An Introduction to DSL (Digital Subscriber Line)

DSL -- High Data Rate Copper Wire Technology

From "Get to know the xDSL family!" by Carina Björklind

"Running data and voice traffic over copper wires is nothing new, but during the 90s new technology has been applied that has made copper into gold - within the existing copper wire network that is used for telephony lies the promise of speed that we used to dream about. ………

"The symmetric branch HDSL: The Grandmother in the xDSL family is HDSL (High-bit-rate Digital Subscriber Line). It was developed in the early 90s as an alternative way to achieve T1 and E1 speeds (1.5 to 2 Mbps). The idea was to apply the same modulation technique as was used in ISDN (2B1Q), split the service on two lines (four wires), increase the bits per baud rate and reduce the frequency spectrum needed.

"All this meant higher speed and longer loop reach. The result was an HDSL-based DS-1 service, specifying loops of up to 3.6km (12,000ft) at a wire thickness of 0.5mm (24 gauge). With time, another modulation technique was introduced, CAP (Carrierless Amplitude Phase modulation) which enabled the use of an even lower range of the frequency spectrum, with less attenuation and longer loop reach. These line codes have been endorsed in technical reports by both ANSI and ETSI's HDSL standards committees.

"Further development of HDSL has resulted in a Daughter, Single pair HDSL, which is known under the generic name of SDSL, Symmetric Digital Subscriber Line. The standard on SDSL has just been completed.

 "The trade-off between the four wires of HDSL and the two wires of SDSL is loop reach, but the difference is so small (3.6km to 3.4km) that it is almost a non-issue. Another name for SDSL is HDSL-2, and MDSL is a proprietary variation on SDSL.

"The asymmetric branch ADSL: The other branch of the family tree, the asymmetric applications, was originally developed for video purposes. By experimenting with maximizing loop reach and various line codes it was evident that it was possible to send information at a higher speed from the CO to a remote office or home, than in the opposite direction. This research resulted in the Grandfather, ADSL, Asymmetric Digital Subscriber Line. In the late 80s and early 90s, cable TV operators were moving in on the telecom market, and the telecom market wanted to even the odds.  With time, it became evident to the access industry that many data applications were also asymmetric, with the Internet being a typical example. A request to download will result in huge amounts of data being sent the other way."

 
Some technical details: Of all the xDSL offerings, HDSL probably has the largest installed basebecause it was the first DSL technology to be invented . The technology arose from carriers' problems in extending broadband speeds -- specifically T1(1.544 Mbps) and E1 (2.048 Mbps) services -- over long copper loops.  Because long copper loops distort signal quality, repeaters or amplifiers are installed on copper pairs at prescribed intervals to restore signal quality. In today's T1/E1 networks, they must be installed about every 3,000 to 4,000 feet -- a time consuming and expensive process.

In the late 1980s, Bellcore began research into a new method of T1 and E1 provisioning that would eliminate repeaters and simplify the overall deployment of high bandwidth networks for the so-called "last mile" into the home or office. The technology, called HDSL, was designed to deliver traditional T1/E1 services over unconditioned wires by placing transceivers on each end of two or three twisted pair.

However, HDSL is also being modified to work over a single copper phone line to provide direct premises connections between a customer and its serving central office. The penalty for operating over one pair is performance; current implementations top out at either 384 Kbps or 768 Kbps symmetrical speeds.

In contrast to ISDN, ADSL adopts a passband approach. High and low pass filters divide the line into three frequency channels: bidirectional analog POTS, narrow digital upstream, and wide digital downstream. This technique, which is referred to as Data Over Voice, enables compatibility with existing POTS equipment.

Attenuation vs Frequency	Attenuation (db/100m)	Attenuation (db/100m)	(Attenuation (db/100m)
Frequency (MHz)	Cat 3 UTP	Cat 5 UTP	150W STP
1	2.6	2.0	1.1
4	5.6	4.1	2.2
16	13.1	8.2	4.4
25	-	10.4	6.2
100	-	22.0	12.3
300	-	-	21.4

Limitations of Twisted Pair for  higher frequency signals:

1. skin effect is  main limitation of TP 
- increasing frequency ‡ current flows only on outer-surface of
   wire
- uses less of available cross-section ‡ increases resistance ‡
  higher attenuation

2. @ higher frequencies more signal power is lost to radiation   
    effects  (source)    

