The new ADSL2+

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A number of ISPs are now introducing ADSL2+ as a high speed option for internet connection.

The ADSL2+ standard is defined by ITU (International Telecommunication Union) as G.992.5.

Speeds.
The standard extends the capability of basic ADSL by doubling the number of downstream bits. By definition, the data rates can be as high as 24 Mbit/s downstream and 3.5 Mbit/s upstream depending on the distance from the DSLAM to the customer's home, and the set up of the ISP.

All ISPs are not deploying the same standard, so check with the various ISPs before signing a lengthy contract.
 
Protocols:

The design standards for ADSL2+ are:

ITU G.992.5 ADSL2+ 24 Mbit/s Downstream  1.0 Mbit/s UpStream
ITU G.992.5 Annex L, otherwise known as RE-ADSL2+, 24 Mbit/s Downstream  1.0 Mbit/s UpStream
ITU G.992.5 Annex M ADSL2+ 24 Mbit/s Downstream 3.5 Mbit/s UpStream

These downstream and upstream rates are theoretical maximums.
Because the ISPs' DSLAMs may have been implemented based on differing or incomplete standards some ISPs may advertise different speeds.

For example, Ericsson has several devices that support non-standard upstream speeds of up to 2 Mbit/s in ADSL2 and ADSL2+.
Despite this some ISPs cap the upstream data rate to 1.024 MHz, whilst other ISPs allow unfettered Data rates which allow upsteam speeds considerably in excess ot this.

Thus, The Data Rate at which you can operate depends very much on your distance from your Local Telephone exchange and your choice of ISP.

Standard ADSL (Sometimes called ADSL1) is capable of a Downstream rate of 8 Mbps and upstream of 1024 kbps. However for those on Telstra DSLAMs the upstream rate is limited to 384 kbps

Annex L allows greater distances to be achieved.

Annex M is now available in Australia, however it is only available from a few ISPs. The maximum upstream rate is to the order of 2.5 MHz.


Speed V Distance:

As mentoned in my other guide, ADSL is a distance-sensitive technology. As the connection's length increases, the signal quality decreases and the connection speed goes down, The effect is even more pronounced with ADSL2+

Your anticipated connection speed can be roughly calculated from the following graph. (Ebay limits the size of the picture here, so copy the picture and expand it in your Viewer). If it does not expand well, since eBay does not allow outside links in this guide,  email me through eBay and I shall send you an address where you can view it full size or try here - medicash2.com/ADSLSpeed.gif  (put the www in front)

Attenuation:

You can obtain your line attenuation from your Modem Status Page.

If you do not have this then:

Channel attenuation is determined by the Wire Gauge Telstra uses in their cable and cross connect construction, distance from the exchange, and the frequencies of the signals sent through the channel.

A very rough guestimate is:
1.0km = 14db
1.5km = 21db
2.0km = 28db
2.5km = 34.5db
3.0km = 41db
3.5km = 48db
4.0km = 56db (The Telstra cut-off point)
4.5km = 62db
5.0km = 69db

Remember that cable distance is not "as the crow flies". The cables have to be routed around streets  and corners and available trenches and paths, and so the actual cable lenghth will be greater than the distance that you see on a map.

Difficulties with ADSL2+:

Most problems incurred are associated with line quality, which is the responsibility of Telstra, and in house wiring or filters.

Here are a few pointers regarding your in house wiring.

It must be remembered that the main reason for filters is impedance matching of the telephone line. Noise reductionis secondary.

At Voice and standard ADSL frequencies, impedance mismatches tend not to be significant.
With ADSL2+ frequencies,however, even relatively short lengths of cable can approach the wavelength of the signal or a significant fraction of it. As in quarter wavelength or half wavelength. The wavelength at 24Mhz, the maximum ADSL2+ frequency, is around 10 metres.

Lengths of cable are often added within buildings for extra outlets, telephones, fax etc with no regard given to correct termination, bridging etc. This did not matter greatly at voice frequencies or even at standard ADSL.

The nominal impedance of the transmission line is 600 ohms, however this is only so if the transmission line is terminated with a load equal to its nominal impedance. For maximum power transfer impedances must match, so simply connecting another wire pair across a transmission line will cause an impedance mismatch and consequent signal deterioration.

On top of this, at ADSL2+ frequencies, extra lengths of cable placed in parallel, with no impedance matching, can act as what are called Stubs or bridge taps..  

Depending on their length, these can effectively short out or attenuate the incoming signal by creating Standing Waves along the transmission line, these unterminated extensions can also act as antennae and introduce RF and other noise into the line.

It must be remembered that a standard telephone connected to a telephone does not terminate the line with its natural impedence - an on-hook telephone appears as on open circuit to the line.
This is why all extensions should be terminated with a filter.

If there are problems, just disconnecting everything is not a conclusive test. In fact it can exascerbate the problem - all extensions must be correctly terminated.

A central filter is often recommended by ISPs when problems are encountered and it is a very efective solution,  but note that a central filter does not introduce anything into the line between the incoming telephone line and the modem - it is a direct line. It simply impedance matches and isolates the existing in-house wiring from the incoming line at frequencies above 4KHz .

The main advantage of a central Filter installation is that a new direct line is installed between the filter and the modem and the line is thus terminated correctly for ADSL2+ frequencies, there are no VSWR problems and maximum power transfer is achieved.

This is also why most ISPs recommend installing a Central filter - it's an easy way to isolate any in house wiring problems from the ADSL circuit.

However it is also a very expensive fix. 

Expect to pay around $200 for supply and installation.

The same result can often be achieved by correctly terminating all extensions.

It is worth noting that Telstra uses two pair cabling for in-house wiring. One pair for the first telephone line and the other pair for a second line or other uses. If you are going to the expense of having a Central Filter installed, do not allow the technician to use this second pair for the ADSL modem. Insist on new cabling being run for the ADSL Modem.

ADSL2+ Filters:
If you are upgrading to ADSL2+, your existing filter may not be satisfactory.

The higher data rates of ADSL2+ is obtained by doubling the number of frequency channels with which your ADSL signal is transmitted.
This involves a corresponding increase in the frequency spectrum that the filter must match.

Normal ADSL filters are not capable of this and the result will be a decrease in the speed of your connection.

This may be small and not really noticeable due to the huge increase which ADSL2+ allows or may be dramatic to the extent that it actually slows your connection to below that which can be obtained with standard ADSL.

Ensure that your filters are certified as ADSL2+ capable, many simply labeled as “ADSL Filter” or “Broadband Filter” are not. A good manufacturer will provide complete details.

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