wavelength plan to allow
WDM expansion including adding
video broadcast service

OLT downstream wavelength of
1480–1500 nm specification allowing
wavelength expansion over B-PON
G.983.4 A broadband optical access
system with increased service
capability using dynamic
bandwidth assignment

Improves upstream bandwidth
utilization

Allows flexible provisioning of
bandwidth
G.983.5 A broadband optical access
system with enhanced
survivability

Addresses protection of G.983.1
systems
G.983.6 OMCI specifications for
B-PON systems with
protection features

Enhancements to G.983.2 to support
protected B-PON systems Table 2.1 ðContinued Þ
ITU-T Recommendation Key features
G.983.7 OMCI specification for
DBA B-PON system

Specifies enhancements to G.983.2
to support DBA-capable ONTs
G.983.8 OMCI support for IP,
ISDN, video, VLAN tagging,
VC cross-connections, and
other select functions

Includes enhancements to G.983.2
to support IP, ISDN, video, VLAN
tagging, and VC-cross-connections
G.983.9 OMCI support for wireless
Local Area Network
interfaces

Enhancements to G.983.2 to support
wireless LAN interfaces
G.983.10 OMCI support for Digital
Subscriber Line interfaces

Enhancements to G.983.2 to support
DSL interfaces
Q.834.1 ATM-PON requirements
and managed entities for
the network element and
network views

Describes B-PON information model,
focusing on NML/EML interface
Q.834.2 ATM-PON requirements and
managed entities for the
network view

Describes B-PON information model,
focusing on NML/EML interface
Q.834.3 A UML description for
management interface
requirements for B-PONs

Defines part of the management aspects
for network resources
Q.834.4 A CORBA interface
specification for B-PONs
based on UML interface
requirements

