Atm Passive Optical Networks (pons)
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Asynchronous Transfer Mode (ATM) Passive Optical Networks (PONs) Definition This tutorial discusses the economics, operator and customer benefits, and technological development of optical distribution networks with asynchronous transfer mode passive optical networks (ATM PONs). ATM–PON infrastructure is widely cited by telecommunications carriers and equipment vendors as potentially the most effective broadband access platform for provisioning advanced multimedia services as well as legacy services such as tier 1 (T1). Since 1995, an influential group of worldwide carriers and equipment vendors has been developing requirement specifications for a full-service access network with ATM PON as the core technology.
Overview The deployment of fiber-optic technology to homes and businesses is poised to change the way telecommunications services—primarily voice, data, and video services—will be delivered to the twenty-first century, information-based economy. Interest is high among business and residential consumers for advanced, broadband services such as fast Internet access, electronic commerce, video on demand, digital broadcasting, teleconferencing, and telemedicine, among others. However, the lack of available bandwidth to deliver these services effectively to the last mile of homes and businesses has stifled development of new multimedia applications. An optical distribution network with ATM PON as the core technology promises benefits to end users as well as carriers and service providers. When optical network access is achieved in scale, businesses and consumers will realize opportunities for advanced services at relatively low costs. Because of cost savings inherent with the ATM–PON platform, telecommunications carriers and service providers will realize efficiencies in provisioning future applications and upgrading bandwidth to satisfy customers' demands.
Topics 1. The Case for Fiber-Optic Access 2. How ATM PONs Work Web ProForum Tutorials http://www.iec.org
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3. Benefits of ATM PONs 4. Technology Comparison with xDSL 5. Full-Service Access Network Initiative 6. Major Players 7. The Future of ATM PONs Self-Test Correct Answers Glossary
1. The Case for Fiber-Optic Access Fiber-optic technology, offering virtually unlimited bandwidth potential, is widely considered to be the ultimate solution to deliver broadband access to the last mile. Today's narrowband telecommunications networks are characterized by low speed, service-provisioning delays, and unreliable quality of service. This limits the ability of a consumer to enjoy the experience at home or the ability of workers to be efficient in their jobs. The last mile is the network space between the carrier's central office (CO) and the subscriber location. This is where bottlenecks occur to slow the delivery of services. The subscriber's increasing bandwidth demands are often unpredictable and challenging for telecommunications carriers. Not only must carriers satisfy today's bandwidth demands by leveraging the limits of existing infrastructure, they also must plan for future subscriber needs. A new network infrastructure that allows more bandwidth, quick provisioning of services, and guaranteed quality of service (QoS) in a cost-effective and efficient manner is now required. Today's access network, the portion of a public switched network that connects CO equipment to individual subscribers, is characterized by predominantly twisted-pair copper wiring. Fiber-optic technology, through local access network architectures such as fiberto-the-home/building (FTTH/B), fiber-to-the-cabinet (FTTCab), and fiber-tothe-curb (FTTC) offers a mechanism to enable sufficient network bandwidth for the delivery of new services and applications. ATM–PON technology can be included in all these architectures, as shown in Figure 1.
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Figure 1. ATM–PON Architectures
In general, the optical section of a local access network can either be a point-topoint, ring, or passive point-to-multipoint architecture. This tutorial focuses on the passive point-to-multipoint architecture (PON). The main component of the PON is an optical splitter device that, depending on which direction the light is traveling, splits the incoming light and distributes it to multiple fibers or combines it onto one fiber. The PON, when included in FTTH/B architecture, runs an optical fiber from a CO to an optical splitter and on into the subscriber's home or building. The optical splitter may be located in the CO, outside plant, or in a building. FTTCab architecture runs an optical fiber from the CO to an optical splitter and then on to the neighborhood cabinet, where the signal is converted to feed the subscriber over a twisted copper pair. Typically, the neighborhood cabinet is about 3 kft from the subscriber's home or business. FTTC architecture runs an optical fiber from the CO to an optical splitter and then on to a small curb-located cabinet, which is near (typically within 500 ft) to the subscriber. It is then converted to twisted copper pair. The PON can be common to all of these architectures. However, it is only in the FTTH/B configurations that all active electronics are eliminated from the outside plant. The FTTCab and FTTC architectures require active outside-plant electronics in a neighborhood cabinet or curb. This tutorial will focus on FTTH/B architectures. When fiber is used in a passive point-to-multipoint (PON) fashion, the ability to eliminate outside plant network electronics is realized, and the need for excessive signal processing and coding is eliminated. The PON, when deployed in an FTTH/B architecture, eliminates outside plant components and relies instead on the system endpoints for active electronics. These endpoints are comprised of the Web ProForum Tutorials http://www.iec.org
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CO–based optical line terminal (OLT) on one end and, on the other, the optical network termination (ONT) at the subscriber premises. Fiber-optic networks are simple, more reliable, and less costly to maintain than copper-based systems. As these components are ordered in volume for potentially millions of fiber-based access lines, the costs of deploying technologies such as FTTH, FTTB/C, and FTT/Cab become economically viable. One optical-fiber strand appears to have virtually limitless capacity. Transmission speeds in the terabit-per-second range have been demonstrated. The speeds are limited by the endpoint electronics, not by the fiber itself. For the ATM–PON system today, speeds of 155 Mbps symmetrical and 622 Mbps/155 Mbps asymmetrical are currently being developed. As the fiber itself is not the constraining factor, the future possibilities are endless. Furthermore, because fiber-optic technology is not influenced by electrical interferers such as cross-talk between copper pairs or AM band radio, it ensures high-quality telecommunications services in the present and future. In addition, fiber does not exhibit radio frequency (RF) emissions that can interfere with other electronics and is regulated by the Federal Communications Commission (FCC). While copper-based transport technologies remain ubiquitous, the long-term industry belief holds that it is inevitable that fiber will replace copper throughout the access infrastructure. Because copper infrastructure is embedded in communications systems, this transformation to optical transport is expected to occur over many years. Over time, new builds ("Greenfield") will be all fiber based, and existing builds will be rehabilitated by replacing copper with fiber or by overlaying new fiber on the existing copper infrastructure. Electronic equipment, as well, must be replaced with optical equipment.
2. How ATM PONs Work Recent technological advances and economies of scale have drawn increasing interest to optical-distribution networks with ATM PON. A functional overview of ATM–PON architecture is presented in Figure 2.
