>> According to the display I have an ADSL2+ connection, yet the package I have been given by O2 is a 12meg connection
adsl2+ is rate adaptive and your line will sync as high as it can up to 24Mb. The longer the line then the more the sync speed will reduce and its only the shortest lines that are able to sync at the full 24Mb.
Its possible to have a long line syncing at 2Mb that could be on adsl2+, since this is the specification of the technology used and the limitation of the line length.
>> 'ADSL(1&2) has a maximum available 223 downstream subchannels. 223 x 56kbps = 12Mbps.' Huh?
>> I am not over bothered about this it just confuses me a little.
adsl1 and adsl2 has sufficient channels for a line to theoretically sync
downstream at up to 12 Mb. However overheads needed for such things as Error Correction and limitations of ATM packet sizes mean that there is a physical limitation of 8Mb set on the line at the dslam.
Theres something called S=1/2 mode which makes the Error Correction more efficient and switching to S=1/2 mode on adsl1 will allow a higher sync. S=1/2 mode combines 2 RS codewords into one making Error correction more efficient with regards to overheads.
Both the router and the dslam have to be able to cope with s=1/2 mode though for this to work... and its the reason why some interleaved lines will sync higher than the 7616 often quoted. However the 8128 is then a restriction set on the dslam which is why it wont sync higher than this for adsl1.
adsl 2 & adsl2+ already make use of s=1/2mode so no point fiddling with that any further if you are already using adsl2 or 2+
Just found
this which explains S=1/2 mode a bit more.
Although in that article it says by using this it yeild up to a maximum 16Mbps for adsl2. Therefore Im not 100% certain why adsl2 is still set at 12Mb.. its very likely they are talking about the ideal world situation and no problems with crosstalk and no neighbouring lines to contend with.
Because of crosstalk certain channels cant be used for downstream.. so when that article refers to 16Mb it could mean
total speed.. because adsl2 allows 12Mb downstream and 3.5Mb upstream on the channel allocation. Add those together and also add in the bands that cant be used (the stop bands and for POTs) then you are looking at 16Mb.
In the UK (and other countries) certain bands just simply cant be used for downstream and masks are applied to ensure that they arent used... or else anyone with a strong downstream signal at those frequencies would simply drown out any neighbouring upstream signals and vice versa.
I believe a very simplified way to look at this is
adsl(1) allows up to 8Mb downstream sync
adsl 2 uses S=1/2 mode to increase the downstream sync to 12Mb
adsl 2+ doubles the available frequencies allowing a maximum downstream sync of up to 24Mb