Defines CORBA interface for B-PON 2.3.2 ITU-T G.984 G-PON FOR HIGHER SPEEDS
SBC began deployment with a standards-based B-PON access network. While B-PON meets
SBC’s current needs for PON, G-PON (based on the ITU-T G.984 Recommendation series)
is seen as the best direction for continued full service networks supporting IP video. Table 2.2
gives a brief overview of the G.984 Recommendations. Use of the G-PON Encapsulation
Method (GEM) protocol will allow for highly efficient delivery of Ethernet packets over GPON.
GEM utilizes flexible frame sizes to transport data and also allows frame fragmentation.
Using GEM, a header is applied to each data frame or frame fragment that is destined for or
coming from a user. This header provides information including the length of the attached
frame fragment in order to support delineation of the user data frames and a traffic identifier
used to support traffic multiplexing on the PON. When an Ethernet packet is mapped into a
GEM frame, the Preamble, and Start Frame Delimiter (SFD) bytes are stripped off and no
Inter-Packet Gap is needed. This, combined with GEM’s flexible frame size and support for
frame fragmentation, allows for efficient delivery of Ethernet-based traffic over the PON.
In addition, the G-PON protocol allows for support of native TDM over GEM along with
Ethernet packets. TDM services may also be supported on G-PON via a circuit emulation approach. Support for both Ethernet and TDM on a common access system is a powerful
combination to expand the suite of full-service network applications for G-PON. Enhancements
to G-PON are a near-term active area of work in the FSAN OAN Working Group,
with the intent to finalize in 2005 for possible 2006 deployments.
Another key aspect of G-PON is enhanced security and privacy protection using the
Advanced Encryption Standard (AES). Similar to our B-PON deployment, our network will
require the optical reach and hardened ONT options that are supported by the G-PON
Recommendations. FSAN and ITU-T continue with enhancements to the G-PON standards
to meet evolving requirements of worldwide operators.
2.3.3 THE ROLE OF STANDARDS IN INTEROPERABILITY
A goal of service providers, and a key factor in widespread deployment, is to establish
equipment interoperability that will allow a multi-vendor supply environment. Today, the
OAN group is actively working on issues pertaining to interoperability of B-PON equipment
in a multi-vendor environment and has organized a series of interoperability efforts.
In March 2004, multi-vendor B-PON interoperability was demonstrated during conformance
testing that included the TC layer, optical levels, and OMCI. Following this, in June
2004, an interoperability demonstration showing Ethernet service level interconnectivity
among ITU-T compliant B-PON systems was exhibited by FSAN members during the ITUT
All Star Network Access workshop. Multi-vendor voice interoperability over B-PON
systems was demonstrated in September 2004; four OLT vendors, eight ONT vendors, and
one test vendor participated in this event. The series of interoperability events is described on the FSAN website at http://www.fsanweb.org/news.asp and http://www.fsanweb.org/presentations/
page310.asp.
Figure 2.2 depicts the multi-vendor configuration for the voice interoperability event.
FSAN continues to develop interoperability among OLT and ONT vendors at all layers,
including the service level. The strong support for these interoperability efforts from both the
operator and vendor communities serves as an indicator of the interest within the industry in
developing and deploying standards-compliant B-PON systems capable of interoperating in
a multi-vendor environment.
The operators within the OAN group are working on a document called the Common
Technical Specifications (CTS) for B-PON systems. This document includes specifications
from the physical layer up to the services layer, and is intended to provide additional benefit
to the industry in developing systems beyond the protocol and physical layer of the G.983/
G.984 Recommendations. The development of common specifications worldwide can build
volume and lower costs for fiber access systems as well as provide additional structure to
direct future interoperability efforts.
Along with its contributions towards further enhancements to B-PON and G-PON
specifications and interoperability, FSAN continues to be a vibrant group working on the
future use of fiber in access networks. SBC has realized great benefits from the availability of
FSAN compliant access systems. The mechanism to enhance and maintain in both FSAN
and ITU-T is vital to keep the system expandable to new services in a standards-based
implementation with the level of specification necessary for interoperability. SBC has
continuously contributed to the FSAN work activities since 1997 and will continue to
work in FSAN to develop next-generation access systems.
2.4 PON TECHNOLOGY BACKGROUND
In this section, we review some of the key technology features of PONs that make them so
attractive for FTTH.
2.4.1 UPSTREAM BANDWIDTH ASSIGNMENT
A key feature of PON is the aspect of shared bandwidth, which raises the question of how
individual users will be allocated time/bandwidth on the network. Downstream allocation is
relatively straightforward because there is one transmitter and bandwidth is broadcast to all
ONTs on the PON. In the upstream direction, however, a problem of access control arises
with the multiple upstream transmitters. PON solves this problem with grants from the headend controller to each ONT. Grant timing is communicated in downstream messages to
all ONTs, which inform the ONTs of their time slots.
Utilization of the upstream bandwidth on the PON can be improved through the
implementation of Dynamic Bandwidth Assignment (DBA), which was introduced in
ITU-T Recommendation G.983.4. With DBA, the OLT monitors the upstream bandwidth
requirements of the ONTs and adjusts how it distributes grants accordingly.
G.983.4 introduced the concept of Transmission CONTainers (T-CONTs), each of which
can aggregate one or more physical queues into a logical buffer. When DBA is employed,
grants are associated with individual T-CONTs. Each T-CONT has bandwidth-related
parameters associated with it that are used in the grant assignment process. Four categories
of bandwidth are identified for DBA – fixed, assured, nonassured, and best-effort (listed from
highest to lowest priority in terms of granting). Five T-CONT types are defined with different
combinations of these bandwidth categories. Each ONT can support one or more T-CONTs;
the specific T-CONT type or combination of T-CONT types on a given ONT is tailored to
support the quality of service (QoS) requirements of the traffic flows on the ONT (G.983.4
provides a guide indicating which QoS categories are supported by which T-CONT types).
For example, T-CONT type 5 is the most flexible type, accommodating all four bandwidth
categories, and a single type 5 T-CONT on an ONT can be used to accommodate multiple
traffic flows with a variety of QoS.
There are two ‘flavors’ of implementing DBA – idle cell monitoring and status reporting.
