A winning solution

As carriers sort out how to deploy broadband in the local loop, a unified access platform can reduce the hassle of managing multiple systems - and support narrowband services No single solution is likely to be the best choice for all possible applications. In the short term, local exchange carriers may not be able to cost-justify deploying broadband solutions in the local loop.

Telcos may find that the majority of today's upgradeable narrowband deployment needs can be met by switched wireline infrastructures based on FTTC, asymmetrical DSL (ADSL) and very high bit rate DSL (VDSL) technologies.

Even within this subset of technologies, however, the characteristics of the serving area - including the length of copper plant from the central office or nearest cross-connection point, the quality of the existing plant, the presence of high-rise buildings and the density of the subscribers - will affect the choice of technologies. FTTC deployments with coaxial drop cables are cost-effective for new build/growth areas and rehabilitation scenarios, while areas with good quality copper plant and high-rise buildings will require ADSL/VDSL solutions because of the need to reuse the existing twisted wire drop cables.

The challenge is to support the wide range of deployment scenarios using local loop platforms that can be configured for a variety of technologies, while at the same time providing for unified service and equipment provisioning and management. Unified access platforms are designed to fulfill those requirements.

They also can support a full range of revenue-generating narrowband services today and can be upgraded to support broadband services in the future.

New growth and rehab FTTC technology has matured to the point that it is frequently a cost-effective choice for deployments that are primarily narrowband but have some business data/broadband requirements and where residential broadband service is likely to be deployed in the near future. New growth and rehabilitation areas are prime candidates for FTTC technology.

If drop cables are being installed or replaced, it is possible to use a coaxial drop cable from the fiber-fed terminal to each residence. The coaxial cable provides connectivity to the high-bandwidth fiber optic network to support broadband services. It can also be used to provide narrowband services beyond basic POTS, which is supported by traditional twisted pair drops.

Business users can be served from a universal service access multiplexer installed on the customer premises, which acts as a remote terminal and can provide a full range of telephony interfaces such as POTS and basic rate ISDN (Figure 1).

The universal service access multiplexer can support additional business services including DS-1 (1.5 Mb/s), primary rate ISDN, high-bit-rate DSL (HDSL) and services based on asynchronous transfer mode user-to-network interfaces. HDSL, which provides bandwidth in the DS-1 range, can be deployed from the universal service access multiplexer, eliminating the need to install T-1 lines and cumbersome T-1 repeaters.

For residential and some small business applications, the FTTC solution uses a broadband network unit located within 500 feet of the home, with twisted pair drops for narrowband services and coaxial drops for additional narrowband and broadband services. A passive network interface device is used at the residence to support basic telephony services over twisted pair wiring, while an active network interface device can be located on the network side of the demarcation point to support special services such as digital data services and circuits connecting to state lottery networks.

The active network interface device connects to the broadband coaxial drop cable on the network side and presents a standard regulated service interface on the customer side. It is quite similar to today's digital added main line.

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 a residential gateway that can be used as a centralized service platform for voice, video and data services.

At the CO or remote terminal location, a broadband digital terminal interfaces with the narrowband and broadband networks. Interface requirements include the TR-008 and GR-303 narrowband interfaces over DS-1 and OC-3 (155 Mb/s) connections, as well as ATM interfaces - unidirectional OC-12 (622 Mb/s) links for broadcast-like video services and bidirectional OC-12 links for interactive services. An element management system supports provisioning of narrowband/broadband services and equipment, and interfaces to legacy and emerging operations support systems (OSSs).

The universal service access multiplexer also can serve as a CO terminal - either at the CO or at the remote terminal site where the broadband digital terminal is deployed. When used in this manner, the service access multiplexer can support unbundled local loops and special services.

Multitalented A unified approach to upgrading the local loop would allow carriers to deploy FTTC, ADSL or VDSL from a single access platform, providing increased economies of scale and enabling carriers to use a common set of methods and procedures.

This approach also would support a high level of OSS integration, which should minimize costs and provide a higher level of reliability. A common skill set would be required for OSS personnel, minimizing training requirements.

Key to such a platform is the use of an integrated transport method to support traditional telephony services as well as packet-based video and data services, while providing for a separate quality of service for each data stream. From a mechanical perspective, it is essential that the broadband digital terminal interface with a variety of optical fiber- or electrical cable-fed shelves, which in turn support everything from traditional POTS to FTTC to various DSL flavors.

The use of a single fiber from the broadband digital terminal to outlying terminals is desirable to reduce fiber count and minimize cross-connects, splicing, and installation and connector costs. Fortunately, the availability of inexpensive wavelength division multiplexing components has made single fiber solutions possible at an acceptable cost. The use of multiple fibers can be reserved for applications requiring redundancy or additional capacity.

When a unified access platform architecture is used, a broadband digital terminal may be located in the CO or at a central wire center serving multiple COs (Figure 2). Alternatively, the broadband digital terminal can be installed in a cabinet, controlled environment vault or at the customer premises.