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Figure 2. Functional Overview of ATM–PON Architecture
Figure 2 shows the ONT placed at the customer premises, which suggests FTTH/B architecture. The carrier's demarcation point would be the subscriber side of the ONT, typically in the form of a T1, Ethernet, integrated services digital network (ISDN), plain old telephone service (POTS), etc. For FTTCab and FTTC architecture, an optical network unit (ONU), rather than an optical network termination (ONT), is used. It is placed in the outside plant and must be temperature-hardened and properly enclosed. The final drop to the network termination (NT) at the customer premises may be copper or fiber. The carrier demarcation point is the subscriber side of the NT in the form of a T1, Ethernet, ISDN, POTS, and etc. Access to bandwidth on the PON may be obtained by several methods, including time division multiple access (TDMA), wave division multiple access (WDMA), code division multiple access (CDMA), and subcarrier multiple access (SCMA). TDMA in the upstream and TDM in the downstream were chosen by the FullService Access Network (FSAN) group and submitted to the International Telecommunications Union (ITU) for standardization, based on their simplicity and cost-effectiveness. As shown in Figure 2, the network components supporting ATM PON consist of OLT, ONT, and a passive optical splitter. One fiber is passively split up to 64 times between multiple ONTs that share the capacity of one fiber. Passive splitting requires special actions for privacy and security, and a TDMA protocol is necessary in the upstream direction. The use of the optical splitter in the PON architecture allows users to share bandwidth, thus dividing the attendant costs. Costs are further reduced by a decrease in the number of opto-electronic devices needed at the OLT; one interface may be shared among many ONTs. Web ProForum Tutorials http://www.iec.org
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The ATM–PON system uses a double-star architecture. The first star is at the OLT, where the wide-area network interface to services is logically split and switched to the ATM–PON interface. The second star occurs at the splitter where information is passively split and delivered to each ONT. The OLT is typically located in the carrier's CO. The OLT is the interface point between the access system and service points within the carrier's network. When data content from the network reaches the OLT, it is actively switched to the passive splitter using TDM in the downstream. The OLT behaves like an ATM edge switch with ATM– PON interfaces on the subscriber side and ATM–synchronous optical network (SONET) interfaces on the network side. The ONT will filter the incoming cells and recover only those that are addressed to it. Each ATM cell has a 28-bit addressing field associated with it called a virtual path identifier/virtual channel identifier (VPI/VCI). The OLT will first send a message to the ONT to provision it to accept cells with certain VPI/VCI values. The recovered ATM cells are then used to create the service interface required at the subscriber side of the ONT (see Figure 2). Because TDMA is used in the upstream direction, each ONT is synchronized in time with every other ONT. The process by which this happens is called ranging the ONTs. Basically, the OLT must determine how far away in distance each ONT is so they can be assigned an optimal time slot in which to transmit without interfering with other ONTs. The OLT will then send grant messages via the physical layer operation, administration, and maintenance (PLOAM) cells to provision the TDMA slots that are assigned to that ONT. The ONT will then adapt the service interface to ATM and send it to the PON using the TDMA protocol. Ethernet and T1s are two examples of what can be transported over the ATM– PON. As ATM–PON is service-independent, all legacy services and future services can be readily transported. The basic frame format between the OLT and ONT for the symmetrical 155 Mbps rate is shown in Figure 3.
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Figure 3. ATM–PON Frame Formats
The asymmetrical version of 622 Mbps/155 Mbps downstream/upstream is similar but beyond the scope of this document. As can be seen in Figure 3 the downstream payload capacity is reduced to 149.97 Mbps because of the PLOAM cells. These cells are responsible for allocating bandwidth (via Grant cells), synchronization, error control, security, ranging, and maintenance. In the upstream direction the capacity is reduced to 149.19 Mbps because there are 3 overhead bytes per ATM cell. In addition to the three overhead bytes per cell there are PLOAM cells in the upstream direction, the rate of which is defined by the OLT for each ONT, depending on the required functionality. The minimum PLOAM rate in the upstream direction is one PLOAM every 100 ms. This equates to approximately one PLOAM every 655 frames, which is negligible. Although the maximum PLOAM rate is undefined, it is also expected to be negligible. The 3 overhead bytes contain a minimum of 4 bits of guard time to provide enough distance in time to prevent collisions with cells from other ONTs. This field length is actually programmable by the OLT. The preamble field is used to acquire bit synchronization and amplitude recovery. The Delimiter field is used to indicate the start of an incoming cell. Given that a single fiber is used for both the upstream and downstream paths, two wavelengths of light are used—1550 nm for the downstream and 1310 nm for the upstream. Although one wavelength can also be used, two provide better optical isolation between the laser transmitters and receivers and eliminate the need for expensive beam-splitting devices. Instead, low-cost planar light circuits (PLCs) can be used, which enable low-cost manufacturing techniques to be Web ProForum Tutorials http://www.iec.org
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employed, somewhat similar to the production of silicon chips. ATM cells are directly converted to light and sent to the PON. Because of the broadcast nature of the PON, encryption techniques are employed to prevent security breaches. In the upstream direction, the ONT uses the TDMA protocol and again directly converts ATM cells to light for transport over the PON (see Figure 2). A typical ATM–PON system can furnish up to 64 customer locations on a single, shared strand of fiber running at 155 Mbps. Most, however, will likely utilize 32 locations in the distribution and drop portion of the network in the near term. In the future, the ATM–PON specification does allow for up to 64 locations to be served.
3. Benefits of ATM–PONs The ATM–PON system offers a number of benefits for carriers and end users. Because fiber is less costly to maintain than copper based systems, carriers benefit by being able to reduce costs and thereby increase profit margins or simply lower prices to end users to ward off competitive threats. ATM–PON transmission is conducted through a single strand and thereby conserves fiber. Using a single fiber strand for up to 64 end users provides great cost savings over the current point-to-point architecture. ATM PON conserves optical interfaces at the OLT because a single fiber is used to service as many as 64 end-user locations. Thus, a 64 to 1 reduction in optical interfaces is achieved in comparison to point-to-point optical systems. Another advantage of the ATM–PON system is the aggregation and concentration of ATM cells in the OLT. This concentration allows the carriers to serve many more customers than if only TDM–based techniques are used. At the same time, QoS benefits of ATM allow the carriers to provide service-level agreements (SLAs) and rest assured that service is guaranteed. It is estimated that ATM–PON technology can achieve savings of 20 to 40 percent over circuitbased access systems. ATM PON realizes these savings through the use of ATM concentration and statistical multiplexing in addition to sharing active optoelectronic components through the splitter elements. Because the ONTs share the same fiber and optical splitter, the bandwidth can also be shared. In the future, dynamic bandwidth-allocation protocols will allow the carriers to serve more users by allocating bandwidth on an as-needed basis. These protocols are already part of the FSAN specification as an optional requirement. Therefore, more users can be served with a smaller number of OLTs, leading to additional savings.
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Operational and maintenance savings will be derived from ATM PON. Because the system is based on ATM, a single management system can completely provision the bandwidth end to end. Also, if the service interface is a high-speed local-area network (LAN) such as 10/100Base–T, where the carrier's ATM circuit rather than the physical interface bit rate is the limiting factor to the bandwidth, then bandwidth can be incrementally provisioned over time as needed, up to the limitations of the physical interface. For example, if a small business needs only 1 Mbps capacity at first but will require 2 Mbps a year from now, then the carrier must only provision greater ATM PVC rate, rather than having to do a truck roll to wire more T1 lines (as is currently done). Because the PON system will be ATM–based, it can adapt to virtually any service desired. Telco operators, for instance, can deliver all of their legacy services, such as T1 and T3 lines, or deliver new services, such as transparent LAN service (TLS) over the optical network (see Figure 4). This future-proofs the architecture. New revenue streams are derived by being able to provide transparent LAN services to end users quickly and easily. Figure 4. Transparent LAN over the Optical Network
The ONT is proportioned for small- to medium-sized businesses and costs little. This low cost is achieved because there are more small businesses than large ones. Currently, service providers serve small businesses from synchronous optical network (SONET) ring nodes, and these are costly elements when compared to small ATM–PON ONTs. ATM PONs will mean new business for carriers and services providers, as they can eliminate the need to place small- and medium-sized businesses on SONET rings that exist in most metropolitan area networks.