In idle cell monitoring, the OLT monitors how many idle cells are being sent from each TCONT.
In status reporting, the ONTs send reports to the OLT regarding the queue status/
length of each T-CONT. The OLT then adjusts the allocation of grants based on the
information it obtains regarding the T-CONTs. Particularly for scenarios where heavy
utilization of the PON is found, it is expected that status reporting provides some advantages
over idle cell monitoring in aspects such as cell delay. As such, deployment scenarios with
heavy utilization involving MDUs and small businesses, for example, would be expected to
benefit from implementing the status-reporting method of DBA.
2.4.2 RANGING
The physical distance between the OLT and the ONTs on the PON varies, which means that
signals require different times to get to and from the different ONTs. A technique called
ranging is used to adjust the timing between each ONT and the OLT. The ranging protocol in
ITU-T Recommendation G.983.1 allows placement of an ONT anywhere within a 20 km
distance from the OLT, providing flexibility in ONT placement in the ODN. To initiate
ranging, the OLT sends a specific grant to the ONTs to trigger the ranging process and opens
up a window during which it can receive ranging information from the ONTs. Upon receipt
of this grant, an ONT sends a ranging cell back to the OLT. Based on the elapsed time
between when the OLT sends the ranging grant and when it receives a ranging cell from an
ONT, the OLT can determine the appropriate equalization delay to assign to that ONT.
2.4.3 SPLITTERS
The splitter can be considered a defining feature of PON, since it is the key technology that
allows the access network to be electrically passive. A major cost advantage of PON is the reduced fiber requirements versus a point-to-point architecture with fiber direct from the CO
to each home. This cost reduction is achieved using the splitter to take one fiber from the CO
and serve up to 32 homes in the SBC network.
Significant improvements in splitter technology have occurred in the last 4 years,
including improvement/advances in optical performance, reliability, and cost per port.
These advances contributed to the selection of the PON topology for FTTH at SBC.
Today, the performance of splitters has reduced excess loss to 1–1.5 dB above the ideal loss
of the device and nonuniformity to less than 2 dB over a wide wavelength range and wide
temperature range while achieving satisfactory cost per port.
Advances in fabrication and packaging technology for passive fiber splitters were driven
by market demand for increased optical performance in CATV fiber distribution and optical
networking applications. Reducing splitter excess insertion loss and uniformity of loss
variation across all ports focused supplier investment in large port size (1  16, 1  32)
devices using the planar lightwave circuit (PLC) technology. PLC fabrication involves
creating optical waveguides in a planar substrate such as silica to form a splitting function.
SBC has selected the 1  32 size predominantly with the 1  16 size a second option when
additional fiber reach is required.
PON deployments in Japan were increasing and creating a larger market for splitters for
PON applications. Industry leading suppliers provided improvements in reliability assurance
programs to meet the requirements for splitters placed in the outside plant environment
where temperature and humidity are not controlled.
SBC evaluated the performance of splitters fabricated using the fused biconical taper
(FBT) process and the PLC process starting in 2000. The FBT fabrication process involves
drawing two or more optical fibers together under heat and pressure to achieve the
appropriate coupling ratio. Splitters with larger sizing are made by joining multiple 1  2
devices in a cascading fashion and providing a larger package size than PLC devices. We
review the assessment of splitter optical performance and reliability collected since 2000 in
advance of our early FTTH deployments, trials, and planned rollouts of FTTH.
2.4.3.1 Splitter Performance
SBC splitter requirements for loss and uniformity span the three wavelength bandpass
regions designated for ITU-T G.983.3 B-PON systems, including a WDM overlay option for
video signaling. The three bandpass regions have center wavelengths of 1310, 1490, and
1555 nm. The splitter optical performance is dependent on the splitter fabrication technology.
Figure 2.3 illustrates the optical performance of two different 1  32 devices, one
fabricated with a PLC process and the other with a FBT process. The results illustrate the
variation of loss over the bandpass of interest for PON systems.
Each line represents the loss from the single input to one of the 32 output ports of the
device. While the FBT device loss shown here does provide low loss windows centered on
the commonly used 1310 and 1550 nm bands, the PLC device is more uniform over the
bands and up to 1640 nm. A fiber access network infrastructure with a uniform loss across a
large wavelength range simplifies the optical test and acceptance of the fiber network.
Evolution strategies to additional wavelength bands in the future are simplified by the
selection of PLC-based splitters with loss that has very low dependence on the wavelength. A PON installation with nonuniform loss and wider variance in loss with wavelength
complicates planning due to uncertainty in the loss for any new wavelengths being added to
the PON in the future. SBC has found selection of PLC to be advantageous as we plan for
future WDM expansion on our PON deployments.
2.4.3.2 Splitter Reliability
Splitter devices placed within the SBC footprint require reliability under environmental
extremes ranging from the elevated summer heat and humidity in Southern Texas to the low
winter temperatures in Northern Michigan. Based on a comprehensive review of environmental
and mechanical testing results from several PLC providers, we found that early issues
of reliability with certain PLC devices were no longer a fundamental concern. Reliability
results from PLC suppliers verified the availability of splitters with the required robustness
for placement in uncontrolled environments.
2.4.3.3 Splitter Conclusions
Splitter evaluations have provided SBC with reliable and cost-effective devices achieving
excellent uniformity and low loss over the contiguous bandpass from 1260 to above
1600 nm. Splitters fabricated with PLC technology are the superior choice over splitters
made with FBT technology, and PLC-fabricated splitters were selected for our 2002
construction of our first B-PON deployment in Mission Bay and SBC continues to deploy
only PLC splitters in B-PON deployments.
2.5 THE SBC FTTH NETWORK
Key characteristics contributing to the success of the SBC FTTH network (Figure 2.1(b)) are
the triple play of services transported by the network, detailed design of the optical fiber/
distribution network, and the availability of a family of ONTs optimized for different
applications. 2.5.1 THE OPTICAL FIBER/DISTRIBUTION NETWORK
