The idea
Every home already had a copper telephone line, so the cheapest way to deliver broadband was to reuse it. Dial-up squeezed data into the same narrow voice band the phone used and capped out at modem speeds. xDSL asks a different question: why is the line so slow when the copper itself can carry far more?
How it works
The family of xDSL technologies
xDSL is a family of technologies for supporting high-speed digital communication over the existing copper telephone lines. It allows voice and data at the same time. The lecture lists four members:
- ADSL, Asymmetric Digital Subscriber Line
- SDSL, Symmetric Digital Subscriber Line
- HDSL, High-bit-rate Digital Subscriber Line
- VDSL, Very-high-bit-rate Digital Subscriber Line
The lecture also notes there are multiple xDSL technologies and shows an ITU-T/ETSI standards chart. The chart is a figure and does not survive text extraction, so no standard numbers are given here.
How it works
Asymmetric, and where the bandwidth comes from
In asymmetric DSL, the downstream direction (ISP to user) has more bandwidth than the upstream (user to ISP). This reflects what home users do: mostly pull content in. DSL uses the existing telephone lines, the local loop.
ADSL gets a much higher data rate than traditional dial-up because of this chain of facts, all from the lecture:
- A twisted-pair cable can actually handle bandwidths up to
1.1 MHz. - The filter at the end office of the telephone company limits the bandwidth
to
4 kHz, which is sufficient for voice. - If the filter is upgraded, the entire
1.1 MHzis available for data and voice communications.
So the copper was never the bottleneck. The filter was.
How it works
Limiting factors, and data rate versus distance
Three factors limit xDSL performance:
- Line distance. Signal strength degrades with distance.
- Wire gauge. A bigger wire gives less signal attenuation.
- Bridging tap. It causes undesired interference to DSL through an echoed signal.
The lecture follows this with a slide titled “xDSL: Data Rate vs. Distance”, which is a chart comparing the DSL variants. The chart’s numbers are not in the text dump, so this page does not state data rates per distance. Check the slide in the lecture PDF for the figures before the exam.
How it works
DSLAM
In FTTN and FTTB, a DSLAM (Digital Subscriber Line Access Multiplexer) facilitates the use of xDSL at the premises. The lecture describes a “powered” DSLAM that provides electrical and optical conversion and talks to the modems in apartments over WiFi or Ethernet, with Ethernet carried over fibre. See FTTx.
Where marks get lost
4 kHz is the filter, not the wire
A common slip is to say the telephone line only supports 4 kHz. The lecture
says the cable supports up to 1.1 MHz and that the 4 kHz limit is imposed by
the filter at the telephone company’s end office. ADSL’s gain comes from
upgrading that filter.
In the exam
- Expand all four names: A for asymmetric, S for symmetric, H for high-bit-rate, V for very-high-bit-rate.
- Quote both numbers when explaining ADSL versus dial-up:
4 kHzfor the filter,1.1 MHzfor the cable. - Asymmetric means downstream is wider than upstream, and the reason is home-user demand.
- Three limiting factors: line distance, wire gauge, bridging tap, each with its effect.
- The shortfall in the numbers is a source gap, not a fact about DSL. Check the “data rate vs. distance” slide yourself.
Aside
The slide credits ARS Technica for the bandwidth-portion figure and an NTT paper for the G.fast standardisation chart. Neither figure’s values are part of the extracted text, so none are repeated here.
Check yourself
- xDSL reuses the telephone local loop and carries voice and data together.
- The four variants are ADSL, SDSL, HDSL and VDSL.
- Twisted pair handles up to
1.1 MHz, but the end-office filter held it to4 kHz. Upgrade the filter and the full band is available. - Asymmetric means more downstream than upstream.
- Limits are line distance, wire gauge and bridging tap.