High-bit-rate Digital Subscriber Line
tHigh-bit-rate digital subscriber line (HDSL) is a form of digital subscriber line (DSL) developed to reduce the cost of implementing T-1 transmission systems. It is a form of symmetric digital subscriber line (SDSL), which means that it supports the same transmission rate in both directions.
Nobody meant for T-1 and E-1 to have any drawbacks at all. Both were considered state-of-the-art technology for their time. The trouble is, that time was the early 1960s! In fact, only the familiar EIA-232 serial interface is as old as T-1 and E-1 when it comes to telecommunications.
The end electronics have come a long way since the 1960s. Consider the changes in the desktop PC with increased processing power, the low cost and availability of a lot of memory, and overall advances in digital signal processing (DSP) chip sets, which make it easy to massage bits almost any way desired.
The philosophy for the 1980s was more along the lines of “don’t adapt line conditions to end electronics, adapt end electronics to line conditions.” Modern analog modems do their own equalization across the frequency range (called self-equalization) instead of relying on a technician to do it for the whole line (which would be difficult on a dial-up facility!). Echo cancellation to minimize near-end crosstalk (NEXT) circuits were added, and so on. Applying this approach to T-1 and E-1 results in high-bit-rate DSL (HDSL).
A T-1 operates at 1.544 Mbps. They are deployed using two twisted pair (4 wire) with one pair for transmit and the other for receive. Originally, the T-1 used a line code called Alternate Mark Inversion (AMI). Later, when these circuits were used for data as well as voice, the B8ZS line code came into common use. The maximum distance a T-1 can be extended without repeaters is 6,000 feet (6 kilofeet). The T-1 repeaters have to be powered, are typically located in the outside plant, are susceptible to damage and failure, and represent a significant percentage of the cost of deploying the circuit. The digital nature of the T-1 is such that tolerance for bridged taps and load coils on the twisted pair is nonexistent.
HDSL emerged as a strategy for lowering the cost of T-1 deployment by reducing the need for repeaters in the circuit. The goal was to be able to extend the cable plant to 12 kilofeet (kfeet) without a repeater. This is effectively double the repeater separation for the traditional T-1, but when you consider that the first repeater in a T-1 is required to be within 3 kilofeet of a central office (CO) or serving wire center (SWC), the gain is even more significant. A T-1 extending 12 kfeet will need two repeaters. The same service deployed over HDSL requires none.
To achieve these greater distances, modifications needed to be made in the transmission system. The first HDSL technologies used the 2B1Q line code (borrowed from ISDN BRI). This modulation scheme allowed for a bidirectional, 784 kbps transmission rate over a single twisted pair cable. Using two pair in parallel, a full 1.544 Mbps can be achieved. That technology was adequate for deployment in the United States, but was less attractive to parts of the world that use the E-carrier system. The ITU-T standardized a different version implementing the Carrierless Amplitude Phase (CAP) modulation line code, which was capable of reaching 1.168 Mbps on one pair. Using two pair it could support the 2 Mbps rate of the E-1.
HDSL can even tolerate some number of bridged taps, as long as the total length of the bridged taps does not exceed 2500 feet. It cannot, however, tolerate the presence of load coils.
HDSL gave the carriers what they wanted, a lower cost for deploying T-1 circuits. Naturally, changing the T-1 infrastructure would not have been acceptable if it required wholesale change of the customer interface. To avoid this, each end of the HDSL circuit terminates in a device that maps the HDSL signal to T-1. On the subscriber end this is called an HDSL terminal unit - remote user (HTU-R). At the provider end is the HDSL terminal unit - central office (HTU-C). The latter may be integrated into a DSLAM.
To achieve distances greater than 12 kfeet (assuming 24 gauge cable), HDSL repeaters can be used. HDSL supports up to four repeaters for a maximum reach of 60 kfeet (about 20 km). However, because a significant number of T-1s are less than 12 kfeet (e.g., metro-based circuits), HDSL is often dubbed "repeaterless T-1," although the term is not strictly accurate.
Another benefit of HDSL is its comparatively lower bit error rate (BER), when compared to traditional T-1 technology.
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