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.
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.
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.
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.
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.
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.
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 x 4 and 1 x 8 splitters, which form a logical 1 x 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
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
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.
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.
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)
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 exteriorpedesta), and hence must be environmentally hardened. Powering for the MTU ONT will be provided by either a local UPS or a -48 Vdc supply.
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. 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).
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:
1. 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).
2. 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.
3. 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.
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 infranstructure 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-to the-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 x 32 splitter and the ONT are located inside the apartment/ condominium building. Key elements of the system are:
1. The OLT is an Alcatel 7340 P-OLT (Packet OLT) system, which supports up to 36 PON interfaces and up to 1052 SFU-ONTs.
2. 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.
3. The Voice Gateway is the General Bandwidth G6 Packet Telephony Platform, which supports up to 3360 simultaneous calls and 26 880 ONTs.
4. The 1 x 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.
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 SDV on PON as well. Thus, initial plans to use the video overlay wavelength were abandoned in favor of SDV.
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.
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.