Broadband Optical Access Technologies and FTTH Deployment in NTT
The NTT access network encompasses NTT central offices and customer terminals and networks, and consists of transmission devices for individual customers in each NTT central office, transmission lines, and corresponding devices on the customers’ premises. Unlike trunk relay lines between NTT central offices, access networks are not designed to carry concentrated communication traffic. Since cost reduction strategies such as multiplexing and task splitting cannot be employed, it is important that the component parts are as inexpensive as possible.
Providing an access network for ordinary customers that covers the length and breadth of the nation represents an enormous undertaking. For instance, the process of upgrading facilities to alleviate congestion in telephone services necessitated a substantial 20-year investment program that started in the 1950s, when NTT was known as the Nippon Telegraph and Telephone Public Corporation. Given the scale of the task, it is especially important to develop simple, well-designed technologies and systems for the construction, operation, and maintenance of the access network.
FROM MULTI-MODE FIBER TO SINGLE-MODE FIBER
Multi-mode fiber was originally used as subscriber optical fiber. In 1988, NTT decided to introduce single-mode fiber, which had originally been developed for trunk networks, into the optical access network as well. This decision followed research into the best way to introduce the fiber into the subscriber network .
The decision to go with the potentially more costly single-mode fiber in the subscriber system, where cost considerations were paramount, was indeed a bold one, especially considering that most other countries were considering multi-mode fiber for subscriber systems. However, given the low-loss characteristics of single-mode fiber and its suitability for broadband applications, it was definitely the correct decision for the future. The introduction of single-mode fiber was made possible through the development of new technology in a number of areas. In particular, these developments related to high-precision
OPTICAL SYSTEMS AT METAL-WIRE COSTS
In 1994, NTT announced its intention to accelerate the development of optical fiber access networks and provide an upgraded communication infrastructure for future broadband expansion. To achieve the objectives set out in the published statements, it would be necessary to achieve further economies with FTTH technology. To this end, in 1995 NTT embarked on research into cost reduction initiatives, particularly in relation to distribution systems and user-end optical distribution line systems.
The newly developed technology first appeared in the optical access system,
commonly known as the p system [10], which essentially involved grafting an optical access network onto an existing metal-wire telephone infrastructure. The optical network was brought as close as possible to subscriber homes (usually to the nearest telephone pole or the outer wall of an apartment building). It was terminated at a p-ONU device carrying the traffic of 10–20 subscribers, which was then linked to the existing metal cables. In July 1996, NTT announced its plan to deploy the p system to upgrade the access network.
Optical Access Installation
We now consider optical access installation technology. By creating small holes in the fiber, it is possible to change the fiber properties [20,21]. This technology enables to develop fiber with a very small bending radius and substantially reduced bending losses. The configuration and bending loss characteristics of this fiber, which is known as Hole-Assisted Fiber, is shown in Figures 1.13 and 1.1.4, respectively. Because such a fiber could easily be bent with a radius of less than 15 mm, fiber cords containing such fiber could be laid quickly indoors in any required configuration without compromising aesthetic requirements.
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:
1. The Optical Network Terminations (ONTs), which interface the system to customers’ home telephony, data, and video networks.
2. 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.
3. The Voice Gateway (VGW), which interfaces the system to the legacy PSTN/TDM network.
4. 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.)
5. The Element Management Systems (EMSs), which interface the different network elements to SBC’s core operations network(s).
6. The ATM network, which aggregates/switches ATM traffic from multiple core networks to the OLT(s).
7. 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.
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.
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.
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.
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.
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 he 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.
PON TECHNOLOGY BACKGROUND
In this section, we review some of the key technology features of PONs that make them so
attractive for FTTH.
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.
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.
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 X 16, 1 X 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 X 32 size predominantly with the 1 X 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 X 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.
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 X 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.
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.
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.