Wireline local loop

Incumbent carriers traditionally use copper wire for local loop transmissions. Twisted copper wire can carry narrowband voice traffic to a central office for switching. As ISDN (Integrated Service Digital Network) technology became available in the mid-1980s, voice and data (128 Kb/s) can be simultaneously transmitted on copper wire between CPE (customer premise equipment) and a central office. Recently, xDSL (digital subscriber line) technology has further enabled copper wire to provide simultaneous broadband data (megabit) and voice services. Among xDSL technologies, ADSL has been the most widely deployed DSL technology for broadband service. Since the existing copper loop can be upgraded to provide broadband service via DSL technology, and in light of the applications stemming from both the Internet and the world wide web, incumbent carriers have been aggressively deploying ADSL equipment to meet customer demands for broadband service. Under the current ADSL network design, voice and data services are carried via different frequencies on a copper wire. Therefore, it is possible to split the ADSL line into voice and data traffic and allow subscribers to pick and choose different carriers to provide voice and data services for them. The arrangement, called line sharing, occurs when two carriers offer service over different frequencies of the same copper wire.

Other potential local loop access technologies and architectures include the following:

• FTTx: FTTx is a network where an optical fiber runs from the telephone switch to the subscriber's premises - business or home. For the past few years, telecom companies have been working diligently to provide us with pseudo-broadband Internet connections over copper (DSL) and cable (cable modem), with practical speeds of up to 1.5 Mbps. No doubt improvements will be made over the next few years to squeeze more out of copper

and cable. However, fiber which can provide download speeds of up to 155 Mbps remains a viable candidate for the access network. Such speeds will enable instantaneous data transfer and video on demand. It also provides a future-proof network in that one does not have to upgrade from ADSL to xDSL to digital co-ax to digital wireless. Optic fiber has higher reliability and does not need electric powering and is immune to lighting and other transients. These properties of the fiber lead to lowest powering costs and operational costs (such as maintenance, provisioning, and facilities planning). However, challenges such as making FTTx cost effective will create obstacles along the way to providing FTTx.

Also the lack of product standardization and the lack of greater bandwidth demand from consumer and business applications have so far made FTTx deployment impractical.

Several new technologies are leading to cost reduction of FTTH. Previously a single fiber was needed to connect to each home separately. Technology advances permit N-way distribution of the bits to many homes through resource sharing circuits (Passive Optic Network based FTTx). Recent advances in loop lasers, fiber and other components, and chips for compressing digital video greatly reduce system costs and will bring FTTx to the forefront of broadband access technology. It will provide service providers with the potential to increase revenues from applications such as interactive access, pay-per-view, video on demand, and subscription services (www.xilinx.com).

• PON: A passive optical network (PON) is a system that brings optical fiber cabling and signals all or most of the way to the end user. Depending on where the PON terminates, the system can be described as FTTx. The passive simply describes the fact that optical transmission has no power requirements or active electronic parts once the signal is going through the network. Essentially, carriers want to connect each customer site with a wavelength of light, but they want to avoid having to dedicate a fiber to every wavelength.

PONs address this issue by bundling together multiple wavelengths (up to 32 at present)

so they can be carried over a single access line from the carrier's central office (CO) to a manhole or controlled environmental vault close to a cluster of customer sites. At that point, the wavelengths are broken out and each one is steered into a different short length of fiber to an individual site. A different scheme is used for collecting traffic traveling in the opposite direction - from user sites to the CO. In this case, each site is given a specific time slot to transmit, using a polling scheme similar to the one used in old networks.

PONs share the costs of fiber and much of the equipment located with the service provider among several customers, while also eliminating expensive, powered equipment between the service provider and these customers. The optical path is "transparent" to bit rate, modulation format (e.g., digital or analog), and protocol (e.g., SONET/SDH, IP, Ethernet).

Such transparency results from nothing being installed between the service provider and the customer, which is specific to the bit rate, modulation format, etc., allowing services to be mixed or economically upgraded in the future as needed. New services and/or new customers can be added by changing service-specific equipment only at the ends of the network, and only for those customers affected. Such flexibility is not the case in most of today's other access network architectures. Despite their advantages, PONs face significant obstacles on the road to success. The fact that PONs share bandwidth among multiple subscribers lowers service costs and helps carriers efficiently amortize the equipment and operations expenses. However, any amount of upstream bandwidth transmitted over a PON will be divvied up among the number of users at the customer site.

Therefore, on a 155-Mbit/s PON link with four splits, each subscriber will receive 38.75 Mbps. Addition of splitters to links that have already been split leads to lowering of the final available bandwidth. Also, the fact that PONs do not regenerate or convert optical signals mid-network makes them cheaper, but it also limits their reach. Without regeneration, light signals lose power quickly, consequently losing transmission capability.

Due to these disadvantages and the availability of other broadband access alternatives the market for PONs will remain relatively small for the next few years. Currently, ATM-based PON and Ethernet-ATM-based PON are the two main PON technologies that are available in the market (www.xilinx.com).

• Cable TV: Cable TV (CATV) Access refers to the use of CATV network facilities to provide "always-on" connections to the Internet from a PC. The key components of this system are the cable modem, the Hybrid Fiber Coaxial (HFC) Cable link and the Cable Modem Termination System (CMTS). The cable modem takes data from a PC and modulates it for transmission over the HFC line. The signal then travels all the way to a service provider's office where it gets terminated at a CMTS along with other HFC cables.

HFC lines have inherently higher bandwidth capacity than an ordinary phone line, up to a 1000 times, as they are designed to carry video. CATV Access technology therefore provides very fast access speeds typically ranging from 500 kbps to 10 Mbps, depending on the number of users sharing the link. The performance of the cable network is inversely proportional to the number of users accessing the network in the neighborhood.

In North America, Data Over Cable Service Interface Specification (DOCSIS) is the CATV Access standard and in Europe, Digital Audio Video Council (DAVIC) standards are used. In addition to video transmission, cable operators have been upgrading their networks to provide voice communications to their subscribers. A cable modem (set-top box) is used to integrate voice and data traffic for two-way communications at a customer site (www.xilinx.com).

Although FTTH and Gigabit Ethernet network access technologies can offer gigabits of bandwidth to subscribers, the high construction fee of laying fiber makes it economically unfeasible for local loop deployment in the near future. Meanwhile, cable networks are now

competing directly with ADSL technology for Internet broadband local access in most countries.