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Active components of the ATM–PON system are located at the customer premises or CO, rather than at remote outside plant terminals. Thus, costs associated with outside plant–battery backup systems and active electronics that must incur severe temperature variations are eliminated. Battery backup systems can be placed indoors at the customer premises and thus last much longer between maintenance intervals. In addition, the option of having the end user provide the battery backup from low-cost computer UPC systems can be offered on a per-user basis. With typical outside-plant systems (such as DLC or FTTCab) that are shared between many users, this option is simply not available. As ATM–PON architecture and processes mature, end users will benefit by being able to provision their own services, whenever they are needed, through an automated process. This process will either link the carriers' service management system (SMS) with the customers' network management system or allow the customer access to the SMS through a secured Web-browser interface. The CO then updates network elements and provisions the new bandwidth.
4. Technology Comparison with xDSL This section will compare ATM–PON systems with xDSL technologies and describe the issues associated with each. ATM is an ultrahigh-speed, one-size-fits-all, cell-based data transmission protocol that may be run over many physical-layer technologies such as xDSL modems. These are attached to twisted-pair copper wiring and transmit data at speeds of 1.5 Mbps to 9 Mbps downstream to the subscriber and 64 Kbps to 1.5 Mbps upstream, depending on the condition and distance of the copper line. Asymmetric digital subscriber line (ADSL), for instance, offers users an alwayson service, but its maximum downstream and upstream speeds are ultimately limited by distance and the aging copper infrastructure; typically, only speeds of 1.5 Mbps over 12 kft are achieved. If the customer is not directly connected to a CO–based digital subscriber line access multiplexer (DSLAM), then an expensive upgrade to an existing outside-plant DLC system is usually the only solution. Very-high-speed DSL (VDSL) extends ADSL downstream speed to a potential 52 Mbps, with a proportionately lower upstream speed, but offers a shorter distance range (1 kft to 3 kft) than ADSL. However, this too requires expensive outside plant electronics installed in a cabinet that must survive severe temperature variations. In addition to the distance problem, xDSL technology has inherent interference problems, a liability with copper-based technology. ATM PONs cannot be interfered with by AM band radio and other radio frequency interference (RFI)/electromagnetic interference (EMI) sources. XDSL is largely considered to be a short-term broadband solution; since it can be easily installed without an Web ProForum Tutorials http://www.iec.org
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expensive outside-plant infrastructure build, the existing copper plant can be used. The PON system, however, is believed to offer an ultimate, end-to-end broadband platform that is future-proofed.
5. Full-Service Access Network Initiative Overseeing the development of passive optical networks as part of fiber-optic backbones is the Full-Service Access Network (FSAN) Initiative. FSAN is a group of 20 telecommunications companies working collaboratively with equipment suppliers to agree on a common broadband access system for provisioning both broadband and narrowband services. Since June 1995, the FSAN group has been working on the international initiative and recognizing that each member has differing needs, depending on regulatory, business, and structural environment in each country. FSAN is not a standards body, but rather submits specifications to standard bodies such as the International Telecommunications Union (ITU). Existing standards are incorporated where applicable. In October of 1998, the ITU adopted the G.983.1 broadband optical access system based on PON. Members of the initiative throughout the process have intended to introduce elements of their results to appropriate standards bodies. On June 22, 1999, four FSAN members—NTT, British Telecom, BellSouth, and France Telecom—issued a common technical specification for ATM subscriber systems. Because each initiative member understood the need to develop future access networks, the group realized that industry-wide benefits could be achieved through adopting a common set of specifications. The consortium determined that the per-line cost of producing a full-service access network will decrease slowly with the production volume. The group concluded that as volume increases, the development of new technologies will enable significant reductions in per-line equipment and installation costs. Fiber-based broadband networks could be cost-effective to deploy if their component part were built in bulk quantities for tens of millions of access lines, rather than according to today's typical 300,000-line system order. The group's work has occurred in two phases. First, its task was to identify technical and economic barriers to the introduction of a broadband access network. It was determined that an ATM PON was the most promising technology to achieve large-scale, FSAN work deployment that could meet the evolving service needs of network users. The consortium felt that ATM PON was the best means of supporting a range of architectures such as FTTH, FTTB/C, and FTTH/CAB. Members have recognized that all operators require the same elements in their access network. The major differences come from the positioning of the optical network unit (ONT). All members see the need for a Web ProForum Tutorials http://www.iec.org
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PON system. Second, the group's work was to devise a common set of specifications for full-service access networks. Six working teams—systems engineering/architecture; optical access networks; home network/network termination; operation, administration, and maintenance (OAM&P); VDSL; and component technology—undertook the development process.
6. Major Players Japan's Nippon Telegraph and Telephone Corporation (NTT) is recognized as a leading telecommunications carrier in the creation of high-speed optical-network access systems. Its leadership is demonstrated by its involvement in the FSAN Initiative as well as by its own cutting-edge research and development and collaboration with other carriers. NTT already has deployed narrowband and video-distribution FTTH and broadband ATM–PON systems. In 1999, it will introduce a fully FSAN–compliant, FTTB/C ATM–PON system. According to a press release issued by NTT and BellSouth in June 1998, the two companies announced that they would work together. NTT and BellSouth announced they would deliver a high-speed optical-network access platform, pooling their respective research and development resources to advance the availability of affordable FTTH technology. In June 1999, BellSouth unveiled plans to install a FTTH system to the Atlanta area using FSAN–compliant ATM– PON technology. In the news release about the Atlanta installation, BellSouth announced that suburban Atlanta residents will be the first in North America to experience the nearly unlimited speed and bandwidth of passive optical networking delivered directly to their homes. BellSouth's vision for FTTH is for customers to buy communications appliances for voice, video, data or imaging applications at retail stores and plug them into their home optical telecommunications network. The BellSouth fiber network, by talking to the appliance, would deliver the necessary provisioning. Both BellSouth and NTT believe that customer orientation and demand will drive down the cost of FTTH equipment and accelerate its worldwide availability. Historically, both BellSouth and NTT have pioneered fiber-optic technology. In the late 1980s, BellSouth launched an FTTH trial near Orlando, Florida. Historically, NTT has actively promoted FTTH, particularly in the area of interface specifications for high-speed optical access systems. NTT's FTTB/C project, to be launched in 1999, will replace copper cable with fiber throughout most of NTT's subscriber system.
