As network operators begin to roll out full-service access networks, the FSAN committee shifts its focus to deployment-related issues.
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|G.983.1 family of ATM-PON architectures|
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Recently, the work of the FSAN committee has been extended by a subset of its members. This work underlies upcoming deployments of standards-compliant fiber-based access networks in Europe, Japan, and the United States.
For the past four years, an international group of network operators has worked to define a family of fiber-based broadband access architectures. The following companies are currently participating in the FSAN initiative: Bell Canada, BellSouth, British Telecom (BT), Chungwa (Taiwan) Telecom, Deutsche Telekom, Dutch PTT, Telecom Eireann, France Telecom, GTE, Israel Telecom, Korea Telecom, Nippon Telegraph & Telephone (NTT), SBC Communications, SingTel, Swisscom, Telecom Italia, Telefonica, Telia, Telstra, and US West.
The 20 members of the FSAN initiative shared an interest in providing a full spectrum of broadband and narrowband services but faced diverse deployment geographies, regulatory environments, service timeframes, and installed infrastructures. The FSAN operators also recognized the importance of mass production, and its associated cost reductions, in achieving an economically viable broadband access network. Toward that end, an open interface would stimulate competitive pressures, which would further minimize costs.
The FSAN committee first defined a common set of requirements and identified the key cost drivers in fiber-based access networks. At an early stage, it was recognized that a splitter-based passive optical network (PON) transporting ATM cells would support multiple service offerings and offer deployment flexibility. An ATM-PON architecture can support multiple deployment options depending on where the optical link is terminated.
With the assistance of a number of key equipment suppliers, the FSAN committee produced a set of technical specifications defining open optical interfaces to the ATM-PON. These technical requirements have now been accepted by the International Telecommunication Union-Telecommunication (ITU-T G.983.1), the ATM Forum, and the European Telecommunication Standards Institute (ETSI)1.
The family of ATM-PON access architectures identified by FSAN is depicted in the Figure. The ITU-T G.983.1 ATM Passive Optical Network Specification primarily defines the physical and logical interfaces between the optical-line terminal (OLT) and optical-network units (ONUs).
All network services are transported in ATM cells, upstream and downstream, on the PON. ATM transport provides key service features, such as multiple quality-of-service (QoS) guarantees, which are generally not well implemented in other formats. ATM can provide a guaranteed-bandwidth connection with low cell delay and cell-delay variance, enabling time-sensitive applications to be carried over the ATM-PON architecture. It also permits statistical multiplexing gains for bursty traffic like Internet access and data transfers.
In the downstream direction, a continuous stream of ATM data cells and occasional physical-layer operations, administration, and maintenance cells (PLOAM cells--used to operate and administer the PON itself) is sent from the OLT to a set of subtending ONUs. Each ONU identifies the cells intended for it out of the shared stream of ATM cells by inspecting the ATM header. Two downstream line rates (155- and 622-Mbit/sec) are defined in G.983.1.
The PON aspect of the system is the splitting of the optical system. The optical signal from a single transmitter in the OLT is passively split to multiple (up to 32) ONUs. Part of the economy associated with the ATM-PON architecture is that one OLT transceiver and the fiber connection from the OLT to the optical splitter can be shared by several ONUs. The G.983.1 specification calls for a minimum logical reach of at least 20 km, an optical power budget consistent with that reach, and a maximum split ratio of 32.
Depending upon where the ONU is situated, the architecture is identified as fiber-to-the-exchange (FTTEx), fiber-to-the-cabinet (FTTCab), fiber-to-the-curb (FTTC), fiber-to-the-business (FTTBus), or fiber-to-the-home (FTTH). When the ONU is somewhere other than at the home or business, some form of high-speed digital-subscriber-line (DSL) transport can be used over existing metallic pairs to provide the final link to the end user. This reuse of existing metallic facilities is a critical economic factor driving the use of shared ONUs such as in FTTCab and FTTC. On the other hand, increased sharing of the ONU generally corresponds with greater distance between the end user and the ONU and an associated reduction in the sustainable data rate to the end user.
In the upstream direction, a time-division multiple access (TDMA) protocol is defined in G.983.1 to ensure that the ATM cells from different ONUs are interleaved without interference into a composite bit rate of 155 Mbits/sec, regardless of the downstream rate. The OLT determines when each ONU is permitted to transmit upstream and consequently how much bandwidth it is allocated. The central role played by the downstream PLOAM cells is the delivery of transmission grants to the ONUs to regulate the time and rate at which each can send cells upstream. A large part of the G.983.1 standard is involved in defining ranging and granting protocols so that OLTs can successfully talk to and control ONUs, even when the equipment is from different vendors.
G.983.1 supports a 2-fiber alternative in which the upstream and downstream optical signals are transmitted on physically different PONs. The preferred option, however, calls for a single-fiber PON on which the downstream and upstream signals are transported on different wavelengths--1550 and 1310 nm, respectively.
Because the shared optical signal from the OLT is delivered to every connected ONU, there is a possibility of eavesdropping on other downstream signals when the ONU is deployed at the home. The G.983.1 PON specification includes a simple form of data scrambling known as "churning" to provide a level of privacy protection for signals dedicated to a single ONU. But encryption operating at a higher layer of the system, such as between the service provider and video set-top box, is expected to protect broadcast services such as video.