The design of the optical fiber network is dependent on the transmission system planned for
the desired services. Video service design can have a big impact on the fiber network. SBC
initially intended to use the video overlay wavelength, and we discuss some of the
challenges of designing and constructing a PON for that approach so that other providers
can potentially benefit.
In 2001, our direction was to implement a video service using the readily available video
headend equipment and Set-Top Boxes (STBs) used in CATV networks. The video service
system transmits a multi-channel signal with a mixture of both analog and digital modulated
RF carriers. The transmission of analog video would allow a video service to be provided
without a digital STB at the televisions in the residence.
The video signal is broadcast downstream on the PON on a separate wavelength band
compliant to the ITU-T G.983.3 specification. Support for analog video over a PON network
with 32 splits and sufficient optical reach requires systems supporting the Class B optics
specified in G.983.3 to provide an optical budget of 25 dB. To achieve maximum reach and
contain cost of the analog video transport equipment, the passive optical network had to be
built with products and methods for loss control not required of digital transmission systems
commonly being deployed in other SBC fiber networks. Analog modulated RF carriers for
video transmission were well known in the CATV industry to require the control of optical
loss and optical reflection. Papers published in the early 1990’s detailed the issues as the
emerging Hybrid Fiber Coax (HFC) networks were being designed and evaluated.1
Extending analog video to a PON with significantly greater optical budget than HFC
networks was needed. SBC developed fiber design and construction guidelines for a passive
optical network capable of supporting analog video transmission.
The design and construction methods to support analog video over a passive optical
network require consideration of optical loss control, loss variation control, and reflection
control. We report on the successful analog video service trial delivered over the SBC
deployment in San Francisco using fiber products and methods enabling analog video
transmission over a PON.
2.5.1.1 Loss Control
Operators deploying passive optical networks must consider fiber products and construction
methods that lower the fiber network losses and provide sufficient reach. Fiber products with
reduced optical loss include the following: lower loss optical splitters, low loss fiber cable,
lower loss fusion splicing rather than mechanical splicing, and low loss fiber connectorization
products. Construction methods to reduce optical loss include minimizing the use of
fiber connectors, enhanced training to clean and inspect fiber endfaces for lower connector
mating loss, and fiber management practices to reduce cabling loss from excessive bending
in closures and cabinets.
Testing end-to-end loss within the required range of the PON system is a necessary
verification of the fiber network before service activation. Three loss control guidelines promoted to meet the requirements are the following: splicing using fusion techniques only,
greater attention to connector cleaning and inspecting, and the specification of lower loss
splitters.
SBC studies have found fiber reach to be insufficient for active sites for trials and planned
deployments using 32-way splitters. The reach limitations occur even with greater attention
to additional loss control measures undertaken for PON when compared to point-to-point
fiber systems used. B-PON reach with 32-way splitters becomes limited at distances
exceeding 10 km and well short of the 20 km reach available by the ranging protocol in
B-PON systems. Surveys of new housing developments have found 20 % of potential FTTH
locations to be in the range of 10–20 km. Extending the reach to support these longer loops
from the CO has become a significant issue in the use of FTTH to new housing
developments. New developments in the SBC footprint are typically found in the undeveloped
regions of cities which are further from existing central offices in the older part of a
city. Improvements in the optical budget from advances in optical devices have occurred to
provide on the order of 1–2 dB in recent years. SBC has specified Class B optics with a
25 dB maximum budget with enhancements to as high as 28 dB.
However, further advances in budget cannot be expected without cost impacts. SBC
expects an ongoing requirement for extending reach, and several extended reach alternatives
are considered. The two design approaches for extended reach include using 16-way splitters
with lower splitter loss and applying budget for greater fiber reach or placing remote OLT
cabinets. SBC plans to place remote OLTs due to the higher cost penalties from a lowered
split to 16-way for the larger number of homes in new builds in our region. Implementation
of loss control measures continues to a significant issue to support the deployment of PON
throughout the region.
2.5.1.2 Loss Variation Control
Loss variation control is a unique requirement for RF-video fiber transmission with analog
modulated signaling. Signal levels arriving at all ONTs sharing the PON must be within the
dynamic range of the RF video optical receiver to provide adequate video quality. Figure 2.4
illustrates the key methods for the passive optical network to support analog video. The
design of the distribution area fiber network to minimize loss variation includes the
following: using optical splitters with enhanced uniformity over all outputs, minimizing
the number of optical connectors that can each contribute to higher loss variation, and
limiting a PON to a single distribution area thereby limiting the differential length to an area
which is commonly 1 km but not more than 2 km in SBC. The optical loss testing and
verification after fiber construction, prior to services delivery, will ensure that an analog
video signal can arrive to each ONT on a PON.
Several methods, including the method to test for loss and install optical attenuators at
specific points to lower loss variation in the network during construction, were detailed in a
paper provided at the NFOEC in 2003.2 The combination of analog-friendly PON design and
construction guidelines, deployment of low uniformity splitters, amplifiers with low
variation in power, and video receivers with wider dynamic range provide a viable approach
to delivering analog video signals in the operating range to each ONT. 2.5.1.3 Reflection Control
Methods to control reflection in fiber joints are readily available by using fusion splicing
exclusively and angled end-face fiber connectors. The target for reflectance of any fiber joint
in the fiber network to support analog video was 50 dB to eliminate elevated noise from
multi-path interference that degrades the viewing quality of analog video systems. Mechanical
splicing can produce elevated optical reflections worse than 50 dB that occur at
construction or degrade during environmental exposure in the outside plant. Angled fiber
