A panacea for DSL access

The days of high-priced ISDN connectivity may soon be over. A new, readily available transmission technology-ISDN digital subscriber line-offers a painless way for ISDN users to exit the dial-up world while saving money and increasing access.

Although ISDN is a widely used technology, it has severe limitations for many business-oriented applications, especially those that rely on dedicated network connections such as Webcasting and push or pull technologies. Not only can dedicated ISDN access be quite expensive, it is not always possible to get full 128 kb/s throughput. And because ISDN is a dial-up solution that uses the voice infrastructure, congestion problems can arise.

IDSL has generated a lot of interest not only because it is the easiest and least expensive to deploy variety of DSL, but also because its technological similarity to ISDN makes it easier to evaluate. Like ISDN, IDSL offers a choice of bandwidths: the standard 64 and 128 kb/s as well as a higher bandwidth of 144 kb/s. Because IDSL is a symmetrical service, this bandwidth is available in both directions. Also like ISDN, IDSL employs 2B1Q line coding. Finally, IDSL can run on ISDN-ready local loops-up to 18,000 feet on a single pair without a repeater and up to 36,000 feet with a standard "U" loop mid-span ISDN repeater.

Yet IDSL can cost far less than ISDN, making it an excellent fit for small to medium-sized businesses, a typically underserved market. And it offers a number of advantages to carriers, service providers and end users alike because it solves many of the problems associated with ISDN.

Leased line, low cost ISDN can be an expensive service for small to medium-sized businesses that require frequent or permanent network access. One 64-bit B channel typically costs 2 cents a minute. A small business with a Web server may require two B channels 24 hours a day, seven days a week at a monthly cost of $1780 or more. ISDN is so expensive because it uses the dial-up voice infrastructure, which employs time division multiplexing (TDM). Using TDM means that every time an incumbent or competitive local exchange carrier adds a new subscriber, the infrastructure needs to be expanded for end-to-end circuit switching. This continual upgrading of switches is an expensive proposition.

The existing alternate for an "always-on" application-fractional T-1, which sells in increments such as 56 and 128 kb/s-is not cost-effective either. Fractional T-1 requires conditioned T-1 circuits with repeaters every 3000 to 6000 feet and costs service providers $300 to $900 a month. Fractional T-1 is thus very costly, even though the full T-1 capability may not be used.

IDSL bypasses the voice network infrastructure, guaranteeing a full 144 kb/s across a single pair in an unconditioned loop. With a typical cost of about $15 a month per local loop when leased from incumbent LECs, IDSL is much less expensive than a fractional T-1 line. In addition, IDSL employs statistical multiplexing, which cuts the infrastructure cost. The service provider simply needs to deploy a DSL concentrator equipped with IDSL line cards. These technology differences add up, enabling carriers to offer an always-on IDSL connection at a low flat rate.

Where no DSL has gone before... ISDN has its share of congestion problems that stem from its use of the voice infrastructure. When using ISDN, customers have to contend with voice calls for the same ports in the same switch. Whoever dials in first is connected; if lines are congested, the user may get a busy signal. The network congestion created by long data calls can potentially block voice calls for emergency services.

Calls from both ISDN and dial-up customers go to the Class 5 voice switch at the central office and then are directed through the voice network. IDSL data calls do not go through the voice infrastructure; they are routed through a DSL access multiplexer and then directly to a data network such as the Internet. This alleviates congestion in the voice network and in the process provides better connectivity (Figure 1).

Another major benefit of IDSL is that it is the only DSL solution that works from the CO when digital loop carriers are on the local loop (Figure 2). A DLC extends the reach of the CO by terminating copper wires from the CO and transmitting the signals over a T-1 or fiber optic line. At the other end of the line, which may be several miles away, another DLC sends the signals out over the local copper loop to the subscriber. It is not easy for CLECs to gain access to DLCs in the field and install a DSL card; existing DLCs may not even have enough backhaul bandwidth to accommodate DSL or enough physical space to add many DSL cards. Therefore, IDSL is the only DSL technology that CLECs can use to remain transparent to DLCs and offer high-speed data service for users served by DLCs.

