The Unified Access Platform for the Local Loop


Bill Weeks, Thomas Eames and Chuong Vu

Next Level Communications

6085 State Farm Drive

Rohnert Park CA 94928





When most of us get online from home today, we dial up an Internet Service Provider (ISP) on our phone line. If you are the one always with the latest toys, you probably have a 56Kbits/s modem at home and dialing into an ISP with a 28.8Kbits/s or 56Kbits/s connection. At these rates, you are already ahead of many people. However, you would still find them not fast enough to download large files or graphics, or to do voice on the Internet. Then there are trials in various parts of the globe by telephone companies (telco’s) to provide digital video broadcast and video-on-demand (VoD) to the subscribers’ homes.


The inevitable demand for high bandwidth and high quality digital services, using new drop technologies, requires the telco’s to upgrade their equipment. Various technologies have been explored to increase the network transport rate for the above applications. They include wireless, Hybrid Fiber Coax (HFC), Fiber-to-the-Curb (FTTC), Fiber-to-the-Home (FTTH), Asymmetrical Digital Subscriber Line (ADSL) and Very high rate Digital Subscriber Line (VDSL).


Therefore, any equipment deployed today must be capable of being incrementally upgraded to provide broadband services, including digital video and high-speed Internet access, at high penetration rates.


FTTC, ADSL, and VDSL technologies would best satisfy the needs of these deployments. Which of these technologies to use depends on the characteristics of the serving area. The determining characteristics are the length of copper plant from the central office or nearest cross connection point, the quality of the existing copper plant, the presence of high-rise buildings and the density of the subscribers. FTTC deployments with coaxial drop cables are cost-effective for new build or new growth areas and rehabilitation scenarios. ADSL/VDSL solutions can be deployed for areas with good quality, re-usable copper plant and high-rise buildings.


The challenge for the telco’s then is to support the wide range of deployment scenarios using local loop platforms which can be configured to use a variety of drop technologies, while at the same time providing for unified service and equipment provisioning and management. Unified Access Platforms (UAPs) are local loop systems which meet these goals and which can provide the full range of revenue generating narrowband services today and be upgradable for broadband services in the future.


I will now discuss the FTTC, and ADSL/VDSL equipment and deployment scenarios for the UAP.


Fiber-to-the-Curb (FTTC) and Fiber-to-the-Business (FTTB) for New Growth and Rehabilitation


New growth and rehabilitation areas are prime candidates for the matured FTTC technology. As new drop cables are installed or replaced, a coaxial drop cable from the fiber-fed terminal to each residence can be installed (Figure 1). The coaxial cable provides connectivity to the high-bandwidth fiber optic network for broadband services, and can also be used to provide narrowband services beyond the basic POTS (Plain Old Telephone Service) lines. POTS are still supported by traditional twisted pair drops. FTTC is frequently a cost effective choice for deployments which are primarily narrowband, but have small business data and /or broadband requirements, or where residential broadband service is likely to be deployed in the future.


Figure 1. FTTC Architecture


For residential applications, the Next Level Communications (NLC) FTTC solution brings a single fiber, bi-directional 155Mbits/s (OC-3c) from the Broadband Digital Terminal (BDT) in the central office to the 8 or 16 home level. It utilizes a Broadband Network Unit (BNU) located within 500 feet of the residence, at the curbside (hence the term FTTC). Using different narrowband and broadband linecards, the BNU provides twisted pair drops for narrowband services, and point-to-multipoint coaxial drops for both broadband and additional narrowband services. One BDT supports 64 BNU’s. The distance from the BDT to the BNU can be up to 10 miles.


A passive Network Interface Device (NID) is used at the residence for basic telephony services over twisted wire pair, as in all POTS installations today. Broadband devices are connected directly to the coaxial wiring in the residence, and provide a variety of data, video, and telephony services. An alternative to having multiple connections to the coaxial cable in the residence is the use of a residential GateWay, which can be used as a centralized service platform for voice, video and data services (Figure 2).


The rates on the coaxial drop are 51.84 Mbits/s downstream, and 19.44 Mbits/s upstream. When some portion of the drop consists of twisted pair, a lower bit rate signal is utilized.


The BDT is a native ATM machine. It transports isochronous TDM services (POTS, ISDN, DS1) with ATM services (IP data and MPEG2 video) together on the fiber cable.