connectors have highly repeatable reflectance of <50 dB with very low possibility of
degradation even during exposure to hostile environments. In practice, angled fiber
connectors’ failure mechanism is elevated loss and not elevated optical reflection due to
contaminated endfaces or separation and loss of contact.
Operators who choose to provide analog video service over a PON network can select
fusion splicing and angled connectors to control optical reflection, eliminate one potential
source of video picture quality degradation, and simplify optical layer troubleshooting and
live optical testing on a working PON fiber system. Use of mechanical splicing and/or
nonangled fiber connectors for an analog video service delivery over a PON will impose a
greater need to test with OTDR to verify the control of reflection.
2.5.1.4 Optical Fiber Network Results at Mission
Bay Deployment
The SBC Mission Bay deployment was the first deployment of PON at SBC (Mission Bay
described further below). The Mission Bay fiber network was constructed using the optical
design and construction methods developed by SBC to ensure an analog video transmission
capability. Fiber connectors were used only at the CO location and the ONT connection inside the living unit at the ONT location. No connectors were deployed at the splitter
location or at the building entrance location in the high-rise building. The design used only
fusion splicing and only angled fiber connectors (SC/APC) to minimize optical fiber
reflections from the video headend to the serving office and to the residence. End-to-end
loss was tested and recorded from the CO cable termination to each ONT location. The
range of losses measured was 19.9 to 17.1 dB over a total fiber distance of 2.2 km. The
variation in loss was 2.8 dB for the first building constructed with FTTH in the Mission Bay
deployment. The low variation was achieved by using high-performance splitters with low
loss variation between the 32 output ports, by limited use of fiber connectors, use of core
alignment splicers, troubleshooting measures to locate and repair excessive losses, and the
attention to fiber cleaning including inspection of fiber endfaces. The network losses were
tested and verified, and repairs done in advance of any services applied to the fiber network.
The superior results for loss and loss variation were obtained with a skilled fiber construction
crew and additional attention to transmission to analog video. The use of angled connectors
and fusion splicing minimized the concerns over multiple optical reflections as verified by
low optical return loss and back reflectance measurements taken at Mission Bay. After voice
and data services were operational, the video wavelength was inserted into the working fiber
network at the central office using a previously installed WDM coupler. No voice and data
services were impacted during the insertion of the 1550 nm video overlay signal and video
service activation. Video service quality for the analog signals was measured and verified to
meet the analog and digital video service quality requirements. No adjustment to the optical
network to adjust the optical level reaching the ONT optical receiver was required to be in
the operating range of the video receiver. Our experience in Mission Bay showed that proper
control of loss, loss variation, and reflection on the fiber network can successfully deliver
analog video services over a passive optical network.
2.5.1.5 Evolving Optical Design at SBC
Since the Mission Bay deployment, SBC has given greater attention to lowering construction
costs of FTTH. The Mission Bay successes in fiber network loss control and loss variation
control were largely achieved due to the use of construction crews with previous experience
in fiber handling, and the extra troubleshooting time to find and repair excessive losses in
Mission Bay. SBC is investigating methods and products with improvements in fiber
handling to allow a reduction in the construction costs for FTTH. A key construction cost
driver for SBC is the cost of fusion splicing in the distribution area. An approach to lowering
splicing cost was the introduction of additional fiber connectors in the FTTH trials following
our first deployment in Mission Bay. SBC has trialed a fiber cross-connect cabinet where the
PON splitters are placed.
The cabinet, which is pictured in Figure 2.5, is called a Primary Flexibility Point (PFP).
The PFP serves as a single point for multiple splitters and serves a typical distribution area of
200–400 residences. In comparison, some operators are using separated 1  4 and 1  8
splitters, which form a logical 1  32 total splitting ratio. The PFP allows for higher
utilization of the splitter and attached CO electronics since each new residence taking the
PON services can be sequentially added using the fiber jumper flexibility in the PFP. Each of
the 32 PON splitter outputs can then be dedicated and filled for the first 32 customers taking
the service in the distribution area. In this design, troubleshooting is enhanced with fiber connector access for test equipment
insertion which can be necessary to locate faults towards the subscriber from the PFP
location. The PFP concept also has the advantage of allowing simplified replacement of
splitters at one centralized location.
SBC has concluded that the SC type connector is the superior connector type with the
best reliability when exposed to testing consistent with placement in the hostile environments
in the PFP and near the residence. Smaller form factor fiber connectors would be an
advantage over the SC connector due to the smaller size PFP, but must have improvements
in reliability. Future fiber connector improvements in physical size, reliability, and better
immunity to airborne contaminants are needed to keep connectors a benefit and not a
liability for network reliability. SBC now specifies the Ultra-Polish Connector (UPC)
endface polish for FTTH deployments planning. The SC/APC connector with an angled
endface is no longer an SBC requirement to eliminate fiber reflections from connector
pairs which is consistent with the SBC removal of analog video as a requirement for
delivery going forward.
2.5.1.6 Optical Network Summary
SBC developed design and construction guidelines for the optical distribution network for
FTTH deployments that supported the transmission of analog-modulated video signaling.
The results from the Mission Bay deployment showed an analog video service can be
delivered successfully over a PON network with proper attention to the guidelines developed
by SBC. For PON networks without RF-video signaling, the requirements for loss variation
and reflection control are greatly relaxed, and loss control becomes the primary design
concern. The relaxation of the optical network design and construction requirements without
analog video will be leveraged by SBC to reduce the deployment costs for FTTH in the
future. 2.5.2 FTTH ONTs
The ONT is one of the highest cost components of the FTTP system because it is located at
the customer end of the loop and is thus shared by the fewest customers. In addition, it