7. The Future of ATM PONs As a future-proof technology, ATM PON will serve as a framework for applications yet to be developed or advanced. While commercial deployments of Web ProForum Tutorials http://www.iec.org
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ATM PON—with the exception of examples in limited areas in Japan by NTT— have yet to occur, trials have been increased in 1999 and are expected to accelerate in 2000. Perhaps the biggest advantage for ATM PON is that interest in the technology exists on a global scale, a situation that may be attributed in great part to the collaboration of the FSAN Initiative. Carriers and equipment makers believe that extensive collaboration on ATM–PON physical-layer interoperability will lead to an increase in the production volume of silicon chipsets that can be created to the global specification. Interoperability among the technology's management layers will depend on alliances among strategic vendors. Settling on the core framework, however, is what will propel the technology forward. Carriers and service providers are expected to focus initially on business uses through FTTB, as real revenue streams typically originate in these areas. As production accelerates, operators will increasingly look to the mass residential market. Through the Internet age, small- and medium-sized businesses have been characterized as being on the down slope of technology. However, ATM PON, with its cost savings and flexibility, is capable of bringing more of these businesses on-line quickly. Future applications aimed at FTTH scenarios include asymmetric broadband services (such as digital broadcast, video on demand, distance learning, and fast Internet), symmetric broadband services (such as telecommunications services and teleconferencing opportunities), and narrowband telephone services (such as the public switched telephone network [PSTN] and integrated services digital network [ISDN]).
Self-Test 1. The access network, which is the portion of a public switched network that connects access nodes to individual subscribers, is predominantly characterized today by which of the following? a. fiber-optic cable b. hybrid-fiber coaxial cable c. twisted-pair copper wiring d. electrical wiring 2. Fiber to the home (FTTH), fiber to the building/curb (FTTB/C), and fiber to the cabinet (FTTCab) are examples of which of the following? a. local access network architectures
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b. digital loop carriers c. transport protocols d. fiber-optic components 3. A single optical-fiber strand's capacity lies in what range, according to recent demonstrations? a. Mbps b. kbps c. virtually limitless d. Gbps 4. Asynchronous transfer mode (ATM) is ____________. a. a cell-based data transmission protocol b. an opto-electronic component c. a circuit-switched access systems 5. ATM PON is attractive to telecommunications carriers because it contains __________. a. active electronics b. no active electronics in outside plant c. SONET rings d. copper-based wiring 6. A typical ATM PON system can furnish up to _____________. a. 64 customer locations on a single, shared strand of fiber b. 72 customer locations on a single, shared strand of fiber c. 96 customer locations on a single, shared strand of fiber d. 128 customer locations on a single, shared strand of fiber
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7. The use of the splitter in the PON architecture allows network users to ___________. a. share bandwidth b. provision bandwidth c. increase bandwidth d. ensure privacy and security 8. The optical line termination (OLT) in the ATM PON system is typically located _____________. a. at the customer premises b. in a curbside cabinet c. in a residential gateway device d. in the carrier's CO or POP 9. It is estimated that an ATM–PON system can achieve savings of ___________. a. 20 percent to 40 percent over circuit-based access systems b. 40 percent to 60 percent over circuit-based access systems c. 60 percent to 80 percent over circuit-based access systems d. 80 percent to 100 percent over circuit-based access systems 10. Full-service access network (FSAN) is ____________. a. a standards body that regulates broadband networks b. an access network for delivering broadband services c. a group of 20 global telecommunications equipment manufacturers collaborating on specification requirements for broadband access systems d. a group of 20 global telecommunications carriers collaborating on specification requirements for broadband access systems
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11. The main advantages afforded to the carriers of ATM PON are _________________. a. cost savings due to lower maintenance costs than copper b. cost savings due to use of single fiber for up to 64 users c. cost savings due to easy bandwidth upgrading with no truck rolls d. cost savings due to aggregation and concentration in the OLT e. all of the above
Correct Answers 1. The access network, which is the portion of a public switched network that connects access nodes to individual subscribers, is predominantly characterized today by which of the following? a. fiber-optic cable b. hybrid-fiber coaxial cable c. twisted-pair copper wiring d. electrical wiring See Topic 1. 2. Fiber to the home (FTTH), fiber to the building/curb (FTTB/C), and fiber to the cabinet (FTTCab) are examples of which of the following? a. local access network architectures b. digital loop carriers c. transport protocols d. fiber-optic components See Topic 1. 3. A single optical-fiber strand's capacity lies in what range, according to recent demonstrations? a. Mbps
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b. kbps c. virtually limitless d. Gbps See Topic 1. 4. Asynchronous transfer mode (ATM) is ____________. a. a cell-based data transmission protocol b. an opto-electronic component c. a circuit-switched access systems See Topic 2. 5. ATM PON is attractive to telecommunications carriers because it contains __________. a. active electronics b. no active electronics in outside plant c. SONET rings d. copper-based wiring See Topic 2. 6. A typical ATM PON system can furnish up to _____________. a. 64 customer locations on a single, shared strand of fiber b. 72 customer locations on a single, shared strand of fiber c. 96 customer locations on a single, shared strand of fiber d. 128 customer locations on a single, shared strand of fiber See Topic 2. 7. The use of the splitter in the PON architecture allows network users to ___________. a. share bandwidth b. provision bandwidth Web ProForum Tutorials http://www.iec.org
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c. increase bandwidth d. ensure privacy and security See Topic 2. 8. The optical line termination (OLT) in the ATM PON system is typically located _____________. a. at the customer premises b. in a curbside cabinet c. in a residential gateway device d. in the carrier's CO or POP See Topic 2. 9. It is estimated that an ATM–PON system can achieve savings of ___________. a. 20 percent to 40 percent over circuit-based access systems b. 40 percent to 60 percent over circuit-based access systems c. 60 percent to 80 percent over circuit-based access systems d. 80 percent to 100 percent over circuit-based access systems See Topic 3. 10. Full-service access network (FSAN) is ____________. a. a standards body that regulates broadband networks b. an access network for delivering broadband services c. a group of 20 global telecommunications equipment manufacturers collaborating on specification requirements for broadband access systems d. a group of 20 global telecommunications carriers collaborating on specification requirements for broadband access systems See Topic 4.
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11. The main advantages afforded to the carriers of ATM PON are _________________. a. cost savings due to lower maintenance costs than copper b. cost savings due to use of single fiber for up to 64 users c. cost savings due to easy bandwidth upgrading with no truck rolls d. cost savings due to aggregation and concentration in the OLT e. all of the above See Topic 3.
