Exercise 2.1: General Description of xDSL

From LNTwww

xDSL medium

The need for digital subscriber lines to improve transmission quality and increase customer convenience was recognized as early as the 1970s. After the ISDN specification at the beginning of the 1980s, the actual development and standardization of ${\boldsymbol{\rm xDSL}}$ began, with the "x" as a placeholder for one letter each. A distinction is made between the following system variants:

  • ${\boldsymbol{\rm HDSL}}$ – High–bit–rate Digital Subscriber Line
    First concept in 1986, first prototypes in 1989, first publications by "ANSI" in 1992 (Technical Report E1T1/92-002R1) and "ETSI" (Technical Report ETR 152), final definition in 1998 by "ITU"–Recommendation G.991.1.

  • ${\boldsymbol{\rm ADSL}}$ – Asymmetric Digital Subsciber Line
    Start of conceptual preliminary work in 1989, first prototypes in 1992, first publications in 1995 by ANSI ('Technical Report' T1.413), in 1996 by ETSI ('Technical Report' ETR 328), in 1999 ITU recommendations G.992.1 and G.992.2, further development to ADSL2 (2002, ITU recommendations G.992.3 and G.992.4) and to ADSL2+ (2003, ITU recommendation G.992.5).

  • ${\boldsymbol{\rm VDSL}}$ – Very–high–speed Digital Subscriber Line
    First approaches and preliminary tests in 1994, first publication of specifications in 1998 (ANSI Draft Technical Document T1E1.4/98-043 R1 or ETSI Technical Specification TS 101 270-1), first publication of ITU Recommendation G.993.1 in 2001, and finally publication of ITU Recommendation G.993.2 (further development) in 2006.

The questions for this exercise are limited to basic things such as the question:  "Is the transmission medium copper or air?" According to the model chosen here, both would be possible.


"ISDN"  (Integrated Services Digital Network),
"GSM"  (Global System for Mobile Communications),
"UMTS"  (Universal Mobile Telecommunications System),
"WiMax"  (Worldwide Interoperability for Microwave Access).



What other communication systems have similar function as xDSL?



Which other systems use the same medium as xDSL?



What does "asymmetric" refer to in connection with xDSL?

It says that xDSL is only used on the "last mile".
It makes statements about the interconnection of the dual wires.
It indicates different rates in upstream and downstream.


Which statements are true regarding downstream rates?

VDSL is faster than ADSL.
VDSL(1) is faster than VDSL(2).
The data rate depends on the length of the copper twisted pair.


(1)  Correct are solutions 3 and 4:

  • The essential task of xDSL is to provide high-speed Internet access. In areas with no or inadequate DSL coverage, wireless technology-based "WiMax" is often used as a substitute.
  • ISDN, on the other hand, is a pure telephone service. The main task of the European mobile communications standard GSM is also telephony; data can thus only be transported at a very low data rate.
  • With the UMTS mobile communications standard, data rates of up to $374 \rm kbit/s$ are possible in the downlink (FDD mode), and there is even talk in specifications of $2 \rm Mbit/s$, so that this system could (but need not) be compared to DSL in terms of functionality.

(2)  Correct is the proposed solution 1:

  • DSL is provided over copper twisted pairs. This transmission medium is also used by analog telephone systems (Plain Old Telephone Services - POTS) and the digital fixed network standard "ISDN" (Integrated Services Digital Network) is used.
  • In contrast, the other three (mobile) communications systems are all based on radio technology.

(3)  Correct is the last proposed solution:

  • For example, in ADSL, the data rate to the subscriber is $8 \ \rm Mbit/s$ and in the opposite direction it is only $1 \ \rm Mbit/s$.
  • The reason for this is that the majority of DSL users "download" much more data than they send themselves.
  • VDSL is also asymmetrical with $25$ ... $50 \ \rm Mbit/s$ downstream and $5$ ... $10 \ \rm Mbit/s$ upstream.
  • In contrast, with HDSL - the first proposed xDSL variant - the data rate is the same in both directions.
  • It is true that xDSL is only used in the local loop up to the local exchange, after which a conversion to optical systems takes place. However, this is independent of whether a symmetric or asymmetric xDSL variant is present.

(4)  Correct are solutions 1 and 3:

  • According to the downstream data rate, the following order applies as long as the cable length does not become too long:
HDSL:Between $1.54$ and $2.04 \rm Mbit/s$ symmetrical,
ADSL: Downstream $8 \ \rm Mbit/s$, upstream $1 \ \rm Mbit/s$,
ADSL2+: Downstream up to $25 \ \rm Mbit/s$, upstream $1 \ \rm Mbit/s$,
VDSL(1): Downstream $25 ... 50 \ \rm Mbit/s$, upstream $5 ... 10 \ \rm Mbit/s$,
VDSL(2): Downstream up to $200 \ \rm Mbit/s$.
  • The second statement is incorrect. VDSL(2) is currently (2009) the fastest xDSL connection with up to $200 \ \rm Mbit/s$ according to specification.
  • The systems in Korea and Japan operate at $100 \ \rm Mbit/s$, while in Germany and Denmark data rates of up to $50 \ \rm Mbit/s$ are achieved at somewhat longer ranges (statement also from 2009).
  • The last statement is also true: The shorter the copper line as seen from the subscriber, the faster his DSL can be configured.