determines to a great extent the type and quality of service available to the customer. A
number of different ONT types for different applications could be specified, ranging from a
single-family residential ONT that provides two voice and one data ports to a multi-tenant
business ONT that provides multiple voice, data, and video service ports. It is unlikely that
any company will deploy all the different types of ONT because inventory-management/
volume-discount issues prescribe a smaller number of types. Table 2.3 provides a list and
description of the ONTs that are planned for use in SBC.
2.5.2.1 SFU ONT
The SFU ONT is intended for use in residential applications with single family/detached and
small (e.g., 2–4 unit) multi-dwelling/attached homes. It will provide, as a minimum, four
POTS interfaces and one 10/100-bT Ethernet interface. (Current versions of the ONT also
provide a coax interface intended for video over the B-PON overlay wavelength, but this will
not be used/required in future designs.) The SFU ONT is environmentally hardened and will
be installed on the outside of the home – replacing the current passive Network Interface
Device (NID). Powering for the SFU ONT will be provided locally by an DC Uninterruptible
Power Supply (UPS), which will be installed inside the customer’s home or garage.
Table 2.3 FTTH ONT types and characteristics.
Service interfaces
ONT/ONU type POTS Data Video DS1 Description
Single-Family Unit (SFU)
SFU:ONT (Triple Play) 4 1 Ethernet 1 – Hardened. Maximum dimensions
1300  1300  600.
SFU-P-D:ONT 4 1 Ethernet – – Locally powered by a separate
12-Vdc UPS, with min backup
for 4-hr POTS & 1-hr data.
Multiple-Dwelling
Unit (MDU) MDU:ONT 24 12 Ethernet 1 – Locally powered. Data to be
and/or xDSL supplied in modular units of 4.
Small Business Unit (SBU)
SBU-V:ONT 8 1 Ethernet 1 2 Hardened. Locally powered by a
separate 12-Vdc PSU. Number of
Service Interfaces indicated are
minimum values.
Multi-Tenant/Shared
Business Unit MTU:ONT 24 8 Ethernet 1 4 Hardened. Locally powered.
Number of Service Interfaces
indicated are minimum values.
THE SBC FTTH NETWORK 33
2.5.2.2 MDU ONT
The MDU ONT is intended for use in apartment complexes, condominiums, and townhouses
that contain five3 or more living units and house long-term residents. (In the future, these
ONTs may also be used for short-term resident applications, such as university dormitories
and hotels.) Each MDU ONT will be capable of serving 12 living units, and provide a
minimum of 24 voice interfaces, 12 VDSL interfaces in modular units of 4, and 1 RF video
interface with addressable tap. MTU ONTs may be installed in several different types of
locations (e.g., inside a communication closet or terminal room, on the outside building wall,
or in an exterior pedestal/enclosure), and hence these ONTs must be environmentally
hardened. Powering for the MTU ONT could be provided either by a local DC UPS or by an
existing 48 Vdc supply (e.g., in existing buildings).
2.5.2.3 B-ONT
Two types of B-ONTs are required for the FTTP system: the SBU ONT, which is intended to
provide service to one business; and the MTU ONT, intended to provide service to four or
more small businesses. The SBU-ONT is similar to the triple-play SFU ONT, in that it is
intended to provide triple-play services for use by one small/home office. It will provide, as a
minimum, eight POTS interfaces, one 10/100-bT Ethernet interface, and two DS1 interfaces.
Like the SFU ONT, it will be environmentally hardened for installation on the outside of the
office/home (replacing the current passive NID), and will be powered locally by a DC UPS,
which will be installed inside the home/office. The MTU ONT is intended to serve four to
eight small businesses, and will typically be located in a small business park or strip mall. It
must provide, at a minimum, 24 POTS interfaces, 8 10/100-bT Ethernet interfaces, and
4 DS1 interfaces. Like the MDU ONTs, the MTU ONT may be installed in several different
types of locations (e.g., communication closet, terminal room, exterior wall, or exterior
pedesta), and hence must be environmentally hardened. Powering for the MTU ONT will be
provided by either a local UPS or a 48 Vdc supply.
2.5.2.4 FTTH ONT Powering
While one of the advertised advantages of the PON architecture is a wholly passive plant, in
actuality power must be provided to the ONT located at the customer end of the network.
Power has long been considered the ‘Achilles’ heel’ of fiber to the home because the same
fiber that brings megabits per second of information to a customer separates that customer
from the typical ‘always on’ power plant familiar from standard POTS service.
Over the years, many powering schemes have been proposed, tested, and deployed. In
general, these can be categorized into ‘centralized’ and ‘local’ powering schemes (Figure 2.6).
In centralized powering schemes, power is provided to several/many ONTs from a central
network site such as a CO, RT, or remote power node. The primary source is generally
commercial AC, rectified and converted to DC at the site, and the backup source is typically
batteries and engine generators. Both primary and backup power are transmitted to the ONT
over metallic media, typically conventional twisted copper pair. Variations to the basic centralized power scheme that have been explored over the years include use of solar energy,
wind energy, and fuel cells as the primary source, use of flywheel energy storage and various
new/evolving battery technologies as the backup source(s), and even the use of the fiber as
the power transport medium.4
In local powering, power is provided to an ONT from its own dedicated source, which is
located near/at the ONT. The typical primary source is again commercial AC, rectified and
converted to low-voltage DC by the source/supply, and the typical backup source is batteries.
Variations to this basic scheme include use of solar energy and fuel cells as the primary
source, and use of flywheel energy storage, mechanical power converters, and various new/
evolving battery technologies as the backup source(s).
2.5.2.4.1 Recommended Powering Architecture
Centralized powering can provide high power reliability and can be easier to maintain and
operate than local powering. However, it experiences much high power loss (e.g., transmission
loss and multiple power conversions), presents a single point of failure, and mitigates/
removes many of the reliability, maintainability, and operational advantages of an all-passive
optical network. Because of this, SBC chose a local powering architecture for its FTTH
deployments. This architecture is illustrated in Figure 2.7 for the SFU ONT.
As indicated in the figure, the ONT will be powered from a DC UPS, which can be located
as much as 100 feet away from the ONT. The UPS will provide low-voltage DC power to the
ONT; obtain primary input power from a commercial 120 VAC power connection in the customer’s premises; and obtain secondary/backup input power from a rechargeable battery
located within the UPS housing. Key features of this powering scheme include:

