science

High-bit-rate digital subscriber line

High-bit-rate digital subscriber line is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand High-bit-rate digital subscriber line rather than just read about it. In short: High-bit-rate digital subscriber line (HDSL) is a telecommunications protocol standardized in 1994. It was the first digital subscriber line (DSL) technology to use a higher frequency spectrum over copper, twisted pair cables.

Key takeaways

High-bit-rate digital subscriber line belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect High-bit-rate digital subscriber line to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of High-bit-rate digital subscriber line from memory before moving on to harder problems.

Reference excerpt

High-bit-rate digital subscriber line (HDSL) is a telecommunications protocol standardized in 1994. It was the first digital subscriber line (DSL) technology to use a higher frequency spectrum over copper, twisted pair cables. HDSL was developed to transport DS1 services at 1.544 Mbit/s and 2.048 Mbit/s over telephone local loops without a need for repeaters. Successor technology to HDSL includes HDSL2 and HDSL4, proprietary SDSL, and G.SHDSL.

Standardization HDSL was developed for T1 service at 1.544 Mbit/s by the American National Standards Institute (ANSI) Committee T1E1.4 and published in February 1994 as ANSI Technical Report TR-28. This American variant uses two wire pairs with at a rate of 784 kbit/s each, using the 2B1Q line code, which is also used in the American variant of the ISDN U interface. First products were developed in 1993. A European version of the standard for E1 service at 2.048 Mbit/s was published in February 1995 by the European Telecommunications Standards Institute (ETSI) as ETSI ETR 152. The first edition of ETR 152 specified the line code 2B1Q on either three pairs at 784 kbit/s each or two pairs at 1,168 kbit/s each. A second edition of ETR 152, published in June 1995, specified trellis coded carrierless amplitude/phase modulation (CAP) as an alternative modulation scheme, running on two pairs at 1,168 kbit/s each. A third version of ETR 152, published in December 1996, added the possibility of using a single CAP-modulated pair at 2,320 kbit/s. Later, an international HDSL standard was published by Study Group 15 of the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T) on 26 August 1998 and adopted as recommendation ITU-T G.991.1 on 13 October 1998.

Comparison to legacy T1 Legacy T1 carriers operated using the alternate mark inversion (AMI) line code, more recently also B8ZS, on two wire pairs. Each wire pair was operated in simplex, that is, one wire pair was used for transmission in each direction. The Nyquist frequency of a 1.544 megabaud signal is 772 kHz. Higher frequencies are attenuated more strongly than lower frequencies, motivating the use of technologies that reduce the signal bandwidth. In HDSL, full duplex by means of echo cancellation is used, enabling simultaneous transmission in both directions on each of the two wire pairs, effectively reducing the symbol rate by a factor two. Through the use of 2B1Q encoding, two bits are combined to one symbol, further reducing the symbol rate by a factor of two. For this two-pair 2B1Q variant of HDSL, framing increases the bitrate from 1.544 Mbit/s to 1.568 Mbit/s, resulting in a symbol rate of 392 kilobaud and a Nyquist frequency of 196 kHz. Legacy T1 required repeaters every 35 dB of attenuation, equivalent to 1 to 1.2 miles (1.6 to 1.9 km), depending on conductor gauge and other circumstances. Originally marketed as "non-repeated T1", HDSL increased the reach to 12,000 feet (3.7 km) on an AWG24 local loop. To enable longer HDSL lines, up to four repeaters can be used for a reach of 60,000 feet (18 km).

Comparison to other DSL variants Unlike ADSL, HDSL operates in the baseband and does not allow POTS and ISDN to coexist on the same wire pairs. Unlike ADSL, the proprietary SDSL, and G.SHDSL, HDSL is not rate adaptive: the line rate is always 1.544 Mbit/s or 2.048 Mbit/s. Lower rates at multiples of 64 kbit/s are offered to customers by using only a portion of the DS0 channels in the DS1 signal, referred to as channelized T1/E1. HDSL gave way to new symmetric DSL technologies, HDSL2 and HDSL4, the proprietary SDSL, and G.SHDSL. HDSL2 offers the same data rate over a single pair of copper; it also offers longer reach, and can work over copper of lower gauge or quality. SDSL is a multi-rate technology, offering speeds ranging from 192 kbit/s to 2.3 Mbit/s, using a single pair of copper.

References

External links Gareth Marples (11 September 2008). "The History of DSL Internet Access – A Race for Technological Speed". {{cite web}}: |access-date= requires |url= (help)CS1 maint: deprecated archival service (link) Sean Martin and Alan G. Hutcheson. "High-bit-rate Digital Subscriber Line (HDSL)". ADC Telecomm. Retrieved 23 June 2013. Joseph W. Lechleider (August 1991). "High Bit Rate Digital Subscriber Lines: A Review of HDSL Progress". IEEE Journal on Selected Areas in Communications. 9 (6): 769–784. doi:10.1109/49.93088.

Worked examples

Example 1 — a first encounter with High-bit-rate digital subscriber line

Start with the simplest possible case. Write down what High-bit-rate digital subscriber line claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to High-bit-rate digital subscriber line before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about High-bit-rate digital subscriber line ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of High-bit-rate digital subscriber line

In research: High-bit-rate digital subscriber line appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses High-bit-rate digital subscriber line in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: High-bit-rate digital subscriber line is common in secondary-school and first-year university syllabi. It links to neighbouring topics Digital subscriber line, ITU-T G Series Recommendations, ITU-T recommendations, so understanding it makes those chapters shorter.
In everyday life: Look for High-bit-rate digital subscriber line outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study High-bit-rate digital subscriber line in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what High-bit-rate digital subscriber line means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain High-bit-rate digital subscriber line out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is High-bit-rate digital subscriber line in simple terms?

Why does High-bit-rate digital subscriber line matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study High-bit-rate digital subscriber line?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on High-bit-rate digital subscriber line.