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DSL - Broadband in the Local Loop By Rex Cardinale, Chief Technology Officer, Covad Communications. There are actually several types of DSL technology, with each technology having advantages for particular market segments. ADSL Asymmetric DSL is designed to support higher transfer rates in the downstream
direction (toward the end user) than in the upstream direction. This matches
the typical profile of residential consumers, whose most common network
application is web access. ADSL uses frequency splitting, in which the
upstream and downstream signals use different frequency bands, both of
which sit above the existing voice frequencies. Symmetric DSL provides the same data-rate in both directions. This provides
a good match for the needs of most business users, who frequently need
to send as well as receive large amounts of data. SDSL uses the same frequency
band for both the upstream and downstream directions, using echo cancellation
techniques. Current SDSL technology uses baseband frequencies and hence
cannot share loops with existing voice services ISDN DSL uses existing ISDN technology to provide a lower speed version of DSL. By combining the 2B+D ISDN channels, IDSL provides 144 Kbps symmetric data rates. Unlike ISDN, however, IDSL runs as an always-on, un-switched service Design of DSL services requires consideration of a number of factors that affect the actual data rates achievable over the local loop. These factors include loop length, wire gauge, presence of bridged taps, and potential crosstalk from other loops in the same bundle. Service providers must engineer all of these factors into their DSL performance specifications. DSL Equipment In order to provide DSL service over a local loop, DSL equipment must be installed at both ends of the loop. The customer premises equipment, or CPE, may be a simple DSL modem attached to a single PC, or it may be a more complex router or bridge capable of connecting multiple PC's to the DSL line through a local area network. Terminating the other end of the local loop, in the ILEC central office, is a DSL access multiplexer, or DSLAM. The basic function of the DSLAM is to terminate large numbers of DSL lines and aggregate the traffic for all of these lines onto higher speed trunk circuits within the provider's data network. Most of the popular DSLAMs accommodate multiple types of DSL within the same chassis. This gives the service provider the flexibility to serve multiple markets with the same network. DSLAMs typically provide slots for a variable number of line cards, with each line card supporting multiple DSL lines. Many service providers also install automated loop test equipment within
the central office. This allows them to streamline both the provisioning
and troubleshooting processes. Loop test equipment diagnoses the electrical
characteristics of the local loop, and sends the results back the provider's
network operations center (NOC) for centralized network management. Some
loop test equipment also provides a cross-connect function, allowing a
loop to be patched to a different line card in the DSLAM in the event
of a card failure. The CPE and DSLAMs are key components of the data networks that are deployed by DSL service providers. The majority of these data networks are based upon ATM technology, employing carrier-grade ATM switches at network hub sites. Each ATM hub site terminates the high-speed trunks from up to several dozen DSLAMs. A large metropolitan region will typically have several hub sites distributed within the region. Today's trunks are typically DS-3 , growing to OC-3 and higher as subscriber counts increase. One of the benefits of this hub and spoke architecture is that it scales extremely easily and economically, since trunk speeds can be optimized for the actual number of subscribers on the network, and can be increased when necessary. In addition to terminating the DSLAM trunks, the regional ATM switches provide the connection points for the upper layer service providers. A well-designed DSL network also provides a centralized network management
capability. Separate, lower speed management circuits are typically deployed
between each central office and the regional hubs, and the hubs are connected
via redundant management networks back to the NOC, allowing centralized
management of a large-scale network. In addition to solving the Internet access bottleneck, the DSL network architecture also provides a platform on which to provide high-quality voice services at reduced costs. This application has high potential within the small/medium business market segment. The concept is very straightforward: use the high bandwidth of the DSL line to carry not only Internet data, but several toll-quality voice connections as well. A 1 Mbps SDSL line is capable of carrying over a dozen voice lines in addition to data. This voice over DSL (VoDSL) application is a direct extension of the network architecture described earlier. It involves two additional pieces of network equipment, a VoDSL gateway, and an integrated access device, or IAD. The function of the VoDSL gateway is to interconnect the DSL access network and the existing Public Switched Telephone Network (PSTN). In a model analogous to the connection of the ISP to the DSL provider for data, the VoDSL gateway connects a voice services provider to the DSL access network. The gateway connects to the Class 5 voice switch using the GR-303 interface, which was designed for connecting digital loop carriers (DLC's) to voice switches. The VoDSL gateway connects into the DSL ATM network with a DS3 or OC-3 circuit. Using the AAL2 ATM adaptation layer, multiple voice lines may be efficiently carried over the DSL line to the end user. A separate PVC is provisioned to carry all the voice lines between the customer premise and the gateway. The second new piece of equipment required for VoDSL is the IAD, which resides at the customer premise. The IAD provides three primary functions: (1) it serves as the DSL modem (normally SDSL for this application); (2) it provides the IP routing function for the local area network connection to the DSL line; and (3) it provides the interface for multiple voice lines. In the small business market, this voice interface typically connects to a key system or small PBX, although phones can be linked directly to the IAD if desired. Standard analog phones are used with VoDSL, and the IAD performs the analog-digital conversion and the packetization between the analog phone lines at the customer premise and the ATM network of the DSL service provider. Status and Future of DSL DSL will continue to ride the technology curve in the years ahead. Continued improvements in digital signal processing technology will drive DSL modem cost and power down while driving performance up, just as with every other modem technology to date. CPE will continue to drop in price and increase in function. As increasing numbers of homes add second and third computers and new computing devices, home networking will become a requirement, and every computer will need high-speed access. In addition, expect wireless LAN technology to drop in price and become integrated with DSL CPE, providing DSL access to all devices in the home. VoDSL will eventually migrate to the residential market, and cordless phone base stations will be integrated into the CPE as well, supporting multiple phone lines within the home. While there are sizable challenges to meeting the large-scale demand and providing high-quality customer service, the application of modern software technology and innovative business models appears to be a winning combination for DSL providers.
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