Local loops that use DLCs represent a large customer base, making IDSL an attractive technology to CLECs and other service providers that don't want to be shut out from the large portion of the market served by DLCs. An estimated 20% of existing local loops in the U.S., or approximately 27.3 million lines, are provisioned through DLCs. The use of DLCs varies widely. In California, for example, about 4% of lines go through DLCs, while about 40% of lines in cities in Texas go through DLCs. The percentage may be even higher in other parts of the country.

Another important factor in IDSL's favor is that many local loops already have ISDN repeaters-and IDSL is the only type of DSL that is transparent to these repeaters. Even if an ISDN-ready loop has no repeater, the loop will only work with IDSL if a DLC is already on the loop. Of course, symmetrical DSL also works on ISDN-ready loops that don't have DLCs and repeaters.

The full bandwidth Although ISDN can theoretically provide a total throughput of 128 kb/s, it is more realistic to assume it supports 64 kb/s per B channel. Although incumbent LECs may guarantee two B channels, they need to bond the terminal points of the channels at a central site to get an aggregate throughput of 128 kb/s. Not all service providers have implemented the technology needed to bond the channels, especially when they terminate in different access servers. This happens quite frequently because most carriers offer ISDN as a bandwidth-on-demand service, so the second channel is completely separate from the first. The result: Even when two 64 kb/s B channels are available, the customer may never get an aggregated throughput of 128 kb/s. IDSL, on the other hand, supports bandwidths of 64, 128 and 144 kb/s over one line-the bandwidth is never split into two channels.

IDSL allows carriers to reach a large number of customers that otherwise could not use DSL, either because of the loop reach limitations of DSL or because they are served by loops that incorporate DLCs. Because IDSL can run over lines with ISDN repeaters, which provide a longer loop reach, it can reach users who live beyond the 12,000- to 18,000-foot limit of other DSL services. Carriers can also use IDSL to reach those customers traditionally served only by DLCs. IDSL thus enables carriers to expand their customer bases into the bandwidth-hungry suburban and rural areas not otherwise served by DSL, where customers may be many miles apart.

IDSL is not only compatible with ISDN-ready loops, it is also compatible with existing ISDN routers and bridges that support leased-line functionality with frame relay or PPP for an always-on connection. Therefore, moving to IDSL is easy and cost-effective for existing ISDN customers because they do not have to purchase new customer premises equipment-their old equipment can interoperate with IDSL as soon as the line is up. No other DSL type provides that degree of interoperability for ISDN users, who can migrate to IDSL using existing CPE for Internet access, telecommuting and branch office connectivity (Figure 3).

IDSL is also a cost-effective choice for new customers who have never used ISDN. Because the ISDN market is more than a decade old, users can choose from a wide range of existing CPE with different functions and options. And because the technology is mature, ISDN routers are less expensive than other DSL CPE with the same functionality.

The easy DSL migration IDSL is an appropriate DSL choice for several different types of user organizations. For small and medium-sized businesses with Web servers, IDSL provides the symmetrical bandwidth to support both heavy traffic coming into the Web server and outgoing traffic from corporate users. Customers who already have ISDN routers and bridges and don't want to spend more money on CPE are also ideal customers for IDSL. Tapping into the installed base of ISDN users with IDSL opens the market to millions of customers, making the business case for carriers all that more attractive. Finally, IDSL is a good choice for small and remote offices that want to connect to headquarters but don't need the higher bandwidth of other DSL implementations.

As these customers grow, they may need to move to a higher-speed version of DSL. IDSL provides an easy migration path; the service provider reconnects the customer loop to a new high-speed DSL line card at the CO, and the customer switches to the appropriate CPE. With the right type of DSL equipment and support in the CO, users can reap the benefits of migrating seamlessly to other DSL flavors. And carriers can cost-effectively expand the market without expensive truck rolls to user premises to install new equipment.

Carriers that offer IDSL stand to gain hefty benefits, including a broader customer base, a wide range of interoperable CPE and an easy migration path for current end users to a more flexible and profitable service offering.