Glossary ADSL asymmetric digital subscriber line ATM asynchronous transfer mode CDMA code division multiple access CO central office DLC digital loop carrier DSL digital subscriber line FTTB/C fiber-to-the-business/curb FTTCab fiber-to-the-cabinet FTTH fiber-to-the-home FSAN full-service access network Web ProForum Tutorials http://www.iec.org
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HDSL high bit-rate digital subscriber line ISDN integrated services digital network LAN local-area network NT network termination OAM&P operation, administration, and management protocol OLT optical line terminal ONT optical network termination/terminator ONU optical network unit PLC planar light circuit PON passive optical network POP point of presence POTS plain old telephone service QoS quality of service SDMA subcarrier division multiple access SLA service-level agreement SMS service management system Web ProForum Tutorials http://www.iec.org
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SONET synchronous optical network TDMA time division multiple access VDSL very high speed digital subscriber line WDMA wave division multiple access
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Overview The deployment of fiber-optic technology to homes and businesses is poised to change the way telecommunications services—primarily voice, data, and video services—will be delivered to the twenty-first century, information-based economy. Interest is high among business and residential consumers for advanced, broadband services such as fast Internet access, electronic commerce, video on demand, digital broadcasting, teleconferencing, and telemedicine, among others. However, the lack of available bandwidth to deliver these services effectively to the last mile of homes and businesses has stifled development of new multimedia applications. An optical distribution network with ATM PON as the core technology promises benefits to end users as well as carriers and service providers. When optical network access is achieved in scale, businesses and consumers will realize opportunities for advanced services at relatively low costs. Because of cost savings inherent with the ATM–PON platform, telecommunications carriers and service providers will realize efficiencies in provisioning future applications and upgrading bandwidth to satisfy customers' demands.
Topics 1. The Case for Fiber-Optic Access 2. How ATM PONs Work Web ProForum Tutorials http://www.iec.org
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3. Benefits of ATM PONs 4. Technology Comparison with xDSL 5. Full-Service Access Network Initiative 6. Major Players 7. The Future of ATM PONs Self-Test Correct Answers Glossary
1. The Case for Fiber-Optic Access Fiber-optic technology, offering virtually unlimited bandwidth potential, is widely considered to be the ultimate solution to deliver broadband access to the last mile. Today's narrowband telecommunications networks are characterized by low speed, service-provisioning delays, and unreliable quality of service. This limits the ability of a consumer to enjoy the experience at home or the ability of workers to be efficient in their jobs. The last mile is the network space between the carrier's central office (CO) and the subscriber location. This is where bottlenecks occur to slow the delivery of services. The subscriber's increasing bandwidth demands are often unpredictable and challenging for telecommunications carriers. Not only must carriers satisfy today's bandwidth demands by leveraging the limits of existing infrastructure, they also must plan for future subscriber needs. A new network infrastructure that allows more bandwidth, quick provisioning of services, and guaranteed quality of service (QoS) in a cost-effective and efficient manner is now required. Today's access network, the portion of a public switched network that connects CO equipment to individual subscribers, is characterized by predominantly twisted-pair copper wiring. Fiber-optic technology, through local access network architectures such as fiberto-the-home/building (FTTH/B), fiber-to-the-cabinet (FTTCab), and fiber-tothe-curb (FTTC) offers a mechanism to enable sufficient network bandwidth for the delivery of new services and applications. ATM–PON technology can be included in all these architectures, as shown in Figure 1.
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Figure 1. ATM–PON Architectures
In general, the optical section of a local access network can either be a point-topoint, ring, or passive point-to-multipoint architecture. This tutorial focuses on the passive point-to-multipoint architecture (PON). The main component of the PON is an optical splitter device that, depending on which direction the light is traveling, splits the incoming light and distributes it to multiple fibers or combines it onto one fiber. The PON, when included in FTTH/B architecture, runs an optical fiber from a CO to an optical splitter and on into the subscriber's home or building. The optical splitter may be located in the CO, outside plant, or in a building. FTTCab architecture runs an optical fiber from the CO to an optical splitter and then on to the neighborhood cabinet, where the signal is converted to feed the subscriber over a twisted copper pair. Typically, the neighborhood cabinet is about 3 kft from the subscriber's home or business. FTTC architecture runs an optical fiber from the CO to an optical splitter and then on to a small curb-located cabinet, which is near (typically within 500 ft) to the subscriber. It is then converted to twisted copper pair. The PON can be common to all of these architectures. However, it is only in the FTTH/B configurations that all active electronics are eliminated from the outside plant. The FTTCab and FTTC architectures require active outside-plant electronics in a neighborhood cabinet or curb. This tutorial will focus on FTTH/B architectures. When fiber is used in a passive point-to-multipoint (PON) fashion, the ability to eliminate outside plant network electronics is realized, and the need for excessive signal processing and coding is eliminated. The PON, when deployed in an FTTH/B architecture, eliminates outside plant components and relies instead on the system endpoints for active electronics. These endpoints are comprised of the Web ProForum Tutorials http://www.iec.org
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CO–based optical line terminal (OLT) on one end and, on the other, the optical network termination (ONT) at the subscriber premises. Fiber-optic networks are simple, more reliable, and less costly to maintain than copper-based systems. As these components are ordered in volume for potentially millions of fiber-based access lines, the costs of deploying technologies such as FTTH, FTTB/C, and FTT/Cab become economically viable. One optical-fiber strand appears to have virtually limitless capacity. Transmission speeds in the terabit-per-second range have been demonstrated. The speeds are limited by the endpoint electronics, not by the fiber itself. For the ATM–PON system today, speeds of 155 Mbps symmetrical and 622 Mbps/155 Mbps asymmetrical are currently being developed. As the fiber itself is not the constraining factor, the future possibilities are endless. Furthermore, because fiber-optic technology is not influenced by electrical interferers such as cross-talk between copper pairs or AM band radio, it ensures high-quality telecommunications services in the present and future. In addition, fiber does not exhibit radio frequency (RF) emissions that can interfere with other electronics and is regulated by the Federal Communications Commission (FCC). While copper-based transport technologies remain ubiquitous, the long-term industry belief holds that it is inevitable that fiber will replace copper throughout the access infrastructure. Because copper infrastructure is embedded in communications systems, this transformation to optical transport is expected to occur over many years. Over time, new builds ("Greenfield") will be all fiber based, and existing builds will be rehabilitated by replacing copper with fiber or by overlaying new fiber on the existing copper infrastructure. Electronic equipment, as well, must be replaced with optical equipment.
2. How ATM PONs Work Recent technological advances and economies of scale have drawn increasing interest to optical-distribution networks with ATM PON. A functional overview of ATM–PON architecture is presented in Figure 2.
