High-Bit-Rate Digital Subscriber Line (HDSL)

In 1958, Bell developed a voice multiplexing system that uses the 64-Kbps pulse code modulation (PCM). Using the PCM techniques, voice calls were sampled 8,000 times per second and coded using a n8-bit encoding.

These samples were then organized into a framed format, using 24 time slots to bundle and multiplex 24 simultaneous conversations onto a single four-wire circuit. Each frame carries 24 samples of 8 bits and 1 framing bit, 8,000 times a second.

This produces a data rate of 1.544 Mbps. We now refer to this as a Digital Signal Level 1 (DS-1) at the framed data rate. This rate of data transfer is used in the United States, Canada, and Japan.

Throughout the rest of the world, standards were set to operate using an El with a signaling rate of 2.048 Mbps. The differences between the two services (Tl and El) are significant enough to prevent their seamless integration.

However, in the digital arena, Tl required that the provider install the circuits to the customer's premises. The local provider installs a four-wire circuit. Repeaters are spaced at every 5,000 to 6,000 feet. When installing the Tl on the local loop, limitations of the delivery mechanism get in the way.

Alternate mark inversion (AMI) consumes all the bandwidth and corrupts the surrounding cable spectrum quickly. Consequently, the providers can only use a single Tl in a 50-pair cable. This inefficient use of the wiring makes it impractical to install T1s to small office and residential locations. Further limitations require the providers to remove bridge taps, clean up splices, and remove load coils from the wires to get the Tl to work.

To circumvent these cabling problems, HDSL was developed. HDSL does not require the repeaters on a local loop of up to 12,000 feet. Bridge taps will not bother the service,and the splices are left in place.

This means that the provider can offer HDSL more efficiently for 1.544 Mbps. The modulation rate on the HDSL service is more advanced. Sending 768 Kbps on one pair and another 768 Kbps on the second pairof wires splits the Tl.

Originally, HDSL used two pairs at distances of up to 15,000feet. HDSL at 2.048 Mbps uses three pairs of wire for the same distances. The most recent version, HDSL-2, uses only one pair of wire and is more acceptable to the providers. Nearly all providers today deliver Tl capabilities on some form of HDSL.