The BDT also interfaces to the narrowband and broadband networks (Figure 2.1). Narrowband interface requirements include Bellcore TR-008 and GR-303 over DS1and OC-3 connections (V5.1 and V5.2 over E1 for ITU requirements). Broadband ATM interfaces include both unidirectional OC-12 links for broadcast video services, and bi-directional OC-12 links for interactive services. An Element Management Layer (EML) supports provisioning of narrowband/broadband services and equipment, and interfaces to legacy and emerging Operational and Support Systems (OSS).


Small and medium businesses can be supported by the deployment of a Universal Service Access Multiplexer (USAM) on the premises. The USAM acts as a Remote Terminal (RT) or Customer Premises Equipment (CPE) and can provide the full range of telephony interfaces including POTS, ISDN, DS1, and services based on ATM UNI interfaces. With 16 hex linecards, the USAM can support up to 96 POTS or Basic Rate ISDN lines.


The USAM can also serve as the means for interfacing specials and additional (e.g. unbundled) loops by serving as a Central Office Terminal (COT) at the central office or remote terminal site where the BDT is deployed.



Figure 2. FTTC for business and residential services.


Figure 2.1. High Level View of an Access System’s Interfaces to Narrowband and Interactive Broadband Networks


ADSL/VDSL Support and the Unified Access Platform Approach


We just saw how the BDT is used with the BNU for a FTTC deployment, using twisted pair drop for narrowband, and coaxial drop for broadband and additional narrowband services to the home. The USAM is used to provide narrowband and broadband services to the businesses. But as a flexible equipment, the USAM can also be used where re-using existing twisted pairs in the distribution area is needed to support ADSL/VDSL to deliver narrowband and broadband services.


In such deployments, the ADSL/VDSL signal can be placed on the twisted pair at a Serving Area Interface (SAI) if the USAM is located in an outside plant cabinet (Figure 3), or it can be derived from the basement of an (Multi-Dwelling Unit) MDU or high-rise apartment (Figure 4). In this latter case the USAM is placed in a basement rack, configured with ADSL/VDSL linecards (up to 16 dual linecards or 32 xDSL circuits).

Figure 3. The NLevel3 ADLS/VDSL Application from the FDI


Figure 4. USAM VDSL Application from the MDU Basement


The ADSL/VDSL signal terminates at the customer premises at one of the ADSL/VDSL termination products provided by NLC. In other words, one ADSL/VDSL modem resides in the USAM, at one end of the twisted pair. One modem resides at the other end of the twisted pair at the customer premise.


So with the use of the same BDT, the deployments of FTTC, FTTB and ADSL/VDSL technologies in the local loops can be achieved with the installations of BNU’s and USAM’s. This unified approach would allow the telco’s to deploy all these technologies from a single access platform, which leads to lower costs and better reliability and low trouble rates.


The single BDT provides an integrated, ATM transport method for the unified access platform (UAP). It allows the transport of isochronous (telephony type) services as well as packet-based video and data services, while allowing for a separate Quality of Service (QoS) for each data stream. The host terminal (BDT) must be able to support a variety of optical fiber or electrical cable fed shelves which in turn support everything from traditional POTS, to Rate Adaptive, Asymmetric, or Very high speed Digital Subscriber Line (xDSL), FTTB and FTTC. Furthermore, the use of a single fiber from the host to outlying terminals is desirable to reduce by half the fiber count, cross-connect, splicing, size, installation, and connector costs. Multiple fibers may be used when redundancy or capacity is required.


Figure 5. A Universal Access Platform (UAP) architecture.


Figure 5 illustrates a UAP architecture, in which a host system - located in a serving central office, a central wire center serving multiple central offices, at a cabinet or controlled environmental vault, or on a large customer premises - is capable of supporting ADSL, VDSL as well as FTTC and FTTB. The system is somewhat of a hybrid between Bellcore’s vision of a combined ATM and TDM TA-909 FTTC and the ATM based Service Access Multiplexor, by combining the functions and best features from both architectures.,


Another benefit which emerges from this approach is the ability to integrate telephony services with advanced data services. Whereas deployment of an overlay, no-host, Digital Subscriber Loop Access Muliplexer (DSLAM) such as a non-integrated ADSL add-on system precludes integration of data services with telephony.


