Variants of ADSL

Annexes
ITU standards often  includes annexes that define how xDSL works under various conditions.  ADSL, or G.992.1, also include annexes.  The most commom annexes are:

• Annex A - ADSL over POTS.  This is the basic ADSL among all other ADSL infrastructure, as we discussed in the previous sections.  It is most common in north america, but also in many other places around the globe.
• Annex B - ADSL over ISDN.  Most common in Europe.  Places who already have ISDN infrastructure installed, or anybody who wants to enjoy the benefits  ISDN capabilities offer in the LAN/telephony system (integrated services ,multiple user channels in a single line, high quality telephony capabilities) , but also connect the LAN to the internet, can choose this configuration. This way,  ADSL Complements ISDN on the “Last Mile”.   For more details see ADSL&ISDN section.
  The implementation is done using frequencies seperators called "splitters", which combines both services at the CO and then split them back at the customer premises.
• Annex C - ADSL over TCM-ISDN.  This is the common infrastructure in japan, in which the existing communication lines are based on a system which called TCM-ISDN, a variation of ISDN.

G.lite: Splitterless ADSL
ITU Recommendation G.992.2 G.lite (there is no ANSI spec)  is quite similar to the basic ADSL (aka G.992.1 G.DMT ) with two distinctions which influence ADSL deployment:  splitterless operation and power management.

• Splitterless operation - standard ADSL operation requires the installation of special filter box, known as a "splitter", at the customer premises.  The splitter will seperate the DSL part of the line for the DSL modem from the part used for telephony services.  This installation requires a technician which will install the splitter in a location away from the modem, typically where the phone wires entered the home.
  G.lite modems however, can operate fine without splitters.  At most some micro filter are needed near individual phones to reduce noise from the DSL and the telephony parts and vice versa (but they are still optional).  The installation is simple and can be done directly by the customer without the need of a technician. The technology behind this feature are fast retrain techniques which allow the DSL modems to change their operating characteristics very quickly in order to adapt to events which might cause interference such as the telephone ringing or someone placing a telephone call.

Figure 1. ADSL filter

• Power management - active DSL modems consume a lot of electrical power while transmitting.  However, the typical customer doesn't transmit data all the time, and the modem stays idle for some long periods of time.  Power management is therefore an effective tool for reducing power, and thus reducing operation costs.  It enables the DSL modem to go into "sleep" mode, similar to the power management tools found in PCs.  The DSL modem can then be "woken up" on demand and resume transmitting data in a very short amount of time. 

G.lite can also extend longer reach then the G.dmt.  However, all of those benefits do have a cost, in the form of a lower bandwidth that G.lite can offer - 1.5Mbps downstream/512kbps upstream in contrast to 8Mbps downstream/864Kbps upstream in G.dmt.  That is one of the reasons why most service providers today prefer the full-rate G.dmt over G.lite.

G.lite contains several Annexes as G.dmt does.  The following table summarizes both G.dmt and G.lite annexes.

RADSL
The Rate-adaptive ADSL is a non standard version of ADSL, in which the DSL modem have the additional capability of adjusting bandwidth to the quality of the phone line not only at the start of the connection but also at any time during the data transmission .
RADSL increases the maximum distance supported from 3.5 to around 5.5km (18000ft) which makes it ideal for suburban neighborhoods.  However, once again we pay the cost of reduced data rate.

Next: Some applicative examples