PDF of this tutorialThe three main DSL service types—asymmetric DSL (ADSL), symmetrical DSL (SDSL), and integrated services digital network (ISDN) DSL (IDSL)—each enable different applications and serve differentiated markets.
ADSL fulfills the needs of the mass market of residential users, as its asymmetry is ideal for using the World Wide Web (WWW). Full-rate ADSL provides roughly 8 Mbps downstream and 0.64 Mbps upstream, while lite versions will provide roughly 1.5 Mbps and 0.5 Mbps. As a result of its mass-market appeal and its ability to counter the cable-modem threat, ADSL is expected to take the largest share of the three DSL varieties. End-user pricing for ADSL service generally runs from $40 to $100 per month, depending on the speed.
While ADSL is likely to be the most attractive option for casual Internet users, SDSL is the most popular with businesses and teleworkers. SDSL meets the requirements of these segments, because symmetric bandwidth up to 1.5 Mbps mimics LAN connectivity. This enables workers to send and receive large files from corporate servers with high speed in both directions. As the business and teleworker segments are predicted to be early adopters of DSL services, SDSL is an important offering. SDSL is also generating significant interest in the telco industry because symmetry provides an advantage over cable modems. Because SDSL serves the less price-sensitive and more service-conscious business segment, it is priced higher than ADSL—usually around $150 per month.
IDSL serves a unique market segment as a result of its greater reach. Typical IDSL speeds are 128 or 144 kbps, symmetric, and the technology is ideal for any customers too far from the central office (CO) for ADSL or SDSL, as well as any customers wanting to preserve their existing ISDN CPE.
The Multiple Benefits of Multiservice DSLAMs
To offer a menu of DSL services cost effectively, carriers must deploy these services from a single integrated DSLAM platform. Unfortunately, most of the DSL systems available today cannot accommodate the full suite of DSL technologies. This leaves a service provider with the following three choices:
- deploy a limited DSLAM and ignore some market segments
- serve the whole DSL market by deploying multiple DSLAMs from different vendors
- deploy a multiservice DSLAM with the ability to serve the entire broadband-access marketplace
A multiservice DSLAM is a broadband-access network element (NE) that combines support for multiple DSL transmission types. When coupled with high-capacity asynchronous transfer mode (ATM) switching, multiservice DSLAMs deliver scalability, port density, and a redundant architecture for reliability. Multiservice DSLAMs, together with various CPE elements, can enable the relatively efficient deployment of broadband networks for high-speed Internet access as well as voice and video applications. Such DSLAMs often allow for full ATM switching, traffic management, and quality of service (QoS), in addition to the delivery of a full range of services. These services include analog, ISDN, IDSL, SDSL, rate-adaptive DSL–competive access provider (RADSL–CAP), and RADSL–discrete multitone (DMT) on a single platform.
The multiservice DSLAM can also be configured to add value in the form of routing and security functionality. The device is intended to enable service providers to optimize the bandwidth of existing infrastructure as well as deliver high-speed, integrated services over a single physical-access medium.
DSLAM Scalability: Avoiding the Hidden Costs of DSL Deployment
One of the costs of scalability is in network management. The costs associated with network management will be roughly proportional to the number of NEs. A carrier with an evolving DSL network may have to do four software upgrades each year: two scheduled upgrades and two bug fixes. Suppose that each upgrade takes two hours to download and verify correct operation at each NE. At a fully loaded cost of $100 per hour for the network manager’s time, this represents an annual cost of $800 per element. Software upgrades are only one category of management cost; others would include installation of added work stations, alarm monitoring, and periodic maintenance. The total annualized network-management costs could easily approach $2,000 per element.
Trunking—the Largest Hidden Cost
Even more significant than network management will be trunking costs. In the year 2002, when there are an average of 2,000 xDSL lines deployed at each CO, Carrier A’s DSLAMs would require an optical carrier (OC)–3 trunk for each group of 500 lines, while Carrier B’s DSLAMs would need only one trunk for each group of 2,000 lines. In addition to needing fewer trunks, Carrier B can also aggregate more traffic onto each and therefore utilize higher-speed trunks as well. The prices of trunking are not linearly proportional to bandwidth, so the higher-speed trunks will save significant costs for Carrier B. Suppose that digital signal (DS)–3 trunks cost $3,000 per month, and OC–3 trunks have a value of $6,000 per month. These values represent cash costs to a competitive local exchange carrier (CLEC) and opportunity costs to an incumbent local exchange carrier (ILEC). In either case, they are real and affect the bottom line. In our example, Carrier A will experience annual trunking costs of $144 million, while Carrier B will only have costs of $36 million. Even if Carrier A only uses DS–3 trunks for each 500-line group, its annual trunking expense would still be $72 million—twice what Carrier B must pay.