The unified access platform may incorporate features from several different Bellcore specifications, including the ATM-based service access multiplexer and fiber in the loop (FITL). Quality-of-service monitoring and the ability to establish virtual channel connections have emerged from Bellcore's vision of transporting voice, video and data from an integrated platform.

One benefit of the unified access platform approach is the ability to integrate telephony services with advanced data services. In contrast, deployment of an overlay DSL access multiplexer (DSLAM) precludes integration of data services with telephony.

That approach would require complex wiring to and from the cross-connect frame and would make it more difficult to isolate problems between the telephone line and DSLAM or other systems. Switch-generated noise on the analog connection might impair video or data services. DSL filters at the DSLAM or VDSL multiplexer could complicate mechanized loop testing. And this approach also would require additional lightning and power protection.

The DSLAM also does not permit the economies of scale that a unified access platform provides. The unified access platform supports deployment of a single broadband digital terminal and multiple, low-cost remote shelves. The DSLAM requires deployment of a full system at every CO.

Upgradeability is also an issue. The broadband digital terminal for a unified access platform can support a DSL upgrade by moving the shelf in the CO used for ADSL services to the serving area interface box located between the CO and the customer premises. By reducing the distance between the shelf and the customer - and upgrading the modem on the customer premises - higher VDSL line speeds can be offered.

The ability to perform this upgrade is facilitated by the use of ATM as a transport mechanism for the unified access platform. DSLAMs, in contrast, typically are based on switched multimegabit data services or DS-0 services, which provide less bandwidth flexibility.

Traditional digital loop carrier (DLC) products may permit upgrades to DSLAM-based ADSL if the DLC system was installed within carrier serving area rules, but classical non-broadband DLC architectures have the inherent limitation of supporting neither ATM transport nor an interface to the ATM network. The unified access platform serves as a broadband-ready DLC that shares a large broadband digital terminal and provides a cost-effective narrowband switch interface as well as ATM switch interfaces. A typical broadband digital terminal used with a unified access platform can support up to 64 DLCs.

Telephony interfaces The problem with the traditional method of bridging a DSL line from an existing copper pair is that there are two twisted wire pairs from the cross-connect frame, two sets of lightning protection and unknown characteristics in terms of the trip ring, and other impulse noise on the POTS line that could be detrimental to the DSL signal (Figure 3).

An alternate approach is to derive the POTS signal locally on the DSL line card, as opposed to having it originate at the switch (Figure 4). This approach minimizes the amount of lightning protection required, as well as ensuring that the impulse noise generated by the POTS circuit is characterized and controllable.

In addition, the displaced feeder pair and its switch port can be recovered and reused to relieve second lines or for growth. By moving the line to the switch-integrated environment, it should be possible to have lower maintenance and trouble reports because the feeder pairs and cross-connects have been removed.

The DSL bridging method allows the line to be tested by a local loop tester similar to that proposed in Bellcore's TA-909, which covers FITL. This method eliminates the risk that mechanized loop testing will be unable to work through the POTS filter or will give false results from the distribution section beyond the line card.

By removing the feeder section, testing that portion of the customer line becomes unnecessary. Because the DSL line has continuous performance monitoring, the need for a full mechanized loop test is minimized, often eliminating the need for a remote tester.

Another method for delivering telephony services using DSL and a unified access platform is to derive the telephony signal from the DSL signal at the residence. This is possible with a unified access platform because the TR-008 or a TR-303 interface to the Class 5 switch is incorporated into the system.

Telephony services such as POTS or ISDN can be derived at an active network interface device on the side of the residence or in customer premises equipment that is either standalone or integrated into computer equipment, as is the case with an ISDN modem.

All three of these options - overlay POTS, DSLAM-derived POTS and premises-derived POTS - are desirable in different scenarios. Overlay POTS works well when a carrier wants to deliver digital data services immediately to a limited number of customers. DSLAM- derived POTS works well in a rehabilitation scenario in which new POTS technology is being deployed but the existing drop cable is being reused. Premises-derived POTS works well when a customer needs a second phone line in addition to broadband services.

Although telecommunications equipment manufacturers and LECs have come to think of competition between local loop technologies as normal and healthy, it is likely that technology cooperation - rather than competition - will be the winning solution in the deregulated broadband telecommunications world.

The industry has been staging field trials of FTTC technologies for more than 15 years. Carriers can now cost-effectively deploy FITL systems for narrowband services knowing that these systems can support future upgrades for broadband services.

Intense competition among DSL component manufacturers will result in cost-effective, single-chip solutions in the near future. A unified access platform architecture allows these technologies to be co- deployed on a single platform. Simultaneously, it can also support FTTC as well as a traditional DLC.

Integrated OSS functionality for all the deployment technologies will also be an important factor in the battle for customers that deregulation has ignited.

Thomas Eames is President, William Weeks is Vice President of Technology and Charles Eldering is Director of Advanced Systems Engineering for Next Level Communications, Rohnert Park, Calif. Eames' e-mail address is tre@nlc.com.