Some technical issues are still being addressed within FSAN, but the major focus has shifted from the development of ATM-PON specifications toward deployment-related issues. A Deployment Working Group (WG) was formed last January to address these issues and help focus the direction of the following technical working groups:
- Optical Access Network (OAN)
- Operations, Administration, and Maintenance (OAM)
- Services, Capabilities, and Performance (SCP)
- Digital Subscriber Line (xDSL)
The Deployment WG also serves as a forum for network operators to discuss their business needs and determine where their equipment requirements are similar.
While the G.983.1 specification addresses the open optical interfaces for the OLT and ONU to the PON, it stops short of defining an end-to-end access system. Four of the FSAN network operators--BellSouth, British Telecom (BT), France Telecom, and NTT--have released a set of technical specifications that extend G.983.1 to define an end-to-end system2. The common technical specifications (CTS) can be found on the FSAN Web site (www.labs.bt.com/profsoc/access).
These additional specifications focus on defining technical details, including the number and types of user-to-network interfaces (UNIs) on the subscriber side of the ONU, the interfaces to ATM switches and operations systems, and the classes of service supported by the system. They also address more mundane matters such as defining the electrical voltage and power plug to be provided on the ONU, which can differ from country to country.
The subset of specifications also includes some operator-specific requirements. These specifications reflect the anticipated demand of system features. For example, BellSouth and France Telecom require 10/100Base-T UNIs this year, whereas BT and NTT do not foresee need for this interface until 2000 or later. Similarly, NTT, BT, and France Telecom expect to use ATM-25 interfaces this year, while BellSouth does not foresee it as a need until the year 2000 or later.
Each of the four contributors to the CTS has considerable experience with the deployment of fiber-access architectures prior to ATM-PON systems. BellSouth deployed its first FTTH system in 1986 and currently has more than 300,000 lines served by FTTC systems. BT offers telephony over PON. France Telecom conducted FTTH trials as early as 1979 and is currently conducting trials of FTTC and FTTH on a Tamaris platform. NTT tested a Synchronous Transfer Mode (STM) PON system in 1996 and has deployed an aerial FTTC system (FTTPOLE) in large volumes since 1997. Each of these FSAN members has also announced ATM-PON deployment plans.
BellSouth is currently planning an ATM-PON FTTH system with first-generation equipment provided by Lucent Technologies and Oki Electric. Deployment is expected in the fourth quarter. This first-generation equipment will not support telephony or video service on the ATM-PON. It will primarily serve as an aerial overbuild providing data-only service in an area of 400 homes already linked to copper facilities. Since this system will not initially support lifeline telephony service, the indoor-mounted, locally powered ONU will not require backup power. Conventional analog and digital video broadcast channels will be provided on a second PON.
BT's intended application of an FSAN-compliant ATM-PON system is to provide services to the small-to-medium business enterprise (SME). Based on an analysis of the deployment economics of ATM-PON for SMEs, BT reached these conclusions:
- An ATM-PON is more cost-effective when used as a means of access, rather than as a local-exchange bypass.
- Savings relative to Synchronous Digital Hierarchy (SDH) alternatives come from reductions in the outside-plant costs associated with remote placement of the ATM-PON splitters.
- Access-fiber savings depend primarily on service penetration, not service type, bit rate, or circuit destination.
Even when limited to the business niche, considerable cost savings can accrue as a result of properly deploying an ATM-PON access network.
France Telecom is interested in deploying ATM-PONs for building or office applications in an overlay setting. The ATM-PON will provide interfaces for existing services along with ATM interfaces to support new offerings. A second phase, the extension to SME and residential customers, requires further growth in broadband demand. By that time, it is anticipated that narrowband services--telephony and Integrated Services Digital Network (ISDN)--will be supported by the ATM-PONs. France Telecom produced a request-for-proposal (RFP) on an FSAN-compliant system in the fourth quarter of 1998, with trials slated to occur this year to pave the way for first deployments in 2000.
NTT plans to deploy a symmetric 155-Mbit/sec FSAN-compliant ATM-PON system this year. Designed to serve business customers, the network will support ATM and non-ATM data services (no telephony or ISDN). Like France Telecom, NTT expects deployment to residential and small- and home-office settings will occur later when telephony and ISDN are integrated with broadband services on the ATM-PON network.
As demand for broadband optical access networks steadily increases, initial deployments of FSAN-complaint networks are underway in Europe, Japan, and the United States. Work on the FSAN specifications is now focused on deployment-related issues. The main goal of the FSAN initiative remains the same--establishing international standards that will promote vendor interoperability, mass production of compatible optical equipment, and ultimately competitive and cost-effective solutions for broadband and narrowband services in the fiber-access network.
- ETSI DTS/TM-03024, "Optical access networks (OANs) for evolving services"; "ATM passive optical networks (PONs) and the transport of ATM over digital subscriber lines."
- Dan Spears, Brian Ford, Jeff Stern, Alan Quayle, Jacques Abiven, Sophie Durel, Kenji Okada, and Hiromi Ueda, "Common Technical Specifications for atm pon System," European Conference on Networks and Optical Communications (NOC '99).
Dan Spears is research director and Brian Ford is a member of the technical staff at BellSouth Science and Technology (Atlanta). Paul Shumate, Jr. is executive director, broadband access and premises networks, and Ron Menendez is a senior scientist at Telcordia Technologies (Morristown, NJ).