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Figure 2. Functional Overview of ATM–PON Architecture
Figure 2 shows the ONT placed at the customer premises, which suggests FTTH/B architecture. The carrier's demarcation point would be the subscriber side of the ONT, typically in the form of a T1, Ethernet, integrated services digital network (ISDN), plain old telephone service (POTS), etc. For FTTCab and FTTC architecture, an optical network unit (ONU), rather than an optical network termination (ONT), is used. It is placed in the outside plant and must be temperature-hardened and properly enclosed. The final drop to the network termination (NT) at the customer premises may be copper or fiber. The carrier demarcation point is the subscriber side of the NT in the form of a T1, Ethernet, ISDN, POTS, and etc. Access to bandwidth on the PON may be obtained by several methods, including time division multiple access (TDMA), wave division multiple access (WDMA), code division multiple access (CDMA), and subcarrier multiple access (SCMA). TDMA in the upstream and TDM in the downstream were chosen by the FullService Access Network (FSAN) group and submitted to the International Telecommunications Union (ITU) for standardization, based on their simplicity and cost-effectiveness. As shown in Figure 2, the network components supporting ATM PON consist of OLT, ONT, and a passive optical splitter. One fiber is passively split up to 64 times between multiple ONTs that share the capacity of one fiber. Passive splitting requires special actions for privacy and security, and a TDMA protocol is necessary in the upstream direction. The use of the optical splitter in the PON architecture allows users to share bandwidth, thus dividing the attendant costs. Costs are further reduced by a decrease in the number of opto-electronic devices needed at the OLT; one interface may be shared among many ONTs. Web ProForum Tutorials http://www.iec.org
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The ATM–PON system uses a double-star architecture. The first star is at the OLT, where the wide-area network interface to services is logically split and switched to the ATM–PON interface. The second star occurs at the splitter where information is passively split and delivered to each ONT. The OLT is typically located in the carrier's CO. The OLT is the interface point between the access system and service points within the carrier's network. When data content from the network reaches the OLT, it is actively switched to the passive splitter using TDM in the downstream. The OLT behaves like an ATM edge switch with ATM– PON interfaces on the subscriber side and ATM–synchronous optical network (SONET) interfaces on the network side. The ONT will filter the incoming cells and recover only those that are addressed to it. Each ATM cell has a 28-bit addressing field associated with it called a virtual path identifier/virtual channel identifier (VPI/VCI). The OLT will first send a message to the ONT to provision it to accept cells with certain VPI/VCI values. The recovered ATM cells are then used to create the service interface required at the subscriber side of the ONT (see Figure 2). Because TDMA is used in the upstream direction, each ONT is synchronized in time with every other ONT. The process by which this happens is called ranging the ONTs. Basically, the OLT must determine how far away in distance each ONT is so they can be assigned an optimal time slot in which to transmit without interfering with other ONTs. The OLT will then send grant messages via the physical layer operation, administration, and maintenance (PLOAM) cells to provision the TDMA slots that are assigned to that ONT. The ONT will then adapt the service interface to ATM and send it to the PON using the TDMA protocol. Ethernet and T1s are two examples of what can be transported over the ATM– PON. As ATM–PON is service-independent, all legacy services and future services can be readily transported. The basic frame format between the OLT and ONT for the symmetrical 155 Mbps rate is shown in Figure 3.
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Figure 3. ATM–PON Frame Formats
The asymmetrical version of 622 Mbps/155 Mbps downstream/upstream is similar but beyond the scope of this document. As can be seen in Figure 3 the downstream payload capacity is reduced to 149.97 Mbps because of the PLOAM cells. These cells are responsible for allocating bandwidth (via Grant cells), synchronization, error control, security, ranging, and maintenance. In the upstream direction the capacity is reduced to 149.19 Mbps because there are 3 overhead bytes per ATM cell. In addition to the three overhead bytes per cell there are PLOAM cells in the upstream direction, the rate of which is defined by the OLT for each ONT, depending on the required functionality. The minimum PLOAM rate in the upstream direction is one PLOAM every 100 ms. This equates to approximately one PLOAM every 655 frames, which is negligible. Although the maximum PLOAM rate is undefined, it is also expected to be negligible. The 3 overhead bytes contain a minimum of 4 bits of guard time to provide enough distance in time to prevent collisions with cells from other ONTs. This field length is actually programmable by the OLT. The preamble field is used to acquire bit synchronization and amplitude recovery. The Delimiter field is used to indicate the start of an incoming cell. Given that a single fiber is used for both the upstream and downstream paths, two wavelengths of light are used—1550 nm for the downstream and 1310 nm for the upstream. Although one wavelength can also be used, two provide better optical isolation between the laser transmitters and receivers and eliminate the need for expensive beam-splitting devices. Instead, low-cost planar light circuits (PLCs) can be used, which enable low-cost manufacturing techniques to be Web ProForum Tutorials http://www.iec.org
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employed, somewhat similar to the production of silicon chips. ATM cells are directly converted to light and sent to the PON. Because of the broadcast nature of the PON, encryption techniques are employed to prevent security breaches. In the upstream direction, the ONT uses the TDMA protocol and again directly converts ATM cells to light for transport over the PON (see Figure 2). A typical ATM–PON system can furnish up to 64 customer locations on a single, shared strand of fiber running at 155 Mbps. Most, however, will likely utilize 32 locations in the distribution and drop portion of the network in the near term. In the future, the ATM–PON specification does allow for up to 64 locations to be served.
3. Benefits of ATM–PONs The ATM–PON system offers a number of benefits for carriers and end users. Because fiber is less costly to maintain than copper based systems, carriers benefit by being able to reduce costs and thereby increase profit margins or simply lower prices to end users to ward off competitive threats. ATM–PON transmission is conducted through a single strand and thereby conserves fiber. Using a single fiber strand for up to 64 end users provides great cost savings over the current point-to-point architecture. ATM PON conserves optical interfaces at the OLT because a single fiber is used to service as many as 64 end-user locations. Thus, a 64 to 1 reduction in optical interfaces is achieved in comparison to point-to-point optical systems. Another advantage of the ATM–PON system is the aggregation and concentration of ATM cells in the OLT. This concentration allows the carriers to serve many more customers than if only TDM–based techniques are used. At the same time, QoS benefits of ATM allow the carriers to provide service-level agreements (SLAs) and rest assured that service is guaranteed. It is estimated that ATM–PON technology can achieve savings of 20 to 40 percent over circuitbased access systems. ATM PON realizes these savings through the use of ATM concentration and statistical multiplexing in addition to sharing active optoelectronic components through the splitter elements. Because the ONTs share the same fiber and optical splitter, the bandwidth can also be shared. In the future, dynamic bandwidth-allocation protocols will allow the carriers to serve more users by allocating bandwidth on an as-needed basis. These protocols are already part of the FSAN specification as an optional requirement. Therefore, more users can be served with a smaller number of OLTs, leading to additional savings.
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Operational and maintenance savings will be derived from ATM PON. Because the system is based on ATM, a single management system can completely provision the bandwidth end to end. Also, if the service interface is a high-speed local-area network (LAN) such as 10/100Base–T, where the carrier's ATM circuit rather than the physical interface bit rate is the limiting factor to the bandwidth, then bandwidth can be incrementally provisioned over time as needed, up to the limitations of the physical interface. For example, if a small business needs only 1 Mbps capacity at first but will require 2 Mbps a year from now, then the carrier must only provision greater ATM PVC rate, rather than having to do a truck roll to wire more T1 lines (as is currently done). Because the PON system will be ATM–based, it can adapt to virtually any service desired. Telco operators, for instance, can deliver all of their legacy services, such as T1 and T3 lines, or deliver new services, such as transparent LAN service (TLS) over the optical network (see Figure 4). This future-proofs the architecture. New revenue streams are derived by being able to provide transparent LAN services to end users quickly and easily. Figure 4. Transparent LAN over the Optical Network
The ONT is proportioned for small- to medium-sized businesses and costs little. This low cost is achieved because there are more small businesses than large ones. Currently, service providers serve small businesses from synchronous optical network (SONET) ring nodes, and these are costly elements when compared to small ATM–PON ONTs. ATM PONs will mean new business for carriers and services providers, as they can eliminate the need to place small- and medium-sized businesses on SONET rings that exist in most metropolitan area networks.