(
           

DSL -- Spectral Considerations:

To create multiple channels, ADSL modems divide the available bandwidth of a telephone line in one of two ways -- Frequency Division Multiplexing (FDM) or Echo Cancellation.  FDM assigns one band for upstream data and another band for downstream data. The downstream path is then divided by time division multiplexing into one or more high speed channels and one or more low speed channels. The upstream path is also multiplexed into corresponding low speed channels. Echo Cancellation assigns the upstream band to over-lap the downstream, and separates the two by means of local echo cancellation, a technique well know in V.32 and V.34 modems. With either technique, ADSL splits off a 4 kHz region for POTS at the DC end of the band.

An ADSL modem organizes the aggregate data stream created by multiplexing downstream channels, duplex channels, and maintenance channels together into blocks, and attaches an error correction code to each block. The receiver then corrects errors that occur during transmission up to the limits implied by the code and the block length. The unit may, at the users option, also create superblocks by interleaving data within subblocks; this allows the receiver to correct any combination of errors within a specific span of bits. This allows for effective transmission of both data and video signals alike.

DSL -- System Topology:

Comparison of DSL Technologies

 

DSL Comparison (Source)

DSL Type	Description	Data Rate	Distance Limit	Application
IDSL	ISDN Digital Subscriber Line	128 Kbps	18,000 feet on 24 gauge wire	Similar to the ISDN BRI service but data only (no voice on the same line)
CDSL	Consumer DSL from Rockwell	1 Mbps downstream; less upstream	18,000 feet on 24 gauge wire	Splitterless home and small business service; similar to DSL Lite
DSL Lite (same as G.Lite)	"Splitterless" DSL without the "truck roll"	From 1.544 Mbps to 6 Mbps downstream, depending on the subscribed service	18,000 feet on 24 gauge wire	The standard ADSL; sacrifices speed for not having to install a splitter at the user's home or business
G.Lite (same as DSL Lite)	"Splitterless" DSL without the "truck roll"	From 1.544 Mbps to 6 Mbps , depending on the subscribed service	18,000 feet on 24 gauge wire	The standard ADSL; sacrifices speed for not having to install a splitter at the user's home or business
HDSL	High bit-rate Digital Subscriber Line	1.544 Mbps duplex on two twisted-pair lines; 2.048 Mbps duplex on three twisted-pair lines	12,000 feet on 24 gauge wire	T1/E1 service between server and phone company or within a company; WAN, LAN, server access
SDSL	Symmetric DSL	1.544 Mbps duplex (U.S. and Canada); 2.048 Mbps (Europe) on a single duplex line downstream and upstream	12,000 feet on 24 gauge wire	Same as for HDSL but requiring only one line of twisted-pair
ADSL	Asymmetric Digital Subscriber Line	1.544 to 6.1 Mbps downstream; 16 to 640 Kbps upstream	1.544 Mbps at 18,000 feet; 2.048 Mbps at 16,000 feet; 6.312 Mpbs at 12,000 feet; 8.448 Mbps at 9,000 feet	Used for Internet and Web access, motion video, video on demand, remote LAN access
RADSL	Rate-Adaptive DSL from Westell	Adapted to the line, 640 Kbps to 2.2 Mbps downstream; 272 Kbps to 1.088 Mbps upstream	Not provided	Similar to ADSL
UDSL	Unidirectional DSL proposed by a company in Europe	Not known	Not known	Similar to HDSL
VDSL	Very high Digital Subscriber Line	12.9 to 52.8 Mbps downstream; 1.5 to 2.3 Mbps upstream; 1.6 Mbps to 2.3 Mbps downstream	4,500 feet at 12.96 Mbps; 3,000 feet at 25.82 Mbps; 1,000 feet at 51.84 Mbps	ATM networks; Fiber to the Neighborhood

References:

Connecting Homes to the Internet: An Engineering Cost Model of Cable vs. ISDN: access to Sharon Eisner Gillet's excellent thesis (ps).

Digital Subscriber Lines and Cable Modems: by Paul Sabatino of Ohio State.

ADSL Tutorial: Twisted Pair Access to the Information Highway

ADSL: Speeding up the local loops

Line Simulator (LiSi) for Asymmetric and Very High-Speed Digital Subscriber Line

Twisted Pair Goes Supersonic

ADSL2 boosts data rate and reach