The Applications of HDSL

The first such application involves services provided by the phone companies, generically labeled public network access or data network access. This involves harnessing the power of a T1/E1 line, for business or residential uses, to move voice, data, or video traffic. The local phone company provides that service and charges you for it, based on applicable local tariffs.

In addition to the so-called public applications of HDSL, there is a burgeoning opportunity for HDSL to add value in the private sector, mainly as an augmentation to emerging enterprise networks . In this setting there is a natural role for HDSL to play.

Outside the telco setting, HDSL is becoming increasingly effective in campus area networks. In this context, a campus is defined as a setting where multiple locations or buildings are located a few miles or kilometers apart, a single building with multiple floors or a single building spread over an expanse of real estate (e.g.: a large manufacturing facility). The implied commonality in any of these campus situations in an embedded copper cable plant.

Traditionally, campus area networks are prevalent in a number of industries, including local, state and federal governments and utility companies. In addition, HDSL has been particularly well accepted within hospital, university/college, and military bas settings. HDSL is being used, and is especially well-suited, for a number of specific campus applications, including:

• LAN connectivity/extension - allowing networks and users in different locations to easily exchange data.
• PBX network/channel extensions - to distribute voice traffic from a centralized PBX to users throughout the campus.
• Video conferencing/distance learning - enabling multiple video hook-ups, allowing a subject at one site to be seen and interact with people at various distributed sites.
• Fiber backbone extension - using HDSL and traditional copper circuits to connect with high-speed fiber optic backbones across a campus network.

Fiber access
In many larger metropolitan areas, cities have gone to great lengths, and spent millions of dollars to install vital fiber backbones using SONET (Synchronous Optical Network) in the U.S., or SDH (Synchronous Digital Hierarchy) in countries where E1 is the standard. SONET/SDH provides gigabytes of available bandwidth for mission-critical applications. Due to cost and logistical factors, the scope of these backbones is limited, with only a relatively small percentage of sites being able to tap directly into the backbone. There often remains a great many associated city buildings and end users that could benefit from access to this fiber, but direct fiber connection is prohibitive. This situation is a perfect fit for HDSL.

With HDSL, satellite locations can be linked quickly, transparently and inexpensively, using existing copper to produce fiber optic-quality transmission (BER 10^-10).

Traditionally, fiber is employed within a campus area network on a more conservative basis, usually being used to connect a small number of buildings or to connect critical users at just one site. In this environment, FDDI (Fiber Distributed Data Interface, a network backbone technology that uses a fiber ring to connect critical resources and transmitting at 100 mbps) has gained significant acceptance. The most convenient and versatile way to give users outside the scope of the FDDI ring access to fiber is through HDSL. Again, users can use existing copper cable plants to link buildings and connect to the central routes that enable access to the fiber ring. The cost to accomplish this is small, roughly 80 percent less than that of fiber, and can virtually happen in under one hour.

Other campus area network HDSL applications
HDSL provides a unique capability for campus area connectivity in several other scenarios. First, it can provide instant line-speed enhancement - from 56 kbps to 2.048 mbps, 30 times faster - for building-to-building connection. Second, HDSL can significantly extend the potential operating distances between sites. Connectivity using DSU/CSU connection or T1/E1 line drivers is limited to 4,000 feet (1.2 km) and 2,000 feet (.6 km) respectively. With HDS, you can facilitate T1/E1 connections over four times the distance with no repeaters. And finally, HDSL is ideal for system redundancy and disaster recovery. In mission-critical situation using fiber or another medium, copper-based HDSL provides a natural back-up, with the capability to instantly get networks up and running when fiber links fail.