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Active components of the ATM–PON system are located at the customer premises or CO, rather than at remote outside plant terminals. Thus, costs associated with outside plant–battery backup systems and active electronics that must incur severe temperature variations are eliminated. Battery backup systems can be placed indoors at the customer premises and thus last much longer between maintenance intervals. In addition, the option of having the end user provide the battery backup from low-cost computer UPC systems can be offered on a per-user basis. With typical outside-plant systems (such as DLC or FTTCab) that are shared between many users, this option is simply not available. As ATM–PON architecture and processes mature, end users will benefit by being able to provision their own services, whenever they are needed, through an automated process. This process will either link the carriers' service management system (SMS) with the customers' network management system or allow the customer access to the SMS through a secured Web-browser interface. The CO then updates network elements and provisions the new bandwidth.
4. Technology Comparison with xDSL This section will compare ATM–PON systems with xDSL technologies and describe the issues associated with each. ATM is an ultrahigh-speed, one-size-fits-all, cell-based data transmission protocol that may be run over many physical-layer technologies such as xDSL modems. These are attached to twisted-pair copper wiring and transmit data at speeds of 1.5 Mbps to 9 Mbps downstream to the subscriber and 64 Kbps to 1.5 Mbps upstream, depending on the condition and distance of the copper line. Asymmetric digital subscriber line (ADSL), for instance, offers users an alwayson service, but its maximum downstream and upstream speeds are ultimately limited by distance and the aging copper infrastructure; typically, only speeds of 1.5 Mbps over 12 kft are achieved. If the customer is not directly connected to a CO–based digital subscriber line access multiplexer (DSLAM), then an expensive upgrade to an existing outside-plant DLC system is usually the only solution. Very-high-speed DSL (VDSL) extends ADSL downstream speed to a potential 52 Mbps, with a proportionately lower upstream speed, but offers a shorter distance range (1 kft to 3 kft) than ADSL. However, this too requires expensive outside plant electronics installed in a cabinet that must survive severe temperature variations. In addition to the distance problem, xDSL technology has inherent interference problems, a liability with copper-based technology. ATM PONs cannot be interfered with by AM band radio and other radio frequency interference (RFI)/electromagnetic interference (EMI) sources. XDSL is largely considered to be a short-term broadband solution; since it can be easily installed without an Web ProForum Tutorials http://www.iec.org
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expensive outside-plant infrastructure build, the existing copper plant can be used. The PON system, however, is believed to offer an ultimate, end-to-end broadband platform that is future-proofed.
5. Full-Service Access Network Initiative Overseeing the development of passive optical networks as part of fiber-optic backbones is the Full-Service Access Network (FSAN) Initiative. FSAN is a group of 20 telecommunications companies working collaboratively with equipment suppliers to agree on a common broadband access system for provisioning both broadband and narrowband services. Since June 1995, the FSAN group has been working on the international initiative and recognizing that each member has differing needs, depending on regulatory, business, and structural environment in each country. FSAN is not a standards body, but rather submits specifications to standard bodies such as the International Telecommunications Union (ITU). Existing standards are incorporated where applicable. In October of 1998, the ITU adopted the G.983.1 broadband optical access system based on PON. Members of the initiative throughout the process have intended to introduce elements of their results to appropriate standards bodies. On June 22, 1999, four FSAN members—NTT, British Telecom, BellSouth, and France Telecom—issued a common technical specification for ATM subscriber systems. Because each initiative member understood the need to develop future access networks, the group realized that industry-wide benefits could be achieved through adopting a common set of specifications. The consortium determined that the per-line cost of producing a full-service access network will decrease slowly with the production volume. The group concluded that as volume increases, the development of new technologies will enable significant reductions in per-line equipment and installation costs. Fiber-based broadband networks could be cost-effective to deploy if their component part were built in bulk quantities for tens of millions of access lines, rather than according to today's typical 300,000-line system order. The group's work has occurred in two phases. First, its task was to identify technical and economic barriers to the introduction of a broadband access network. It was determined that an ATM PON was the most promising technology to achieve large-scale, FSAN work deployment that could meet the evolving service needs of network users. The consortium felt that ATM PON was the best means of supporting a range of architectures such as FTTH, FTTB/C, and FTTH/CAB. Members have recognized that all operators require the same elements in their access network. The major differences come from the positioning of the optical network unit (ONT). All members see the need for a Web ProForum Tutorials http://www.iec.org
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PON system. Second, the group's work was to devise a common set of specifications for full-service access networks. Six working teams—systems engineering/architecture; optical access networks; home network/network termination; operation, administration, and maintenance (OAM&P); VDSL; and component technology—undertook the development process.
6. Major Players Japan's Nippon Telegraph and Telephone Corporation (NTT) is recognized as a leading telecommunications carrier in the creation of high-speed optical-network access systems. Its leadership is demonstrated by its involvement in the FSAN Initiative as well as by its own cutting-edge research and development and collaboration with other carriers. NTT already has deployed narrowband and video-distribution FTTH and broadband ATM–PON systems. In 1999, it will introduce a fully FSAN–compliant, FTTB/C ATM–PON system. According to a press release issued by NTT and BellSouth in June 1998, the two companies announced that they would work together. NTT and BellSouth announced they would deliver a high-speed optical-network access platform, pooling their respective research and development resources to advance the availability of affordable FTTH technology. In June 1999, BellSouth unveiled plans to install a FTTH system to the Atlanta area using FSAN–compliant ATM– PON technology. In the news release about the Atlanta installation, BellSouth announced that suburban Atlanta residents will be the first in North America to experience the nearly unlimited speed and bandwidth of passive optical networking delivered directly to their homes. BellSouth's vision for FTTH is for customers to buy communications appliances for voice, video, data or imaging applications at retail stores and plug them into their home optical telecommunications network. The BellSouth fiber network, by talking to the appliance, would deliver the necessary provisioning. Both BellSouth and NTT believe that customer orientation and demand will drive down the cost of FTTH equipment and accelerate its worldwide availability. Historically, both BellSouth and NTT have pioneered fiber-optic technology. In the late 1980s, BellSouth launched an FTTH trial near Orlando, Florida. Historically, NTT has actively promoted FTTH, particularly in the area of interface specifications for high-speed optical access systems. NTT's FTTB/C project, to be launched in 1999, will replace copper cable with fiber throughout most of NTT's subscriber system.