The power supply and batteries will be located inside the customer’s home in a more
weather-controlled environment to enhance battery capacity and life (SBC is currently
investigating a hardened power supply to facilitate installation).

During AC power outages, a power-down scheme is used to disable nonessential services.
This will promote longer life for voice services as the backup battery will last longer.

The UPS will alert the customer of various powering events (i.e., an AC power outage and
a missing, failed, and discharged battery) to help ensure uninterruptible powering of the
ONT.
2.5.3 SBC’S MISSION BAY TRIAL
Mission Bay is a 4-billion dollar redevelopment project in San Francisco, CA that will
convert over 300 acres of former landfill (from the 1906 earthquake/ fire) and rail-yards into
a virtual ‘city in a city.’ The development is located south of downtown and is equal in size
to San Francisco’s entire downtown business district.
At completion (expected to take 10–20 years), Mission Bay will include 6000 residential
housing units, 6 million square feet of office/life science/technology commercial space, a
new University of California research campus, 800 000 square feet of retail space, a 500-
room hotel, 49 acres of public open space/parks, a new public school, and new fire and
police stations. The project was spearheaded by the Catellus Development Corporation, who envisioned
an innovative, state-of-the-art community supported by a ‘broadband technology infrastructure
that will provide homes with voice, video, and data’ (Catellus).
To provide the broadband infrastructure, Catellus issued a competitive RFP to telecommunications
carriers/providers in 1999.
Pacific Bell/SBC won the proposal, in large part because of its offer to provide a ‘fiber-tothe-
home/apartment’ technology to the residential units. On the basis of the RFP, SBC and
Catellus signed a comarketing relationship in January 2002, in which SBC is the preferred
provider for all voice, high-speed internet access (HSI), and video services for Mission Bay.
Figure 2.8 shows the FTTH system that was presented to Catellus and is now deployed in
Mission Bay. It is the same as the generic B-PON FTTH architecture discussed in previous
sections, except that the 1  32 splitter and the ONT are located inside the apartment/
condominium building. Key elements of the system are:

The OLT is an Alcatel 7340 P-OLT (Packet OLT) system, which supports up to 36 PON
interfaces and up to 1052 SFU-ONTs.

All ONTs used to date are the Alcatel 7340 H-ONT, which supports up to four separate
POTS interfaces, one 10/100baseT Ethernet interface, and one RF-video interface. In the
future, MDU-ONTs may be used in some installations; these ONTs will serve up to 12
apartments/condominiums and provide POTS, VDSL, and RF video interfaces.

The Voice Gateway is the General Bandwidth G6 Packet Telephony Platform, which
supports up to 3360 simultaneous calls and 26 880 ONTs.

The 1  32 splitters are housed inside the buildings in a cabinet provided by Tyco. The
splitters are made by NEL of Japan. A single fiber is routed from the splitter cabinet to
each living unit using fusion splicing and a single SC/APC connector in each living unit to
connect to the ONT. Residential voice and data service over B-PON in Mission Bay launched in April 2003.
Since then, growth in these services has followed the Mission Bay build and occupancy
rates. As of October 2004, there were about 500 voice lines on B-PON in Mission Bay, with
a penetration rate of over 80 % for the HSI access service.
Video services in Mission Bay are not currently offered over the B-PON system. However,
SBC performed a limited technology trial of these services using the video overlay
wavelength from June 2003 through July 2004. In the trial, video services were provided
only to customers in one 32-unit condominium. Services included analog and digital video,
interactive and broadcast video (over 300 broadcast channels were offered including local,
commercial-free digital music, Pay-per-View movies and sports, and digital premium and
multiplex channels), and Standard-Definition (SD) and High-Definition (HD) video. The
trial was very successful and demonstrated/verified the vast potential of the B-PON video
enhancement band.
2.6 SBC FIBER TO THE NODE (FTTN) NETWORK
The SBC plan will make FTTN the dominant triple-play network in terms of homes served.
This is due to economic evaluation favoring FTTN and the convergence of several technologies
allowing the support of video. These technologies include advanced video compression,
standard VDSL, and carrier class Gigabit Ethernet.
The fiber feed for FTTN is Gigabit Ethernet. Gigabit Ethernet meets the bandwidth
requirements for feeding video to up to 200 homes, allowing a lower cost network for video
distribution. At the node, there is a VDSL DSLAM that handles switching of all the video
and other services to DSL ports that supply VDSL to the home at distances up to about 5000
feet on a single pair.
The DSLAM implements IGMP processing to allow replication of channels to homes for
a single video stream on the GigE link. On the VDSL link, packet mode is also used, so ATM
has been eliminated from the system. This lowers cost and eliminates unused overhead.
The other technology advancement that makes the whole solution viable is advanced
video coding, reducing the total bandwidth of an HDTV channel eventually to perhaps
6 Mbps. This allows the support of four channels, with one or even two HDTV streams, over
the VDSL link with a bandwidth between 20 and 25 Mbps.
The planned deployment of FTTN did end up having a significant impact on the SBC
FTTH solution. FTTN dictated the use of Switched Digital Video (SDV) in that architecture
due to the approximate 20–25 Mbps of bandwidth available. In order to have a single video
solution, it was determined that the best option was to also use SDVon PON as well. Thus,
initial plans to use the video overlay wavelength were abandoned in favor of SDV.
2.7 THE HOME NETWORK
The final stage in the delivery of services for both FTTH and FTTN is the home network. A
major goal of all this high-speed networking of course is the delivery of triple-play services,
including a full complement of entertainment video. When that video arrives at the house as
high-speed data, some new solutions are called for. However, we desire to use standards and
industry trends as much as possible An advantage of more or less simultaneous implementation of FTTH and FTTN is that
similarities in home networking can be optimized. Thus, the two solutions have the same
design once the respective physical layer is terminated. The full solution set for a FTTH
subscriber who has video service and high-speed data is to deliver all the traffic out of the
100 Mbps Ethernet port on the ONT and run it over CAT5 to the Residential Gateway (RG).
The RG then routes the traffic either to a STB or to a PC on the home LAN.
The solution for FTTN is exactly the same after the data is delivered to the RG. The only
difference is that FTTN of course will have VDSL as the physical layer input to the RG and
this is carried from the telephone interface into the house via CAT3 or COAX.
The demands of distribution from the RG are challenging. Communication to multiple PC
locations may be required as well as multiple TVs. SBC plans to provide service for up to
four TVs and the bandwidth needed for video is high. A key element is to minimize cost by
re-using existing inside wire if at all possible, so this eliminates approaches like running all
new CAT5. Unfortunately, no existing wireless scheme works well enough for video and a
wired solution is a must.
For video distribution we will use the technique for Ethernet over COAX promoted by the
Multimedia over COAX Alliance (MoCA). This supports the bandwidth required, and in
many cases, the wiring to the TV location is already in place. Of course, if the customer
desires the TV at a new place, some wiring may have to be done.
For data distribution the best bet seems to be a combination of 802.11 wireless and HPNA,
which reuses existing telephone twisted pair.
The home architecture will further support VoIP, allowing full conversion of all services
to IP. VoIP traffic will be given the highest priority for both downstream and upstream
handling.
2.8 MOTIVATING THE NEW NETWORK – IPTV
A fundamental goal for building these new network capabilities is to give consumers new
options in video entertainment delivery, in particular a full offering of digital entertainment
TV carried in IP packets throughout the network. We review here some of the additional
basic network features that support this exciting new way of video delivery.
Multi-casting is a key feature the network must support, even as planning allows for
substantial migration to video-on-demand (VOD) as customers expand desires to view what
they want when they want. To conserve bandwidth for basic TV service, the network should
carry only a single channel as far as possible from the acquisition point to the subscriber. To
support this, each node in the path needs to be multi-cast enabled. This includes at least four
points in a typical case: the home router, the Access Node, the first aggregation switch, and
the first router. This would allow two or more TVs in the home to be watching the same
channel and only one version of that channel appears on the SBC network.
With the multi-casting approach one of the main concerns is sizing each component for
the required number of multi-cast streams. Special consideration may be needed when this
number exceeds 4–8 per home, which can easily be the case when supporting multiple TVs.
Managing quality of service (QoS) for video is also key. The network is multi-service of
course, carrying voice and data as well as video. The video needs to get priority treatment
over the data and in such a way as to maintain a very high-quality viewing experience by
the customer. This QoS can be supported with appropriate Ethernet tagging resulting in high-priority treatment and excellent loss, jitter, and delay results. Multiple queues for video
in the Access Nodes for handling normal video versus VOD will also allow better guarantees
for the most watched programs.
The separation of services via VLAN tags is also important to overall service control.
Options for VLAN assignment include per service and per customer. In per service VLAN
tagging for example, IPTV would have one VLAN assignment and VoIP a different one but
all traffic of a given type to the home would have the same assignment.
More complicated models also allow per service VLANs to be mapped to per subscriber
assignments at different layers in the network.
Finally, to enable IPTV an appropriate set-top box (STB) is required. IP STBs have
emerged on the market, but the selected one must run the appropriate middleware and
applications for the service as well as handling the selected video compression coding. For
example, STBs able to support HDTV with MPEG-4 are early stage at this time though now
becoming available.
SUMMARY
In this chapter we have reviewed the experience and plans for extending fiber in SBC to
provide customers with choices for advanced services. It is truly an exciting time for SBC
and the industry as a whole, with the ultimate payoff being great new options for customers.
Customers will be served by FTTH or FTTN and be offered the same or similar services.
These networks will meet the demands of customers for more advanced television, including
HDTV and VOD, as well as growing desires for higher speed data and IP services.