Kitz Forum
Broadband Related => Broadband Technology => Topic started by: digitalnemesis on May 13, 2016, 09:10:25 AM
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Since 8 bits = 1 byte, does this apply to VDSL2?
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It applies to everything. Sure you weren't querying how many bits can be carried per symbol / Hz though? That's more relevant to VDSL 2 as it's at heart an analogue technology, not a digital one, even if it does carry digital data encapsulated in the analogue signals. :)
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It applies to everything. Sure you weren't querying how many bits can be carried per symbol / Hz though? That's more relevant to VDSL 2 as it's at heart an analogue technology, not a digital one, even if it does carry digital data encapsulated in the analogue signals. :)
Yes how many bits per tone? I'm trying to work out how much sync I could get on a 3dB noise margin.
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I graph I made based off WWWombats rule of thumb.
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Yes how many bits per tone? I'm trying to work out how much sync I could get on a 3dB noise margin.
Each tone can carry a maximum of 15bits. However, not all tones are fully loaded.
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I don't know how it is in VDSL2 but on ADSL2+ if some tone has got one bit more, the synchronization rate is 4 kbps higher.
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Each tone can carry a maximum of 15bits. However, not all tones are fully loaded.
15 bits in a fully loaded tone = how many bytes? 1.875 bytes?
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15 bits in a fully loaded tone = how many bytes? 1.875 bytes?
Yes. So 8 "fully loaded" tones will provide 15 bytes.
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Yes. So 8 "fully loaded" tones will provide 15 bytes.
There are 4096 tones for VDSL2, so if all tones were fully loaded with 15 bits, 1.875 bytes x 4096 = 7680 bytes?
Doesn't seem right. I still don't get it. :lol:
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Ahhh... I get what youre trying to do now. One bit as in bit-loading != 1kbps.
It takes 3dB of SNR to load 1 bit in each carrier bin.
Then the QAM rate determines sync speed. See Bit Allocation (http://www.kitz.co.uk/adsl/adsl_technology.htm#bit_allocation).
The QAM rate is said to be 4,000 symbols per second, therefore each 3dB of SNR available in the sub channel over the base line will give approx 4kbps of sync speed, subject to a maximum of 60kbps (15 x 4kbps) per carrier.
Youre a brave man trying to figure it out for vdsl. I attempted it once on adsl2+ and it worked out more or less correct.. it was an easier calculation for my line as most of the bins were fully loaded.
Vdsl2 has lots of tones that arent in use such as the guard bands and then 4 bands for upstream. Add on PCB and PSD masks which are never going to fully load.... and... well good luck :)
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I don't know how it is in VDSL2 but on ADSL2+ if some tone has got one bit more, the synchronization rate is 4 kbps higher.
That fits with the QAM rate of 4000, I think because VDSL2 uses DMT it will still be the same.
I've never fully looked into DMT v QAM only systems, other than knowing DMT uses a 'narrow' QAM signal per bin.
I'd try to check... well I have tried to check the QAM rate for vdsl, but atm I cant seem to load anything google related. It was working earlier.
C:\Users\kitz>ping google.co.uk
Pinging google.co.uk [212.56.71.40] with 32 bytes of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Ping statistics for 212.56.71.40:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
C:\Users\kitz>tracert google.com
Tracing route to google.com [212.56.71.98]
over a maximum of 30 hops:
1 <1 ms <1 ms <1 ms 192.168.1.1
2 11 ms 11 ms 11 ms lo0.16.Central16.ptw-bng01.plus.net [195.166.130
.217]
3 11 ms 11 ms 12 ms irb.16.ptw-cr02.plus.net [84.93.249.210]
4 * * * Request timed out.
5 * * * Request timed out.
6 * * * Request timed out.
7 * * * Request timed out.
8 * * * Request timed out.
9 * * * Request timed out.
10 * * * Request timed out.
11 * * * Request timed out.
12 ^C
C:\Users\kitz>tracert google.co.uk
Tracing route to google.co.uk [212.56.71.98]
over a maximum of 30 hops:
1 <1 ms <1 ms <1 ms 192.168.1.1
2 11 ms 11 ms 11 ms lo0.16.Central16.ptw-bng01.plus.net [195.166.130
.217]
3 11 ms 11 ms 11 ms irb.16.ptw-cr02.plus.net [84.93.249.210]
4 * * * Request timed out.
5 ^C
C:\Users\kitz>
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Ahhh... I get what youre trying to do now. One bit as in bit-loading != 1kbps.
At 0230 hours this morning I was puzzling over what DN was trying to do and had a suspicion that I had misunderstood . . . even confusing things with my reply! :-[
. . . atm I cant seem to load anything google related. It was working earlier.
Have you discovered Google's "black hole"? Puzzling. ???
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Yes, VDSL2 uses the same symbol rate as ADSL - 4000 symbols per second.
The raw bitrate comes from summing the bit-loading on each tone in the two directions, and multiplying by 4000.
There are roughly 2750 tones used to carry downstream data. If you can add one extra bit to the loading of every single tone, you add (2750*4000) bits per second, or 11Mbps (Hence my rule-of-thumb mentioned earlier).
There isn't really a concept of 8 bits per byte that deep down in the VDSL spec. It is, instead, just a bitstream - a steady stream of bits, allocated to be sent on the tones according to the bit loading allocations.
At higher levels, the packet of user data - somewhere between 64 and 1500 bytes that we understand to be 8 bits per byte - is converted into a bitstream at 8 bits per byte. However, the user data ends up embedded in surrounding overhead control and protection data, and gets encoded in various ways, which ends up padding out the bitstream somewhat. FEC is one such additional overhead, and interleaving is one such form of encoding, though there are more.
The end result is that, for every byte of user data - every 8 bits - you need just over 8 bits in the low-level bitstream being allocated to tones. At best, it might be 8.01 bits in the bitstream for each byte. With high levels of FEC turned on, it could easily reach 9 or 10 bits for each byte.
Unless, of course, you are the lucky recipient of @kitz's data. If it isn't arriving at her modem, it must be appearing as extra overhead on someone else's. ;) I wonder if there's a tagging issue going on, with VLAN tags going wrong somewhere.