7. The Future of ATM PONs As a future-proof technology, ATM PON will serve as a framework for applications yet to be developed or advanced. While commercial deployments of Web ProForum Tutorials http://www.iec.org
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ATM PON—with the exception of examples in limited areas in Japan by NTT— have yet to occur, trials have been increased in 1999 and are expected to accelerate in 2000. Perhaps the biggest advantage for ATM PON is that interest in the technology exists on a global scale, a situation that may be attributed in great part to the collaboration of the FSAN Initiative. Carriers and equipment makers believe that extensive collaboration on ATM–PON physical-layer interoperability will lead to an increase in the production volume of silicon chipsets that can be created to the global specification. Interoperability among the technology's management layers will depend on alliances among strategic vendors. Settling on the core framework, however, is what will propel the technology forward. Carriers and service providers are expected to focus initially on business uses through FTTB, as real revenue streams typically originate in these areas. As production accelerates, operators will increasingly look to the mass residential market. Through the Internet age, small- and medium-sized businesses have been characterized as being on the down slope of technology. However, ATM PON, with its cost savings and flexibility, is capable of bringing more of these businesses on-line quickly. Future applications aimed at FTTH scenarios include asymmetric broadband services (such as digital broadcast, video on demand, distance learning, and fast Internet), symmetric broadband services (such as telecommunications services and teleconferencing opportunities), and narrowband telephone services (such as the public switched telephone network [PSTN] and integrated services digital network [ISDN]).
Self-Test 1. The access network, which is the portion of a public switched network that connects access nodes to individual subscribers, is predominantly characterized today by which of the following? a. fiber-optic cable b. hybrid-fiber coaxial cable c. twisted-pair copper wiring d. electrical wiring 2. Fiber to the home (FTTH), fiber to the building/curb (FTTB/C), and fiber to the cabinet (FTTCab) are examples of which of the following? a. local access network architectures
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b. digital loop carriers c. transport protocols d. fiber-optic components 3. A single optical-fiber strand's capacity lies in what range, according to recent demonstrations? a. Mbps b. kbps c. virtually limitless d. Gbps 4. Asynchronous transfer mode (ATM) is ____________. a. a cell-based data transmission protocol b. an opto-electronic component c. a circuit-switched access systems 5. ATM PON is attractive to telecommunications carriers because it contains __________. a. active electronics b. no active electronics in outside plant c. SONET rings d. copper-based wiring 6. A typical ATM PON system can furnish up to _____________. a. 64 customer locations on a single, shared strand of fiber b. 72 customer locations on a single, shared strand of fiber c. 96 customer locations on a single, shared strand of fiber d. 128 customer locations on a single, shared strand of fiber
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7. The use of the splitter in the PON architecture allows network users to ___________. a. share bandwidth b. provision bandwidth c. increase bandwidth d. ensure privacy and security 8. The optical line termination (OLT) in the ATM PON system is typically located _____________. a. at the customer premises b. in a curbside cabinet c. in a residential gateway device d. in the carrier's CO or POP 9. It is estimated that an ATM–PON system can achieve savings of ___________. a. 20 percent to 40 percent over circuit-based access systems b. 40 percent to 60 percent over circuit-based access systems c. 60 percent to 80 percent over circuit-based access systems d. 80 percent to 100 percent over circuit-based access systems 10. Full-service access network (FSAN) is ____________. a. a standards body that regulates broadband networks b. an access network for delivering broadband services c. a group of 20 global telecommunications equipment manufacturers collaborating on specification requirements for broadband access systems d. a group of 20 global telecommunications carriers collaborating on specification requirements for broadband access systems
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11. The main advantages afforded to the carriers of ATM PON are _________________. a. cost savings due to lower maintenance costs than copper b. cost savings due to use of single fiber for up to 64 users c. cost savings due to easy bandwidth upgrading with no truck rolls d. cost savings due to aggregation and concentration in the OLT e. all of the above
Correct Answers 1. The access network, which is the portion of a public switched network that connects access nodes to individual subscribers, is predominantly characterized today by which of the following? a. fiber-optic cable b. hybrid-fiber coaxial cable c. twisted-pair copper wiring d. electrical wiring See Topic 1. 2. Fiber to the home (FTTH), fiber to the building/curb (FTTB/C), and fiber to the cabinet (FTTCab) are examples of which of the following? a. local access network architectures b. digital loop carriers c. transport protocols d. fiber-optic components See Topic 1. 3. A single optical-fiber strand's capacity lies in what range, according to recent demonstrations? a. Mbps
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b. kbps c. virtually limitless d. Gbps See Topic 1. 4. Asynchronous transfer mode (ATM) is ____________. a. a cell-based data transmission protocol b. an opto-electronic component c. a circuit-switched access systems See Topic 2. 5. ATM PON is attractive to telecommunications carriers because it contains __________. a. active electronics b. no active electronics in outside plant c. SONET rings d. copper-based wiring See Topic 2. 6. A typical ATM PON system can furnish up to _____________. a. 64 customer locations on a single, shared strand of fiber b. 72 customer locations on a single, shared strand of fiber c. 96 customer locations on a single, shared strand of fiber d. 128 customer locations on a single, shared strand of fiber See Topic 2. 7. The use of the splitter in the PON architecture allows network users to ___________. a. share bandwidth b. provision bandwidth Web ProForum Tutorials http://www.iec.org
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c. increase bandwidth d. ensure privacy and security See Topic 2. 8. The optical line termination (OLT) in the ATM PON system is typically located _____________. a. at the customer premises b. in a curbside cabinet c. in a residential gateway device d. in the carrier's CO or POP See Topic 2. 9. It is estimated that an ATM–PON system can achieve savings of ___________. a. 20 percent to 40 percent over circuit-based access systems b. 40 percent to 60 percent over circuit-based access systems c. 60 percent to 80 percent over circuit-based access systems d. 80 percent to 100 percent over circuit-based access systems See Topic 3. 10. Full-service access network (FSAN) is ____________. a. a standards body that regulates broadband networks b. an access network for delivering broadband services c. a group of 20 global telecommunications equipment manufacturers collaborating on specification requirements for broadband access systems d. a group of 20 global telecommunications carriers collaborating on specification requirements for broadband access systems See Topic 4.
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11. The main advantages afforded to the carriers of ATM PON are _________________. a. cost savings due to lower maintenance costs than copper b. cost savings due to use of single fiber for up to 64 users c. cost savings due to easy bandwidth upgrading with no truck rolls d. cost savings due to aggregation and concentration in the OLT e. all of the above See Topic 3.
Glossary ADSL asymmetric digital subscriber line ATM asynchronous transfer mode CDMA code division multiple access CO central office DLC digital loop carrier DSL digital subscriber line FTTB/C fiber-to-the-business/curb FTTCab fiber-to-the-cabinet FTTH fiber-to-the-home FSAN full-service access network Web ProForum Tutorials http://www.iec.org
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HDSL high bit-rate digital subscriber line ISDN integrated services digital network LAN local-area network NT network termination OAM&P operation, administration, and management protocol OLT optical line terminal ONT optical network termination/terminator ONU optical network unit PLC planar light circuit PON passive optical network POP point of presence POTS plain old telephone service QoS quality of service SDMA subcarrier division multiple access SLA service-level agreement SMS service management system Web ProForum Tutorials http://www.iec.org
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SONET synchronous optical network TDMA time division multiple access VDSL very high speed digital subscriber line WDMA wave division multiple access
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