Traditional Digital Loop Carrier (DLC) products may permit upgrades to DSLAM-based ADSL if the DLC system was installed within CSA rules. However, classical non-broadband DLC architectures have the inherent limitation of not supporting ATM transport nor the interfaces to the ATM network. The UAP serves as a broadband-ready DLC that shares a large host (typically up to 64 DLCs) and provides a cost-effective narrowband switch interface as well as the ATM switch interfaces.


ADSL/VDSL Modem Technology


I have described the deployment scenario where ADSL/VDSL can be supported with the UAP. With a limited time, I will briefly describe the modem technology used for the ADSL/VDSL twisted pair drops between the USAM and the CPE. The NLC-proposed solution for ADSL/VDSL involves carrying the POTS signals on the twisted pair at baseband and separation of the POTS service at the entry to the customer premise in a passive NID. Therefore, the upstream and downstream signals are implemented in a way so that they do not interfere with POTS signals on the same twisted pair. Note that although the USAM’s ADSL/VDSL solution is proposed for existing wiring, this wiring must be of adequate quality in order for ADSL/VDSL to perform reliably.


The ADSL/VDSL modem being utilized initially by NLC is the most advanced single chip, low power device currently available. The device is a bi-directional native ATM device, which utilizes t = 8 Reed Solomon forward error correction, a powerful 96 tap equalizer (for AM ingress immunity) and fully integrated A/D and D/A converters. It is able to operate with true rate adaptability by selecting any modulation from QPSK (Quadrature Phase Shift Key) to QAM/CAP 256 (Quadrature Amplitude Modulation/Carrierless Amplitude/Phase) and allows fine granularity in the selection of carrier frequencies and symbol rates, permitting a very fine adaptation to the twisted pair conditions and spectral demands. Carrier frequency and rate adaptability are both critical advantages to a copper based transmission scheme for high quality and high reliability broadband service delivery. NLC intends to support DMT (Discrete MultiTone) when that technology matures, or becomes standard.


The NLC ADSL/VDSL modem is frequency agile, allowing the center frequencies and symbol rates of the upstream and downstream signals to be picked with flexibility. The rates used by the ADSL/VDSL modem are adaptable to allow the best performance for a given twisted pair quality and noise ingress. The design is also based on a point-to-point topology. Topologies that attempt point-to-multipoint from twisted pair to inside coax, as for video connection, are viewed by NLC as very risky due to the spectral incompatibility of the coax and the twisted pair and the high ingress potential of the inside coax for an ADSL/VDSL signal.


The various data rates currently chosen by NLC for ADSL/VDSL solutions are listed in Table 1 and 2. The loop reaches are expected values based on simulations, not measured. These are not finalized but provide a good view of both the rates and reach achievable by the ADSL/VDSL technology. The rate adaptive nature is seen as essential in order to get the maximum reach and rate from each different loop.


Downstream Rate

Upstream Rate

Loop Reach 24ga. AWGN

6.144 Mbps

640 Kbps



Table 1. Preliminary ADSL Rates



Upstream Rate

Loop Reach 24ga. AWGN

6.48 Mbps to 51.84 Mbps

1.08 Mbps to

3.24 Mbps

4 Kft down to > 2Kft


Tabel 2. Preliminary VDSL Rates


In high-rise deployments, the NLC VDSL modem will vary both the upstream and downstream signal’s symbol rate and modulation format in order to adapt to the conditions of the riser cable.




As more people demand high bandwidth and high quality digital services for new applications, such as high speed Internet access and Video on Demand, new drop technologies will have to be deployed by the local telephone companies. It is likely that a mix, rather than competition, of technologies will be the winning solution in the deregulated broadband telecommunications world. FTTC (Fiber-to-the-Curb) technologies have been used on a field trial basis for well over 15 years, and are cost-effectively deployed for narrowband services today. But these equipment must be upgradable for broadband services in the future. The intense competition among the xDSL (Digital Subscriber Line) component manufacturers will result in cost-effective single chip solutions in the near future. The Unified Access Platform allows these technologies to be co-deployed in a single platform.


The Unified Access Platform approach is based on a transport system with the ability to deliver isochronous voice services over an ATM local loop network which also supports the full range of broadband services, and the ability to deploy xDSL solutions simultaneously with FTTC solutions as well a traditional DLC (Digital Loop Carrier).